✍ Scribed by R.S. Toth; H. Sato
No coin nor oath required. For personal study only.
An exact knowledge of the structure of martensites is essential in understanding the transformation mechanisms. Rapid progress in the knowledge of the structure of martensites in the p phase region of non-ferrous alloys is being made irrespective of the difficulties in the structural analyses, due mainly to the inhomogeneity in the transformation and the high density of faults involved in the transformation products. In most cases, the transformation is found to take a basic b.c.c. structure into some form of close packed structure, and in general, at least two stacking variants of close packed structures, 3R and ZH,(l) appear with increasing electron-atom ratio in an alloy system, the exact transition composition depending also on the geometrical relations of the transformation.(2) In the case of the Cu-Al alloys near 25 at. y0 Al, the transformation is from a /3' ordered structure (Fe,Al type) to products which are considered to be 3R and 2H variants of the M = 1 structure(3) (a long period superlattice with antiphase boundaries at each unit cell distance of a CusAu type structure, often referred to as DO,,). These results, however, have not been reached without some controversy, since a high density of imperfections (stacking faults) have made the structure analysis of the pi martensite difficult. Originally, Swann and Warlimont assumed &' to have a 1R structure with an extremely high density of stacking faults,t4) but Nishiyama et LzZ.(~) concluded that it had a 3R structure with 18 layers in a structural unit, while Wilkens and Warlimont proposed a 11H structure with 22 layers in the structural unit.@) Even the concept of whether such a stacking order with long period is an approximate one due to the periodic distribution of ordinary stacking faults or is a definite crystallographic form with a long period stacking order, has not been established. This basic point, however, is an important one in the investigation of the mechanisms of martensite transformations.(4-6)
In order to resolve these controversies, the present authors made a systematic examination of the electron diffraction data of this marteniste structure,(3' based on a general method of analysis of close packed structures with long period stacking order previously developed by us,(') and concluded that the structure of pi' should be basically a 3R variant of the M = 1
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