DO CRACKS MELT THEIR WAY THROUGH

Abstract

Real-time, in situ fracture studies in the high-voltage electron microscope (HVEM) show that mi- croscopically thin regions of amorphous NiTi form ahead of moving crack tips in the B2-NiTi in- termetallic compound during tensile straining at temperatures equal to or below 600K. The upper cutoff tempemture of 600K for this stress-induced melting (or amorphization) is identical to the upper cutoff temperatures reported in the literature for both heavy-ion-induced amorphization of the intermetallic NiTl and ion-beam-mixing-induced amorphization of N1and TI muhilayem. These results, together with the fact that the higher crystallization temperatures (-1300K)of unrelaxed amorphous NiTi alloys obtained by rapid quenching can also be reduced to, but not lower than 600K, by heavy-ion irradiation, strongly suggest that structural relaxation processes enhanced or induced by dynamic atomic disordering allow the for&ation of a unique, fully-relaxed glassy state which is characterized by a unique isothermal crystallization temperature. We believe that this unique temperature is the Kauzmann glass-transition temperature, corresponding to the ideal glass having the same entropy as the crystalline state. As the glassy state with the lowest global free en- ergy, the preferential formation of this ideal glass by disorder-induced amorphization processes can be understood as the most energetically-favored, kinetically-constrained melting response of crystalline materials driven far from equilibrium at low temperatures.