High Temperature Shape Memory Alloys Research Articles

Structural Changes of Tipt High-Temperature Shape Memory Alloys Induced by High Pressure Torsion

Structural Changes of Tipt High-Temperature Shape Memory Alloys Induced by High Pressure Torsion

Study of martensitic transformation in TiNiHfZr high temperature shape memory alloy using in situ neutron diffraction

The thermoelastic martensitic transformation in the polycrystalline Ti29.7Ni50.3Hf10Zr10 alloy was studied using high-resolution and in situ neutron diffraction in the temperature range of 25–250 °C. The thermal expansion tensors of the B19′ martensite and austenite were determined. B19′ martensite is characterized by a strong anisotropy of the thermal expansion coefficients. The analysis of the diffraction peak integrated intensities suggests that nearly 10% of austenite, which is partly disordered, does not take part in martensitic transformation. The volume effect of the transformation in the Ti29.7Ni50.3Hf10Zr10 alloy was 0.79%. The simultaneous addition of Hf and Zr atoms into the TiNi matrix leads to the deterioration of the crystallographic compatibility between martensite and austenite, which may be responsible for the large thermal hysteresis and volume changes upon transformation.

γ-phase evolution in aged Co–Ni–Ga shape memory alloy

A synchrotron X-ray diffraction study of high-temperature (HT) shape memory alloy 49Co–21Ni–30Ga (in at. pct.) was performed. The volume fraction, cell parameter and temperature evolution of the different secondary phases were analyzed. This study reports reliable experimental data on these parameters to be used as a future reference to adjust the composition of the material.

Development and testing of a Ni50.5Ti27.2Hf22.3 high temperature shape memory alloy

A NiTiHf alloy with a composition of Ni50.5Ti27.2Hf22.3 (at%) was heat treated at various temperatures between 400 and 550 °C at times ranging from 1 to 51 h to determine the effect of aging on its thermomechanical properties. Uniaxial constant-force thermal cycling was performed to determine the transformation temperature, transformation strain, and dimensional stability. Through appropriate heat treatment, the initial austenite finish temperature of 126 °C could be increased to over 200 °C, and the dimensional stability improved to near 0% residual strain through precipitation strengthening, without a significant loss in transformation strain.

In-situ synchrotron X-ray diffraction analysis of the elastic behaviour of martensite and H-phase in a NiTiHf high temperature shape memory alloy fabricated by laser powder bed fusion

• In-situ macroscopic elastic behaviour of a dual phase NiTiHf high temperature alloy obtained by laser powder bed fusion was studied. • Anisotropic behaviour of martensite is evidenced. • The micromechanical behaviour of H-phase, a strengthening phase, is shown for the first time. High temperature shape memory alloys of the Ni-Ti-Hf system are potential candidates for aerospace applications where powder bed additive manufacturing technologies are being increasingly used. In this work, a Ti-rich NiTiHf high temperature shape memory alloy powder was processed by laser powder bed fusion. The standard heat treatment of 550 ⁰C for 3 hours was imposed to promote H-phase precipitation. At room temperature, the material has a dual-phase microstructure composed of martensite, the matrix, and H-phase, as a strengthening precipitate. High energy synchrotron X-ray diffraction is used to evaluate, in-situ, the elastic behaviour of the fabricated part. The deformation anisotropy of several (h k l) families of planes of both phases is evidenced. No major texture changes were observed upon macroscopic elastic loading. We illustrate the potential of using high energy synchrotron X-ray diffraction for detailed analyses of minority phases in additively manufactured components.

The microstructure and martensitic transformation of Ti-13 V-3Al light weight shape memory alloy deformed by high-pressure torsion

Ti-13 V-3Al (at%) high temperature shape memory alloy was considered attractive material due to its low density. The demand for the better shape memory performance is proposed to widen the scope of application. In the present study, Ti-13 V-3Al is processed by high-pressure torsion and annealing. The microstructure, martensitic transformation and mechanical properties were investigated. The severe plastic deformation introduced high density defects and decreased the stability of the α″ phase. The phase constitution is the sole β phase after high-pressure torsion, which was related to the Gibbs-Thomson effect. After the annealing treatment at 700 °C, numerous small parallel α″ martensite variants were observed, which was affected by the residual defects. More nucleation sites were provided, resulting that the reverse martensitic transformation was accomplished in a narrow temperature range. Moreover, the formation of (111) type I twinning become easier due to the assistance of the (111) stacking faults. The α″ martensite variants became larger and evolved to the V-shaped and the triangular self-accommodated morphologies as the annealing temperature increased to 800 °C. The twinning relationship in the different annealing temperature both were the (111) type I twinning. The properties were characterized by the tensile loading-unloading tests and microhardness tests. The shape memory effect was optimized and 4% full recoverable strain was obtained in the alloy annealed at 700 °C. The apparent hardness decreased from 323HV and 263HV as the annealing temperature raised from 700 °C to 800 °C.

Enhancement of machining and surface quality of quaternary alloyed NiTiCuZr shape memory alloy through ultrasonic vibration coupled WEDM

NiTiCuZr shape memory alloys (SMA) outperform ternary and binary SMA alloys in terms of functional fatigue and higher temperature performance due to their high cyclic stability and transformation temperatures. Owing to the impairment of the shape memory effect during processing, it is difficult to select a manufacturing process for obtaining design functionality with the required dimensions and surface roughness. In this work, a high-temperature NiTiCuZr SMA was machined using an ultrasonic vibration assisted wire electric discharge machine (USV-WEDM). The machining was conducted using various parameters with a constant ultrasonic vibration of 20 kHz provided on a wire-electrode to evaluate surface roughness (Ra) and material removal rate (MRR). Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray analysis (EDX) were utilized to examine the surface integrity and chemical composition of the machined surfaces. MRR increased by 62% with a steady increase in pulse-on time ( Ton) and applied current ( I), whereas increasing levels of parameters such as pulse-off time ( Toff) and servo voltage (SV) reduced surface roughness ( Ra) by 69%. The results reveal that tool vibration at ultrasonic frequency reduces the surface roughness and improves the material removal rate of the machined NiTiCuZr SMA as compared to that of non-ultrasonic assisted machining conditions. SEM-EDX investigation reveals that the formation of re-solidification and oxide layers during NiTiCuZr machining at high machining parameters results in increased hardness and surface roughness. USV-WEDM is a suitable process for machining SMA alloy without adversely impacting SMA properties.

Effect of Solution Treatment Temperature on Microstructures and Mechanical Properties of (35.5-x)Ti-49.5Ni-15Hf-xNb (x = 3, 10) (at. %) High Temperature Shape Memory Alloys

In this study, the effects of solution treatment temperature on the microstructures and mechanical properties of (35.5-x)Ti-49.5Ni-15Hf-xNb (x = 3, 10) (at.%) high-temperature shape memory alloys were studied by means of scanning electron microscopy, differential scanning calorimetry, and tensile tests. The microstructures of the solution treated with a 1123 K Ti-49.5Ni-15Hf-3Nb alloy consisted of (Ti, Hf, Nb)2Ni and Nb-rich phases and a B19′ martensite matrix. The solution treated at 1123 K Ti-49.5Ni-15Hf-10Nb alloy consisted of the B19′ martensite matrix and Nb-rich, Hf-rich, and Ni-rich phases. The Ni-rich phases were eliminated by increasing the solution treatment temperature. When the solution treatment temperature was increased from 1123 K to 1273 K, the fracture strain increased from 8.4% to 10.4% and from 11.2% to 22.1% for the Ti-49.5Ni-15Hf-3Nb and Ti-49.5Ni-15Hf-10Nb alloys, respectively, which was ascribed to the elimination of the Ni-rich phase. Ms (B2-B19′ transformation start temperature) also increased from 421.3 K to 439.7 K and from 192.6 K to 299.2 K for the 3Nb and 10Nb alloys, respectively. Superelastic recovery ratios of 62.7% and 78.7% were observed in the 3Nb and 10Nb alloy solutions treated at 1123 K, respectively. The superelastic recovery ratio increased with an increase in solution treatment temperature from 1123 K to 1273 K for both the 3Nb and 10Nb alloys.

Novel Ti–20zr–49.7pd High Temperature Shape Memory Alloy with Facilitated Detwinning and Precipitation Strengthening

It is desirable to develop high temperature shape memory alloys with a martensitic transformation start temperature (Ms) above 100°C and a recoverable strain of about 4‐6% in the shape memory effect. The latter property is achieved with low variant reorientation stress due to easy detwinning of martensite and high plastic deformation stress due to precipitation strengthening. We have previously demonstrated the facilitation of detwinning for Ti–Zr–Ni–Pd quaternary alloy systems through controlling the crystal structure of martensite, and proposed that Ti–(15‐20)Zr–49.7Pd (at.%) and surrounding Ni‐containing compositions are candidates. In this study, aging temperature was optimized for a Ti–20Zr–49.7Pd alloy, and an excellent shape memory effect with Ms above 130°C, low reorientation stress around 200 MPa, high plastic deformation stress around 1800 MPa, and large recovery strain of 4.5% was achieved. On the other hand, short‐range ordering of solute atoms occurs just above the reverse transformation temperature and decreases Ms.

Phase transformation and viscoplasticity coupling in polycrystalline nickel-titanium-hafnium high-temperature shape memory alloys

The objective of this study was to investigate the interactions between phase transformation and viscoplasticity during uniaxial constant force thermal cycling (UCFTC) of a Ti-rich Ni-Ti-20Hf (at.%) high-temperature shape memory alloy (HTSMA). These tests were conducted (up till failure) at 1, 10 and 50 ∘C/min, to vary the duration of exposure to high temperatures, viz. the amount of viscoplasticity, and to examine the rate-dependency of actuation. The macroscopic results from the tests were used to investigate the evolution of transformation temperatures, hysteresis, transformation and irrecoverable strains for the cycles in which the effect of potential damage mechanisms could be assumed to be negligible. The phenomena that affected the behavior were: viscoplasticity at 1 ∘C/min, transformation-induced plasticity (TRIP) at 10 and 50 ∘C/min, and accumulation of retained martensite at all the three rates. More interestingly, the response at 1∘C/min indicated a unique interplay between the effect of viscoplasticity over phase transformation and static recovery. The retained martensite was identified through a series of DSC and XRD analyses, and its contribution to TRIP strain was quantified through a UCFTC test. Furthermore, a test involving alternating isothermal creep and UCFTC at 10 ∘C/min was conducted to investigate an effect of viscoplasticity produced by creep on the behavior, while reducing the viscoplasticity during thermal cycling. The alternating test revealed an effect of phase transformation over the viscoplastic strain rate. The experimental investigations demonstrated a rate-dependent phase transformation behavior, and a two-way coupling between phase transformation and viscoplasticity, bringing out the importance of understanding viscoplastic deformations in phase-transforming materials.

Actuation fatigue performance of NiTiZr and comparison to NiTiHf high temperature shape memory alloys

The actuation fatigue performance of a Ni-rich Ni50.3Ti29.7Zr20 (at.%) high temperature shape memory alloy (HTSMA) was evaluated and compared to previous results for Ni50.3Ti29.7Hf20 HTSMA. The shape memory properties of these alloys are known to improve significantly after proper aging heat treatments. Yet, their widespread implementation in high temperature applications requires a thorough characterization and a detailed understanding of their actuation fatigue performance. Consequently, actuation fatigue tests were performed by thermal cycling the NiTiZr alloy under constant loads from 200 to 500 MPa until failure. The results revealed that NiTiZr exhibits a shorter fatigue life with lower recoverable strain levels than a comparable NiTiHf alloy at all applied stress levels and the difference in fatigue life between the two alloys became more distinct with increasing stress. Building upon the extensive research on the microstructure, mechanical and functional behaviors of NiTiHf and NiTiZr HTSMAs during the last decade, this work is the first fundamental study that directly compares actuation fatigue behaviors of Ni-rich NiTiZr and NiTiHf alloys.

Facilitation of Detwinning Through Controlling Crystal Structure in Ti-Zr-Ni-Pd High Temperature Shape Memory Alloys

Ti-(Zr, Hf)-Ni alloys, in which Zr or Hf is added to Ti-Ni shape memory alloys, are expected to be excellent high temperature shape memory alloys because they have martensitic transformation temperatures above 100°C and high strength at elevated temperatures by utilizing H-phase precipitation. However, during reorientation of martensite variants, deformation stress increases with applied strain without showing a plateau region, making it difficult to obtain a large shape recovery strain. In the martensite phase of Ti-(Zr, Hf)-Ni alloys, thin and dense (001)B19′ compound twins are introduced during the martensitic transformation, and the detwinning of martensite variants becomes difficult to occur, resulting in the increase in deformation stress during the variant reorientation. Therefore, we propose a new alloy design concept to control the crystal structure by adding a fourth element in order to facilitate detwinning. In this paper, we first consider the cause of (001)B19′ compound twinning in Ti-(Zr, Hf)-Ni alloys, and then select Pd as the fourth element to change the crystal structure for preventing the formation of (001)B19′ compound twins. Experimental investigations of the effects of alloy composition on the crystal structure, transformation temperatures, and precipitate formation realized the concept with presenting a candidate composition range of Ti-(Zr, Hf)-Pd-based alloy systems for new high temperature shape memory alloys which readily undergo detwinning.

Effect of thermo-mechanical treatment on Ti–Ta-Hf high temperature shape memory alloy

The microstructure, martensitic transformation and mechanical/functional properties of Ti–Ta-Hf alloys with various thermo-mechanical treatments were investigated. The results reveal that the hot-rolling could refine the grain size and introduce a certain number of defects, resulting in the disappearance of martensitic transformation. The as-casted and solution treated Ti–Ta-Hf alloys were composed of α'' martensite phase and smaller volume of β phase. In contrast, the grain size of solution treated Ti–Ta-Hf alloy was slightly less than that of as-casted Ti–Ta-Hf alloy. This should be responsible to the higher yield stress and superior strain recovery characteristics for solution treated Ti–Ta-Hf alloy. The yield stress for the dislocation slip and the maximum recoverable strain of solution treated Ti–Ta-Hf alloy were 723 ​MPa and 5.06%, respectively.

High-temperature shape memory properties of Cu15Ni35Ti25Hf12.5Zr12.5 high-entropy alloy

The major challenges hindering the application of high-temperature shape memory alloys are their decreased mechanical strength and thermal stability. Herein, we present high-temperature shape memory properties of the Cu15Ni35Ti25Hf12.5Zr12.5 high-entropy alloy with improved mechanical properties and superior shape memory characteristics. Under an immense stress of 1400 MPa, the alloy exhibited a shape recovery ratio of 88.7%. Using an in-situ high-temperature X-ray diffractometer, the alloy was established to undergo phase transformation from B19′ to B2 phase. Additionally, the alloy exhibited a martensitic transformation temperature higher than 100 °C. The fracture strength and ductility were 1670 MPa and 24.7%, respectively, as measured by compression tests. The deformation mechanism and shape recovery phenomenon of this shape memory alloy are also discussed. This study demonstrates the potential of high-entropy shape memory alloys for high-temperature applications.

Investigating the effect of hot extrusion and annealing to the functional fatigue behavior of Ni50Ti30Hf20 high temperature shape memory alloy

This study reveals the functional fatigue properties of hot extruded and the effect of post annealing process to the cyclic stability and fatigue properties of Ni50Ti30Hf20 (at.%) alloy. Samples were cut from the hot extruded billet and three of them were randomly selected for functional fatigue experiments. Variation in the shape memory properties such as transformation temperatures, actuation and accumulated irrecoverable strain values was observed. Additionally, the samples showed 580, 754 and 1738 thermal cycles under 200 MPa in fatigue experiments. Six more samples were cut through the cross section of another piece of the extruded billet and enumerated for further fatigue testing to investigate the reason of the variation in the properties. The edges of the gauge sections of enumerated fatigue test samples were ground to remove the surface roughness. Although the fatigue life times were increased by 3–6 times with the edge grinding the differences in the values of the shape memory properties were still observed and this observation was attributed to the deformation variation and the residual stresses induced by hot extrusion. To overcome this problem, hot extruded samples were annealed and the fatigue experiments were repeated. All annealed samples exhibited the same transformation temperatures, actuation and accumulated irrecoverable strain values. Moreover, the transformation temperatures were stabilized and the fatigue life was increased to almost 12 000 cycles.

Processing, Preaging, and Aging of NiTi-20 at.% Hf High-Temperature Shape Memory Alloy from Laboratory to Industrial Scale

High-temperature shape memory alloys (HTSMAs) are a growing area of research in active materials that exhibit recoverable transformation strain and shape memory responses. They show promise for applications such as actuators in the aerospace, automotive, and energy exploration industries. NiTiHf is a cost-effective HTSMA solution with increased transformation temperatures above 115 °C. In this study, processing and aging treatments were performed on an industrially produced, hot-extruded Ni50.3Ti29.7Hf20 (at%) HTSMA. Portions of the extruded material were hot-rolled, pre-aged for 12 h at 300 °C, aged for 3 h at temperatures ranging 450–750 °C, or a combination. The goal of the aging treatments is to establish coherency between the interface of the matrix and H-phase nano-precipitates in the HTSMAs. After processing, the samples were characterized by differential scanning calorimetry, Vickers hardness testing, scanning electron microscopy, and synchrotron radiation x-ray diffraction. Previous studies have shown that pre-aging is effective for controlled nucleation of H-phase precipitates in low Hf content NiTiHf high-temperature shape memory alloys, this study shows that pre-aging of high Hf containing NiTiHf high-temperature shape memory alloys plays a small contributing role on thermal or mechanical properties, but is essentially ineffective and unnecessary at the pre-aging conditions examined here. Additionally, it is shown that hot-rolling can be used to control the orientation of H-phase in NiTiHf HTSMAs.

Effect of Sm Doping on the Microstructure, Mechanical Properties and Shape Memory Effect of Cu-13.0Al-4.0Ni Alloy.

The effects of rare earth element Sm on the microstructure, mechanical properties, and shape memory effect of the high temperature shape memory alloy, Cu-13.0Al-4.0Ni-xSm (x = 0, 0.2 and 0.5) (wt.%), are studied in this work. The results show that the Sm addition reduces the grain size of the Cu-13.0Al-4.0Ni alloy from millimeters to hundreds of microns. The microstructure of the Cu-13.0Al-4.0Ni-xSm alloys are composed of 18R and a face-centered cubic Sm-rich phase at room temperature. In addition, because the addition of the Sm element enhances the fine-grain strengthening effect, the mechanical properties and the shape memory effect of the Cu-13.0Al-4.0Ni alloy were greatly improved. When x = 0.5, the compressive fracture stress and the compressive fracture strain increased from 580 MPa, 10.5% to 1021 MPa, 14.8%, respectively. When the pre-strain is 10%, a reversible strain of 6.3% can be obtained for the Cu-13.0Al-4.0Ni-0.2Sm alloy.

Effect of annealing temperature on microstructure and mechanical response of sputter deposited Ti-Zr-Mo high temperature shape memory alloy thin films

The shape memory thin films are most suitable candidates as actuators in MEMS applications than other smart materials due to its ability to produce larger recovery strains at low operation voltages. Recently, Ti based shape memory alloys are gaining attention due to its ability to produce stable shape memory properties at high temperatures. In current study, Ti-Zr-Mo high-temperature shape memory thin films were fabricated using multi-target DC/RF magnetron sputtering system on Si (100) substrate. Deposition time and target power were varied to obtain the desired composition. Post-annealing was carried out at 500 °C, 600 °C and 700 °C for 15 min each to crystallize the thin films that were amorphous in the homogenized condition. The annealing temperature was varied to observe the structural and morphological changes. The total thickness of the film was found to be around 300 nm measured from the cross-sectional micrographs. The results suggest that flatness of surface topography is attributed to higher ad-atom mobility with increasing the annealing temperature. The martensite phase (α’) was observed after annealing at 500 °C and the formation of silicides was observed with annealing at higher temperatures. The transformation temperature of 220 °C was obtained from electrical resistivity measurement. The enhanced mechanical properties with maintaining structural stability was observed in Ti-Zr-Mo thin film annealed at 500 °C for 15 min.

A unified description of mechanical and actuation fatigue crack growth in shape memory alloys

The damage tolerance approaches to fatigue rely on fracture mechanics concepts to explicitly account for the fatigue crack growth process. These approaches typically adopt the Paris equation, which relates the crack growth per cycle to the range of variation of the stress intensity factor, ΔK, during cyclic loading by means of experimentally calibrated parameters. Conditions on the validity of a single parameter description of the near crack tip stress and strain fields restrict the applicability of ΔK to fatigue crack growth resistance in Shape Memory Alloys (SMAs) under general thermomechanical loading paths. These conditions are substantially relaxed for the cyclic ΔJ–integral approach, which can achieve similitude over a wider range of loading conditions and geometric configurations. Thus, ΔJ qualifies as a potential unified descriptor of thermomechanical fatigue crack growth resistance in SMAs, an assertion, which is tested in this work for data from both purely mechanical and actuation fatigue crack growth experiments of a high temperature SMA, Ni50.3Ti29.7Hf20. In the latter, compact tension specimens are subjected to thermal cycling under constant load between an upper and a lower cycle temperature that allows for the stable phase to alternate in each cycle. It is shown here that a Paris-type power-law crack growth expression based on the ΔJ-integral can fit fatigue crack growth rate data from both types of experiments with a single set of parameters. This new analysis method for both actuation and mechanical fatigue crack growth rates provides a unified description for fatigue crack growth in SMAs and can allow estimating actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.

Transformation behavior and superelasticity of TiZrHfNiCoCu multi-component high-temperature shape memory alloys

Multi-component (TiZrHf)1Ni1.50Co0.60Cu0.90, (TiZrHf)1Ni1.48Co0.58Cu0.88, (TiZrHf)1Ni1.45Co0.57Cu0.86, and (TiZrHf)1Ni1.40Co0.54Cu0.82 high-temperature shape memory alloys (HTSMAs) were prepared by arc-melting and then microstructures, transformation temperatures, phase constituents and superelasticity were investigated by scanning electron microscope observation, differential scanning calorimetery, X-ray diffraction and dynamic mechanical analysis in tensile mode, respectively. The microstructure of solution treated TiZrHfNiCoCu HTSMAs specimens consisted of (TiZrHf)(NiCoCu)-type matrix and (TiZrHf)2(NiCoCu)-type second phase. The area fraction of the second phase increased from 1.7% to 17.2% with the increased in (TiZrHf) content from 50 at% to 52 at%. Martensitic transformation start temperature of the solution treated specimens increased from 53.5 °C to 188.7 °C with the increase in (TiZrHf) content from 50 at% to 52 at%. TiZrHfNiCoCu HTSMAs showed clear superelasticity in the solution treated state. The superelastic recovery strain of solution treated TiZrHfNiCoCu HTSMAs was in the range of 4.2–4.6% depending on the alloy composition.