Heterogenity of Microstructure Evolution in NiTi (50 at% Ni) Alloy Severely Deformed by High Pressure Torsion

An equiatomic B2 NiTi alloy, severely deformed by high-pressure torsion, was investigated. Nanocrystallization of the as-deformed amorphous matrix has been examined. The formation of internal porosity after long-time annealing was established by the radiotracer technique. Such a porosity prohibited reliable measurements of grain boundary diffusion of the ⁴⁴Ti tracer in the nanocrystallized NiTi alloy. The percolating porosity does not exist in the as-deformed state of NiTi alloy; its formation was affected by free volume redistribution during nanocrystallites' growth under residual stresses. The atomic force microscopy study reveals a highly modified surface with average roughness of about 50 nm after annealing the samples at 600 K for 7 d.