Influence of microstructure on phase transformation behavior and mechanical properties of plasma arc deposited shape memory alloy

Recently, additive manufacturing have gradually become an attractive processing technology for TiNi alloy. In this work, the Ti50Ni50 alloy was successfully prepared by plasma arc deposition (PAD) technology. The microstructures, phase transformation characteristics and mechanical properties were investigated in detail. The PAD-TiNi alloy mainly comprised of TiNi (B2), TiNi (B19') and Ti2Ni phases. The TiNi cellular crystals, TiNi dendrite crystals and TiNi equiaxed crystals were presented at the bottom, middle and top area of PAD-TiNi alloys, respectively. Some irregular Ti2Ni phases were distributed in the inter-dendritic areas. The peritectic reaction of L (Ti) + TiNi →Ti2Ni led to the formation of irregular Ti2Ni phases. The coherence of Ti2Ni phases and TiNi matrix phases drove the two-step phase transformation (B2→ R → B19') during the cooling process. The as-deposited samples showed favorable hardness and high strength, especially possessing special quasi-linear superelasticity (up to 3.8%) with narrow hysteresis, which was derived from the quick appearance and disappearance of deformation microtwins under loading and unloading condition. The newly formed interface between Ti2Ni precipitates and TiNi matrix phase offered the nucleation sites for deformation microtwins and promoted the formation of microtwins.