Asymmetric behavior of elastic scattering appearing prior to the transformation in Au-47.5at%Cd

The shape memory effect of alloys are caused by martensitic transformation. AuCd alloy is one of the typical alloys which shows martensitic transformation. AuCd alloy has CsCl type structure as a parent phase and two distinct martensite called γ2' around composition of Au-47.5at%Cd and ζ2' around composition of Au-50at%Cd. Mechanism of the martensitic transformation from microscopic point of view has not been completed yet. Crystal structure of γ2' phase was obtained with alternative motion of (110) plane of parent phase. Inelastic neutron scattering experiments showed softening behavior at the Brillouin zone boundary in addition to ζ ∼ 1/3 of [ζζ0]TA2 phonon branch. Single crystal of Au-47.5at%Cd were prepared and small fragments of the sample were electro-polished for X-ray diffraction experiments. Intensity measurements with four circle diffractometer (Rigaku AFC-5R) were performed around 1/3 and 1/2 of [ anti 110]* at several temperatures above the transformation temperature (Ms) to observe the precursor phenomenon of the martensitic transformation. Extra reflection at 1/2, which corresponds to γ2' martensite reflection after the transformation, was not observed. Although the 1/3 of [ anti 110]* reflections were not expected from structural study of γ2' martensite, 1/3 reflection was observed prior to the transformation temperature (Ms). They appeared asymmetrically around the Bragg reflection of the parent phase. (orig.)