Effects of HVEM irradiation on ordered phases in Ni-Ti

Various ordered phases in the Ni-Ti system were subjected to electron irradiation in the Berkeley HVEM. Austenitic NiTi (B2 structure) disorders and turns amorphous with room-temperature irradiations at accelerating potentials between 1 and 1.5 MeV. Total doses for the onset of amorphiticity range between 0.7 x 1022 and 3 x 1022 e.cm-2 (0.4 to 1.0dpa). At 90K the dose requirement decreases to 4 x 1020e.cm-2 (approx. 10-2dpa). Martensitic NiTi (distorted monoclinic structure) readily detwins and transforms to austenite when irradiated for short times (approx. 10 seconds). Vapor-deposited amorphous films were crystallized to produce NiTi, Phase X (ordered nickel-rich phase with unknown structure) and Ni3Ti (DO24 structure). Upon electron irradiation, NiTi and Phase X disorder and become amorphous, while Ni3Ti disorders but does not turn amorphous with doses up to 4 x 1022e.cm-2 at 90K. These results are discussed in terms of the requirement of a critical concentration of defects and their relative mobilities. Brimhall's solubility criteria for amorphization of ordered alloys by ion bombardment is apparantly applicable to electron-induced crystalline to amorphous transitions in this alloy