Strengthening Mechanism of TiNi Shape Memory Sintered Alloy using Elemental Mixtures of Pre-alloyed TiNi Powder and TiO_2 Particles

The mechanical properties and the strengthening mechanism of the extruded and heat-treated titanium-nickel (TiNi) shape memory alloys (SMAs) prepared by powder metallurgy (P/M) were investigated. TiNi pre-alloyed powder was mixed with titanium dioxide (TiO_2) particles by a planetary ball mill, and consolidated by spark plasma sintering (SPS) at a sintering temperature of 1173 K in vacuum (6 Pa). Subsequently, the SPSed TiNi alloy compacts were extruded at a pre-heated temperature of 1373 K, and shape memory heat-treated at a holding temperature of 773 K. Ti_4Ni_2O phases formed by reaction between the TiNi matrix and the additive TiO_2 particles caused an increase of the soluted Ni content of the matrix, and resulted in a decrease of the martensitic transformation temperature of the TiNi matrix. Consequently, the extruded and heat-treated TiNi powder alloy with 1.0 vol.% TiO_2 particles showed a remarkably high plateau stress of 524 MPa and ultimate tensile strength (UTS) of 1298 MPa at room temperature (298 K), respectively. It also revealed a good shape recovery rate of 89.3 % when 8% tensile strain was applied