MARTENSITIC TRANSFORMATIONS IN beta PHASE ALLOYS.

Abstract

The trigonal structure of the zeta prime martensite in Au-50% Cd has been confirmed and shown to exhibit P31m symmetry (space group 162). This structure has 18 atoms in the unit cell and a formal description of the transformation involves shuffling of six symmetry related atoms of each type in 100 directions of the parent phase. This same structure appears to arise as a transition phase in the premartensitic transformation in Ti-Ni. In this case, however, evolution of the structure into fully developed martensitic plates is not observed and the transition phase appears to exist as long-lived dynamic fluctuations analogous to those that characterize the omega transformation in Ti and Zr alloys. The martensitic structure in TiNi is considered to be a monoclinic distortion of the B19 ( gamma prime ) martensite that forms in Au-47. 5% Cd. The monoclinic distortion alone, however, does not produce the shuffle deduced from the diffraction pattern.