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Thermo-mechanical characterization of a Ni45.3Ti39.7Hf10Pd5 polycrystalline shape memory alloy after aging at 550°C for 3 h was conducted. Thermal cycling under constant stress, superelasticity and loading till failure tests were employed to reveal the shape memory and material properties. Recoverable shape memory strain of 3.8% was achieved under compressive stress of 500 MPa. The sample showed almost perfect superelasticity above 70°C with total recoverable strain of about 4%, but failed at 2340 MPa with compressive strain > 13% at 35°C, and aging was not found to significantly affect its shape memory behavior because of the low volume fraction of precipitates. [ABSTRACT FROM AUTHOR]

The phase transformation characteristics and isothermal oxidation behavior of ternary Ni50Ti49Ta1 (at.%) and Ni50Ti47Ta3 (at.%) alloys were investigated through differential scanning calorimetry and thermogravimetric analysis techniques in addition to surface morphology captured by scanning electron microscopy. It was found that the ambient temperature and chemical composition are important factors in the oxidation behavior of the alloys in oxygen atmosphere. The oxidation constants were linearly increased with oxidation temperatures ranging between 850 and 1150 °C. The activation energies were determined to be 283.54 kJ/mol and 249.38 kJ/mol for the Ni50Ti49Ta1 (at.%) and Ni50Ti47Ta3 (at.%) alloys, respectively. [ABSTRACT FROM AUTHOR]

The effects of aging under compressive stress of 300 MPa on the shape memory properties and microstructure of [ 1 ¯ 17]-oriented Ni 45.3 Ti 29.7 Hf 20 Pd 5 single crystals were studied. It was revealed that stress-assisted aging resulted in 4 variants of the H-precipitates in contrast to 6 variants observed after stress-free aging. It was revealed that precipitate properties and martensite morphologies strongly depend on aging conditions. As a consequence of the aligned precipitates and generated internal stress, two-way shape memory effect (with up to 0.55–0.8% strain) and higher recoverable strains were observed in Ni 45.3 Ti 29.7 Hf 20 Pd 5 single crystals after stress-assisted aging. [ABSTRACT FROM AUTHOR]

Effects of heat treatment temperature and time on the microstructure and shape memory behaviors (e.g. transformation temperatures, load-biased shape memory effect, superelasticity, two-way shape memory effect, and related properties) were investigated in a Ni 45.3 Ti 29.7 Hf 20 Cu 5 (at%) high temperature polycrystalline shape memory alloy. Heat treatments could be used to control the TTs and to a lesser extent recoverable and irrecoverable strains. The Ni 45.3 Ti 29.7 Hf 20 Cu 5 alloy was capable of recovering shape memory strains of up to 2% at temperatures above 100 °C under high compressive stresses (700 MPa) and up to 0.8% TWSME strain was possible after a non-intense stress-cycling training process. However, due to high Clausius–Clapeyron slopes, large temperature hysteresis, and a strong dependence of transformation stress on temperature, fully recoverable superelastic behavior was not observed because plastic deformation occurred concurrently with the stress-induced martensitic transformation. [ABSTRACT FROM AUTHOR]

The shape-memory properties of NiTiHfPd and NiTiHfPd polycrystalline alloys were determined through superelasticity and shape-memory tests in compression. It has been revealed that the NiTiHfPd has a maximum transformation strain of 3.8 % and work output of up to 30 J cm, while the NiTiHfPd has a maximum transformation strain of 2.6 % and work output of up to 20 J cm at 700 MPa. Two-way shape-memory strains of 0.6 and 0.85 % were obtained in NiTiHfPd and NiTiHfPd alloys, respectively. The NiTiHfPd showed superelasticity at 90 °C with recoverable strain of 3.1 %, while high hardening of NiTiHfPd limited its superelastic behavior. Microstructure of the NiTiHfPd alloy was revealed by transmission electron microscopy, and effects of composition on the lattice parameters of the transforming phases and martensite morphology were discussed. [ABSTRACT FROM AUTHOR]

The shape memory properties of Ni45.3Ti29.7Hf20Pd5 single crystals aged at 550 °C for 3 h and at 600 °C for 48 h were investigated along the [111], [011] and [-117] orientations in compression. The material was stronger along the [-117] orientation compared to the [111] and [011] orientations based on load-biased thermal cycling experiments. The shape memory properties such as reversible strain, temperature hysteresis, critical stress for stress-induced martensite transformation and Clausius-Clapeyron relations were also strong functions of orientation and aging condition (precipitate characteristics). Shape memory effect with no or negligible irrecoverable strain was observed under stress levels as high as 1000 MPa. After aging at 550 °C for 3 h, the maximum reversible strains were 2.2%, 2.7% and 0.7% along the [111], [011] and [-117] orientations, respectively. Aging at 600 °C for 48 h resulted in maximum reversible strains of 2.3%, 3.2% and 0.9% along the [111], [011] and [-117] orientations, respectively. In both cases, similar levels of transformation strain, as a function of orientation, were observed during superelastic testing. The maximum work output reached 27 J/cm3 in the [011] orientation after aging at 550 °C for 3 h. [ABSTRACT FROM AUTHOR]

NiTiHf-based shape memory alloys have been receiving considerable attention for high temperature, high strength and two-way shape memory applications since they could have transformation temperatures above 100uC, shape memory effect under high stress (above 500 MPa) and superelasticity above 100uC. Moreover, their shape memory properties can be tailored by microstructural engineering. However, NiTiHf-based alloys have some drawbacks such as low ductility and high slope in stress induced martensite transformation region. In order to overcome these limitations, studies have been focused on microstructural engineering by aging, alloying and processing. It has been revealed that microstructural control is crucial to govern the shape memory properties (e.g. transformation temperatures, matrix strength, shape recovery strain, twinning type, etc.) of NiTiHf-based alloys. A summary of the most recent improvements on selected NiTiHf-based systems is presented to point out their significant shape memory properties, effects of alloying, aging and microstructure of transforming phases and precipitates. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ni45.3Ti39.7Hf10Pd5 alloys have transformation strain of up to 4.6% and work output of up to 29Jcm−3. [•] The alloy showed good superelastic behavior at 90°C with recoverable strain of 4.3%. [•] The alloy exhibited 1.6% two-way shape memory strain after a simple training procedure. [•] The formation of 〈011〉 type II twins in martensite plates results in large transformation strain. [ABSTRACT FROM AUTHOR]

Abstract: Systematic characterization of the shape memory properties of a quaternary Ni45.3–Ti29.7–Hf20–Pd5 (at.%) polycrystalline alloy was performed in compression after selected aging treatments. Precipitation characteristics were revealed by transmission electron microscopy. The effects of aging temperature and time on transformation temperatures, recoverable and residual strains, and temperature and stress hystereses were determined by differential scanning calorimetry, constant-load thermal cycling experiments and isothermal strain cycling (superelasticity) tests. The crystal structure and lattice parameters of the transforming phases were determined from X-ray diffraction analysis. It was revealed that precipitation hardening significantly improved the shape memory properties of the NiTiHfPd alloy. Under optimum aging conditions, shape memory strains of up to 4% under 1GPa were possible, and superelasticity experiments resulted in full strain recovery without any plastic deformation, even at stress levels as high as 2GPa. The NiTiHfPd polycrystalline alloy exhibited very high damping capacity/absorbed energy (30–34Jcm−3) and work output (30–35Jcm−3), which were attributed to the ability to operate at high stress levels without significant plastic deformation and to a high mechanical hysteresis (>900MPa) at temperatures ranging from 20°C to 80°C. [Copyright &y& Elsevier]

Abstract: Shape memory properties of [111] oriented NiTiHfPd single crystals aged at 600°C for 48 and 72h were investigated through isobaric thermal cycling and superelasticity experiments in compression. A maximum transformation strain of 2.6% at 1000MPa compressive stress was attained above 100°C. It was observed that precipitate size and interparticle distances were strong functions of aging time and subsequently govern the shape memory responses of the NiTiHfPd single crystals. Moreover, it was revealed that the elastic energy storage highly depends on precipitate characteristics. [Copyright &y& Elsevier]

The superelastic behavior of an NiTiHfPd single-crystal shape memory alloy in compression along the [111] orientation was investigated as a function of temperature. In the range of −30 to 70°C, a perfect superelastic loop with 4.2% transformation strain was attained. In addition, this material exhibited an ultrahigh yield strength of more than 2500MPa and an exceptional damping capacity of 44Jcm−3. [ABSTRACT FROM AUTHOR]

• Shape memory behavior of Ni 45,3 Ti 34,7 Hf 15 Pd 5 polycrystalline alloys are strongly tailored by post-aging cooling rates. • Thermal hysteresis in the WQ-alloy were higher compared to the FC-alloy. • Maximum of 70 °C thermal hysteresis was observed in the WQ-alloy while it was at most 54 °C at 1000 MPa. • The strain-temperature curves depend on the cooling rate and related microstructures. Effects of post-aging cooling rate (water quenched, WQ, and furnace cooled, FC) on the shape memory properties of polycrystalline Ni 45.3 Ti 34.7 Hf 15 Pd 5 alloys were studied. Their functional properties such as transformation temperatures, hysteresis, elastic energy, superelastic behavior and recoverable strain were found to be closely related to the post-aging cooling rate. Martensite start temperature decreased from 77 °C for WQ sample to 22 °C for FC sample. Impressive shape memory effect was observed under a huge compressive stress of 1 GPa with recoverable strains of 2.2 % and 2.5 % for the water quenched and furnace cooled samples, respectively. While both WQ and FC alloys showed perfect superelasticity for a wide temperature range, the WQ sample showed superelasticity at higher temperatures above 100 °C. [ABSTRACT FROM AUTHOR]

Temperature dependent compressive response and two-way shape memory effect (TWSME) of Ni 50.8 Ti 49.2 (at.%) shape memory alloys were systematically investigated. It was found that solutionized Ni 50.8 Ti 49.2 alloys could show positive and negative TWSME if they are plastically deformed in martensite and austenite (above M d), respectively. The maximum positive TWSME strain was 1.4% after martensite deformation of 8% at −90 ° C and maximum negative TWSME strain of 0.5% was obtained after austenite deformation of 8% at 200 ° C. Image 1 • Two-way shape memory effect of solutionized Ni50.8Ti49.2 (at.%) was studied. • Positive TWSME was observed after it is deformed in martensite. • Negative TWSME was observed after it is deformed in austenite (above Md). • The maximum positive TWSME strain was 1.4%. • The maximum negative TWSME strain was 0.5%. [ABSTRACT FROM AUTHOR]

The effects of chemical composition on the structural, functional and mechanical properties of hafnium palladium (HfPd) alloy system were investigated. Five selected compositions of Hf50-xPd50+x (x = − 0.5, 0, 0.5, 2.5 and 5) alloys were studied to reveal their potentials as high-temperature shape memory alloys to be employed in the applications to operate above 400 °C. All the alloys showed reversible phase transformations at temperatures above 400 °C. The large hysteresis of more than 120 °C was observed for HfPd alloys which is higher than most of the other conventional high-temperature SMAs. Thermal hysteresis increases with Pd content, as it was 124 °C for Hf51.5Pd49.5 alloy and 158 °C for the Hf45Pd55 alloy. The alloys do not show superelasticity and have very low recoverable strain. It was also shown that hardness increased (to above 400 HV) with increasing Pd content. The alloys fail when they are compressed up to 1500 MPa. [ABSTRACT FROM AUTHOR]