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Strain markings in alpha-brass
764
ACTA
METALLURGIC.-\,
the case of complete mixing are equally valid in the case of partial mixing, except for a short transition region at the beginning of the ingot, provided that the appropriate value of It,,, be used. It is not, as the authors imply, necessary that complete mixing be achieved for such equations to be valid. Theory and experiment for keft are described by Burton, Slichter and Prim [3], and by McFee [4]. In this connection the work of Hall [5] on solutes in germanium suggests that transport processes at the solid-liquid interface as well as those in the liquid influence the value of kerf. With respect to single-pass zone-melting, the liquid distribution CL of Figure 14 appears to neglect diffusion at the leading interface of the molten zone as CL should have the value Co at this point. With respect to zone-refining, we question the statement that the optimum zone length for purification is given by conditions (18), for the reason that these conditions were derived for the poorest possible experimental situation for doing zone-refining, namely, the absence of convection or stirring. P. PFANN Bell Telephone Laboratories, Inc. Murray Hill New Jersey References 1. TILLER, W. A., JACKSON, K. A., RUTTER, J. W. and CHALMERS, B. Acta Met. 1 (1953) 428. 2. PFANN, W. G. Trans. A.I.M.E. 194 (1952) 747. 3. BURTON, J. A., SLIGHTER, W. P., and PRIM, R. C. “Distribution of Solutes in Crystals Grown from the Melt,” J. Chem. Phys. (November, 1953). 4. MCFEE, R. H. J. Chem. Phys. 15 (1947) 856. 5. HALL, R. N. Phys. Rev. 88 (1952) 139.
Strain
Markings
in Alpha-Brass*
In a recent paper, D. K. and H. Wilsdorf [I] suggest that the possibility of producing strain markings on a deformed crystal depends on whether or not the original slip markings had an “elementary structure.” They argue that strain markings cannot be etched out on pure metals because the existence of the elementary structure makes the slip-line density so high that the etch is nonselective. Alpha-brass does not exhibit an elementary structure and therefore its relatively widely spaced slip-bands can be selectively etched. The above explanation of strain-markings as*Received
October
1, 1953.
\‘OL.
1,
1953
sumes that the material at slip planes in both pure metals and alpha-brass is somehow different from the unslipped matrix. However, this assumption does not seem to be entirely justified in the case of pure metals. On the other hand it does seem to be justified in the case of alpha-brass and thus explains why etching produces strain markings in this and probably other alloy crystals. As Fisher [2] has pointed out, the act of slip in an alloy crystal changes the short-range order pattern across the slip plane. This accounts for part of the strength of alloy crystals and, since it would be expected to change the chemical potential at a slipped plane. it may account for the appearance of strain markings upon etching. J. J. GILMAN Metallurgy Research Department General Electric Company Schenectady, New York References 1. WILSDORF, D. K. and WILSDORF, H. 394. 2. FISHER, J. C.
Nuclear
Acta Met. 1 (1953)
Acta Met. 2 (1954).
Composition-A Factor in Nucleation*
of Interest
Nucleation in the solid state is a very complicated process. Therefore, theoretical investigations in this field deal with simplified models. Some factors are examined while other factors are neglected. A factor that is usually neglected is the uncertainty of the nucleus composition. It is not necessarily true that the nuclei which form most readily have the composition appropriate for the precipitate, when equilibrium has been established. In the present note this point will be examined, on the assumption that many other factors can be neglected. Gerlach [I] has found that the precipitate in an Au-rich Au-Ni-alloy has a higher Au-content at the beginning of precipitation than later. Scheil [2] has explained this by starting from the assumption that the surface tension of a nucleus depends on its composition in a specified manner. A thermodynamic treatment could be carried out, which showed that the probability should be greatest for formation of nuclei with higher Aucontent than that of the equilibrium precipitate. Thus, under this assumption about the surface *Received
August 31, 1953.
ACTA
METALLURGIC.-\,
the case of complete mixing are equally valid in the case of partial mixing, except for a short transition region at the beginning of the ingot, provided that the appropriate value of It,,, be used. It is not, as the authors imply, necessary that complete mixing be achieved for such equations to be valid. Theory and experiment for keft are described by Burton, Slichter and Prim [3], and by McFee [4]. In this connection the work of Hall [5] on solutes in germanium suggests that transport processes at the solid-liquid interface as well as those in the liquid influence the value of kerf. With respect to single-pass zone-melting, the liquid distribution CL of Figure 14 appears to neglect diffusion at the leading interface of the molten zone as CL should have the value Co at this point. With respect to zone-refining, we question the statement that the optimum zone length for purification is given by conditions (18), for the reason that these conditions were derived for the poorest possible experimental situation for doing zone-refining, namely, the absence of convection or stirring. P. PFANN Bell Telephone Laboratories, Inc. Murray Hill New Jersey References 1. TILLER, W. A., JACKSON, K. A., RUTTER, J. W. and CHALMERS, B. Acta Met. 1 (1953) 428. 2. PFANN, W. G. Trans. A.I.M.E. 194 (1952) 747. 3. BURTON, J. A., SLIGHTER, W. P., and PRIM, R. C. “Distribution of Solutes in Crystals Grown from the Melt,” J. Chem. Phys. (November, 1953). 4. MCFEE, R. H. J. Chem. Phys. 15 (1947) 856. 5. HALL, R. N. Phys. Rev. 88 (1952) 139.
Strain
Markings
in Alpha-Brass*
In a recent paper, D. K. and H. Wilsdorf [I] suggest that the possibility of producing strain markings on a deformed crystal depends on whether or not the original slip markings had an “elementary structure.” They argue that strain markings cannot be etched out on pure metals because the existence of the elementary structure makes the slip-line density so high that the etch is nonselective. Alpha-brass does not exhibit an elementary structure and therefore its relatively widely spaced slip-bands can be selectively etched. The above explanation of strain-markings as*Received
October
1, 1953.
\‘OL.
1,
1953
sumes that the material at slip planes in both pure metals and alpha-brass is somehow different from the unslipped matrix. However, this assumption does not seem to be entirely justified in the case of pure metals. On the other hand it does seem to be justified in the case of alpha-brass and thus explains why etching produces strain markings in this and probably other alloy crystals. As Fisher [2] has pointed out, the act of slip in an alloy crystal changes the short-range order pattern across the slip plane. This accounts for part of the strength of alloy crystals and, since it would be expected to change the chemical potential at a slipped plane. it may account for the appearance of strain markings upon etching. J. J. GILMAN Metallurgy Research Department General Electric Company Schenectady, New York References 1. WILSDORF, D. K. and WILSDORF, H. 394. 2. FISHER, J. C.
Nuclear
Acta Met. 1 (1953)
Acta Met. 2 (1954).
Composition-A Factor in Nucleation*
of Interest
Nucleation in the solid state is a very complicated process. Therefore, theoretical investigations in this field deal with simplified models. Some factors are examined while other factors are neglected. A factor that is usually neglected is the uncertainty of the nucleus composition. It is not necessarily true that the nuclei which form most readily have the composition appropriate for the precipitate, when equilibrium has been established. In the present note this point will be examined, on the assumption that many other factors can be neglected. Gerlach [I] has found that the precipitate in an Au-rich Au-Ni-alloy has a higher Au-content at the beginning of precipitation than later. Scheil [2] has explained this by starting from the assumption that the surface tension of a nucleus depends on its composition in a specified manner. A thermodynamic treatment could be carried out, which showed that the probability should be greatest for formation of nuclei with higher Aucontent than that of the equilibrium precipitate. Thus, under this assumption about the surface *Received
August 31, 1953.