The investigation of the mechanical properties of graded high-temperature shape memory Ti-Ni-Pd alloy

Diffusion annealing of palladium into two Ti-rich alloys of Ti-47 at.% Ni and Ti-49.8 at.% Ni were performed to produce a compositionally graded high-temperature Ti-Ni-Pd shape memory alloy (SMA). A (Ni + Pd)-rich layer was observed throughout the diffusion zone in both alloys. The Ti-47 at.% Ni/Pd alloy failed to show high-temperature characteristics. However, high-temperature peaks with wide transformation temperature intervals were observed on DSC curves of the Ti-49.8 at.% Ni/Pd alloy, which confirmed the success of the method to obtain a graded high-temperature SMA. This was attributed to the formation of Ni-rich precipitates within the diffusion zone of this alloy. The shape recovery studies were performed by micro-hardness, nano-indentation and tension tests. It was shown that, Ti-49.8 at.% Ni/Pd alloy exhibits the higher shape recovery ability than Ti-47 at.% Ni/Pd alloy on both macroscopic and microscopic scales. Furthermore, despite the reports on destructive effect of Pd addition on shape recovery properties of NiTi alloys, Pd diffusion led to an improved shape recovery in Ti-47 at.% Ni/Pd system. •Diffusion annealing of Pd in TiNi alloys of Ti53Ni47 and Ti50.2Ni49.8 were performed.•A (Ni + Pd)-rich layer was observed in the diffusion zones of both alloys.•A compositionally and microstructurally graded HTSMA of Ti-Ni-Pd was produced.•Ti50.2Ni49.8/Pd exhibits better shape recovery abilities compared to Ti53Ni47/Pd.•The maximum shape recovery abilities were obtained for Pd contents less than 8 at.%.