The role of shear instability in amorphization of cold-rolled NiTi

Abstract Alloy sheets of Ni–49·2at% Ti were cold rolled at room temperature. The microstructure of a 60% reduced sample showed the formation of amorphous bands within the finely twinned crystalline matrix. Sample fracture was observed for reductions of more than 65%. The fracture surface extended straight and inclined 30° to 50° to the compression direction. Similarities in morphology to the shear bands observed in heavily cold-rolled metals suggest that an amorphous phase forms in the shear band, caused by the mechanical instability against shear stress.