SELF HEALING BY CU PRECIPITATION IN
DEFORMED FE-CU-B-N-C
S. Zhang 1, G. Langelaan 1, H. Schut 1, E. Brück 1, S. van der Zwaag 2 and N.H. van Dijk 1
1
Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629JB Delft, The Netherlands – e-mail:S.Zhang-1@tudelft.nl; G.Langelaan@tudelft.nl;
H.Schut@tudelft.nl; E.H.Bruck@tudelft.nl; N.H.vanDijk@tudelft.nl, 2
Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands – e-mail:S.vanderZwaag@tudelft.nl
Keywords: Self-healing, Cu precipitation, deformation-induced defects, carbon addition, positron annihilation spectroscopy
ABSTRACT
Steels are among the most widely used construction materials as their mechanical properties can be tuned over a very wide range of desired combinations of strength formability and other properties. However, when exposed for long times to high temperatures steel components can exhibit premature and low-ductility creep fracture, which arises from the formation, growth and coalescence of (initially) nanoscale pores. Self-healing of such defects is regarded as a promising new approach to enhance the component lifetime. In principle, it could be achieved by nanoscale precipitation on the creep cavity surface preventing further growth. Earlier work has shown that Cu may be a suitable alloying element to induce such a healing behaviour in stainless steels.
In the present work the high temperature precipitation behaviour of Cu on deformation inducted defects is studied for a number of high-purity Fe-Cu-B-N-C alloys using positron annihilation spectroscopy (PAS) and hardness tests. Samples with 0% and 8% cold pre-strain are utilized to study the influence of prior tensile deformation on the precipitation kinetics of copper. The time evolution of the S-W points derived from Coincidence Doppler Broadening spectra indicates that deformation-induced defects enhance the Cu precipitation kinetics. A clear reduction in open volume defects is accompanied by a strong increase of Cu signature during the initial stage of aging, demonstrating the self-healing potential in the Fe-Cu-B-N-C alloy. A comparison between the hardness behaviour of Cu, Cu-B-N, and Fe-Cu-B-N-C indicates the added carbon counteracts the acceleration of Cu precipitation caused by the addition of B and N.
1. INTRODUCTION
Self-healing is a promising new approach to extend the lifetime of steel components operating at high temperatures. Recently, Shinya and coworkers have demonstrated that creep damage can be self-healed in austenitic stainless steels [1,2]. The enhanced creep resistance is proposed to be due to the formation of nano-size precipitates and the surface segregation of solute atoms (i.e. copper, boron) on cavities, which fill the defects and prevent further growth of the creep cavities. In our previous work, the influence of deformation-induced defects on copper precipitation
during aging was studied in Fe-Cu and Fe-Cu-B-N model alloys to evaluate the self-healing potential of Fe alloys with Cu, B and N added [3,4]. It was demonstrated that in deformed Fe-Cu-B-N, the addition of B and N accelerated the formation of spherical nanoscale Cu precipitates and strongly reduced the Cu precipitation along dislocations during thermal aging at 550 oC. Carbon, the primary alloying element in steel, is widely used to improve the mechanical properties of the steel. It is desirable to investigate the influence of carbon on the self-healing behavior and the Cu precipitation kinetics.
2. MATERIALS
The chemical composition of the studied Fe-Cu-B-N-C alloy produced by Goodfellow is shown in Table 1. Dog-bone shaped samples with a thickness of 0.5 mm were machined by spark erosion from the rolled sheet material. The samples were solution treated at 800 oC for 3 h in evacuated silica tubes filled with 200 mbar ultrahigh purity argon, and subsequently quenched into water at room temperature. The Fe-Cu-B-N-C alloy samples were studied in two conditions: with and without an applied pre-strain of 8%. The pre-strain was applied at room temperature by tensile deformation using a 2 kN microtensile tester (Deben).
Table 1: The chemical composition of Fe-Cu-B-N-C alloy (in wt.%) with balance iron. The Ce concentration amounts to the nominal concentration.
Cu B N C S Ce
1.00 0.050 0.019 0.087 0.002 0.015
3. METHODS
The coincidence Doppler broadening (CDB) measurements were carried out by using a sandwich of two samples with a 22Na positron source in between. Recording of >106 annihilation events results in a distribution of the energy difference. By measuring the Doppler shift in the energy of the two 511 keV annihilation -rays, one obtains the information on the momentum distribution of the electrons involved in the annihilation process. The overall energy resolution was about 1 keV at 511 keV (full width at half maximum, FWHM), corresponding to a momentum resolution of 4×10-3 m0c (FWHM) with m0 the electron rest mass and c the light speed. The samples were
heated to a fixed temperature of 550 oC in a vacuum furnace (< 5×10-5 Pa) in separate aging steps. The CDB spectra were recorded between the heat treatments. Vickers microhardness testing was carried out using a load of 500 g on samples that were aged at 550 oC for 0-96 h in a vacuum chamber (< 5×10-5 Pa).
4. RESULTS
In order to clarify the Cu precipitation kinetics and the evolution of defects, we evaluate the evolution of two parameters (S and W) derived from the Doppler broadening spectrum during aging up to 96 h. The S parameter reflects the annihilation with valence electrons and is calculated as the ratio of the counts in a
fixed low-momentum interval ( 3 0
3.1 10
L
p m c). Similarly, the W parameter is obtained from the contribution of annihilations with high momentum core electrons (
3 3
0 0
9.2 10 m c pL 24.3 10 m c) [3].The copper precipitates can be regarded as a
potential well for positrons and result in a high W parameter. Figure 1 shows the evolution of the S-W points for the as-quenched and the 8% deformed samples. All
S-W points are normalized to those for annealed pure iron (SFe = 0.4609, WFe =
0.1198). For the as-quenched Fe-Cu-B-N-C alloy, the W parameter is comparable to that of annealed pure iron. The relatively high S value for the sample with 8% pre-strain is due to the deformation-induced defects (such as dislocations). During the initial aging stage, the S-W points shift toward the Cu position for both the 0% and 8% pre-strain samples. The data points approach the value of the annealed pure Cu after an aging time of 2-4 h for the Fe-Cu-B-N-C alloy without deformation and 1-2 h for the Fe-Cu-B-N-C alloy with 8% pre-strain. A comparison of the time evolution of the S-W points for the undeformed and deformed samples indicates that the deformation-induced defects accelerate the Cu precipitation kinetics. For longer aging times, a shift in S-W points is observed toward a high S and low W position which is attributed to the misfit between the copper precipitates and the matrix.
0,98 1,00 1,02 1,04 1,06 1,08 1,10 1,12 0,7 0,8 0,9 1,0 1,1 1,2 1h 0.25h 12h 24h 48h 96h 24h 48h 96h AQ Fe Defect Cu Fractured 8% Normalized S N orm aliz ed W 0% prestrain 8% prestrain
Figure 1 : Time evolution of the S and W parameters for the as-quenched Fe-Cu-B-N-C alloy with 0% and 8% pre-strain.
In Figure 2, the Vickers hardness is shown as a function of the aging time for the Fe-Cu-B-N-C alloy. For comparison the data for the Fe-Cu and Fe-Cu-B-N alloys are also shown [3]. For the samples without pre-deformation, the behaviour of Fe-Cu-B-N-C is close to that of the Fe-Cu alloy, i.e. the hardness gradually increases and reaches the peak hardness at about 6 h, followed by a continuous decrease (Figure 2a). For the shortest annealing time of 0.1 h, the hardness of the Fe-Cu-B-N alloy increases sharply to 130 HV0.5, a much higher level than observed for the Fe-Cu
and Fe-Cu-B-N-C alloys. Subsequently, the age-hardening kinetics of the Fe-Cu-B-N alloy is slightly slower, compared to that of the Fe-Cu-B-N-C alloy. For the deformed samples, the addition of trace elements does not noticeably change the age-hardening behaviour for copper precipitation. In the initial aging stage the hardness decreases due to the recovery of dislocations, subsequently it rises with increasing aging time till reaching a peak age and finally decreases again in the over-aged stage. The peak aging time for the Fe-Cu and Fe-Cu-B-N-C alloys is 6 h
while the Fe-Cu-B-N alloy reaches the aging peak within 4 h, as shown in Figure 2b. Thus addition of carbon counteracts the effect of boron and nitrogen on the Cu precipitation. 0,1 1 10 100 100 120 140 160 180 HV 0.5 Aging time (h) Fe-Cu Fe-Cu-B-N Fe-Cu-B-N-C AQ 0,1 1 10 100 100 120 140 160 180 Fe-Cu Fe-Cu-B-N Fe-Cu-B-N-C HV 0.5 Aging time (h) 8%
Figure 2 : Vickers Hardness HV0.5 as a function of aging time at 550 oC of Fe-Cu,
Fe-Cu-B-N, and Fe-Cu-B-N-C alloys (a) without deformation, and (b) with 8% pre-strain (data for the Fe-Cu and Fe-Cu-B-N alloys from Ref. [3] ).
5. CONCLUSIONS
The isothermal copper precipitation in high-purity Fe-Cu-B-N-C alloy with 0% and 8% pre-strain was studied as a function of the aging time at 550 oC. Self-healing of
nano-size defects is demonstrated. In the S-W plots, the deformed alloy exhibits a sharp reduction in open-volme defects accompanied with a strong copper signature durig the initial aging stage. This is explained by the closure of open volume defects by copper precipitation. The Cu precipitation in the Fe-Cu-B-N-C alloy is comparable to that in the Fe-Cu alloy, which is due to the incorporation of boron and nitrogen in carbides.
ACKNOWLEDGEMENTS
This research was financially supported by the innovation-oriented research program (IOP) on self-healing materials of the Dutch Ministry of Economic Affairs, Agriculture and Innovation (IOP project SHM01017).
REFERENCES
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[3] S. M. He, N. H. van Dijk, H. Schut, E. R. Peekstok, S. van der Zwaag, Thermally activated precipitation at deformation-induced defects in Fe-Cu and Fe-Cu-B-N alloys studied by positron annihilaiton spectroscopy, Physical Review B (2010) 094103. [4] S. M. He, N. H. van Dijk, M. Paladugu, H. Schut, J. Kohlbrecher, F. D. Tichelaar, S. van der Zwaag, In situ determination of aging precipitation in deformed Fe-Cu and Fe-Cu-B-N alloys by time-resolved small-angle neutron scattering, Physical Review B (2010) 174111.
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