Stacking Sequence Changes in A 3 B-Type Close-Packed Structures: a One-Dimensional Stacking Model
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STACKING SEQUENCE CHANGES IN A3B-TYPE CLOSE-PACKED STRUCTURES: A ONE-DIMENSIONAL STACKING MODEL Shiyou Pei* and T. B. Massalski Department of Physics, and Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213 ABSTRACT Structural modifications resulting from changes in the sequence of stacking of close-packed layers are very common in A3B-type transition-metal alloys. In the present work, a one-dimensional model of stacking has been explored. We found that the stacking changes observed in these alloys can be interpreted in terms of interactions between a few adjacent layers. I. INTRODUCTION Ordered close-packed structures are most frequently encountered in A3B-type transition metal alloys.[1] The IVA-VA elements usually substitute for the Bcomponent in such alloys and the VIIIA elements for the A-component. The closepacked layers involved in stacking can be either the triangularly-ordered type (Ttype, e.g., like the {111 ) plane of Cu 3Au), or the rectangularly-ordered type (Rtype, e.g., like the {112] plane of Ni3V). Other intermediate ordered types, such as TR, TR 2 and TR4, though having been discovered in systems such as Au-In-Cd[2], are not known to occur in the transition metal alloys. Because of the absence of these intermediate layers, both the T- and R-layers can remain stable over quite a
wide composition range during substitution of the A- or the B-component. It has been shown both experimentally and theoretically that the T-layers tend to be more stable compared with the R-layers in the composition range where the average group number (AGN) does not exceed 8.65.[3-4] Even with a fixed layer-type, further structural modifications can still be made by changing the stacking characteristics of the close-packed layers. In addition to the simplest stacking arrangements 10 (fcc-type) and 11 (hcp-type), many other arrangements such as 33, 22, 31, 21 are also encountered in alloys#. The stability of these sequences appears to depend strongly on composition. Usually several intermediate sequences can occur before any layer-type change takes place. As an example, in the alloy systems "Fe3V"-Co 3V-Ni 3V and Co 3Ti-Ni 3Ti-"Cu 3Ti", a series of structures based on T-layers is observed: 10
->
33
-ý
22 (or 31)
->
21
->
11. It is possible to correlate empirically the general trend of such stacking changes with the AGN or the size difference between the A- and B-atoms.[1,3,5] * Present address: Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 # Here, we use the Zhdanov symbol to denote a stacking sequence. A "+" is assigned if the order between a layer currently being stacked and its previous partner follows the relationship to be expected between any two contacting layers as in the fcc-type sequence, i.e, A -* B, B -C, or C -ý A. Otherwise, a "-" is assigned. For example, the sequence "++ + - - -", or 33 for short, is equivalent to ABCACB. Mat. Res. Soc. Symp. Proc. Vol. 133. c1989 Materials Research Society
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The aim of the present work w
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