SOhlE Z I S C AILLoYs BY B. E. CURRY
Antimony-Zinc Considerable v o r k has been done on the antimon>--zinc yeries of alloys but even the latest efforts leave much t o be desired in accuracy and completeness. -1number of definite compounds ha\-e been credited to thi\ series. Cooke’ v a s the firit to Ivork n i t h these alloj-. IIe iiolated compounds to ~ ~ h i hc c h qa1-e the formula> %n,Sti,ind ZnSh Herschkoxitqch’ made ‘1 iei-ies of electroniotil c iorce determinations from TI hich hc deduced tlie esiztcncc of the conipouncl ZnSbl. The freezinq point curve n a \ tlctcrinined bj Rolland-Gosselin.’ Thii c u r w consisted of four liranches and indicated t u o compounds of uncertain compo\itioi;. He>-cock a n d Sei-ille deterrnined only a small portion of the freezing point curie and on this account their data are not of \-slue here More recentl!- JIonkeniej-er5 liai redetermined the freezing point curve and has worked o1-u the series microscop~call~-. This freezing point curve consists of four branches mith two maxima and three eutectics. The maxima appear at 3.j and 4 j percent zinc nhile the eutectics appear a t 2 2 , 3 ; and 9;. j percent zinc. The diagram is shown in Fig. I . The four phases separating from the melt are pure antimony ZnSb, Zn,Sb, and pure zinc. The inversion point noted in pure zinc a t 321 O introduces a fifth phase which would seem t o indicate an allotropic form of zinc. If not a second form of zinc, the data are incomplete. Also the fact that the inversion temA ~ I Jour. . Sci. ( z ) , 18,229 ( I S L j l ) ; 20, 2 7 6 ( I 8 j j ) ; 30, 194 (1860). Phil. Nag. [4],49, 4oj (1860). Zeit. phys. Chem., 27, 123 (1888). 3Bull. SOC.d’Encouragement ( j ) , I, 13o1, 1310 (1896). Cf. Gautier: Contributions B l’ktude des alliages, 101, I I O (1901). Jour. Cheni. SOC., 71, 394, 402 (189;). 5 Zeit. anorg. Chernie, 43, 182 (190j). . -
B. E . Curry
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perature varies over a range of 30' indicates inaccuracies or error due t o unstable conditions. Theoretically across the Zn field this inversion temperature must be constant. Zn,Sb,
+
1
Fig.
I
Independently Zemczuznyj duplicated the work of Monkemeyer. He found similar freezing point data except that no maximum appeared at 3 j percent zinc and therefore no minimum between this and the -1.5 percent composition. -Us0 his work proved that the inversion or modification which Monkemeyer found in zinc at about 320' was due t o a change in the Zn,Sb, compound and not to a change in the zinc. The diagram is shown in Fig. 2 . This work also leaves the Zn,Sb, inversion point determinations incomplete because the line KIM representing the inversion temperature has no logical ending. Theoretically this line should have a constant temperature across the Zn,Sb, field and should end somewhere beyond the Zn,Sb, composition. The cause of the wide range of the inversion temperature is evidently due to supercooling. In this work I have redetermined the melting point curve. In the main it is coincident with the curve obtained Zeit. anorg. Chem., 49, 384 (1906)
Some Zinc Alloys
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by Zemczuznyj. The observations were made on heating curves and the difficulties which are met with on cooling curves were thus eliminated. As a further precaution the observations were made on ingots that had been annealed and which had therefore reached equilibrium. On this
Fig.
2
Fig. 3
account the heat changes which Zemczuznyj found over the ZnSb + Sb field did not appear in these observations. The heating curve data follow in Table I. These data are represented graphically in Fig. 3. In the melting point curve,
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which consists of five branches, only one maximum and two eutectics appear, one at 2 2 percent and the other a t 97.5 percent zinc. The series forms a melting point curve with five branches. It appears, however, as will be explained later, that only four phases actually separate from the melt. Along the branch AB pure antimony separates from the melt; along BC the stable phase is ZnSb; along CDE the solid solution a is in equilibrium with the melt. The ,8phase separates from the melt along the new branch EF; along FG the stable phase is pure zinc. TABLEI Percent zinc
Liquidus
IO0
85
419O 411 43 7 457 470
--
480 488
70 65 60
496
97.5 95 90 80 I3
--
33
50
45 43 41 39 37 35 32 30 25 23
I9 15 IO
5 0
505 5 10 5 30 530
565
562 559 555 548 545 542 537 522 515 517
Inversion
405O 405 405 405 405 405 405
405 405 405
405 40 5 43 5 485 485 48 5 -
-
437 O 437 43 7 437 43 7 437 437 437 43 7 437 480 500
500 j 00
-
542 57 0 600 63 1
The alloys containing 0-35 percent zinc show but two heat absorptions on heating curves. These occur at the
So me 2iw c A 1loys
593
liquidus and at the eutectic temperature jogo. The heat evolution which Zemczuznyj noted below the eutectic on cooling curves across this range does not appear in the heating curves. The 35 percent zinc alloy shows but one heat absorption on the heating curve and that a t 546O, the melting point. The alloys containing from 3\5-45 percent zinc show four breaks in the heating curves. These represent two inversion temperatures, the solidus and the liquidus. The horizontal portion of the solidus represents the teniperature at which the phase changes into ZnSb and vice versa. On cooling the melted alloys, the partial inversion of a into ZnSb may not occur until many degrees below the solidus. This \vas noted by Zemczuznyj. The fact that the horizontal lines e’f’ and a’b’ can not be followed beyond 42 percent zinc shows something concerning the width of the a, ,3 and r fields. These inversions do not appear on cooling curves and have been overlooked until this time. The lines F’c and b’c’ lie between single phase fields and therefore are not limited to a constant temperature. Heating curves on alloys containing from 45-9 j percent zinc show four heat absorptions. These represent two inversion points, the solidus and the liquidus. The first inversion point occurs a t a temperature of 405’ along the line c’d’ and 6’ below the solidus. This represents the inversion of the r phase into p. The solidus occurs a t 411’ along the line fF. The second inversion temperature occurs a t 437’ along the line eE. At this temperature p changes over into a. The change which comes just below the solidus or eutectic is likely to be obscured by the eutectic heat absorption. On cooling, this change may not appear until as much as 75’ below the stable temperature. On reheating to the eutectic and recooling, the inversion again takes place far below where it appears on the heating curve. KO evidence of the change of a into 8? appears on the cooling curve. On this account a crystals continue to separate from the melt below 437’ unless crystals of be added for nuclei. Along the branch EF or from 95-97.5 percent zinc 9 crystals separate
594
B. E. Curry
from the melt. Over this range three breaks occur in the heating curves of .the inversion temperature of y into p, the solidus and the liquidus. From 97.5 percent zinc t o pure zinc the heating curves show breaks only at the solidus and liquidus. The mass of eutectic masks the heat absorption of the inversion of the small quality of 7 into p. The thermal data show that the compound ZnSb does not combine with more antimony, and form a solid solution. The eutectic is readily traced from pure antimony t o the compound ZnSb. The same is true of the eutectic formed between p and pure zinc. The thermal data taken below the CD portion of the CDE branch show that the phase which separates here is a solid solution rather than a definite compound. This series of alloys shows the inadequacy of cooling curve determinations. The freezing .point curve for this series may be determined very accurately by this method. The same may be said for the solidus. On the other hand when one solid phase should change into another, it does not change. As a result the temperature a t which the change does occur may be many degrees below where it should appear. In other instances the change does not appear a t all on the cooling curve. Neither Monkemeyer nor Zeniczuznyj noted the inversion points in the alloys with 35-45 percent zinc. If equilibrium were established readily, breaks would have appeared in the cooling curve. In the go percent zinc alloy they found an inversion point a t about 330'. This should have appeared at 406' or j6O higher. In one of these instances we have seen an error of j6O and the other the nonappearance of the second phase. At present we have no reason t o doubt the appearance of inversion points on heating curves. While we know that super-cooling is very common, the phenomenon of superheating is quite rare. The inversion points found across the concentration between 35 and 45 percent zinc, as has been stated, do not appear on any of the cooling curves. They do appear, however, on heating curves. In making the determinations
Sowe Zinc Alloys
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the rise in temperature was made at the rate of about 4' per minute. Under these conditions the breaks in the heating curve appear sharply and definitely. The inversion temperature below the eutectic with 45-100 percent zinc is best located by heating at a rate of only about 2 or 3' per minute. Otherwise the heat absorption at the eutectic is likely t o obscure the other. When an ingot is heated from room temperature there is no evidence of any heat absorption below 4 0 j 0 , thus showing that the inversion located there by Monkemeyer and Zemczuznyj was inaccurate. When this same ingot is cooled from the eutectic or from above 405' the cooling curve shows a definite break, depending on the composition and rate of cooling, from 20--Soo below the stable temperature. This series of alloys affords excellent examples of what errors the investigator of alloys is likely to make if the data are collected by means of cooling curves alone. Microscopically the equilibrium diagram for the antimony-zinc alloys consists of six phases. Two of the phases are the pure components, antimony and zinc. All alloys, containing less than 35 percent of zinc, contain crystals of ZnSb. When these alloys are etched with alcoholic ferric chloride in hydrochloric acid, the antimony crystals etch white and the ZnSb crystals appear as the dark phase. Across this range, the amount of the antimony crystals decreases as the composition ZnSb is approached. The alloy containing 35 percent zinc etches dark with the above solution and shows only faint traces of the white antimony crystals. This alloy is homogeneous when chill-cast. This structure is not changed by annealing. The crystals appear as large plates. These alloys have no inversion points and the only effect of annealing is to increase the size of the crystals. These alloys come. within the ZnSb +Sb field in Fig. 3. As the zinc content is further increased, a new white phase appears. The 36.9 percent zinc alloy contains large amounts of a second phase which etches white. This phase is only slightly attacked b y the dilute acids and is much less soluble than the phases with which it comes into equilibrium. As the zinc
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content is further increased to 39. j or 40 percent, the amount of the new phase increases and the ZnSb crystals diminish in amount and disappear a t about 40 percent. Alloys with a zinc content of 40 to 4j percent are homogeneous when chillcast and when annealed. n'hen these alloys crystallize from the melt, the n phase appears first; a t lower temperature under equilibrium conditions, the (L phase breaks down into the ,3 and then the i' modifications. Both the microscopical and thermal data disprove the formation of the compound Zn,Sb?. So far as the microscopical data are concerned, these three phaieb h a w no distinct differmces. ,Annealing the ingots at difierent temperatures does not change the crystalline qtructure. Seither do any of the ctching solutions employed differentiate betn ec i i these three pliaies. Except for the thermal data there ii 110 waj- of differentiLitinq one from t h e otlier. \\+henthe (1 pliaic i i cooled, the cooling c u r w does not indicate the appearance of the otlier phases at the ion-er temperature. In the irrcgular fields desiqnated a, ,3, and 7 these phases appear a i large, bright crystal
