Do cracks melt their way through solids?

Real-time, in-situ fracture studies in the high-voltage electron microscope (HVEM) show that microscopically thin regions of amorphous NiTi form ahead of moving crack tips in the B2-NiTi intermetallic compound. This occurs du ring tensile straining at temperatures equal to or below 600 K. The cut-off temperature of 600 It for this stress-induced melting (or amorphization) i s identical to that reported in the literature for heavy-ion-induced amorph ization of the intermetallic NiTi and for ion-beam-mixing-induced amorphiza tion of Ni and a multilayers. The higher crystallization temperatures (simi lar to 800 K) of NiTi glasses produced by rapid quenching can also be reduc ed by heavy-ion irradiation to 600 It (but not lower than). This strongly s uggests that ion-beam-induced relaxation processes allow the formation of a unique, fully relaxed glassy state characterized by a unique isothermal cr ystallization temperature. We believe this unique temperature is the Kauzma nn isentropic glass-transition temperature of an ideal glassy state having the same entropy as the crystalline state. The formation of this ideal glas sy state via solid-state disordering processes can be understood as the mos t energetically-favoured, kinetically-constrained melting response of cryst alline materials driven far from equilibrium at very low temperatures.