By Francis W. Rowe, B.Sc. (Research Dept., David Brown and Sons (Huddersfield), Ltd.).
T hough th e w eig h t of c astin g s in non-ferro u s alloys p ro d u ced y early is only a sm all p ercen tag e of th e to ta l m etal c a stin g s m ade, y e t if th e re sp e c tiv e value of th e c a stin g s in th e differen t m a te ria ls—n o n -ferro u s alloys, c a st iro n an d steel
— is ta k e n , i t will be seen t h a t a v ery larg e sum of m oney is involved a n d t h a t a m uch g re a te r c a p ita l o u tla y an d a g re a te r v a lu e p e r to n o b tain s w hen m ak in g n o n -ferro u s castin g s.
I f fo r th is reason alone, i t is necessary fo r th e n o n -ferro u s fo u n d ry to o p e ra te o n even m ore econom ical lines th a n th e iro n an d steel founder.
F u rth e rm o re , ow ing to th e p re s e n t s ta te of th e e n g in e e rin g an d allied in d u s trie s as few p a r ts as possible a re m a d e in brasses an d bronzes, owing to th e i r re la tiv e ly h ig h cost; an d th e w eig h t of th e se a re c u t to th e m inim um . T hus i t is essen
ti a l t h a t th e castin g s should be as sound and s tro n g as possible as th e la titu d e in s tre n g th an d th e fa c to rs of sa fe ty a re sm aller th a n form erly.
C onsequently, all th e re s u lts of m etallu rg ical re s e a rc h an d all th e help t h a t m etallu rg ical in s tru m e n ts can give should be em ployed by th e bro n ze fo u n d er to e n su re t h a t his c astin g s shall be as so u n d a n d re lia b le as possible.
I n th is re sp e c t th e research w ork t h a t has been done in u n iv e rsitie s an d in d u s tria l lab o rato ries th ro u g h o u t th e w orld h as proved of im m ense value to th e bronze fo u n d in g in d u s try in h elp in g to p ro duce n o t on ly sounder castin g s, b u t c astin g s of reg u la r com position an d s tre n g th an d m ore fitted fo r p a r tic u la r d u tie s in service.
M e tallo g rap h y em braoes th e stu d y of th e in te rn a l s tr u c tu r e an d c o n s titu tio n of m etals and alloys.
S in ce m uch of th e in te rn a l s tru c tu r e of m etals and alloys c a n n o t be stu d ie d w ith th e nak ed eye, th e m icroscope m u st be larg ely used in ex a m in in g th e ir fo rm a tio n an d c o n stitu tio n .
I t is n o t th e a u th o r ’s in te n tio n in th is P a p e r to discuss th e th e o ry of th e com pound microscope, n o r
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Fig. 1.—MetallographicOutfitinResearchDepartment, DavidBrown& Sons (Huddersfield), Limited.
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describe in d e ta il th e mode of p re p a r a tio n of m etal- lo g rap h ic specim ens, as such in fo rm a tio n is a v a il
able in m ost text-books. I t should be m entioned, how ever, t h a t as m uch useful in fo rm a tio n can be g ain e d f.rom a m e ta llu rg ic a l microscopic o u tfit cost
in g £20 o r £30 as from th e m ore expensive research outfits such as t h a t in F ig . 1, w hich re p re sen ts probably th e la te s t an d best achievem ent in
Fi g. 2.—Un e t c h e d Sp e c i m e n—6 p e r c e n t. Le a d e d Br o n z e x 200 d i a s.
m icroscopes fo r m e ta llu rg y . I t is only w hen th e d esire or necessity arises for p re se rv in g records, in th e shape of p h o to g rap h s of th e v ario u s s tru c tu r e s , t h a t such a n eq u ip m e n t is essential.
M e ta llu rg ic a l specim ens for th e microscope are polished on successive g rad es of fine em ery p a p e r a n d finally buffed on a revolving w et p a d u sin g a su p erfin e ab rasiv e such as alu m in iu m o r m ag
nesium oxide. T his rem oves th e scratches le ft by th e finest em ery p a p e r. Such a specim en is th e n ex am in ed u n d e r th e microscope (before w h a t is know n as etohing) to o b ta in an idea of th e sound
ness o r o th erw ise of th e specim en. P o ro sity ie th u s easily discernible in specim ens which to th e
1/4
nak ed eye in th e polished s ta te o r in th e f r a c tu re d s t a te a p p e a r p e rfe c t an d is shown u p m ore clearly th a n a f te r th e specim en is etched, as th e coloura
tio n of th e specim en m ay m ask th e holes. In brasses an d bronzes most n o n -m etallic inclusions such as san d , slag, e tc ., a re e a sily seen in th e u netohed s ta te , an d th e p resen ce of lead in such alloys is in d ic a te d . L e a d is p ra c tic a lly insoluble
Fi g. 3 . — Un e t c h e d Sp e c i m e n— Le a d Fr e e Br o n z e s h o w i n g Po r o s i t y i n In t e r s t i c e s o f
De n d r i t e s x 2 0 0 d i a s.
in th e o rd in a ry alloys, a n d th u s a p p e a rs in th e solid m e ta l as p atch e s (sm aller o r la r g e r a cco rd in g to circum stances) of th e p u re m etal. T h is b ein g so ft is u su ally picked o u t d u rin g th e final polishing, a n d th u s th e m e ta l a p p e a rs to h av e sm all black p its over th e specim en.
As p o ro sity shows in a som ew hat s im ila r m a n n e r, it m ig h t be th o u g h t t h a t confusion would o ccu r as to w hich of th e holps w ere d u e to lead an d w hich to p o ro sity , b u t a c tu a lly th is seldom occurs, as th e lead a p p e a rs in fa irly re g u la rly sh ap ed p a tc h e s, w hilst p o ro sity shows as ja g g e d , u neven holes.
F igs. 2 an d 3 illu s tra te th i s d iffe re n tia tio n .
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Revealing the Constituents.
A fte r such e x a m in a tio n th e specim en is etched by im m ersing i t in a chem ical solution. The tw o m ost used etch in g re a g e n ts for brasses an d bronzes a re an acid so lu tio n of fe rric chloride an d am m onium p e rs u lp h a te solution.
T he effect of th e e tch in g re a g e n t is to corrode or colour th e d ifferen t cry stals or c o n stitu e n ts
Fi g. 5 . — S o l i d S o l u t i o n x 1 0 0 d i a s . G r a i n B o u n d a r i e s a n d D i f f e r e n t l y O r i e n t a t e d
C r y s t a l s .
d iffe re n tly o r unevenly. T hus a fte r etch in g one is able to see th e d ifferen t c o n stitu e n ts clearly.
T he d iffe re n t classes of c o n stitu e n ts m et w ith in n o n -ferro u s alloys will be generalised sep arately .
(a) Solid solutions cofisist of crystals which are composed of v a ria b le p ro p o rtio n s of one m etal dis
solved in a n o th e r. Solid-solution cry stals are n o t necessarily of definite com position, b u t m ay vary betw een th e lim its of so lu b ility of th e com ponent alloys. Such cry stals b ein g tr u e solutions, i t is im possible to d is tin g u ish th e tw o com ponents u n d e r th e microscope.
An alloy composed of tw o or m ore m etals in such p ro p o rtio n s t h a t th e y dissolve com pletely in
17e
each o th e r p re se n ts u n d e r th e m icroscope in its no rm al s ta te a s im ila r a p p e a ra n c e to t h a t of a p u re m etal. T h a t is, a n u m b er of cry stals o f th e sam e s tr u c tu r e th ro u g h o u t, only th e g ra in or c ry s ta l b o u n d aries show ing.
F ig . 1 4a shows a p h o to m ic ro g ra p h a t 2 0 0
d ia m e te rs of a solid so lu tio n in w hich th e se c h a r
a c te ristic s a re visible, w h ilst F ig . 5 shows one
F i g . 6 . — S o l i d S o l u t i o n x 5 0 d i a s . C o r i n g E f f e c t a n d D e n d r i t e s .
ex actly sim ila r ex cep t t h a t th e com position of th e alloy is such t h a t th e d iffe re n t cry sta ls a p p e a r of d iffe re n t co lo u r d u e to th e lig h t b ein g reflected fro m th e c ry s ta ls in a d iffe re n t m a n n e r. M an y alloys in th e c a st co n d itio n , how ever, th o u g h solid solutions, show a d iffe re n t a p p e a ra n c e from th o se in F ig s. 1 4a a n d 5 .
F ig . 6 shows a p h o to m icro g rap h of a n alloy of
8 0 p e r cen t, copper a n d 2 0 p e r c e n t, nickel. The s e p a r a te la rg e g ra in s a re visible, b u t th e r e is also a n in te r n a l s tr u c tu r e w ith in th e g ra in s. T his fir- tr e e or d e n d rite s tr u c tu r e of w h a t a re know n as
“ cored ” cry sta ls is comm on to m an y solid solu
tio n s in th e c a st c o n d itio n , a n d is due to th e m eth o d of solidification of th e alloy. Solid
soln-tio n s do n o t freeze a t an y definite te m p e ra tu re b u t over a ra n g e w hich m ay v a ry from 10 to 100
? eS ; I 11 th is in sta n c e th e first solid p o rtio n s to freeze o u t of th e m etal h ave been ric h e r in nickel an d th e s u rro u n d in g layers w hich have been b u ilt ro u n d th e nuclei a re progressively low er in n ickel, an d th u s th e difference in th e r a te of a tta c k by th e e tch in g re a g e n t. F ro m th e photom
icro-F ig . 7 . — Ph o s p h o r Co p p e r ( 9 p e r c e n t. P) x 5 0 D I A S .
g ra p h can be seen how th e solidification has p ro gressed in each g ra in . T hus a n alloy is obtain ed c o n sistin g e n tire ly of cry sta ls of th e sam e average com position b u t v a ry in g slig h tly in them selves.
Such c o rin g effects m ay be rem oved by a n n ealin g th e alloy, a lth o u g h i t m u st he p o in ted o u t t h a t w ith som e classes th is in te rn a l coring is extrem ely difficult to rem ove—n o tab ly in th e alloys of which th e p h o to m icro g rap h is ta k e n —th e copper-nickel g ro u p .
Eutectics and Eutectoids.
(6) E u te c tic signifies alloy w ith th e lowest m elt
in g p o in t. T he e u te c tic of an y alloy of tw o or m ore m etals is th a f definite com position which
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solidifies a t th e low est te m p e ra tu re . The solidi
fications of a © utectic ta k e s p lace a t one d efin ite te m p e ra tu re , a n d n o t as w ith solid so lu tio n s over a ra n g e o f te m p e ra tu re .
E u te c tic s, co n sistin g as th e y do, of tw o con
s titu e n ts in tim a te ly m ixed to g e th e r, alw ays p re s e n t a d u p lex s tru c tu r e , th o u g h in m an y in stan ces th e h ig h e r pow ers of th e m icroscope a re necessary to show th is fe a tu re . T he “ p a tt e r n ” of th e
F r o . 8.— Ti n Br o n z e Qu e n c h e d 650 d e g. C. x 400
D I A S .
d u p lex s tr u c tu r e of a e u te c tic v a rie s w ith d iffe re n t alloys an d d iffe re n t m ethods of cooling, b u t th e y c an u su ally be easily id en tified as th e y possess sim ila r g e n e ra l c h a ra c te ristic s.
F ig . 7 shows a p h o to m icro g rap h of a n alloy c o n ta in in g 9.0 phosp h o ru s an d 91.0 p e r c e n t, copper. T he m a jo r p o rtio n consists of th e e u te c tic of copper a n d copper phosphide w ith one d e n d ritic c ry s ta l of copper phosphide. T he alloy c o n ta in s slig h tly m ore phosphorus th a n is re q u ire d fo r th e fo rm a tio n of all e u te c tic (8.2 p e r c e n t.), a n d th u s some copper p hosphide re m a in s free. I t w ill be n o ticed t h a t th e e u te c tic is co arser in som e p a r ts th a n in o th ers, t h a t is a t th e edges of th e g ra in ,
th o u g h th e g ra in b o u n d aries a re n o t u sually dis
tin g u is h a b le except by th is fe a tu re .
E u te c to id s, as th e ir nam e im plies, a re sim ilar to eu tec tics. The essen tial difference is t h a t w h ilst eu tec tics a re form ed by th e sim u ltan eo u s fre e z in g from the liq u id s ta te of tw o c o n stitu e n ts of a n alloy in definite p ro p o rtio n s, e u te c to id s are form ed by th e decom position in th e solid a t a d efin ite te m p e ra tu re of a solid solution.
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Fi g. 9 . — T i n B r o n z e a s C a s t x 4 0 0 d i a s .
T he fo rm a tio n of a e u tec to id m ay be p erhaps best illu s tra te d by a specific exam ple. An alloy of 80 p e r c en t, copper, 20 p e r cen t, t i n commences to so lid ify a b o u t 880 deg. C. A t 790 deg. C. th e alloy is q u ite solid an d consists of a m ix tu re of tw o solid so lutions know n as th e a lp h a an d th e b e ta . T his s tr u c tu r e co n tin u es u n til th e te m p e ra tu r e reaches 525 deg. C. F ig . 8 shows th e s tru c tu r e of such a n alloy in th is ra n g e being obtained by q u en ch in g a sam ple a t 700 deg. C. an d th u s re ta in in g th e s tr u c tu r e a t t h a t te m p e ra tu re .
A t 525 deg. C. th e b e ta solid-solution b reak s up in to a e u te c to id composed of a lp h a solid-solution an d d e lta solid-solution as its c o n stitu e n ts. The
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a p p e a ra n c e of th e alloy in th is co n d itio n is shown in F ig . 9. F ig . 10 shows th e e u te c to id a t a h ig h e r m ag n ificatio n w hen th e tw o c o n s titu e n ts a re seen m ore clearly.
Intermetallic Compounds.
(c) In te r m e ta llic com pounds a re com pounds of tw o or m ore m etals of d efin ite chem ical com posi
tio n . T hey com bine w ith th e o th e r p o rtio n s of an alloy sim ila rly to a p u re m e ta l o r a solid so lu tio n , an d m ay fo rm solid so lutions o r e u te c tic s w ith o th e r solid so lu tio n s or th e p u re m etals. C e rta in classes of m etallic com pounds a re easily identified
Fi g. 10.—Al p h a- De l t a Eu t e c t o i d i n Ti n Br o n z e
x 2,000 D IA S .
by th e fa c t t h a t th e y solidify a t one d efin ite te m p e r a tu r e an d n o t over a ra n g e , a n d also th e fa c t t h a t th e y show no c o rin g effect in th e c a s t con
d itio n like m an y solid solutions.
P e rh a p s one of th e com m onest in te rm e ta llic com pounds of th is d efin ite class t h a t is m e t w ith in o rd in a ry bronze fo u n d in g p ra c tic e is th e com
po u n d copper p hosphide C u 3P .
T he p h o to m ic ro g ra p h F ig . 7 shows th is com pound -—a d e n d rite o r c ry s ta l of th e com pound a n d th e m ix tu re of th e com pound w ith p ra c tic a lly fine copper as a e u te c tic . F ig . 11 shows a n alloy con
ta in in g 4.5 p e r cen t, t i n ; 1.5 p e r c e n t, phosphorus,
a n d 94 p e r c en t, copper. The s tru c tu r e consists of a g ro u n d m ass of solid so lu tio n (alpha) of tin in copper, w hilst th e nodules are phosphide of copper C u3P .
General Characteristics of the Different Constituents.
I t is difficult to g en eralise on th e physical pro p e rtie s of v ario u s ty p es of c o n stitu e n ts, as exceptions a re fa irly fre q u e n t. G enerally sp eak ing, how ever, solid solutions a re com paratively so ft an d d u c tile a n d capable of b ein g w orked cold.
T hey a re sim ila r in th is resp ect to th e m ajo rity of p u re m etals. E u te c tic s a re h a rd —generally
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Fi g. 1 1 . — F r e e C o p p e r P h o s p h i d e N o d u l e s i n L o w - t i n P h o s p h o r B r o n z e x 1 0 0 d i a s .
m uch h a rd e r th a n th e ir c o n s titu e n t m etals— and a re u su ally v ery b r ittle a n d find very little com
m ercial a p p lic a tio n as castin g s, th o u g h h ere again exceptions m u st be m ade as one of th e most p ro m isin g of th e a lu m in iu m c a stin g alloys—.Alpax
— is th e e u te c tic alloy of alu m in iu m an d silicon.
E u te c to id s a re g en erally m uch h a rd e r th a n solid solutions b u t n o t so h a rd n o r so b r ittle as eutectics an d m an y com m ercial alloys—n o tab ly th e bronzes an d g u n m etals— c o n tain a n ap p reciab le an d im por
t a n t p ro p o rtio n o f a e u te c to id .
I n te rm e ta llic com pounds in th e ir p u re s ta te are h a rd e r a n d even m ore b r ittle th a n eutectics,
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p a rtic u la rly those w hich show th e c h a ra c te ris tic s of an in te rm e ta llic com pound in a m a rk e d m an n er.
Examination of Raw Materials.
I t m u st he confessed th a t , chiefly ow ing to th e p e rfe c tio n of m a n u f a c tu r in g co n d itio n s, th e scope of th e m icroscope in th e e x a m in a tio n of v irg in m etals is lim ite d . M etals such as elec tro ly tic
copper, ti n , lead a n d s p e lte r a n d nickel u su ally come in to th e m a rk e t in such a h ig h s t a te of p u r ity t h a t th e ir p erfo rm an ce fo r c a stin g purposes m ay be relied upon a n d v ery little save a n a ly tic a l co n tro l is needed, a n d t h a t only occasionally if s ta n d a r d b ra n d s a re p u rch ased . P e rh a p s th e one exception is in g o t copper.
“ B est Selected ” in g o t cop p er alw ays c o n ta in s an ap p reciab le a n d im p o r ta n t a m o u n t of oxygen.
C opper w hich is fre e fro m oxygen w ill n o t cast soundly in to in g o ts, a n d th e re fo re , in th e refin in g processes fo r copper, some oxygen is le f t in to en su re clean-looking in g o ts w hich w ill b re a k w ith a nice fibrous fra c tu re .
Fi g. 1 4 . - Ad m i r a l t y Gu n m e t a l, a s Ca s t x 2 0 0 d i a s,
As th e re is no ty p e of n on-ferrous alloy which is b e tte r f o r c o n ta in in g oxygen, i t is of im p o rt
ance to keep a c a re fu l w atch on th e possibility of in tro d u c in g th is c o n s titu e n t.
A good q u a lity of B .S . copper should n o t con
ta in m ore th a n 0.1 p e r c en t, oxygen, a n d excess of th is a m o u n t will re s u lt in poor castin g s being o b tain ed w ith m any alloys. The estim atio n chem ically of oxygen in copper is a len g th y and in tr ic a te process, b u t th e d etec tio n an d rough a p p ro x im a tio n u n d e r th e microscope is simple.
O xygen in copper ap p e a rs as cuprous oxide as a e u te c tic w ith copper, an d very sm all am ounts are easily discernible.
N eedless to say, elec tro ly tic copper does not suffer from th is im p u rity an d is w orth th e e x tra cost fo r c e rta in classes of alloys.
Alloys of the Bronze Class.
The basis of all alloys of th e bronze class is th e system co p p er-tin . T he alloys w hich find use in e n g in e e rin g p ra c tic e contain" up to 16 p e r cent, t i n a n d v a ry in g am o u n ts of zinc, lead or phosphorus.
P u r e c o p p e r-tin alloys—t h a t is, those m ade w ith o u t an y d e o x id a n t or a d d itio n of o th e r m etal—
a re seldom used now adays. C o p p er-tin alloys are am ong th o se w hich a re very susceptible to th e presence of oxide. O xide in th ese alloys ren d ers th e m ex trem ely sluggish w hen m olten an d liable 1» c a stin g defects a n d serious w eakness in s tr e n g th due to lack of cohesion in th e crystals.
T he oxide in th e se alloys is t i n oxide—one which it is ex trem ely h a rd to rem ove from th e m etal w hen once form ed.
C onsequently, th e g en eral p ra c tic e is to p rev en t th e fo rm a tio n of oxide by a d d in g e ith e r zinc or phosphorus to th e m e ta l a t th e r ig h t tim e —i.e., b efo re alloying th e tin .
C o p p er-tin alloys form a n extrem ely in te re s tin g an d im p o rta n t stu d y from a m etallo g rap h ic p o in t of view . T h eo retically copper is able to dissolve 12 to 13 p er cen t, of tin . T h a t is, alloys of copper w ith 12 to 13 p e r cen t, tin should form a homo
geneous solid solution. I n a c tu a l p ra c tic e th is does n o t occur, as th e co m p arativ ely ra p id ra te of cooling p rev en ts th e alloy from a tta in in g w hat is know n as its stab le s ta te , and v ery prolonged
a n n e a lin g is re q u ire d before th e alloys a t t a i n th is final s ta te . A ctu ally , in o rd in a ry b ro n ze-fo u n d in g p ra c tic e , th e lim it of so lu b ility is betw een 6 an d 7 p e r c e n t., d e p en d in g on th e c a stin g an d cooling co n d itio n s. T h a t is, a n alloy of 95 copper an d 5 p e r cen t, t i n w ill co n sist w hen solid of homo
geneous solid so lu tio n (save fo r a s lig h t co rin g effect) know n as alp h a , an d will show n o th in g b u t
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x 200 D IA S .
g ra in b o u n d aries u n d e r th e m icroscope. Such an alloy, w ith care, can he rolled an d d raw n .
Above 7 p e r c e n t, t i n i t w ill be seen t h a t a new c o n s titu e n t has m ade its a p p e a ra n c e . This is th e a lp h a -d e lta e u te c to id whose c o n s titu tio n a n d fo rm a tio n was discussed in th e p a ra g r a p h on eu tec to id s. I t is to th is a lp h a -d e lta e u tec to id t h a t th e bronzes owe th e i r w e a rin g p ro p e rtie s. I t is considerably h a rd e r th a n th e b e ta solid so lu tio n from w hich i t is form ed, an d g r e a t c a re m u st be ta k e n to see t h a t i t is form ed w here th e best q u a litie s of th e bronze a re re q u ire d . The a u th o r has know n m an y fa ilu re s a n d sho rtco m in g s in bronzes due to insufficient a p p re c ia tio n of th e fu n c tio n s of th is im p o rta n t c o n s titu e n t.