Mechanics in Steels through Microscopy

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Mechanics in Steels through Microscopy

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 25

th

februari 2013

om 12:30 uur

door

Ganesh Kumar TIRUMALASETTY

Master of Science

Technical University of Munich, Germany

geboren te Hyderabad, India.

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Dit proefschrift is goedgekeurd door de promotor: Professor Dr. H.W. Zandbergen

Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Professor Dr. H.W. Zandbergen, TU Delft, promotor Professor Dr. Ir. J. Sietsma, TU Delft

Dr. Ir. M.A. van Huis, Universiteit Utrecht

Professor Dr. Ing. Habil. D. Raabe, Max-Planck-Institut fuer Eisenforschung Professor Dr. Ir. B.J. Kooi, Universiteit Groningen

Professor Dr. Ir. R. Benedictus, TU Delft Dr. Ir. P. van der Wolk, Tata Steel

This research was carried out under project number MC5.06280a of the Materials innovation institute M2i (www.m2i.nl).

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the author.

ISBN: 978-90-77172-90-2

Printed in the Netherlands by Ipskamp Drukkers, Enschede. A free electronic version of this thesis can be downloaded from: http://www.library.tudelft.nl/dissertations

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§1 Introduction

1.1 State of art 1

1.2 Microstructure-Mechanical properties relationships 4

1.3 Scope of the research 6

§2 Experimental materials and techniques involved

2.1 Materials and processes 11

2.2 Mechanical properties and Testing 23

2.3 X-ray Diffraction 24

2.4 Optical Microscopy 26

2.5 Scanning Electron Microscopy and Electron Back Scattered Diffraction 28

2.6 Electron Probe Micro Analysis 30

2.7 Transmission Electron Microscopy 32

§3 Ductility enhancement through grain rotations

and transformations

3.1 Introduction 42

3.2 Experiments and testing procedures 43

3.3 Results and discussions 45

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§4 Engineering transformations inducing plasticity via local

chemical elements

4.1 Introduction 69

4.4 Conclusions 87

§5 Strengthening by Nano precipitates

5.1 Introduction 91

5.4 Conclusions 109

§6 Strengthening by Ultrafine precipitates

6.1 Introduction 113

6.4 Conclusions 131

§7 Quantum mechanics beneath novel crystal structures

7.1 Introduction 135

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Summary

165

Samenvatting

169

Acknowledgements

175

Publications

177

Curriculum Vitae

179

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Chapter 1 1.1 State-of-the-art advanced high-strength steels

Ever increasing fuel costs, alarming levels of environmental pollution and improved safety regulations have fostered the demand for stronger, tougher, and lighter materials in the automotive, shipping, military, chemical, and aerospace industries. Compared to other materials such as aluminium, magnesium, plastics and composites, steels with higher strengths can lead to significant weight reductions due to the design considerations, and have the advantage that they can be produced on existing production lines, which are similar to those of conventional mild steels. Therefore the overall manufacturing costs will not increase in combination with the benefit of weight savings, thus avoiding the main drawback of all other competing materials [1].

Figure 1: (a) Application of TRIP steel for the high-strength skeleton of an

automobile [i1] (b) B-pillar reinforcement of TRIP steel [i2].

Currently, steels of higher strength are produced with a microstructure consisting of at least two different phases. These so-called multi-phase steels offer attractive combinations of strength and ductility as a result of the excellent mechanical properties of their microstructural components and their interaction. The most prominent multi-phase steels are dual-phase (DP), twinning-induced plasticity (TWIP), and transformation-induced plasticity (TRIP) steels.

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§1 Introduction 2

Dual-phase steels contain two phases with hard grains of a metastable phase called martensite embedded in a soft ferrite matrix [2]. TWIP steels, on the other hand, contain only a single metastable phase called austenite. During deformation, the austenite phase turns into martensite, accompanied by the formation of deformation twins, giving this steel class its name [i3]. In contrast, TRIP steels contain multiple phases with metastable phases called austenite and bainite within a soft ferrite matrix. The austenite phase is retained at room temperature by chemical alloying and thermo-mechanical heat treatments. During deformation, the austenite phase transforms to martensite. The transformation of retained austenite into martensite leads to “transformation-induced plasticity”, which causes extended steady-state work hardening, postponement of necking and unstable deformation [3]. This transformation allows TRIP steels to have a high formability, while retaining excellent strength. This unique combination of ductility, strength and low costs compared to TWIP steels make them an excellent candidate for the body-in-white (the high-strength skeleton) of automobiles and several automotive structural parts as shown in Figure 1.

Figure 2: Schematic strength-elongation range for high-strength steels.

In the processing of steels, a compromise must generally be made between strength and ductility. Figure 2 shows the compromise between strength and elongation for four steel types. The advantage of TRIP steels is that they have much higher ductility than other conventional steels of similar strength. The ductility and strength of TRIP steels makes them an excellent candidate for automotive applications. Indeed, structural components can be made much thinner because TRIP steels have the

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ductility necessary to withstand high-deformation processes such as stamping, while featuring the strength and energy absorption characteristics to meet safety regulations [i4].

Figure 3: Cluster diagram showing the benefits of TRIP steel to the customer, the

manufacturer and the environment.

In general, approximately 55 percent of the mass of an average passenger car is made of steel [i5]. Replacing mild steel with TRIP steel could reduce the weight by 20 percent while maintaining the same stiffness [4]. In other words, the use of TRIP steel could reduce the mass of steel on a vehicle by about 20 percent, and the total vehicle mass by about 11 percent [i4,i6]. This means that using TRIP steels could lead to a reduction in total fuel consumption of 0.64 percent. Considering the above estimate,

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§1 Introduction 4

carbon dioxide emissions of the United States alone could be reduced by 4.18 billion kg or 0.07% of the carbon dioxide emissions worldwide [i4,i6-i8]. Figure 3 shows a cluster diagram highlighting the benefits of TRIP steel to the customer, the manufacturer and the environment. In the automotive industry, TRIP steels are now used in several structural and safety parts such as doors and bumpers [i9], cross members, longitudinal beams, B-pillar reinforcements, sills and bumper reinforcements. To integrate these steels at the design stage, studies based on modeling and experimental tests are currently being carried out [i10].

1.2 Microstructure-mechanical property-application relationships

The core of materials science relates the desired properties and relative performance of a material in a certain application to the structure of the atoms and phases in that material [i11]. The key factors that determine the structure and thus the properties of a material are its constituent chemical elements and the way in which it has been processed into its final form. These factors, along with the laws of thermodynamics and kinetics, govern the material’s microstructure, and thus its properties and performance. Real materials contain defects such as vacancies, dislocations, interstitial atoms, substitutional atoms, precipitates and grain boundaries. These defects can be utilized to create materials with the desired properties [5].

Mechanical properties such as strength, ductility, hardness and wear resistance, which determine the suitability for a given application, are dependent on the microstructure and defects within the material. Macroscopic deformation takes place through dislocation movement and dislocation multiplication during plastic deformation [5]. Restricting the movement of these dislocations will lead to an increase in yield and tensile strength of the material. There are several ways of restricting the dislocations such as increasing the density of dislocations (work hardening) [4], reducing the grain size (grain-boundary strengthening), introducing a second phase (precipitation hardening), introducing interstitial or substitutional atoms (solid-solution strengthening) or utilizing phase transformation (transformation strengthening).

Thorough characterisation is therefore essential to probe the internal structure in order to depict the properties and performance of the material [i12]. Several analysis

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techniques can be used to image a material’s internal structure down to the atomic scale in order to determine the distribution of elements and to investigate their possible interactions.

Figure 4: (a)-(e) Microstructure-mechanical properties-application [i1] relationships.

X-ray diffraction (XRD) provides quantitative insights into the presence of known and unknown phases, preferred grain orientations and stresses within the materials at a bulk level [6]. At the microstructural or nanostructural level, materials are often analysed using microscopes. Optical microscopy with image analysis provides an overview of the microstructural aspects such as grain size, grain shapes and phase fractions [7]. Scanning electron microscopes (SEMs) or electron back-scattered diffraction (EBSD) are used at higher resolutions to reveal microstructural information such as grain boundary characteristics, twins, grain size distribution, orientation relationships, phase transformations and local chemical composition [8].

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§1 Introduction 6

Transmission electron microscopes (TEMs) provide extremely detailed information at a very high resolution, allowing studies such as structural and chemical characterisation of nanoprecipitates, analysis of defects including dislocations, stacking faults, and orientation relationships of grain boundaries [5].

For instance, TRIP steels at micrometer level consists various phases such as austenite (A), ferrite (F) and bainite (B) and these can studied effectively with the help of an SEM/EBSD microstructure as shown in Figure 4a. However, nano-sized strengthening precipitates existing within the ferritic matrix (F) require higher-resolution microscopes such as TEM to understand their sizes, shapes and distributions. Dark field TEM image thereby helps in understanding nano-sized precipitates in ferrite as displayed in Figure 4b. Further, atomic-sized strengthening precipitates within ferrite can also be studied using an atomic resolution TEM image as shown in Figure 4c. In addition, orientation relationships between the atomic-sized precipitates indicated in green and the ferritic matrix indicated in orange can be investigated using this technique. The mechanical stress-strain response of TRIP steel under uniaxial tension in Figure 4d is a consequence of composite deformation of the A, F and B phases and the nano and atomic-sized precipitates that have a direct influence on the automotive application of TRIP steel as shown in Figure 4e.

In addition, systematic microstructural investigations can very much benefit from computational methods based on condensed matter theory, including alloy theory. For instance, in case of Fe(C) precipitates which are extremely small in size and coherent with the matrix as shown in Figure 4c, experimental difficulties will arise in complete structural characterization. No accurate information can be directly be obtained about Fe stacking and C ordering, etc. partially due to the surroundings and the nano-sizes of the precipitates. Additionally, chemical characterization of precipitates is difficult since ferrite which is matrix phase also contains iron (Fe) and carbon contamination in electron microscopes limits an accurate estimate of carbon (C) in the carbide. In this aspect, theoretical methods, especially parameter-free first-principles methods are very helpful. Density functional theory (DFT) calculations provide information about the crystal structures, chemical bonding, optimum stoichiometry, and relative stability of crystals at the ground state [9,10]. [i11]. Understanding the microstructure – both from experiment and calculations – helps not

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only in establishing structure-mechanical property relationships but also aids in optimizing the processing parameters. Therefore, materials with superior mechanical properties can be developed with less optimization cycles and the time-to-market of the material can eventually be reduced.

1.2 Scope and outline of the thesis

Both in everyday applications and in a plethora of industrial applications (automotive, aerospace, military, chemical and biomedical industries, etc.), our society relies heavily on many different kinds of steel products. Major advances in the understanding and design of materials is therefore of direct and eminent importance to the global economy. The recent development of TRIP (transformation-induced plasticity) steels shows that the scientific development of steels is still progressing rapidly, and judicious experiments are required to understand why and how materials can be manipulated efficiently to obtain optimal tailor-made properties.

Chapter 2 gives an overview of steel metallurgy with particular emphasis on TRIP-assisted steels. It also summarises the characterisation techniques used in this study.

Despite the fact that steels are amongst the most extensively investigated materials, many features of the martensitic transformation process during deformation have yet to be unclarified. Therefore, uniaxial straining experiments were performed on a TRIP steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. Chapter 3 shows the transformation behavior of individual metastable austenite grains, both at the surface and inside the bulk of the material, using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD). The deformation-induced changes of individual austenite grains before and after straining were further analyzed with EBSD.

In addition to the austenite-to-martensite phase transformations, the role of alloying elements such as C, Mn, Si, and Al, and their effect on transformations during deformation is poorly understood and there is an increasing need to gain fundamental insight into this topic. Chapter 4 shows a novel approach that combines electron back-scattered diffraction (EBSD) and high-sensitivity electron probe microanalysis (EPMA) used to correlate the changes in microstructural features upon deformation

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§1 Introduction 8

with the local chemical composition. A new cleaning procedure was developed that allowed the completely artifact-free cleaning of the carbon contamination, which is necessary for obtaining interpretable EBSD patterns and reliable local carbon contents. With this experimental procedure, monitoring of the austenite-to-martensite transformation and deformation processes was performed in terms of the local crystal structure, microstructure and chemical composition.

Multiphase steels utilizing composite strengthening may be further strengthened via grain refinement or precipitation by adding microalloying elements. Chapter 5 details the study of Nb microalloyed steel comprising martensite, bainite and retained austenite. By means of transmission electron microscopy (TEM), we investigated the size distribution and the structural properties of novel nano-sized (Nb,Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. The lattice parameters, formation energies, and energetically most favourable stoichiometry of the formed precipitates in these steels were examined by means of atomistic simulation using quantum mechanical density functional theory (DFT) calculations. The role of these precipitates in strengthening the steels will be discussed.

Furthermore, TEM experiments were conducted on precipitates formed in Ti microalloyed TRIP-assisted steels. Chapter 6 reveals the presence of Ti(N), Ti2CS,

and a novel type of ultrafine Fe(C) precipitates in the ferritic matrix. The matrix/precipitate orientation relationships, sizes, shapes, and their effect on the strengthening of steel will be discussed in detail.

Finally, in Chapter 7 we revealed two novel ternary carbides of Cr and Mn that have not been reported previously. Electron diffraction experiments showed that Cr and Mn have formed two distinct crystal structures possessing orthorhombic and monoclinic symmetries. Density functional theory calculations were carried out and excellent agreement was found between calculations and experiments on the lattice parameters and relative atomic positions. The calculations showed that both Mn and Cr have led to these ternary carbides featuring very high thermodynamic stability and local structural relaxations associated with the addition of carbon. Possible implications of

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the novel applications of these carbide phases will be discussed. Figure 5 summarizes the materials, methods and physics theories treated in this thesis.

Figure 5: Tree structure schematic showing the contents of this thesis.

References:

[1] Hulka, K., The Role Of Niobium In Multi-Phase Steel.

[2] Smith W.F. Structure and Properties of Engineering Alloys, second ed., McGraw-Hill, New York, 1993.

[3] Nishiyama, N. Martensitic Transformation Ch 4 (Academic press. New York, 1978).

[4] W. Li & Al., “Application of TRIP Steel to Replace Mild Steel in Automotive Parts”, International Conference on Advanced High Strength Sheet Steels for Automotive Applications Proceedings, 31-36, June 2004

[5] Dieter GE, Mechanical Metallurgy, third ed., New York: McGraw-Hill; 1988. [6] Cullity, BD Elements of X-Ray Diffraction, Addison Wesley, Reading,

Massachusetts; 1956.

[7] Tirumalasetty, G. K., Fang, C.M., Xu Q., Jansen, J., Sietsma J., van Huis, M.A., Zandbergen H.W., (2012), Acta Materialia, 60, 7160–7168.

[8] Tim Maitland and Scott Sitzman, Electron Backscatter Diffraction (EBSD) Technique and Materials Characterization Examples.

[9] C. M. Fang, M. A. Huis and H. W. Zandbergen, Scr. Mater. 63 (2010)418. [10] C. M. Fang, M. A. Huis and H. W. Zandbergen, Scr. Mater. 64 (2011) 296.

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§1 Introduction 10

Internet References:

[i1] http://wallpaperstock.net/ [i2] http://www.msm.cam.ac.uk/phase-trans/2005/TRIP.steels.html [i3] http://www.worldautosteel.org/steel-basics/steel-types/twinning-induced-plasticity-twip-steel/ [i4] http://www.appropedia.org/Transformation_Induced_Plasticity_%22TRIP%22 _Steel [i5] http://www.drivingtoday.com/wcco/news_this_week/1999-09-15-306-driving/index.html [i6] http://www.fueleconomy.gov/FEG/atv.shtml [i7] http://www.bts.gov/publications/national_transportation_statistics/html/table_ 01_11.html [i8] http://www.google.com/publicdata?ds=wb-wdi&met=en_atm_co2e_pc&idim=country:USA&q=us+carbon+dioxide+emi ssions [i9] http://www.posco.com/homepage/docs/eng/html/company/product/s91e60100 10c.html [i10] http://www.arcelormittal.com/automotive/saturnus/sheets/B_EN.pdf [i11] http://en.wikipedia.org/wiki/Materials_science [i12] http://en.wikipedia.org/wiki/Characterization_(materials_science) [i13] http://www.metalravne.com/selector/help/testing/lm.html

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Chapter 2

Abstract

Soaring fuel costs, increased environmental concerns and improved safety regulations have fostered the demand for stronger and tougher materials in the automotive, shipping, military, chemical and aerospace industries. TRIP steels are a promising group of cold formable steels that combine very high ductility and strength. The mechanical properties in these steels are a result of a complex multiphase microstructure consisting of predominantly ferrite grains with a dispersion of bainite, martensite and retained austenite grains. During deformation, the metastable-retained austenite grains transform into martensite, which induces the “transformation-induced plasticity” (TRIP) effect. Understanding the microstructure and the changes in the local chemical concentrations, which determine the stability of retained austenite, is of key importance for establishing structure-property relationships. Furthermore, characterising novel nano-scale and ultrafine precipitates helps assess their strengthening response and thus helps in optimizing the processing parameters. This work gives an overview of steel metallurgy with particular emphasis on TRIP-assisted steels. Moreover, it summarises characterisation techniques used for mechanical testing and microstructural assessment such as tensile testing, x-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM-EBSD), electron probe micro analysis (EPMA) and transmission electron microscopy (TEM).

2.1 Materials and processes

Iron or ferrum is a chemical element signified by the symbol Fe. It is the most common element on our planet, forming much of the earth's outer and inner core [1] and is the fourth most abundant element in the earth's crust. Iron is a metal that has been used since ancient times. Pure iron is very soft, and it is significantly hardened by elements such as carbon that occur during the melting process. Iron with a carbon content of up to 2.1 wt% is called steel, which is up to 1000 times harder than pure

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§2 Experimental materials and techniques involved 12

iron [i8]. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, which has high carbon content. Subsequently, oxygen is used to reduce the carbon content to the desired proportion to make steel. Steels and low-carbon steels with other alloying elements (alloy steels) are by far the most common metals in industrial use, due to their great range of desirable properties and the abundance of applications [i8].

2.1.1 An overview of different phases of steels

The varying amounts of carbon in pure iron results in a considerable difference in its structure, which results in different solid phases having distinct physical and mechanical properties.

Ferrite (α)

The solid solution of carbon in α or body-centred cubic (BCC) iron is denoted

α-ferrite or α-ferrite (Figure 1). It is in the ferromagnetic phase and has a hardness of

approximately 80 Brinell [i9]. In steels, ferrite is stable below 910°C. Between 1390°C, up to its melting point at 1539°C, the body-centred cubic crystal structure is again the more stable form called delta-ferrite (δ-Fe). Above the critical temperature of 771°C, the Curie temperature, ferrite changes from ferromagnetic to paramagnetic. This was formerly referred to as beta-ferrite or beta-iron (β-Fe) [4]. The solubility of carbon in α-ferrite in equilibrium with Fe3C is very low (0.02 wt% at

723°C) and decreases with decreasing temperature to 0.008 wt % at 0°C [3].

Figure 1:

(a) Ferritic stainless steel tubes [i1]. (b) Optical microstructure of ferrite [i2]. (c) Schematic structure showing BCC iron (ferrite). Red spheres represent Fe atoms [i3].

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Ferrite-based steels are used in various applications such as steel tubes, decorative trim, sinks, and automotive applications, particularly exhaust systems [i11].

Austenite (γ)

Austenite is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of 723°C [3]. Its face-centred cubic (FCC) structure (Figure 2) allows it to hold a much higher amount of carbon in solution. It is harder than ferrite and has a room temperature hardness of approximately 220 Brinell [i10]. As it cools, this structure either breaks down into a mixture of ferrite and cementite (pearlite or bainite), or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these phases and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Quenching from the austenite state induces martensitic transformation, whereas subsequent tempering breaks down some martensite and retained austenite and is the most common heat treatment for high-performance steels. The addition of certain other metals, such as manganese and nickel, can stabilize the austenitic structure, facilitating the heat-treatment of low-alloy steels [3]. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, elements such as silicon, molybdenum, and chromium tend to destabilise austenite, raising the eutectoid temperature (the temperature where two phases, ferrite and cementite, transform into a single phase, austenite) [4]. The solubility of carbon in austenite in equilibrium with Fe3C is higher than in ferrite (2.08 wt% at 1148°C) and decreases with decreasing

temperature until 0.8 wt % at 723°C.

Figure 2: (a) Austenitic stainless steel kitchen cabinets [i4]. (b) Optical

microstructure of austenite [i5]. (c) Schematic structure showing FCC iron (austenite). Red spheres represent Fe atoms [i3].

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Applications for austenitic-based steel products include kitchen sinks and architectural applications such as roofing and cladding, gutters, doors and windows, balustrades, heat exchangers, ovens and chemical tanks [i11].

Cementite (Fe3C)

The intermetallic Fe-C compound Fe3C is called cementite. In contrast to ferrite and

austenite, cementite is a hard and brittle compound. It has an orthorhombic crystal structure with 12 iron atoms and 4 carbon atoms per unit cell, as shown in Figure 3. Cementite has a Brinell hardness of about 600 [i9] and exhibits a brilliantly white colour. It is ferromagnetic and a solid non-oxide ceramic that can withstand temperatures up to 1500°C [i12].

Figure 3: Schematic structure showing orthorhombic cementite (Fe3C). Red spheres

represent Fe atoms and turquoise spheres represent carbon.

Pearlite (α+ Fe3C)

Pearlite is a product of a lamellar eutectoid reaction comprised of alternating lamellae of cementite and ferrite phases. Just below the eutectoid temperature, relatively thick layers of both the ferrite and Fe3C phases are produced. This microstructure is called

coarse pearlite and the thin layered structure produced in the vicinity of 540°C is termed fine pearlite [3].

The phases present at various temperatures for very slowly cooled Fe-C alloys up to 6.67 wt % C can be depicted from the metastable phase diagram as shown in Figure 4. However, this is not a true equilibrium phase diagram because cementite under certain conditions can decompose into iron and graphite.

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Figure 4: Iron-carbon phase diagram [i6].

Bainite (α+ Fe3C)

The microstructure of bainite consists of ferrite and cementite phases. Bainite forms needles or plates, depending on the temperature of the transformation [5]. The microstructural details of bainite are so fine that their resolution is only possible by means of electron microscopy. In contrast to pearlite, bainite is a product of a non-lamellar eutectoid reaction comprising needles of ferrite that are separated by elongated particles of the Fe3C phase [3]. Additionally, bainite is generally

surrounded by martensite. The bainite formed at about 350 to 550°C is termed upper bainite and consists of cementite in the form of rods, whereas bainite formed between 250 and 350°C is called lower bainite which has cementite precipitated internally in the ferrite plates [6,7].

In the case of TRIP steels, however, cementite is suppressed by alloying additions and the bainitic ferrite formed in these steels is a carbide-free bainite generally surrounded by austenite instead of cementite [6, 7].

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Martensite (α`)

The supersaturated solid solution of carbon in α or BCC iron is termed martensite (Fig. 5). Figure 5b shows an optical microstructure of martensite etched with Lepera etchant [62]. We obtained this by austenitisation followed by rapid quenching of TRIP 800 steel in a dilatometer. Martensite is a non-equilibrium single-phase structure that results from a diffusionless shear transformation of austenite [4]. The martensitic transformation is not yet completely understood [3]. However, large numbers of atoms experience cooperative movements with only a slight displacement of each atom relative to its neighbours. This occurs in such a way that the FCC austenite experiences a polymorphic transformation to a body-centred tetragonal (BCT)/BCC martensite as illustrated by the Bain distortion model in Figure 6.

Figure 5: (a) Martensitic stainless steel cutlery [i7]. (b) Optical microstructure of

martensite. (c) Schematic structure showing BCC/BCT iron (martensite). Red spheres represent Fe atoms [i3].

Figure 6: Schematic of Bain distortion model. Two adjacent unit cells of FCC iron

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The degree of tetragonality of the martensitic lattice increases with increasing carbon content. The c-axis of the martensitic unit cell increases from 2.86 to 3.08 Å and the axis decreases from 2.86 to 2.83 Å with increasing carbon content to 1.8% [8,95]. The type of martensitic structure obtained depends on the carbon content of the austenite [3]. When the carbon content of the steel is low (i.e. about 0.2% or less) then well-defined lathes are formed. As the carbon content is increased (i.e. to about 0.6%) plates of martensite begin to form and, if the carbon content is increased to over 1.2%, the martensite appears as an array of well-defined plates. Martensitic-based steel products typically include knife blades, cutlery, surgical instruments, fasteners, shafts and springs [i11].

2.1.2 Steel metallurgy and its relation to geological and planetary sciences

Owing to certain similarities between metallic meteorites and engineering alloys, steel metallurgy has been of enduring interest to geologists and planetary scientists. Historically, the invention of metallography was motivated by the study of meteorites [9,17]; in fact, the construction of the first iron–nickel phase diagram was based on information from iron meteorites [10–13, 17]. The cooling rates deduced from the analysis of Widmanstätten structures have been instrumental in determining the thermal history of asteroidal bodies [10, 14–17].

Kamacite (α), a BCC iron–nickel solid solution with up to 7.5 wt% Ni, is a common phase that is found in iron meteorites. The structure as such resembles the ferritic phase in steels. The word comes from the Greek kamas, meaning bar, which was proposed in 1861 by Reichenbach for the ferritic phase in meteorites (before ferrite had been identified as a phase in steel) [18]. Taenite (γ) is another phase of iron with more than 25 percent nickel in solid solution. The atoms of taenite arrange themselves in a cube form with an atom centred in the face. In steels this phase is called austenite. The term taenite was given to this phase in 1861 by Reichenbach, and is derived from the Greek word tainia, meaning band [18]. Taenite transforms in a diffusionless manner into martensite (α`) upon cooling below the Martensitic start (Ms) temperature in the α+γ two-phase region [18]. There is a phase that is very similar to cementite called cohenite occurring in meteorites and on earth in places with very high iron deposits, such as volcanic magma flow trails [i12]. It is also Fe3C, except

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Figure 7: Light microscopic analysis reveals various phases in Gibeon iron meteorite.

a)-b) Kamacite grains are etched in brown, whereas Taenite grains are shown in white. c)-d) shows thin Martensite rims which are in dark brown colour.

An overview of the various kinds of phases that exist in iron meteorites is given in Figure 7. Optical microscopy (OM) was used for the microstructural examination and the colour-etching procedure proposed by De et al. [19] was used to reveal the different phases in Gibeon meteorite.

Table 1: Lattice parameters of various steel and meteorite phases

Lattice parameters Steels Iron

Meteorite

Symbol Unit Cell

a (Å) b (Å) c (Å)

Austenite Taenite γ FCC 3.61 3.61 3.61

Ferrite Kamacite α BCC 2.86 2.86 2.86

Martensite Martensite α` BCT 2.86-2.83 2.86-2.83 2.86-3.08 Cementite Cohenite Fe3C Orthorhombic 4.5165 5.0837 6.7297

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2.1.3 Introduction to TRIP-assisted steels

In comparison to the low-carbon steels commonly used in the automotive industry in the early 1970s, several studies conducted around that time showed that dual-phase steels provide an improved balance of strength and ductility [20-22, 44]. The improvement of its mechanical properties was attributed to the combination of a ductile ferrite matrix with a dispersion of hard martensitic grains. In order to obtain such a microstructure, the steels were intercritically annealed, during which a controlled volume fraction of austenite was formed. This austenite later was subsequently transformed into martensite by water quenching [20-22]. However, these dual-phase steels contained negligible amounts of retained austenite at room temperature [23-24]. In the 1980s, studies demonstrated theoretically that retained austenite can further improve the mechanical properties of dual-phase steels through the transformation-induced plasticity (TRIP) effect, especially if the stability of

tained austenite is increased [25-27].

richment of residual austenite with carbon uring bainite transformation [7,29,43].

re

The early 1990s saw an introduction of a new type of 600 to 1000 MPa grade high-strength dual-phase sheet steel composed of a ferritic matrix along with bainite and retained austenite. These steels, called TRIP-aided dual-phase (TADP) steels, were developed mainly to make various automotive structural parts lighter [28-37]. They possessed excellent press formability properties associated with the transformation-induced plasticity (TRIP) [39] of the retained austenite [31-33]. Many studies [30-34, 37, 40] were subsequently conducted to investigate the formability of TADP steel for applications to automotive underbody parts such as suspension arms and wheel disks. Carbon enrichment of austenite during intercritical annealing was identified as the primary cause of preventing martensitic transformation during quenching. Secondly, particular alloying elements were seen to further enrich the carbon in austenite grains during bainite transformation, leading to the stabilization of austenite at room temperature [41,42]. Both manganese and silicon have made a beneficial contribution to the retention of austenite. Conventional TRIP-assisted steels always contain large concentrations of these elements (from 1.5 to 2.5 wt% for both). Manganese is an austenite stabilizer and silicon is reported to stabilize austenite by inhibiting carbide precipitation, thereby promoting the en

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In general, the mechanical behaviour of TRIP-assisted steel is controlled by the following two factors:

 long-range internal stress resulting from a difference of plastic strain between the ferrite matrix and the second phase, i.e. austenite, ferrite, martensite [35-36], and

 strain-induced transformation of the retained austenite, resulting in both the increases in strain-induced martensite content and the relaxation of the long-range internal stress.

The retained austenite stability is controlled by retained austenite parameters such as carbon concentration and morphology, as well as deformation conditions such as temperature, strain rate, and state of stress [31-36]. Retained austenite grain, which is stable against straining, pose as a hard phase and effectively enhance the internal stress during the early stages of deformation [35-36]. Such stable retained austenite grains tend to transform into martensite over a large strain range, thereby resulting in extremely large elongations by TRIP effects [31-33]. On the other hand, if the retained austenite is unstable, most of the retained austenite particles transform to martensite at an early stage of the deformation process. The resultant strain-induced martensite considerably increases the flow stress with relatively low elongations and relaxed internal stress [35-36]. Thereby the internal stress and retained austenite transformation during early stages is assumed to play an important role in mechanical properties of the TRIP-assisted steel.

2.1.4 Effect of alloying additions in TRIP-assisted steels

Alloy additions other than carbon are mainly added to TRIP-steels for the following reasons:

 To optimise the proportion of retained austenite,  To control cementite precipitation,

 To enhance the hardness of ferrite,

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Carbon

Carbon is an austenite stabiliser and plays a vital role in the formation of martensite [3].

Silicon

Silicon is a ferrite stabiliser and helps to retain carbon-enriched austenite by suppressing cementite precipitation from austenite. Silicon also solid-solution strengthens ferrite and hence can enhance the overall strength of the steel [7,29,43].

Manganese

Manganese is an austenite stabiliser that can compensate for any reduction in silicon and strengthens the ferrite [29,45], but this can limit the amount of bainite formation. Additionally, pronounced banding can occur in steels containing a high manganese concentration [46].

Aluminium

Aluminium is an austenite stabiliser that also inhibits cementite precipitation and hence can also substitute for silicon [47]. However, unlike silicon, aluminium does not strengthen ferrite. Steels in which silicon is replaced by aluminium may therefore be weaker [42,48].

Niobium

The most effective way to improve the strength and ductility as well as weldability of steels is to add microalloying elements such as Nb. The addition of Nb can refine the austenite grain in the hot-rolling process by the formation of NbC carbides [3]. These NbC carbides retard the austenite recrystallization, and in turn refine the final microstructure, which increases the yield strength and tensile strength of TRIP steels [38, 49-51]. It has been suggested that NbC clusters present a dispersion of elastically soft and relatively diffuse obstacles for the movement of dislocations [54, 55]. As a result, the strength is improved while maintaining good toughness properties [54, 55].

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2.1.5 Heat treatment procedures in TRIP-assisted steels

The microstructure of TRIP-assisted steel can be generated after hot rolling or after cold rolling and annealing. Hot rolling is generally carried out at temperatures where the steel is fully austenitic. The material is initially heated to a temperature in the (α+γ) phase region generating a mixture of ferrite and austenite, which partially decomposes to bainite at a lower temperature. After being rolled, the material is cooled to ambient temperature. The cooling rate is controlled so that austenite first transforms into ferrite or allotriomorphic ferrite (an allotriomorph has a shape that does not reflect its internal crystalline symmetry as it tends to nucleate at the austenite grain surfaces, forming layers that follow the grain boundary contours) and then to bainite. However, a two-stage annealing treatment is required to produce the desired microstructure in cold-rolled TRIP-assisted steels (Figure 8). The material is initially heated to a temperature in the (α+γ) phase region, thus generating a mixture of ferrite and austenite, which subsequently decomposes into bainite at lower temperatures.

time Zn bath T (°C) 800 460 -intercritical annealing

ideal TRIP temp, 350-450 C

5-6 mins. 30-120 s time Zn bath T (°C) 800 460 -intercritical annealing

ideal TRIP temp, 350-450 C

5-6 mins. 30-120 s

Figure 8: Schematic illustration of the two methods to generate the microstructure of

TRIP-assisted steel. The typical temperatures and times are indicated. The curve stands for the transformation from a fully austenitic state and intercritical annealing after cold-rolling, respectively.

Microstructural evolution during the isothermal formation of bainite is identical in both hot and cold-rolled TRIP-assisted steels. A typical transformation map is shown in Figure 9 [56]. As the bainite reaction proceeds, the residual austenite becomes enriched with carbon and grains stabilize against martensitic transformation during cooling. When the enrichment is inadequate at low holding times, the austenite may decompose partly into martensite during cooling. On the other hand, the amount of untransformed austenite decreases as more bainite forms with longer holding times. It

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follows that the quantity of retained austenite becomes maximal at an intermediate holding time during bainite reaction.

Figure 9: Typical microstructural evolution map during the bainite reaction [56].

Retained austenite reaches its maximum at an intermediate holding time. The terms γ, α, αb and α` represent austenite, allotriomorphic ferrite, bainitic ferrite and martensite

respectively.

2.2 Mechanical properties and testing

The mechanical properties of a material are those properties that involve a geometrical response to an applied load. The mechanical property of materials determines the range of a material’s usefulness and establishes the service life of the product in use. The most common properties considered are strength, ductility, hardness, impact resistance, and fracture toughness. The most common test to determine a material’s mechanical properties is the tension test, which yields directly measured properties such as strength, ductility, elastic modulus and toughness.

The engineering stress-strain response for TRIP 800, TRIP 700, and TRIP-assisted multi-phase (TAMP) steel with a micro-addition of Nb can be seen in Figure 10. As shown, TRIP steels have a large amount of work hardening. The high work hardening is attributed to the transformation from austenite into martensite during deformation (the TRIP effect), along with the fact that TRIP steels are primarily composed of soft ferrite and hard bainite and martensite phases. This multi-phase nature allows the composite deformation of all these phases while maintaining a high tensile strength. Moreover, Figure 10 shows that the tensile strength is almost twice the value of the yield strength. This means that TRIP steels not only exhibit very stable work

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hardening but also have a delayed onset of necking, which is attributed to their relatively high elongation value. All these factors make TRIP steels ideal for forming operations such as stamping or bending [i16], which are often limited due to an inherent loss of strength of the component during deformation due to wall thinning or rupture as the material reaches its forming limit. TRIP steels are therefore ideal for such operations because they have a high formability limit and a stable yield point elongation, which improves the structural integrity of formed components, making them suitable for automobile body parts.

0

10

20

30

0

250

500

750 Strain (%)

Stress (MPa)

TRIP 800 TAMP 800-Nb TRIP 700

Figure 10: Stress-strain response of TRIP 800, TRIP 700 and TRIP-assisted

multi-phase (TAMP) 800 steel micro-alloyed with Nb.

2.4 X-ray diffraction

Crystallography with x-ray diffraction is another valuable tool to identify unknown materials and reveal the crystal structure of the sample under examination. Quantitative crystallography using x-ray diffraction can be used to calculate the amount of phases present as well as the degree of strain to which a sample has been subjected. XRD in general scans a large area of approximately 1 cm2 and has a greater penetration depth of approximately 11 μm, which allows the bulk properties of the material to be explored further.

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Figure 11: XRD peaks of ferrite and retained austenite content with zero strain and

12.5% strain (red) with expected ferrite (green) and austenite (blue) peak positions indicated on diffractograms.

In particular, x-ray diffraction figures prominently for characterising complex steels such as TRIP steels, which predominantly contain FCC (austenite) and BCC (ferrite, bainite, and martensite) structures. Retained austenite along with ferrite and its role during deformation can be studied using this technique. Figure 11 shows XRD measurements carried out by us on TRIP 800 steel before and after deformation (12.5% strain).The reduction in the intensities of austenite peaks during the straining experiments suggests that the austenite grains have transformed into martensite. Martensite peaks after the deformation could not be observed because the lattice parameters of martensite are very close to those of ferrite and the peaks of martensite could be overlapping the ferritic peaks. The transformation from austenite into martensite in TRIP steels depends on various factors, such as the volume fraction of austenite [31,57], the average carbon concentration in austenite [58], the austenite grain size [59] and the austenite grain orientation [60]. All these factors were quantitatively investigated using this technique [58-61]. Further in situ studies of the deformation and phase transformation behaviour of these steels were also carried out using this method [61].

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2.3 Optical microscopy

The microscopic structure of materials reveals characteristics that have a tremendous influence on their technological utility [i14]. The features that contribute to the strength of materials, and the features that initiate mechanical failure, can be resolved by optical microscopes. Thus, specimen preparation for optical microscopy and the interpretation of micrographs taken with optical microscopes continue to play a vital role in understanding the origin of material properties.

Figure 12: Light optical micrographs of (a) TRIP 800 steel Le Pera etched, (b) TRIP

800 steel-tint etched, (c) TRIP 700 steel Le Pera etched and (d) TRIP 700 steel-De etched.

Unlike conventional steels, which mainly possess a dual-phase microstructure, TRIP steels are composed of multiple different phases (ferrite, bainite, martensite, and retained austenite) resulting in a very complex microstructure, which makes them much more challenging to observe. Therefore, for light microscopy observations, some specific etchants and procedures have been tested during this study. A color

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etching technique adapted from the Le Pera method [62] was employed to reveal the microstructure of TRIP 700 (Al-based TRIP steel) and TRIP 800 (Si-based TRIP steel). In these investigations, the specimen is embedded in a bakelite mount along the rolling direction-normal plane. Classic grinding and polishing stages were performed on the specimens to produce a 0.25 µm diamond paste. The etchant used was a mixture of two solutions (2%Na2S2O5 and 4% Picric acid). It can be observed in

Figures 12a and 12c that ferrite appears brown in colour. The bainitic ferrite, whose morphology is easily distinguishable, is dark grey in colour. Both retained austenite and martensite appear white in colour.

One of the main drawbacks of the Le Pera etching method with TRIP steels is its inability to distinguish between retained austenite and martensite, both of which appear white in colour. To distinguish the retained austenite from martensite, the TRIP 800 is tempered at 180°C for 2 h and etched with Le Pera reagent [63] as shown in Figure 12b. Clearly, retained austenite can be separated from martensite, which appears pale brown in colour. The observed change in colour is due to the precipitation of carbides along the martensite lathes [63]. As the transformation temperature for retained austenite is above 200°C, tempering at 180°C will not significantly alter the retained austenite content in the bulk and would lead to a better distinction of all the four phases with optical microscopy on TRIP steels [63]. Nonetheless, tempering is known to relieve internal stress, which has an influence on the mechanical response of the steel [3] and poses difficulties in the quantification of different phases in these steels.

A stepped color etching technique was then employed using the De etching method [19] to reveal the microstructure of TRIP 700. In this process, the specimen is dipped into a 4% Picric acid solution for 30 seconds, and then rinsed with water. The sample is then placed into a solution of 10%Na2S2O5 for 10 seconds and cleaned with

ethanol. Figure 12d shows the microstructure of TRIP 700 steel in which the large brown areas represent ferrite, and the fine dark and grey areas represent bainite. Martensite is generally etched in a straw-tint colour and retained austenite appears white in the optical micrograph [19]. The proportion of retained austenite was quantified using an image analyzer, constituting an area fraction of 9.5% [19], which is reasonably close to the XRD values of 8%.

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2.5 Scanning electron microscopy and electron back-scattered diffraction

The scanning electron microscope (SEM) uses electrons instead of light to form an image, which results in much higher resolution (closely spaced specimens can be magnified to much higher degrees). It has a large depth of field, which allows more of a specimen to be in focus at one time. As an SEM uses electromagnets rather than lenses, it allows better control over the degree of magnification and leads to strikingly clear images [i17]. Electron back-scatter diffraction (EBSD) or back-scatter Kikuchi diffraction (BKD) are techniques for obtaining microstructural and textural information from bulk samples or thin layers in the SEM. This provides information on the orientation of crystals with a spatial resolution of a few microns. With special software interfaced to the SEM, the electron back-scattered patterns of individual grains can be acquired easily and compared to standard SEM images, allowing crystal structure information to be coupled to the microstructure [i13]. EBSD provides quantitative microstructural information about the crystallographic nature of metals, minerals, semiconductors, ceramics and most inorganic crystalline materials. It reveals the grain size, grain boundary character, grain orientation, texture, and phase identity of the sample under study [65].

Figure 13: Typical multi-phase microstructure of a TRIP-assisted steel with ferrite

(F), bainite (B) and retained austenite (A) grains.

Complex multi-phase microstructures of TRIP steels consisting of a ferritic matrix together with some other constituents dispersed in it such as bainite, metastable austenite and some amounts of martensite were investigated using SEMs [56,63,66]. Figure 13 shows a typical microstructure of TRIP-assisted steel. However, identification as well as quantification was an ongoing task because a correct

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estimation of their volume fraction of the above-mentioned phases and spatial arrangement is required to optimise the microstructure and mechanical properties of the material [67]. In the past decade, the EBSD technique found wide-scale application for the microstructural characterization of complex phase steels together with an accurate evaluation of the local crystallographic texture. Several EBSD investigations have also been carried out so far to quantify the phases in TRIP steels [67-71]. A blind round robin test was performed recently to estimate the retained austenite content on a number of unknown TRIP samples using EBSD [73]. Conventional sample preparation with colloidal silica polishing was used for preparing the samples and EBSD analysis yielded very small retained austenite values (less than 10%) with different cleanup procedures.

Figure 14: Phase identification maps of electropolished TRIP 800 steel.

Colloidal silica polishing on TRIP samples results in a very poor indexing because of mechanically induced transformation occurs due to mechanical polishing [67]. We developed a new electrolyte during this study and optimised the electropolishing parameters, which consistently produced an indexing of above 90% and higher retained austenite values, as can be seen in Figure 14. Details of sample preparation, the composition of the electrolyte and the electropolishing parameters are given in [73] and Chapter 3 of this thesis.

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2.6 Electron probe microanalysis

Electron microprobe analysis or electron probe microanalysis (EPMA) is an analytical technique to establish the composition of small areas of specimens. The electron beam is focused on the surface of a specimen using electromagnetic lenses, and these energetic electrons produce characteristic x-rays within a small volume (typically between one and nine cubic microns) of the specimen. The characteristic x-rays are detected at particular wavelengths, and their intensities are measured to determine the concentrations [74]. All elements (except hydrogen, helium, and lithium) can be detected because each element emits a specific set of x-rays. This analytical technique has a good spatial resolution and high sensitivity, and individual analyses are reasonably short, requiring only a minute or two in most cases. The wavelength dispersive (WD) system can simultaneously analyse up to five chemical elements. Additionally, the electron microprobe can function like an SEM and obtain highly magnified images of a sample. It can also collect back-scattered electron (BSE), secondary electron (SEI) Images. Figure 15 shows a photograph of the EPMA equipment.

Figure 15: JEOL JXA-8230 Super Probe Electron Probe Microanalyser [i18].

The excellent mechanical properties of TRIP steels are closely related to the stability of retained austenite islands during plastic straining. One of the most important factors governing austenite stability is the local carbon enrichment Cc, which can be attained

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austenite is of fundamental interest both for structure/properties studies and for process control. Different techniques are currently available for measuring carbon concentrations in austenite. Among the known techniques, x-ray diffraction (XRD) is traditionally used for obtaining an overall average value. XRD measurements of the residual austenite lattice parameter ac are often used to calculate carbon concentration.

However, there are several limitations to this technique [77]. First, it is an indirect measurement and is therefore subject to all the factors that influence ac such as the

presence of alloying elements or internal residual stresses. Second, it provides an average value of Cc for the volume analysed and thus is not useful for detecting

variations of carbon content between islands or carbon gradients inside the islands. For local and discrete measurements, TEM (discussed in the following Section 2.7) is a very powerful tool because it provides very high spatial resolution, which allows measurements of several grains within the same steel or even in a single austenitic grain. Among the different techniques available to measure carbon concentrations, Kikuchi lines obtained in TEM provide a relatively easy and accurate method [81]. The quantitative analysis of carbon concentrations in steels at high spatial resolution using EELS in a TEM was developed by Scott and Drillet [82]. The procedure was successful in estimating the carbon concentrations of all the phases in TRIP steel, i.e. ferrite, austenite and martensite. Scott and Drillet showed that relative errors using EELS are of the order of 5% in a range of concentrations between 0.2 wt.% and 0.8 wt.% carbon [76]. Nevertheless, the major drawback of carbon quantification becomes evident with the sample thickness. In general, retained austenite grains are too thick in TEM and therefore would yield a poor signal-to-noise ratio, which would result in a wider spread of carbon concentration values. Focused ion beam (FIB) in combination with SEM was therefore used to prepare thin foils for TEM analysis and to perform EELS carbon concentration measurements from a 700 nm retained austenite grain [83]. However, there is growing concern regarding FIB preparation as it induces retained austenite transformation [84]. Recent developments of higher voltage analytical electron microscopes (AEM) now allow EELS analyses to be performed on thicker specimens. However, the most severe limitation of carbon contamination in microscopes could limit the advantage of this technique with realistic carbon quantification.

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Consequently, EPMA emerges as a sound analytical technique to collect and measure compositional information from only a small volume. The beam of electrons interacts with a volume of usually between one and nine cubic micrometers (1×10-18 to 9×10-18 cubic meters) [75]. This volume is known as the interaction volume of the electrons. The small interaction volume of EPMA permits highly localized compositional data to be collected and specimens to be examined that are too small to be studied with other analytical techniques. In addition, it allows the chemical variability over the surface of a sample to be determined. Furthermore, use of an air jet during the measurements helps decontaminate carbon and results in reliable carbon estimation [80]. Thus, EPMA is well-suited to study specimens composed of mixed phases that one wishes to resolve and analyse in situ, leaving the contextual relationships of the phases unaltered and visible. Several studies involving the distribution of the alloying elements and light elements such as carbon in TRIP steels were performed successfully with EPMA [77-80].

2.7 Transmission electron microscopy

With a resolution of 0.1 nanometers [85] (about hundred thousand times thinner than a human hair), TEMs are among the most powerful microscopes to study materials properties. TEM produces high-resolution, two-dimensional images, which allow a wide range of educational, scientific and industrial research. It utilizes high-energy electrons to provide morphological, compositional and crystallographic information on samples. Very detailed information such as structural properties of precipitates, defects such as twins, stacking faults, dislocations, characterizations of grain boundaries (orientation relationships, low-angle misfit etc) can be obtained with this technique.

In general, TRIP steels have a tensile strength of approximately 600 MPa [i16]. Altering the alloy content can result in tensile strengths above 800 MPa. This was first accomplished by raising the carbon content of the alloy to approximately 0.4 % by weight [86]. However, a high carbon content leads to poor weldability and the retained austenite becomes more stable due to the increased carbon content, which reduces the formability of the TRIP steel. Instead of increasing the carbon content, alloying elements such as titanium, niobium, and vanadium can be used to give TRIP steels added tensile strength [87]. Such alloying additions improve the strength of the

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steel through precipitation hardening, while having a minimal effect on weldability and formability [i16]. Several TEM studies involving precipitate investigation [88-91] have been carried out on microalloyed transformation-induced plasticity steel [50,55,64,]. Figure 16a shows a large ferritic matrix with novel ultrafine disc-shaped precipitates in the size range of 2-5 nm. Details of the TEM analysis of these precipitates can be found in Chapter 6 (Figure 10) of this thesis.

Figure 16: (a) Retained austenite grain (A) exhibiting a twin fault surrounded by a

ferritic matrix (F) in TRIP 800 steel. (b) Deformed TRIP 800 steel showing martensitic grains with martensite laths. Lattice fringes can be seen at the bottom right side of this figure.

Complex multiphase TRIP microstructures were studied by several authors using this technique [88,92,93]. Ferrite and austenite can be distinguished easily using electron diffraction because they possess BCC and FCC crystal structures and have distinct projections and d-spacings. Martensite can also be distinguished based on its morphology and distorted Kikuchi patterns. Figure 16b shows a martensite grain with laths in deformed TRIP 800 steel. However, the main difficulty lies in locating the bainitic phase because both the bainitic ferrite and ferrite (allotriomorphic) have a BCC crystal structure with similar lattice parameters. One can overcome this difficulty by using the orientation relationships between ferrite and austenite because bainitic ferrite has a Kurdjmov-Sachs orientation relationship with the filmy-type

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austenite grains [88,92]. Furthermore, defects such as twins and stacking faults, which influence the austenite stability, can be studied effectively with this technique.

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Mechanics in Steels through Microscopy