On the Crystal Structure of the Banded Martensite in Cu-24.5 wt%Sn Alloy

Abstract

The crystal structure and internal defects of the banded martensite in a quenched Cu-24.5 wt%Sn alloy have already been investigated by Nishiyama et al. However, since their results are rather unexpected in some respects, the crystal structure and the morphology were reexamined by means of electron microscopy and selected area electron diffraction. Consequently, the following different results were obtained. (1) The crystal structure of the banded martensites was a 18R type long period stacking order structure with a stacking sequence of AB′CB′CA′CA′BA′BC′BC′AC′AB′. (2) The internal defects of the martensites were stacking faults on the basal plane. (3) The habit plane of the martensites was close to the (\bar1\bar44)_β1 plane, consistent with the result of Greninger and Mooradian, which was determined by the two-surface analysis in bulk specimens. (4) The orientation relationship between matrix and martensite was consistent with the Kajiwara-Nishiyama relationship found in a thermally formed β₁′ Cu–Al martensite. (5) These results were compared with the so-called phenomenological theory, and good agreement was obtained between them.