Synthesis and characterization of nanocrystalline NiTi shape-memory alloy by shock-compression

摘要

Shock synthesis of shape-memory NiTi alloy was investigated in this study, starting with an elemental mixture of Ni+Ti (mixed in equiatomic ratio) as well as pre-alloyed nitinol (Ni50.88Ti49.12) powder. The powders were ball milled using a SPEX 8000 mixer/mill in an Ar atmosphere, to form an intimate mixture of reactants in the case of the Ni+Ti precursor powder, and an amorphous alloy in the case of the pre-alloyed nitinol powder. The ball-milled powders were subsequently shock-consolidated with a three-capsule, plate-impact fixture, using an 80-mm diameter single-stage gas-gun. In the case of the Ni+Ti powder mixture, the recovered compact showed formation of multiple phases including the NiTi B2- and R-phase, NiTi_2, and a trace of Ni_3Ti. The shock-compressed nitinol powder compacts showed partial crystallization in the as-consolidated state, and full crystallization to a nanocrystalline structure upon subsequent annealing. In this paper, the phase-formation mechanisms and shape-memory properties of the shock-synthesized NiTi alloy as a function of grain size, will be described.