~utectic Patterns in Metallic Alloys

Transcription

1 TRANSACTIONS OF THE AMERICAN INSTITUTE'OF GICAL ENGINEERS [SUBJECT TO REVISION: MINING AND METALLUR NO E. ISSUED AITE MINING AND'-METALLURGY. FEBRUA~Y DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presepted in person it ' ; the Ney York Meeting, February. 1925, when an abstract of the paper'will be read. If this is lmpos-,.. s~ble, d~scuss~on~n wrltlng may be sent to the Editor, American Institute of hlining,and Metallurgical Engineers, 29 West 39th Street, New York, N. Y., for presentation by..the Secretary or other representative of its author. Unless s ecial arrangem6nt is made, the discussion of this paper will close April Any,discussion offeref thereafter should preferably be in the form of a. new.paper. ~utectic Patterns in Metallic Alloys (New York Meeting, February, 1925) RECENTLY two papers on the structure of eutectics were read before thc British Institute of Metals, one by F. L. Bradyl and the other by A. Port,evin.Vn the preparation of photomicrographs of laboratory specimens as illustrations for student work, and in connection with various researches, the writer has been working along the same line for some time and, therefore, was much interested in the results given, especially those of Portevin. He has correlated the information on the subject and has outlined a broad scientific basis for future work along this line. It is largely with the idea-of adding something by way of illustration to the work already done that this paper is written. i The eutectic is generally understood to mean the alloy having the lowest melting point of any alloy of the system, and in which the melt becomes supersaturated with reference to the two or more phases which, separate out simultaneously (or alternately). From the surface pattern,, - the eutectic means a structure with no primary crystals. It is not always easy to get the exact eutectic composition it1 making any alloy; in fact, a slight range in composition may be expected and some of the coarser particles may be richer in either of the composing metals. Even when chemical analysis shows the right composition, differences in density of the pure metals may cause segregation, or supercooling resulting from different crystallizing properties may give a surface pattern the appearance of being on one side of the eutectic point. However, the alloys on either side of the eutectic composition, showing primary crystals of one of the phases and the effect of the pro-eutectic form on the surface pattern of the eutectic make a very interesting study, which is slightly. touched upon in this paper. The phases, or microconstituents, of a eutectic may be pure metals (Pb-Sb, T1-Au), solid solutions (Cd-Zn), a pure metal and a solid solution (Ag-Sn), or a pure metal or solid solution with an intermetallic compound (Sb-Mg, Sb-Cu, As-Cu, Bi-Te, etc.). * Assistant in Metallography, University of M~nnesota. Jnl. Inst. Metals (1922) 28, 369. Jn1. Inst. Metals (1923) 29, 239; Engineering (1923), 116,447, 477, 505. Copyright, 1925, by the American Institute of Mining and Metallurgical Engineers, Inc.

2 2 EUTECTIC PATTERNS IN METALLIC ALLOYS PREPARATION OF SPECIMENS Melts were made up in the metallographic laboratory of the ~chool'of Mines'with the usual precautions. All ~~ecimens'were cooled sufficiently slowly to have an annealing effect. No effects of long annealing, or of chill casting have been taken into account. The compositions given are the intended compositions and not the results of chemical analyses. Unless otherwise stated the values are in weight per cent. No attempt has been made to work out constitution diagrams or to find the exact composition of the eutectics, but diagrams given in the literature have been taken as an indication of the constituents present when confirmed by microscopic examination. As fineness of grain or of eutectic.colony depends on rate of cooling, velocity of crystallization, etc., any magnification has been used that will clearly show the pattern, usually X 75, X 100, x 150, or X 200. Usually, several etching reagents were tried on the specimens but only the one finally used to bring out the structure is' listed with the photomicrograph. From the standpoint-of "morphology" of the eutectic, by which is understood the form or shape and relative distribution of the particies or elements, Portevin gives the following classificatim: Type I. Regular crystals. Type 11. Dendrites or skeleton crystals: (a) Regular dendrites; (b) irregular or badly formed dendrites; (c) skeleton crystals or partly formed crystals. Type 111. Arrangement in eutectic colonies or complex grains. 1. A diverging or radiating variety in which the particles thicken out toward the ends... Spherical. These particles in the colonies may be: (a) Rounded, dotted, or spheroidal; (b) flattened, or in more or less wavy- sheets; (c) branching, producing a dendritic appearance in the peripheral zones. 2. The fanlike variety, with slightly divergent particles contained within a narrow cone. Conical. 3. Parallel-clustered variety, occurring as minute rods grouped together in parallel arrangement. Type IV. Granular. Not found in metallic eutectics and later disregarded. Obviously, other types may be discovered in the study of eutectics; the boundaries between these types are not clearly marked as yet and there seem to be transition forms. Apparently, however, every eutectic conforms to some ideal type, determined largely by the properties of the

3 C. H. GREEN ' 3 pure metals making up the system16and when we havecfinally taken into account all the influencing factors and have found the true type of any eutectic, that type is fixed and: th'ough there may be slight temperature and concentration variations during solidification which affect the relative amount of each constituent deposited and the size of the colonies in different parts of the specimen, these variations will be subservient to the typical development of any system and will easily be recognized as such. Further study should also show the effect of chill casting or of long annealing on the pattern in any particular type. Type I. Regular Crystals It is not my intention to discuss every type and variety mentioned, but to add a few illustrations of the types given and to suggest some slight changes in the classification. No illustration of Type I is given. ' Type II. Dendrites or Skeleton Crystals Lead-bismuth and bismuth-tin are given as examples of Type 11. Lead and bismuth (Figs. 1 and 2) form two solid solutions, a a solution bismuth in lead up to some 2G30 per cent. and /3 a solution of leadsin bismuth to approximately 1 per cent. In the photomicrograph, the light constituent /3 is almost pure bismuth. Two fields are shown, both found in one specimen and both typical, but Fig. 2 shows the formation cut at a different angle. In the bismuth-tin alloy (Figs. 3 and 4) the light colored areas are practically pure bismuth which forms skeletons around which the tin-rich solid solution is disposed. The surface patterns of bismuth-lead and bismuth-tin eutectics are very similar, both showing the'same angularity of design and both showing the predominating influence of the bismuth. Brady3 puts these in the angular class but admits that these eutectics may almost be called "dendrit;~" type; Desch4 says they are characteiized by boxed forms or simple polyhedral crystallites divided transversely by parallel bands; Portevin5 calls them a transition form between Types I and 11, showing &me regular dendrites and some areas "regularly formed in such a manner that the shell ar layer reproduces a cubic or pseudo-cubic crystal.') It appears to the writer that Type I1 should cover angular forms, dendrites whether regular or irregular, skeleton forms, and any like formation. Type III. Eutectic Colonies The term "spherulitic" is most applicable to this type of structure even though few perfect spheres are found. The "madreporic" structure of Portevin, named from the resemblance to the growth of a coral polyp \ ', Ibid. iimelallography," 186. Ibid.

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14 14 EUTECTIC PATTERNS IN METALLIC ALLOYS 23 per cent. copper, the eutectic is a mixture of Cu2Sb and of solid solution of copper in antimony or practically pure antimony. Carpenters made a rather complete investigation of the Cu-Sb system and states that the eutectic showed a remarkable lack of orientation, but his,specimen was annealed for six weeks. Brady says it seems to be in the crystalline class, and shows the triangular crystals of antimony. Desch gives a ' like illustration but also shows long plates of antimony. Portevin places it in Type I11 "with slightly divergent particles contained within a narrow cone." It appears that copper tends to form eutectic colonies (Cu-Ag, Cu-Cu20, Cu-Cusp, etc.), but antimony has a strong tendency toward, the angular arrangement. Antimony and CusSb are both strongly crystalline; hence the eutectic form represents a constant struggle between the two phases. Fig. 22 shows eutectic colonies in which the central particles are triangular antimony crystals. Some colonies were found with banded centers, and many areas showed a strong resemblance to the tin-zinc eutectic, see Fig. 23. Morphologically, then, the Sb-CuzSb eutectic might be placed in any one of the types given; probably Type I11 predominates, but further study is needed to decide definitely. After a careful consideration of the literature on eutectics, especially of the two most important recent papers, it appears that there is no disagreement in the results. The morphological classification is far more comprehen'sive but consideration of surface tension gives added weight to certain groups. In the few instances noted, where it seems the form does not correspond to surface tension values, some other property is probably in control. Portevin's Type I, regular crystals, easily covers Brady's crystalline class and is not often found. Every example of Brady's globular and lamellar classes forms eutectic colonies and differs in different parts of the same specimen as to whether the central particles of the colony are globular or appear lamellar. Type 11, dendrites, covers all of Brady's 3A class and probably his 3B class. The following classification is suggested : Type I. Regular crystals. Type 11. Dendrites or skeleton crystals. Angular arrangement. (One metal has a low surface tension or high cohesion. One constituent retains its own crystal form, and acts as the predominant partner. Successive crystallization.) Examples, Bi-Pb, Bi-Sn, Pb-Te, etc. Type 111. Eutectic colonies or complex grains. Spherical or spherulitic. (Metals of high or medium surface tension. Simultaneous crystallization.) Central particles rounded, dotted, parallel straight or wavy sheets, rods; possibly angular or triangular. Int. Jnl. Metnllo,~. (1913), 4, 300.

15 C. H. GREEN 15, Examples, Cd-Sn, Cd-Zn, Te-Sn, Mg-Sn, Pb-Sn, As-Sn, Al-Zn, etc. Type IV. Conical arrangement of one phase in a ground mass of ' the other. I Examples, Sn-Zn, Sb-Te, Bi-Te; possibly Sb-CuzSb. I Acknowledgment is gladly made to Dr. 0. E. Harder, under whose direction the research has been carried out; also to Mr. R. L. Dowdell, of the Department of Metallography, for his assistance in making the specimens.