Development, Characterization and Testing of Nickel Titanium Based High Temperature Shape Memory Alloys

Abstract

TiNi-based shape memory alloys are well known for their excellent shape memory and superelastic properties. TiNiPd alloys are considered as the better high temperature shape memory alloys due to high transformation temperatures, small hysteresis, reasonable strain recovery and comparable workability. However, by further increasing the transformation temperatures i.e. by increasing the Pd content, thermal hysteresis also increases. This has an adverse effect on the actuation behavior of the alloy. At high temperature the critical stress for slip deformation of TiNiPd alloys also decreases, which increases the permanent deformation and reduces the strain recovery in the alloy. In order to prevent increase in thermal hysteresis, reduce permanent deformation and increase strain recovery in TiNiPd alloy, Ni has been replaced by 5 at%, 10 at% and 15 at% Cu. Four alloys; Ti50Ni25Pd25, Ti50Ni20Pd25Cu5, Ti50Ni15Pd25Cu10 and Ti50Ni10Pd25Cu15 (all in atomic %) have been developed and characterized for their microstructure, phase transformation temperatures, mechanical and shape memory properties in solution treated condition. By increasing the Cu content, the transformation temperature of the TiNiPdCu alloys significantly increased, whereas thermal hysteresis decreased. Similarly, the microhardness, yield and fracture strength also increased. Shape memory properties like strain recovery and work output also improved. Therefore, TiNiPdCu alloys showed improved transformation temperatures, strain recovery and critical stress for slip deformation through solid solution strengthening mechanism. The TiNiPdCu alloys were also aged at different aging temperatures i.e. 400°C, 500°C, 600°C and 700°C for 3 hours to investigate their transformation temperatures, mechanical and shape memory properties and compared with the solution treated samples. By aging the Ti50Ni25Pd25 and Ti50Ni20Pd25Cu5 alloys, the transformation temperatures, mechanical and shape memory properties slightly increased. After aging the Ti50Ni15Pd25Cu10 and Ti50Ni10Pd25Cu15 alloys, the transformation temperatures and shape memory properties significantly decreased, however the mechanical properties were improved. viii It can be concluded that aging of Ti50Ni25Pd25 and Ti50Ni20Pd25Cu5 alloys is beneficial to increase their transformation temperatures and shape memory properties. However it has an adverse effect in terms of transformation temperatures and strain recovery by aging the Ti50Ni15Pd25Cu10 and Ti50Ni10Pd25Cu15 alloys.