Shock synthesis and characterization of nanostructured NITINOL alloy

Shock-compression of mechanically amorphized NITINOL powder, followed by crystallization heat treatment of shock-densified compacts, was used to synthesize nanostructured NiTi-shape memory alloy. Thermal analysis of shock-synthesized nanocrystalline compacts revealed a higher martensitic start transformation temperature (Ms) than that of the starting (as-received) NITINOL powder. Factors affecting the variation in the Ms temperature include ultra-fine (nanocrystalline) B2-phase grain size, presence of oxygen and iron contaminations, and the formation of Ni4Ti3 in the compacts. Ni4Ti3 phase formation also alters the Ni content of the compacts, which appears to be the main factor influencing the Ms temperature of the NITINOL compacts. Higher internal stresses due to increased number of different orientations of adjacent grains in nanostructured compacts may also contribute to homogeneous formation of martensites and thus, yield a higher Ms temperature. Compression tests performed on shock-densified nanocrystalline NiTi alloy showed ∼40% higher strength than that of conventional NITINOL alloys, in addition to presence of complete pseudoelasticity with ∼7.6% recoverable strain. In this paper, the microstructural characteristics, martensitic transformation behavior, and mechanical (compression) properties of the shock-synthesized nanostructured NITINOL alloy will be presented.