THE ROLE OF THE MARTENSITE-TRANSFORMATION FOR THE MECHANICAL AMORPHIZATION OF NITI

Mechanical amorphisation by cold rolling and uniaxial deformation of t he intermetallic compound NiTi was studied by TEM and DSC. In cold rol led samples it could be clearly shown that amorphisation occurs and th at the amorphous regions recrystallise at T approximate to 350 degrees C. In uniaxially deformed material a microstructure similar to that o f cold rolled samples and a DSC-peak at the same temperature was found . In the TEM, extended amorphous regions could only be identified afte r a moderate heat treatment at 250 degrees C. After deformation at tem peratures above room temperature, a considerably lower degree of amorp hisation was found, and the fraction of amorphous volume was found to depend on the martensite transition temperature of the material. (C) 1 997 Acta Metallurgica Inc.