STRESS-INDUCED AMORPHIZATION AT MOVING CRACK TIPS IN NITI

In situ fracture studies have been carried out on thin films of the Ni Ti intermetallic compound under plane stress, tensile loading conditio ns in the high-voltage electron microscope. Local stress-induced amorp hization of regions directly in front of moving crack tips has been ob served. The upper cutoff temperature, T-C-A(max), for the stress-induc ed crystalline-to-amorphous transformation was found to be 600 K, iden tical to that for heavy ion-induced amorphization of NiTi and for ion- beam mixing-induced amorphization of Ni and Ti multilayer specimens. 6 00 K is also both the lower cutoff temperature, T-A-C(min), for radiat ion-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, T-X(min), at which stress -induced amorphous NiTi crystallizes. Since T-X(min) should be T-K, th e ideal glass transition temperature, the discovery that T-C-A(max) = T-A-C(min) = T-X(min) = T-K implies that disorder-driven crystalline-t o-amorphous transformations result in the formation of the ideal glass , i.e., the glassy state that has the same entropy as that of the defe ct-free crystal. As the glassy state with the lowest free energy, its formation can be understood as the most energetically-favored, kinetic ally-constrained response of crystalline alloys driven far from equili brium. (C) 1998 American Institute of Physics.