New description of long period stacking order structures of martensites in β-phase alloys

Abstract

The martensitic transformations in {beta}-phase alloys have attracted much attention for many years, since they are associated with the unique shape memory and superelasticity effects, and the lattice instability or unusually soft elastic constant c{prime}[=c11-c12/2]. Besides, the structural changes are all similar to each other; i.e. ordered BCC parent (B2 type or DO{sub 3} type) structures transform to long period stacking order structures, with 2-dimensionally close-packed planes. Furthermore, these alloys in a single crystal state even exhibit martensite-to-martensite transformation among the long period stacking order structures. These structural changes became clear after the crystal structure determination of {beta}{sub 1}{prime} martensite in a Cu-Al alloy near the Cu{sub 3}Al composition by Nishiyama and Kajiwara. The authors discuss the structural change following their scheme. The structure of the martensite in {beta}-phase alloys was extensively re-examined. As a result, the displacements of stacking positions from the ideal a/3 positions and the monoclinicity of 18R(9R) martensites were confirmed in most of the alloys examined, such as Cu-Zn-Ga, Cu-Zn-Al, Au-Cu-Zn, and Cu-Al-Ni alloys. Thus, they can conclude that the structures of the martensites in {beta}-phase alloys are intrinsically monoclinic except for 2H martensites.