I. Karaman, K. C. Atli, M. Haouaoui, Texas A&M University, College Station, TX; C. J. Yu, NAVAIR-Naval Air Systems Command, Patuxent River, MD

Summary: NiTi alloys are ideal for actuator applications in automotive, aeronautics and medicine due to their extraordinary properties such as superelasticity, shape memory, low weight, and high work output. Nevertheless, their transformation temperatures are well below 100°C, restricting their use to low temperature applications. Hence, there is an urgent need to develop high temperature shape memory alloys (HTSMA). TiNiPd alloys have attracted considerable interest as HTSMA since their martensitic transformation temperatures can be varied from room temperature up to 525°C. However, they suffer from embrittlement due to the second phase particles that contain predominantly titanium. In addition, even though the shape memory response of TiNiPd alloys is fairly stable at room temperature, it deteriorates with increasing temperature due to extensive plastic strain accumulation as a result of the decrease in critical shear stress for slip with temperature. In this study, we aim to increase the critical stress for slip (CSS) in NiTiPd alloys by refining the grain size to nanometer range via a severe plastic deformation technique called equal channel angular extrusion (ECAE). Two TiNiPd alloys were ECAE processed at temperatures as low as 400°C. The microstructural evolution of the alloys was monitored using Scanning and Transmission Electron Microscopy. Isothermal monotonic and isobaric thermal cyclic experiments were conducted to evaluate the effect of ECAE-induced microstructural changes on the functional properties. We have observed considerable improvement in thermal cyclic and dimensional stability after ECAE which can be attributed to the increase in CSS. Transmission electron microscopy observations revealed that this improvement is caused by ultrafine-scale grains on the order of few hundred nanometers. ECAE also enhances the ductility of the TiNiPd shape memory alloys by breaking up the Ti-rich second phase particles during processing. It has been determined that the fracture toughness of the TiNiPd alloys was increased after ECAE process.