Microstructure and Strengthening Mechanism of Ni-rich TiNi Shape Memory Alloys via Powder Metallurgy Process

Microstructural and mechanical properties of the Ni-rich TiNi alloys using the elemental mixtures of pure Ti powder and pure Ni powder were investigated. Ti-Ni elemental mixture powders were consolidated by spark plasma sintering (SPS). SPSed TiNi alloy compacts were extruded and heat-treated subsequently. SPSed TiNi compacts consisted of TiNi matrix and Ti4Ni2O phases. The solute Ni content of the matrix could be controlled by Ti-Ni powder mixing ratio. The martensitic transformation temperature of TiNi matrix decreased and the amount of Ti3Ni4 precipitate increased with increasing the solute Ni content. Consequently, the heat-treated Ti-52.0 at.%Ni alloy showed a high plateau stress of 850 MPa and a good shape recovery of 87.3% by applying 8% strain in tensile. The strengthening mechanism of the Ni-rich TiNi alloys was mainly due to a decrease martensitic transformation temperature by an increase soluted Ni content of TiNi matrix.