Aging effect in parent phase and martensitic transformation in Au–47.5at.%Cd alloys

Au–Cd alloy is one of the typical alloys which shows martensitic transformation. There are two martensites close to the 1:1 composition: one is γ′2 martensite and the other is ζ′2 martensite. When the phonon dispersion curve was measured in the composition for Au–47.5at.%Cd which produces γ′2 martensite, phonon softening was observed at the Brillouin zone boundary and at ζ=0.35 of the [ζζ0]TA2 branch and a peculiar behavior was observed. One is that the Ms temperature determined in this experiment was lower than the ordinary value. The other is the time dependence of the 1/3 elastic reflection, which was observed prior to the martensitic transformation. Electrical resistance measurements were performed in this alloy in order to clarify this peculiar behavior. A decrease of the Ms temperature was observed after aging at 393 K, in the parent phase. The lower Ms observed in neutron experiments can be explained by an aging effect in the parent phase. There are two possibilities of explaining the time-dependence of the 1/3 reflection; one is the transformation with diffusion (bainite transformation above Ms) and the other is embryo growing.