Proton-irradiation-induced crystalline to amorphous transition in a NiTi alloy

The crystalline to amorphous (C-A) transition induced by 2 MeV proton irradiation of a NiTi alloy containing 50.95 Ni was investigated at -45deg C and -153deg C using transmission electron microscopy and scanning electron fractography. The fractographic studies indicate that the irradiated region contains two layers, the one closer to the free surface consisting of a mixture of amorphous and crystalline phases, and the other having transformed completely to the amorphous phase. The progress of the C-A transition was dependent on both dose and temperature. The critical dose for complete amorphization was estimated as 0.25 displacements per atom (dpa) at -153deg C and 0.32 dpa at -45deg C, as determined from measurements of the distance from the surface of the sample to the interface between the two layers in the irradiated region. Amorphization commenced well before the crystalline matrix was completely disordered, and the amorphous phase was generally observed to nucleate homogeneously throughout the crystalline matrix. The features of the C-A transition induced by proton irradiation more closely resemble those of ion irradiation than electron irradiation. The stability nonuniform buildup of point defects during proton and heavy-ion irradiation is invoked to explain the different responses of intermetallic compounds to ion and electron irradiation. (orig.)