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Abstract Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 high-temperature shape memory alloys were thermo-mechanically processed and characterized in the present study. The microstructural characteristics of cold-worked alloys showed the presence of shear/deformation bands and defects. TEM observation of cold-worked and annealed (400 to 500 °C) microstructures revealed the emergence of small fraction of newly formed martensite twins at 400 °C. With the increase in the annealing temperature from 400 to 500 °C, the fraction of the martensite twins in the microstructure was found to increase. The Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloys showed martensite finish temperature of ~180 °C and ~136 °C with an hysteresis of ~10 °C and ~9 °C, respectively. Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloys showed a recovery strain of 1.5% and 1.7% at a stress of 175 MPa. During cycling under load, the residual strain for the Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloy was found to be negligible as compared to ~0.2% in the Ni 24.7 Ti 50.3 Pd 25 alloy. The results of the study indicate that these alloys have potential for use as high-temperature thermal actuator materials. Highlights • TEM of cold-worked NiTiPd/NiTiPdSc showed shear/deformation bands and defects. • Cold-worked and annealed microstructures revealed emergence of fine-scale twins at 400 °C. • Fraction of martensite twin-variants increased with increase in annealing temperature. • NiTiPd and NiTiPdSc alloys showed a recovery strain of 1.5% and 1.7% at a stress of 175 MPa. • Thermo-mechanical cycling of NiTiPdSc exhibited lower residual deformation compared to NiTiPd. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ni24.7Ti50.3Pd25.0 showed an Mf of 181°C and low transformation hysteresis of ∼8.5°C. [•] NiTiPd alloy showed high thermal cycling stability, variation in TTs being ±1°C. [•] TEM showed twinless/low twin ratio martensite and antiphase-boundary like structure. [•] Alloy has high activation energy (1931kJ/mol) and low elastic strain energy (0.028J/g). [•] NiTiPd exhibited hardness of ∼4.7GPa and modulus of ∼82GPa in martensite phase. [ABSTRACT FROM AUTHOR]

Vacuum-arc melted Ni24.7Ti50.3Pd25.0 and Ni24.7Ti49.3Pd25.0Sc1.0 (at.%) alloys were investigated to study effect of Sc micro-addition on microstructure and transformation behaviour of NiTiPd alloy. Study showed that microstructure of homogenized NiTiPd alloy consisted of NiTiPd matrix interspersed with Ti2(Ni,Pd) precipitates. In contrast, NiTiPdSc alloy showed a single phase NiTiPdSc matrix with a few scandium oxide particles at isolated places. TEM and X-ray diffraction studies confirmed matrix phase of the alloys to be of orthorhombic B19 structure. TEM observations showed that NiTiPdSc alloy had relatively larger martensite plates with a smaller twin ratio compared to that of NiTiPd alloy. Also, APB (anti-phase boundary) like regions with twinless martensites was observed in both the alloys, area fraction of APB-like regions being more in NiTiPdSc alloy. Thermal analysis showed that transformation temperatures (TTs) of NiTiPd alloy decreased significantly with addition of Sc. The martensite finish temperature (M f ) of 181 °C for NiTiPd alloy lowered to 139 °C upon 1.0 at.% Sc addition. The transformation hysteresis of Ni24.7Ti49.3Pd25.0Sc1.0 (at.%) alloy was measured to be 7 °C, significantly lower than that of 15 °C for Ni24.5Ti50.0Pd25.0Sc0.5 alloy, reported in literature. Alloy purity, lower volume fraction of second phase and presence of twinless/small twin ratio martensite in microstructure is believed to be the reasons for such low transformation hysteresis. The transformation behaviour of the alloys upon stress-free thermal cycling was found stable, variation in TTs being within 1–2 °C. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ti2Ni second phase particles forms in different sizes and shapes in cast ingot. [•] TEM evidences showed shearing/fragmentation of Ti2Ni during processing. [•] Matrix close to Ti2Ni experienced severe plastic deformation lead to amorphisation. [•] Ti2Ni interfaces were mostly faceted and assist in nucleation of martensite. [•] Heterogeneity of microstructure observed near to and away from Ti2Ni. [ABSTRACT FROM AUTHOR]