Effect of Ti$_2$Pd(Ni) on the Transformation Behavior in Sputtered Ti-rich TiNiPd Shape Memory Alloys

TiNiPd based shape memory alloys (SMAs) share similar microstructural features as TiNiCu-based SMAs known for their exceptional resistance to functional fatigue due to their high crystallographic compatibility, nanometer sized grains and coherent precipitates, making them an ideal system to further explore the critical factors influencing cyclic stability. In this study, we investigate the effect of heat treatments (500 {\deg}C, 600 {\deg}C, 700 {\deg}C and 800 {\deg}C) on the cyclic stability and microstructure of free-standing, magnetron-sputtered Ti$_{53.6}$Ni$_{35.2}$Pd$_{11.2}$ films. All heat treatments promote the formation of Ti$_2$Pd(Ni) precipitates and result in a similar grain size (~1-4 $\mu$m). Lower heat treatment temperatures improve the cyclic stability of the stress induced transformation while reducing transformation temperatures and latent heat. Temperature dependent X-ray diffraction reveals a complex microstructure for the martensite phase with Ti$_2$Pd(Ni), Ti$_2$Ni(Pd), TiNiPd(B2), B19/B19$'$ and R-phase. The thermal phase transition changes from a distinct 1st order to a 2nd order like transition, accompanied by increasing amount of remanent austenite and R-phase, with nearly no change for the sample heat treated at 500 {\deg}C. In situ stress dependent X-ray diffraction demonstrates a significant difference between the temperature and stress induced phase transformation for this heat treatment. The observed semi crystalline microstructure, featuring nano domains of Ti$_2$Pd(Ni) precipitates in the sample heat-treated at 500 {\deg}C, leads to a mixture of long range martensitic and strain glass transition. This study highlights the impact of heat treatment and microstructure on the phase transformation behavior and functional fatigue in Ti-rich TiNiPd alloys.