Shock-consolidation and reaction synthesis of nickel titanium intermetallic

The shock synthesis of nanocrystalline NiTi (NITINOL) and the reaction behavior of Ni + Ti as-blended and ball-milled powder mixtures were investigated in this research. Nanocrystalline NiTi intermetallic, synthesized by shock consolidation of pre-alloyed amorphous NITINOL powder revealed an increase in M_S temperature and a 40–80% higher compression strength than that of conventional NITINOL alloys, with recoverable elastic strains of as much as about 11%. The reaction behavior of as-blended Ni + Ti powder mixtures, studied using shock recovery experiments, showed localized region-specific reactions occurring due to thermal effects of shock compression and forming products typical of those produced due to thermally-initiated combustion type reactions. The Ni + Ti powder mixtures ball-milled prior to shock compression showed complete reaction forming products, which were predominately NiTi and Ti₂Ni phases, via shock-induced amorphization and subsequent crystallization. Time-resolved stress measurements performed on the as-blended powder mixtures in the range of 522 m/s to 1046 m/s, showed characteristics of powder densification at input stress of 1.12 GPa, and shock-induced reaction indicated by volume expansion and wave speed increase at stress of 3.22 GPa. Ball-milled powder mixtures also showed similar evidence of reaction at stresses exceeding 3 GPa, with expanded states approaching those calculated using a thermodynamic model for shock-induced formation of reaction products. These results provide clear evidence of shock-induced reactions and possible mechanism of reaction initiation.