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The microstructures of directionally solidified alloys that undergo a peritectic transformation
THE
MICROSTRUCTURES UNDERGO A.
OF DIRECTIONALLY SOLIDIFIED A PERITECTIC TRANSFORMATION*
P.
TITCHENER
and
J. A.
ALLOYS
THAT
SPITTLE?
The microstructures have been examined of unidirect,ionally solidified alloys which sohdify under equilibrium conditions as two phase structures and hare compositions within a peritectic horizontal. It has been demonstrated that steady state solidification can only occur in these allqys when the solidliquid interface is non-planar. When solidification proceeds with a planar solid-liqmd interface the twv phases are deposited alternately. MICROSTRUCTURES On a Btudi6 les 1’Qquilibre en deux que la solidification Lorsque l’interface tirement. DIE
D’ALLIAGES SOLIDIFIES U&NE TRANSFORMATION
DIRECTIONNELLEMEST PERITECTIQUE
PRESEXTAST
microstructures d’alliagges solidifi6s directionnellement: ces alliages se solidifient B phases et leur composition est sit&e sur l’horizontale d’un pbritectique. On montre en r6gime permanent ne peut se produire que si l’interface solide-liquide n’est pas plan. solide-liquide est plan durant la solidification, les deux phases se d6posent alterna-
1\IIKROSTRUKTUR
GERICHTET TISCHEN
ERSTARRTER LEGIERUNGEN UMWANDLUNGES
MIT
PERITEK-
Die Mikrostruktur gerichtet erstarrter Legierungen, die unter Gleichgewichtsbedingungen als Zw-eiphasenstrukturen erstarren und eine Zusammensetzung innerhalb einer peritektischen Horizontalr besitzen, wurde untersucht. Es wird gezeigt, da13 in diesen Legierungen eine stationiire Erstarrung nul erfolgen kann, wenn die fest-fliissig-Grenzflgche nicht-planar ist. Schreitet die Erstarrung mit einel planaren fest -fltissig-Grenzfllche fort, so werden die beiden Phasen alternativ ausgelagert.
INTRODUCTION
(with a planar interface), for the unidirectional freezing of alloys in the range C, - C,. His appraisal was based on the assumption of no diffusion in the solid and, liquid mixing solely by diffusion. For an alloy of composition C, (or any hypoperitectic composition), Fig. 1, the st,eady stat,e was thought to correspond to the simultaneous growth of both the u and B phases producing a structure resembling that of a eutectic. In the case of an alloy of peritectic composition C, it was predicted that the steady &ate corresponds to the formation of p phase and is reached when the composition of the liquid at the interface becomes C,. Likewise for an alloy of composition C, (or any hpperperitectic composition) d! phase would grow initia,lly and p phase would form when the composition of the liquid at the interface reaches C,. Uhlmann and Chadwick(2) unidirectionally solidified alloys in the range C, - C, to examine Chalmers’ predict,ions. They concluded that st,eadp state solidification uit,h a planar interface cannot occur for such alloys and that the a. and B phases cannot freeze simultaneously. It was observed. for the limited range of growth conditions investigated, t’hat the steady state structures, of all alloys within the range C, -+ C,, consisted of dendrites of a phase in a matrix of fi phase. Chalmerst3) subsequently postulated that growth with higher temperature gradients ahead of the solid-liquid interface than those used b- Uhlmann and Chadwick’2) could lead to a reduction in branching of the u dendrites. Goodman(*) considered the solute distribution and microstructure t’hat would result, if alloys within the
There has been considerable speculation regarding the distribution and morphology of t.he phases resulting from the unidirectional solidification of binary liquid alloys whose compositions fall within a peritectic horizontal. In particular, growth under steady state conditions has been the subject of several attempts to int)uitively predict’ the microstructures of directionally grown solids. Before considering some of these predictions it is necessary to differentiate between those alloys solidifying, under equilibrium conditions, to give a single phase solid and those that solidify as two phase structures. In this paper the former (excepting an alloy of exact peritectic composition C,) are referred to as hyperperitectic alloys (compositions within the range C, - C,) and the latter hr_poperitectic alloys (compositions within the range C, -+ CD) as shojn in Fig. 1. Steady state solidification is established, during the unidirectional freezing of alloys, when the solute concentration profile of the boundary layer immediately ahead of the macroscopic solid-liquid int.erface reaches a steady state. A necessary condition for stead>--state growth is the absence of convection in the liquid and, dependent upon alloy composition and gro\Tth conditions, the solid-liquid interface may be planar or non-planar. Chalmers(l) n-as the first to predict the results that might be obtained, under steady state conditions * Received September 16, 1954. + Department of Metallurgy TJniversity College, Cardiff, Wales.
and
ACT_\ METALLURGICA,
23,
VOL.
Materials -4PRIL
1975
Science, 497
ACTA
498
NETALLURGICA,
VOL.
23,
1975
the influence of growth conditions on the microstruc. tures of directionally solidified binary alloys that undergo peritectic reactions and particularly to ascertain if it is possible to obtain steady state composite growth in h~pope~tectic alloys. EXPERIMENTAL
COMPOSITI6N
Fra. 1. Hypothetical
binary periteotic diagram.
range C, + CL were zone melted. It was assumed that there is no diffusion in the solid and perfect mixing in the liquid zone. The microstructures antici~~ should therefore be the same as those predicted by Chalmers. 11) It was argued that the passage of a molten zone along a rod of hyperperitect,ic composition would cause the rod to be divided into two distinct parts a and @, which is in agreement with Chalmers(i) hypothesis. However, it was conject’ured, for hypoperitectic alloys, that the passage of a zone along the rod would lead to the deposit.ion of solids of compositions C, and C,,, in an oscillatory manner. In recent years it has been demonstrated theoretically and experimentally, that alloys of off eutectio composition, l_ying within a eutectic horizontal, can grow as aligned composites (with a planar solidliquid interface) under stringent growth conditions.(~) The necessary conditions are a large G/R ratio (where G is the imposed temperature gradient in the liquid ahead of the solid-liquid interface and R is the growth rate of the solid) and, for steady state growth, the absence of convection in the liquid. In the light of these observations it has again been predicted that, under similar conditions, it should be possible to produce composite structures in peritectic systems(*) and that steady state growth of the a and J?phases at a planar interface is possible in principle for hypoperitectic alloys.(‘) On the other hand Kerr et uZ.(~) have surmised that steady state growth with gradients suficiently high to suppress dendritic growth of the a phase would produce in all alloys in the range C, -+ C, only one phase. The present research is a eont~uation of previously reported studies of the peritectic tra~fo~ation.(s,lO) It was cmried out in order to examine more extensively
PROCEDURE
To examine growth under steady state conditions it is necessary to avoid convection in the liquid induced either thermally or by solute redistribution during freezing. Thermal convection was prevented by solidif_ying the melts vertically upwards. The specimens, 0.7 cm dia. x 12 cm, long, were contained in silica tubes which were lowered at known rates through a stationary furnace. The tubes passed directly into water held at a constant temperature in a constant head container. Extreme care was exercised to avoid any vibrations or temperature variations which might lead to growth rate fluctuations. All the specimens were solidified with high temperature gradients in the melt ahead of the solid-liquid interface. For growth rates greater than IO4 cm/see the complete specimen was directionally solidified. At lower growth rates the specimens were quenched after ~3 cm had solidified. Microstructural examinat,ion was carried out on longitudinal sections. Observations were confined to the study of hypoperitectic alloys. It is reasonable to assume that the steady state rni~r~t~ctu~s of h~~~tecti~ and peritectic alloys will correspond either to those predicted and observed by Uhlmann and Chadwicktz) (for a non-planar interface) or those predicted by Chalmers(‘) (for planar int.erface growth). ALLOY
SYSTEMS
The two systems selected for examination tvere Sn-Sb and Zn-Cu and the compositions of the alloys solidified were within the peritectic horizontals for the reactions C,, + C, -+ Ccsn) (246°C) and C, -!C, + C, (424’C) respectively, Fig. 2. The relatively low invariant temperatures permitted easy establishment of high temperature gradients in the melt during solidification. To prevent convection in the liquid, due to solute redistribution on freezing, the boundary layer which forms at the solid-liquid interface front must be enriched in the heavier component (when solidifying vertically upwards). This is so for the Sn-Sb alloys. However, in the case of the Zn-Cu alloys the lighter component is rejected during solidification. Xevertheless it was felt that it would be in&ructive to also examine the latter system since it is typical of nearly all systems which display peritectic reactions in that
TITCHEXER
200
150
-
DIRECTIOXALLP
SPITTLE:
AX‘D
’ I I I
SOLIDIFIED
PERITECTIC
499
\ \ t 70
It 60
I 00 Wt.%%*
\
t 90
100
phase
diagrams
mPb Zn -;c Cb)
cd) FIU. 2. Portions
ALLOYS
of
the
binary
the Iowest melting point component also has the lowest density. OBSERVATIONS
Sn-Sb system Several alloys were studied in the range 11-15 wt. 7: Sb with temperature gradients ahead of the solidliquid interface varying from 200 to 55O’C/cm. The microstructures described below for a 15 ut. % Sb alloy are typical of those observed in all the alloys. Figure 3 shows the ~cr~tructure for a growth rate of 5 x 1O-6 emfsec and a temperature gradient, of 3OO’C/cm. It consists of aligned j3’ phase, which has solidified in a faceted-cellular manner, in an (Sn) matrix. By solidifying under conditions resulting in a larger G/R ratio non-planar growth of the /Y phase was prevent,ed. Figure 4 shows the struct,ure obtained at a growth rate of 7.1 x 10m6cmjsec with a gradient The 8’ initially solidified with a planar of ~550”C/cm. int.erface. Subsequently ihe structure broke down to one of alternate bands of (Sn) and ,f?‘, It was observed that for the same growth conditions the amount, of /Y’formed. prior t,o the appearance of the first (Sn) band, increased as the antimony content, increased from 11 t.o 15 u-t. 7:.
(a)
Sn-Sb
and
(b)
Zn-Cu.
Fig. 5. In every specimen the dendrites were present throughout the whole length of the ingot. Studies at lower growth rates were restricted to a 3.5 n-t. % Cu alloy. At a growth rate of 1.6 x lo-’ cm/ set the microstructure still consisted of branched s dendrites, with no obvious morphology change, in an
Zn-Cu systerrr Hypoperitectic alloys were unidirectionally sohditied at growth rates in the range 4.6 x 1W cm/set -+ 1.6 :/ lWs cm/set with the temperature gradient G maintained at %25O”C/cm. It was observed, for several alloys in the range 2.7-8 wt. “/d(3.1,that at a growth rate of 1.6 x 10es cm/ see t‘he microst,ructur~ of the solidified specimens consisted of branched E dendrites in an 7 matrix,
FZG. 3. Sn-15
wt..y;t Sb allov undirectionafly
solidified,
at a rate of 6 x lo-& cmisec &th a gmdient of 3W’C/cm,
from position A. The structure of the major part of the length consists of facetedcells of /J’ (black) in an fSn) matrix (grey). Mttgn. x 5.
500
SCT.4
JIETALLVRGICA,
VOL.
23,
1975
q matrix. However, approximately the last 2 cm of the specimen to solidify consisted solely of ‘1 phase. The morphology of the Ephase in a specimen solidified at 2.9 X 1O-5 cm/see was cellular or rod dendritie, Fig. 6. The volume fraction of E in the specilnen cross section was high cf. Fig. 5. Banding was also &dent, the microstructure varying in turn from c rod dendrites plus 751to 9. At 13 iower growth rate 4.6 X lo-” em/set the E phase initially grew with a planar interface and, after some distance, alternate separation of bands of q and E was apparent, Fig. i.
t
FIG 4. Sn-15 wt.O&Sb aIloy nncIirectionaIly solidified,
at a rate of 7.1 x 10-B cm/see with a gradient of 5E03C/cm from position A. ~acrostructure shows bands of 6 (black) and (Sn) (grey). Magn. x .3.
Fro. 6. G-3.6 wt.?;1 Cu alloy undirectionally solidified at 2.9 x lo-& cm/see with a gradient of ?50’C/cm. The microstructure is banded and consists of E rod dendrites (white) in an ‘1 matrix (black). Jlapn. .60.
DISCUSSION
expected, and observed in both s?-stems, increasing the G/lZ ratio suppresses non-planur growth of t~heprimary phase during the unidirectional freezing of hypoperitectic alloys. The individual systems will now be considered in detail. As
Fro. 5. Microstructure of a Zn-3.5 wt.‘j/o Cu alloy, u~di~tionally solidified at a rate of 1.6 x 10-3cmfsec with a gradient of 250’Cfcm, showing aligned branched e dendrites (white) in an q matrix (black). Magn. x60.
The solidification of the alloys in this system took place with no convection in the liquid which is the condition necessary for (but not necessaril? resulting in) steady state growth. Consider first, with respect to Fig. 2(a), the solute redistribution which takes place during the freezing of an alloy of composition C, with a planar interface, and the possible steady stats situation that might develop. The first solid to freeze out is #I’ and as freezing progresses the liquid adjacent to the solid-liquid interface becomes continuously
TITCHESER
AHD
SPITTLE:
DIRECTIOXALLT
SOLIDIFIED
PERITECTIC
ALLOTS
501
anticipated as was observed in the present. investigation. Zn-Cu .system
t-0
f-A
FIG. 7. G-3.5 wt.?& Cu alloy solidified at a rate of 4.6 x 10-O cm/se0 with a gradient of 250’C/cm. Unidirectional freeziug was initiated at position A and the Macrostructure specimen quenched at posit,ion B. shows bands of E (black) and 17 (grey). Magn. x5.
richer in the solvent tin. Ultima~ly the t.iu concentration will reach a value equal to, or more probably greater t,han, 91 wt.% Sn (i.e. CL) whereupon the (Sn) phase uill be nucleated on the 8 surface. This is shown in Fig. S(a). Analogous t’o off-eutectic growth’5) it might be expected that nucleation of the (Sn) phase would lead t,o steady state composite growth of the two phases with a planar interface. The average composition of the composite growing would then be C= as shown in Fig. 8(b). However, when the liquid at the interface attains the composition C, (or some composition slightly richer in Sn) it is in equilibrium with solid of composition C,,. Therefore following nucleation of the (Sn) phase the perit,ectie reaction should take place If it does and as a consequence (Car + Cz - C,,,,). the ,L?’surface is covered by (Sn) subsequent direct freezing of (Sn) from the melt will cause the liquid boundary layer concentration profile t,o change. As the (Sn) phase solidifies the boundary layer will become richer in the solute antimony until nucleat!ion and growth of ,B’ can again occur. Continuous alternat,e deposition of /3’ and (Sn) would therefore be
At the fs&est growth rate, for all the alloys examined the E phase solid&d dendritically and the microstructure of the solid corresponded to that predicted and observed by Uhlmann and Chadwick@) (for a non-planar solid-liquid interface). The growing tip radii of the E dendrites will be small and conaequently most of the zinc will be rejected laterally into the interdendritic regions with little zinc enrichment ahead of the growing tips. The average composition along t,he ingot will be almost uniform and the microstructure therefore corresponds to that, expected under steady-state conditions. There was evidence, from the alloy solidified at 1.6 *: lO-* cm/see of longitudinal segregation at the lower growth rates. This is presumably attributable to convection in the liquid induced by the density inversion created when zinc is rejected by the solid during solidification. The increase in tip radii of the growing E dendrites at the growth rate of 2.9 x 10e5 cm/see will result in a greater amount, of zinc being rejected normally to the
SOLID
I
LIQUID
“--
=d
f DISTANCE
illustration of the probable variation in composition in the solid-liquid interface region, assuming no convection in the liquid, during t,he unidirectional solidification of a hypoperitectic Sn-Sb alloy (a) immediately prior to nucleation of the (Sn) phase and (b) in the esent of the estabhshment of steady-state composite growth. FIG.
8. Schematic
ACTA
502
XETALLURGZCA,
The dendrite macroscopic solid-liquid interface. morphology also changes from a branched to rod type. Growth of the E dendrites will continue until the zinc concentration ahead of the tips reaches a value equal to or greater than C,. The liquid and E phases then react peritectically leading to termination of Egrowth. Growth of q then continues until the boundary layer is sticiently depleted in zinc for the E to again be nucleated on the 7 surface. The cyclic formation of bands of E + 11 and 7 presumably continues until such time that the bulk liquid composition approaches C, when the remaining liquid will solidify as 7 phase. At the slowest growth rate after initial solidification of E with a planar interface a striated structure developed, consisting of bands of 7 and E, for the same reason as previously explained for the Sn-Sb alloys.
VOL.
1975
simultaneous growth of the a and b phases at a planar interface. (3) Hypoperitectic alloys, solidified under conditions similar to those producing steady-state composite structures in off-eutectic allays,(5) display striated structures due to the cyclic separation of (x and b phase bands. ACKNOWLEDGEMENTS
The authors would like to thank Professor H. B Lloyd for permission to use laboratory facilities. One of them (A. P. T.) is indebted to the Science Research Council for financial support. Addendwn The authors wish to comment that since submission of the original manuscript the following paper has appeared in print which arrives at similar conclusions from observations on the &-Cd system.
CONCLUSIONS
The following conclusions have been reached regarding the microstructures of unidirectionally solidified hypoperitectic alloys, i.e. alloys undergoing a peritectic transformation, expressed as C, + C, + C,, and having compositions between C, and C, (where C,, C, and C, correspond to the compositions of the a, liquid and ,!Iphases in equilibrium during the transformation). (1) Steady state solidification with a non-planar solid-liquid interface produces microstructures aa previously predicted,@) i.e. aligned a phase particles in a p matrix. (2) Solidification with G/R ratios sufficiently large to suppress non-planar freezing does not give rise to
23,
W. J. BOETTIXJEI~, Met. Trans. 5, 2023 (19i4). REFERENCES 1. B. CZULMERS, Phytial (1959). 2. D. R. Umxma
835 (1961).
6. i. 8. 9. 10.
Wiley,
Sew
York
and G. A. CHADWICK, dcta -Vet. 9,
3. B. CHALMEBS. Princi&-s 4. 5.
Metallurgy.
of Sdidificalion. Wiley, Sew York (1964). C. H. L. GOODMAN,Research, Land. 7, 163 (1954). I?. R. YOLLARD and M. C. FLEXIN~S, Trans. AINE !289, 1634 (1967). G. A. CHADWICK, The Solidifccation of iUe.@ls. The Iron and Steel Inst. London, Publ. 110, 138 (1966). Y. C. FLEXINOS. Sdidification Proceseivlg. McGraw” Hill, Xew York (1974). H. W. KERR, J. CISSE and G. F. Bourna, dcta -Vet. aa, 877 (1974). J. A. SPIl-lXE, J. 1n8t. .&tiEe 98, 124 (1970). A. P. TITCHENERand J. A. SPITTLE, XelaE Sci. 8, 112 (1974).
MICROSTRUCTURES UNDERGO A.
OF DIRECTIONALLY SOLIDIFIED A PERITECTIC TRANSFORMATION*
P.
TITCHENER
and
J. A.
ALLOYS
THAT
SPITTLE?
The microstructures have been examined of unidirect,ionally solidified alloys which sohdify under equilibrium conditions as two phase structures and hare compositions within a peritectic horizontal. It has been demonstrated that steady state solidification can only occur in these allqys when the solidliquid interface is non-planar. When solidification proceeds with a planar solid-liqmd interface the twv phases are deposited alternately. MICROSTRUCTURES On a Btudi6 les 1’Qquilibre en deux que la solidification Lorsque l’interface tirement. DIE
D’ALLIAGES SOLIDIFIES U&NE TRANSFORMATION
DIRECTIONNELLEMEST PERITECTIQUE
PRESEXTAST
microstructures d’alliagges solidifi6s directionnellement: ces alliages se solidifient B phases et leur composition est sit&e sur l’horizontale d’un pbritectique. On montre en r6gime permanent ne peut se produire que si l’interface solide-liquide n’est pas plan. solide-liquide est plan durant la solidification, les deux phases se d6posent alterna-
1\IIKROSTRUKTUR
GERICHTET TISCHEN
ERSTARRTER LEGIERUNGEN UMWANDLUNGES
MIT
PERITEK-
Die Mikrostruktur gerichtet erstarrter Legierungen, die unter Gleichgewichtsbedingungen als Zw-eiphasenstrukturen erstarren und eine Zusammensetzung innerhalb einer peritektischen Horizontalr besitzen, wurde untersucht. Es wird gezeigt, da13 in diesen Legierungen eine stationiire Erstarrung nul erfolgen kann, wenn die fest-fliissig-Grenzflgche nicht-planar ist. Schreitet die Erstarrung mit einel planaren fest -fltissig-Grenzfllche fort, so werden die beiden Phasen alternativ ausgelagert.
INTRODUCTION
(with a planar interface), for the unidirectional freezing of alloys in the range C, - C,. His appraisal was based on the assumption of no diffusion in the solid and, liquid mixing solely by diffusion. For an alloy of composition C, (or any hypoperitectic composition), Fig. 1, the st,eady stat,e was thought to correspond to the simultaneous growth of both the u and B phases producing a structure resembling that of a eutectic. In the case of an alloy of peritectic composition C, it was predicted that the steady &ate corresponds to the formation of p phase and is reached when the composition of the liquid at the interface becomes C,. Likewise for an alloy of composition C, (or any hpperperitectic composition) d! phase would grow initia,lly and p phase would form when the composition of the liquid at the interface reaches C,. Uhlmann and Chadwick(2) unidirectionally solidified alloys in the range C, - C, to examine Chalmers’ predict,ions. They concluded that st,eadp state solidification uit,h a planar interface cannot occur for such alloys and that the a. and B phases cannot freeze simultaneously. It was observed. for the limited range of growth conditions investigated, t’hat the steady state structures, of all alloys within the range C, -+ C,, consisted of dendrites of a phase in a matrix of fi phase. Chalmerst3) subsequently postulated that growth with higher temperature gradients ahead of the solid-liquid interface than those used b- Uhlmann and Chadwick’2) could lead to a reduction in branching of the u dendrites. Goodman(*) considered the solute distribution and microstructure t’hat would result, if alloys within the
There has been considerable speculation regarding the distribution and morphology of t.he phases resulting from the unidirectional solidification of binary liquid alloys whose compositions fall within a peritectic horizontal. In particular, growth under steady state conditions has been the subject of several attempts to int)uitively predict’ the microstructures of directionally grown solids. Before considering some of these predictions it is necessary to differentiate between those alloys solidifying, under equilibrium conditions, to give a single phase solid and those that solidify as two phase structures. In this paper the former (excepting an alloy of exact peritectic composition C,) are referred to as hyperperitectic alloys (compositions within the range C, - C,) and the latter hr_poperitectic alloys (compositions within the range C, -+ CD) as shojn in Fig. 1. Steady state solidification is established, during the unidirectional freezing of alloys, when the solute concentration profile of the boundary layer immediately ahead of the macroscopic solid-liquid int.erface reaches a steady state. A necessary condition for stead>--state growth is the absence of convection in the liquid and, dependent upon alloy composition and gro\Tth conditions, the solid-liquid interface may be planar or non-planar. Chalmers(l) n-as the first to predict the results that might be obtained, under steady state conditions * Received September 16, 1954. + Department of Metallurgy TJniversity College, Cardiff, Wales.
and
ACT_\ METALLURGICA,
23,
VOL.
Materials -4PRIL
1975
Science, 497
ACTA
498
NETALLURGICA,
VOL.
23,
1975
the influence of growth conditions on the microstruc. tures of directionally solidified binary alloys that undergo peritectic reactions and particularly to ascertain if it is possible to obtain steady state composite growth in h~pope~tectic alloys. EXPERIMENTAL
COMPOSITI6N
Fra. 1. Hypothetical
binary periteotic diagram.
range C, + CL were zone melted. It was assumed that there is no diffusion in the solid and perfect mixing in the liquid zone. The microstructures antici~~ should therefore be the same as those predicted by Chalmers. 11) It was argued that the passage of a molten zone along a rod of hyperperitect,ic composition would cause the rod to be divided into two distinct parts a and @, which is in agreement with Chalmers(i) hypothesis. However, it was conject’ured, for hypoperitectic alloys, that the passage of a zone along the rod would lead to the deposit.ion of solids of compositions C, and C,,, in an oscillatory manner. In recent years it has been demonstrated theoretically and experimentally, that alloys of off eutectio composition, l_ying within a eutectic horizontal, can grow as aligned composites (with a planar solidliquid interface) under stringent growth conditions.(~) The necessary conditions are a large G/R ratio (where G is the imposed temperature gradient in the liquid ahead of the solid-liquid interface and R is the growth rate of the solid) and, for steady state growth, the absence of convection in the liquid. In the light of these observations it has again been predicted that, under similar conditions, it should be possible to produce composite structures in peritectic systems(*) and that steady state growth of the a and J?phases at a planar interface is possible in principle for hypoperitectic alloys.(‘) On the other hand Kerr et uZ.(~) have surmised that steady state growth with gradients suficiently high to suppress dendritic growth of the a phase would produce in all alloys in the range C, -+ C, only one phase. The present research is a eont~uation of previously reported studies of the peritectic tra~fo~ation.(s,lO) It was cmried out in order to examine more extensively
PROCEDURE
To examine growth under steady state conditions it is necessary to avoid convection in the liquid induced either thermally or by solute redistribution during freezing. Thermal convection was prevented by solidif_ying the melts vertically upwards. The specimens, 0.7 cm dia. x 12 cm, long, were contained in silica tubes which were lowered at known rates through a stationary furnace. The tubes passed directly into water held at a constant temperature in a constant head container. Extreme care was exercised to avoid any vibrations or temperature variations which might lead to growth rate fluctuations. All the specimens were solidified with high temperature gradients in the melt ahead of the solid-liquid interface. For growth rates greater than IO4 cm/see the complete specimen was directionally solidified. At lower growth rates the specimens were quenched after ~3 cm had solidified. Microstructural examinat,ion was carried out on longitudinal sections. Observations were confined to the study of hypoperitectic alloys. It is reasonable to assume that the steady state rni~r~t~ctu~s of h~~~tecti~ and peritectic alloys will correspond either to those predicted and observed by Uhlmann and Chadwicktz) (for a non-planar interface) or those predicted by Chalmers(‘) (for planar int.erface growth). ALLOY
SYSTEMS
The two systems selected for examination tvere Sn-Sb and Zn-Cu and the compositions of the alloys solidified were within the peritectic horizontals for the reactions C,, + C, -+ Ccsn) (246°C) and C, -!C, + C, (424’C) respectively, Fig. 2. The relatively low invariant temperatures permitted easy establishment of high temperature gradients in the melt during solidification. To prevent convection in the liquid, due to solute redistribution on freezing, the boundary layer which forms at the solid-liquid interface front must be enriched in the heavier component (when solidifying vertically upwards). This is so for the Sn-Sb alloys. However, in the case of the Zn-Cu alloys the lighter component is rejected during solidification. Xevertheless it was felt that it would be in&ructive to also examine the latter system since it is typical of nearly all systems which display peritectic reactions in that
TITCHEXER
200
150
-
DIRECTIOXALLP
SPITTLE:
AX‘D
’ I I I
SOLIDIFIED
PERITECTIC
499
\ \ t 70
It 60
I 00 Wt.%%*
\
t 90
100
phase
diagrams
mPb Zn -;c Cb)
cd) FIU. 2. Portions
ALLOYS
of
the
binary
the Iowest melting point component also has the lowest density. OBSERVATIONS
Sn-Sb system Several alloys were studied in the range 11-15 wt. 7: Sb with temperature gradients ahead of the solidliquid interface varying from 200 to 55O’C/cm. The microstructures described below for a 15 ut. % Sb alloy are typical of those observed in all the alloys. Figure 3 shows the ~cr~tructure for a growth rate of 5 x 1O-6 emfsec and a temperature gradient, of 3OO’C/cm. It consists of aligned j3’ phase, which has solidified in a faceted-cellular manner, in an (Sn) matrix. By solidifying under conditions resulting in a larger G/R ratio non-planar growth of the /Y phase was prevent,ed. Figure 4 shows the struct,ure obtained at a growth rate of 7.1 x 10m6cmjsec with a gradient The 8’ initially solidified with a planar of ~550”C/cm. int.erface. Subsequently ihe structure broke down to one of alternate bands of (Sn) and ,f?‘, It was observed that for the same growth conditions the amount, of /Y’formed. prior t,o the appearance of the first (Sn) band, increased as the antimony content, increased from 11 t.o 15 u-t. 7:.
(a)
Sn-Sb
and
(b)
Zn-Cu.
Fig. 5. In every specimen the dendrites were present throughout the whole length of the ingot. Studies at lower growth rates were restricted to a 3.5 n-t. % Cu alloy. At a growth rate of 1.6 x lo-’ cm/ set the microstructure still consisted of branched s dendrites, with no obvious morphology change, in an
Zn-Cu systerrr Hypoperitectic alloys were unidirectionally sohditied at growth rates in the range 4.6 x 1W cm/set -+ 1.6 :/ lWs cm/set with the temperature gradient G maintained at %25O”C/cm. It was observed, for several alloys in the range 2.7-8 wt. “/d(3.1,that at a growth rate of 1.6 x 10es cm/ see t‘he microst,ructur~ of the solidified specimens consisted of branched E dendrites in an 7 matrix,
FZG. 3. Sn-15
wt..y;t Sb allov undirectionafly
solidified,
at a rate of 6 x lo-& cmisec &th a gmdient of 3W’C/cm,
from position A. The structure of the major part of the length consists of facetedcells of /J’ (black) in an fSn) matrix (grey). Mttgn. x 5.
500
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q matrix. However, approximately the last 2 cm of the specimen to solidify consisted solely of ‘1 phase. The morphology of the Ephase in a specimen solidified at 2.9 X 1O-5 cm/see was cellular or rod dendritie, Fig. 6. The volume fraction of E in the specilnen cross section was high cf. Fig. 5. Banding was also &dent, the microstructure varying in turn from c rod dendrites plus 751to 9. At 13 iower growth rate 4.6 X lo-” em/set the E phase initially grew with a planar interface and, after some distance, alternate separation of bands of q and E was apparent, Fig. i.
t
FIG 4. Sn-15 wt.O&Sb aIloy nncIirectionaIly solidified,
at a rate of 7.1 x 10-B cm/see with a gradient of 5E03C/cm from position A. ~acrostructure shows bands of 6 (black) and (Sn) (grey). Magn. x .3.
Fro. 6. G-3.6 wt.?;1 Cu alloy undirectionally solidified at 2.9 x lo-& cm/see with a gradient of ?50’C/cm. The microstructure is banded and consists of E rod dendrites (white) in an ‘1 matrix (black). Jlapn. .60.
DISCUSSION
expected, and observed in both s?-stems, increasing the G/lZ ratio suppresses non-planur growth of t~heprimary phase during the unidirectional freezing of hypoperitectic alloys. The individual systems will now be considered in detail. As
Fro. 5. Microstructure of a Zn-3.5 wt.‘j/o Cu alloy, u~di~tionally solidified at a rate of 1.6 x 10-3cmfsec with a gradient of 250’Cfcm, showing aligned branched e dendrites (white) in an q matrix (black). Magn. x60.
The solidification of the alloys in this system took place with no convection in the liquid which is the condition necessary for (but not necessaril? resulting in) steady state growth. Consider first, with respect to Fig. 2(a), the solute redistribution which takes place during the freezing of an alloy of composition C, with a planar interface, and the possible steady stats situation that might develop. The first solid to freeze out is #I’ and as freezing progresses the liquid adjacent to the solid-liquid interface becomes continuously
TITCHESER
AHD
SPITTLE:
DIRECTIOXALLT
SOLIDIFIED
PERITECTIC
ALLOTS
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anticipated as was observed in the present. investigation. Zn-Cu .system
t-0
f-A
FIG. 7. G-3.5 wt.?& Cu alloy solidified at a rate of 4.6 x 10-O cm/se0 with a gradient of 250’C/cm. Unidirectional freeziug was initiated at position A and the Macrostructure specimen quenched at posit,ion B. shows bands of E (black) and 17 (grey). Magn. x5.
richer in the solvent tin. Ultima~ly the t.iu concentration will reach a value equal to, or more probably greater t,han, 91 wt.% Sn (i.e. CL) whereupon the (Sn) phase uill be nucleated on the 8 surface. This is shown in Fig. S(a). Analogous t’o off-eutectic growth’5) it might be expected that nucleation of the (Sn) phase would lead t,o steady state composite growth of the two phases with a planar interface. The average composition of the composite growing would then be C= as shown in Fig. 8(b). However, when the liquid at the interface attains the composition C, (or some composition slightly richer in Sn) it is in equilibrium with solid of composition C,,. Therefore following nucleation of the (Sn) phase the perit,ectie reaction should take place If it does and as a consequence (Car + Cz - C,,,,). the ,L?’surface is covered by (Sn) subsequent direct freezing of (Sn) from the melt will cause the liquid boundary layer concentration profile t,o change. As the (Sn) phase solidifies the boundary layer will become richer in the solute antimony until nucleat!ion and growth of ,B’ can again occur. Continuous alternat,e deposition of /3’ and (Sn) would therefore be
At the fs&est growth rate, for all the alloys examined the E phase solid&d dendritically and the microstructure of the solid corresponded to that predicted and observed by Uhlmann and Chadwick@) (for a non-planar solid-liquid interface). The growing tip radii of the E dendrites will be small and conaequently most of the zinc will be rejected laterally into the interdendritic regions with little zinc enrichment ahead of the growing tips. The average composition along t,he ingot will be almost uniform and the microstructure therefore corresponds to that, expected under steady-state conditions. There was evidence, from the alloy solidified at 1.6 *: lO-* cm/see of longitudinal segregation at the lower growth rates. This is presumably attributable to convection in the liquid induced by the density inversion created when zinc is rejected by the solid during solidification. The increase in tip radii of the growing E dendrites at the growth rate of 2.9 x 10e5 cm/see will result in a greater amount, of zinc being rejected normally to the
SOLID
I
LIQUID
“--
=d
f DISTANCE
illustration of the probable variation in composition in the solid-liquid interface region, assuming no convection in the liquid, during t,he unidirectional solidification of a hypoperitectic Sn-Sb alloy (a) immediately prior to nucleation of the (Sn) phase and (b) in the esent of the estabhshment of steady-state composite growth. FIG.
8. Schematic
ACTA
502
XETALLURGZCA,
The dendrite macroscopic solid-liquid interface. morphology also changes from a branched to rod type. Growth of the E dendrites will continue until the zinc concentration ahead of the tips reaches a value equal to or greater than C,. The liquid and E phases then react peritectically leading to termination of Egrowth. Growth of q then continues until the boundary layer is sticiently depleted in zinc for the E to again be nucleated on the 7 surface. The cyclic formation of bands of E + 11 and 7 presumably continues until such time that the bulk liquid composition approaches C, when the remaining liquid will solidify as 7 phase. At the slowest growth rate after initial solidification of E with a planar interface a striated structure developed, consisting of bands of 7 and E, for the same reason as previously explained for the Sn-Sb alloys.
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simultaneous growth of the a and b phases at a planar interface. (3) Hypoperitectic alloys, solidified under conditions similar to those producing steady-state composite structures in off-eutectic allays,(5) display striated structures due to the cyclic separation of (x and b phase bands. ACKNOWLEDGEMENTS
The authors would like to thank Professor H. B Lloyd for permission to use laboratory facilities. One of them (A. P. T.) is indebted to the Science Research Council for financial support. Addendwn The authors wish to comment that since submission of the original manuscript the following paper has appeared in print which arrives at similar conclusions from observations on the &-Cd system.
CONCLUSIONS
The following conclusions have been reached regarding the microstructures of unidirectionally solidified hypoperitectic alloys, i.e. alloys undergoing a peritectic transformation, expressed as C, + C, + C,, and having compositions between C, and C, (where C,, C, and C, correspond to the compositions of the a, liquid and ,!Iphases in equilibrium during the transformation). (1) Steady state solidification with a non-planar solid-liquid interface produces microstructures aa previously predicted,@) i.e. aligned a phase particles in a p matrix. (2) Solidification with G/R ratios sufficiently large to suppress non-planar freezing does not give rise to
23,
W. J. BOETTIXJEI~, Met. Trans. 5, 2023 (19i4). REFERENCES 1. B. CZULMERS, Phytial (1959). 2. D. R. Umxma
835 (1961).
6. i. 8. 9. 10.
Wiley,
Sew
York
and G. A. CHADWICK, dcta -Vet. 9,
3. B. CHALMEBS. Princi&-s 4. 5.
Metallurgy.
of Sdidificalion. Wiley, Sew York (1964). C. H. L. GOODMAN,Research, Land. 7, 163 (1954). I?. R. YOLLARD and M. C. FLEXIN~S, Trans. AINE !289, 1634 (1967). G. A. CHADWICK, The Solidifccation of iUe.@ls. The Iron and Steel Inst. London, Publ. 110, 138 (1966). Y. C. FLEXINOS. Sdidification Proceseivlg. McGraw” Hill, Xew York (1974). H. W. KERR, J. CISSE and G. F. Bourna, dcta -Vet. aa, 877 (1974). J. A. SPIl-lXE, J. 1n8t. .&tiEe 98, 124 (1970). A. P. TITCHENERand J. A. SPITTLE, XelaE Sci. 8, 112 (1974).