Stress Induced Martensitic Transformation from BCC to FCC in Ag-Zn Alloys

Hokkaido University, Department of Applied Physics, Kita-13, Nishi-8, Kita-ku, Sapporo 060, Japan

Abstract

The martensitic transformation in Ag-Zn alloys of low-Zn content has been studied by optical and electron microscopic observations and by tensile tests. The β1 phase of B2 structure transforms to the thermo-elastic martensite having 9R structure similar to Cu-based alloys upon cooling to temperature below Ms. When the β1 phase is stretched at room temperature, the slip deformation occurs at first and then the stress-induced martensite (SIM) of wedge-like morphology forms. The SIM has the ordered fcc structure containing micro-twins. This direct transformation from bcc to fcc is a unique feature in Ag-Zn alloys. In Cu alloys, martensites of fcc structure appear only after the second transformation from the first transformation product of 9R structure. The critical stress for the martensitic transformation and a degree of order of SIM decrease as the deformation temperature rises. In Ag-Zn alloys, the martensite of disordered fcc is thermally produced also by up-quenching to a higher temperature. In the present study, the relation between martensites of ordered and disordered fcc is discussed through thermodynamical calculations. The condition for the direct transformation from bcc to fcc is also examined.