High Temperature Shape Memory Alloys Research Articles

Study on properties of pre-alloy powder of TiNiHf high temperature shape memory alloy for additive manufacturing by preheating treatment

In this work, pre-alloyed powder high-temperature shape memory alloy (Ti30Ni50Hf20) for additive manufacturing was preheated at 300–800 °C and thermoscycled at 650 °C to examine the influence of preheating temperature and thermal cycle number on the phase composition and phase transformation behavior. The results showed that the H phase precipitation occurred in the matrix with the rising preheating temperature within 300–600 °C, and the phase transformation temperature of the pre-alloyed powders also increased. When the preheating temperature exceeded 600 °C, the H phase dissolved back into the matrix, and the phase transformation temperature decreased. A total of 50 thermal cycles were conducted on the pre-alloyed powders at 650 °C, and the phase transformation temperature and phase composition remained stable after cycling. It can be concluded that the preheating temperature greatly affected the phase composition and phase transformation behavior of Ti30Ni50Hf20 high-temperature shape memory alloy pre-alloyed powders, while the number of thermal cycles delivered a minor effect.

Grain growth in Ni50Ti30Hf20 high-temperature shape memory alloy processed by high-pressure torsion

Grain refinement and precipitation hardening play critical important role for stabilization and improving functional properties of shape memory alloys. However, the relationship between precipitation and nanocrystalline grain growth behavior in NiTiHf alloys is still unclear. This work aims to investigate the role of precipitation in the nanocrystalline grain growth behavior of HPT-processed Ni50Ti30Hf20 high-temperature shape memory alloy. An abnormally low grain growth rate (n = 0.08) was observed after post-deformation annealing (PDA) at 550 °C for 1 h. It was proposed that grain growth suppression may be caused by the presence of relatively large H-phase precipitates, which act as barriers to grain boundary movement. A detailed analysis of the grain growth kinetics during PDA suggests that the coarsening process is controlled by Ni diffusion. Additionally, the dependence of strength and transformation temperatures on grain size in NiTiHf alloy is found to follow the Hall-Petch relation with some exceptions due to H-phase precipitation. The results of this research may be useful for the development of methods and strategies to stabilize the nanocrystalline structure in metallic materials.

Noise-Aware Active Learning to Develop High-Temperature Shape Memory Alloys with Large Latent Heat.

Shape memory alloys (SMAs) with large latent heat absorbed/released during phase transformation at elevated temperatures benefit their potential application on thermal energy storage (TES) in high temperature environment like power plants, etc. The desired alloys can be designed quickly by searching the vast component space of doped NiTi-based SMAs via data-driven method, while be challenging with the noisy experimental data. A noise-aware active learning strategy is proposed to accelerate the design of SMAs with large latent heat at elevated phase transformation temperatures based on noisy data. The optimal noise level is estimated by minimizing the model error with incorporation of a range of noise levels as noise hyper-parameters into the noise-aware Kriging model. The employment of this strategy leads to the discovery of the alloy with latent heat of -36.08 J g-1, 9.2% larger than the best value (-33.04 J g-1) in the original training dataset within another four experiments. Additionally, the alloy represents high austenite finish temperature (481.71°C) and relatively small hysteresis. This promotes the latent heat TES application of SMAs in high temperature circumstance. It is expected that the noise-aware approach can be convenient for the accelerated materials design via the data-driven method with noisydata.

NiTiHf Actuation Fatigue: How Much Do We Really Know?

Abstract The NiTiHf alloy is one of the most extensively studied high-temperature shape memory alloys. This article reviews the key factors affecting the actuation fatigue performance of NiTiHf as well as the current state of research in this area, and it identifies some factors that require further study.

The deformation mechanism of low symmetric Ti–Pt intermetallic compounds containing high density of planar defects

Intermetallic compounds typically exhibit limited plastic deformation capacity due to challenges in activating dislocation slip and deformation twinning, coupled with a lack of alternative deformation mechanisms. Ti–Pt alloys are a prevalent type of intermetallic compound utilized in high-temperature shape memory alloys and as materials for energy applications in electric fields. However, they often exhibit poor deformation capability. Here, we prepared a low-symmetry intermetallic phase, Ti4Pt3, which demonstrates significant plastic deformation capability. This phase features a high density of parallel planar defects, resulting in an exceptionally large lattice periodicity perpendicular to these defects. Through in-situ scanning electron microscope compression tests, we observed substantial plastic deformation in this new phase. Analysis of the deformed Ti4Pt3 phase revealed that the dense planar defects create uniformly distributed sites of internal stress concentration, enabling a rapid increase in back stress within crystals. This phenomenon leads to notable lattice rotation and localized order-disorder transitions, both crucial mechanisms facilitating plastic deformation and enhancing deformation capacity. Our research underscores the potential of leveraging structural asymmetry to enable unconventional deformation mechanisms, thereby enhancing the plasticity of intermetallic materials.

Impact of Hf alloying on the functional properties of Ni-Mn-Ga high temperature shape memory alloys

The paper reports on the functional properties and microstructure of polycrystalline Ni-Mn-Ga-Hf high temperature shape memory alloys, in which different amounts of Hf up to 4 at.% are added in substitution of Mn and Ga, while the nominal Ni content is kept constant. The increase in the amount of Hf promotes a decrease in the valence electron concentration (e/a), tetragonality (c/a) of the martensitic unit cell, and martensitic transformation temperatures, as well as a significant decrease of the transformation hysteresis. The non-modulated tetragonal martensite, typical of Ni-Mn-Ga high temperature shape memory alloys, is formed in all the alloys studied here, though the alloy with 4 at.% of Hf also contains a small fraction of modulated 14M martensite. Hf addition improves the thermomechanical response of the alloys under compressive stress up to 300 MPa and leads to nearly closed cycles with minimal irrecoverable strain and low hysteresis (8 K) for the alloys with 3 and 4 at.% of Hf. These alloys also demonstrate excellent stability under repetitive thermal cycling and little change upon aging at 870 K. A second phase rich in Hf and Ni starts to precipitate at 1 at.% of Hf addition and its volume fraction experiences an abrupt and progressive increase for higher Hf contents. The structure of the second phase looks like the usual f.c.c. γ phase reported in other Ni-Mn-Ga-based alloys, but it has a double lattice parameter. Two structural models based on the A6 face-centered tetragonal unit cell with space group I4/mmm (No. 139), equivalent to the double f.c.c. lattice, are proposed for this new phase.

Composition dependence of phase transformation and shape memory effect of Ti-Zr-Pd-Pt high temperature shape memory alloys

Ti-Zr-Pd-Pt alloys have been considered as potential candidates for high-temperature shape memory alloys (HT-SMAs). In this study, nine alloys were prepared to investigate the effect of multi-component alloying on the phase transformation and shape memory effect. The structural phase diagram of martensite in Ti–Zr–Pd–Pt quaternary alloys was firstly systematically investigated to provide insights and predictions for further research. The phase transformation is divided into three groups: typical martensitic transformation (MT) area, supercooling-controlled phase transformation area and diffusion-assisted phase transformation area. The martensite structure changes from B19 to B33 with the addition of Zr over 25 %. The contribution of Pt contents to raising the martensitic transformation temperature (MTT) became less pronounced with increasing Zr contents. As for shape recovery, over 87 % shape recovery was obtained even under 300 MPa in Ti-10Zr-Pd-Pt alloys, among which Ti-10Zr-15Pt-35Pd presents the highest recovery ratio of over 97 %.

Microstructure and Thermoelastic Martensitic Transformation in Ni-Low and -Rich Ni–Ti–Hf–Nb Hightemperature Shape Memory Alloys

Microstructure and Thermoelastic Martensitic Transformation in Ni-Low and -Rich Ni–Ti–Hf–Nb Hightemperature Shape Memory Alloys

First-Principle Study on Tailoring the Martensitic Transformation of B2 Nb50−xTixRu50 Shape-Memory Alloy for Structural Applications

NbRu has a potential as a high-temperature shape-memory alloy (HTSMA) because it has a martensitic transformation temperature above 1000 °C. However, its shape-memory properties could be improved for consideration in the aerospace and automotive industry. The unsatisfactory shape-memory properties could be associated with the presence of a brittle tetragonal L10 martensitic phase. Therefore, in an attempt to modify the transformation path from B2→L10 in preference of either B2→orthorhombic or B2→monoclinic (MCL), an addition of B2 phase stabiliser, titanium (Ti), has been considered in this study to partially substitute niobium (Nb) atoms. The ab initio calculations have been conducted to investigate the effect of Ti addition on the thermodynamic, elastic, and electronic properties of the Nb50−xTixRu50 in B2 and L10 phases. The results showed that the B2 and L10 phases had comparable stability with increasing Ti content. The simulated data presented here was sufficient for the selection of suitable compositions that would allow the L10 phase to be engineered out. The said composition was identified within 15–30 at.% Ti. These compositions have a potential to be considered when designing alloys for structural application at high temperatures above 200 °C.

LPBF Processability of NiTiHf Alloys: Systematic Modeling and Single-Track Studies.

Research into the processability of NiTiHf high-temperature shape memory alloys (HTSMAs) via laser powder bed fusion (LPBF) is limited; nevertheless, these alloys show promise for applications in extreme environments. This study aims to address this limitation by investigating the printability of four NiTiHf alloys with varying Hf content (1, 2, 15, and 20 at. %) to assess their suitability for LPBF applications. Solidification cracking is one of the main limiting factors in LPBF processes, which occurs during the final stage of solidification. To investigate the effect of alloy composition on printability, this study focuses on this defect via a combination of computational modeling and experimental validation. To this end, solidification cracking susceptibility is calculated as Kou's index and Scheil-Gulliver model, implemented in Thermo-Calc/2022a software. An innovative powder-free experimental method through laser remelting was conducted on bare NiTiHf ingots to validate the parameter impacts of the LPBF process. The result is the processability window with no cracking likelihood under diverse LPBF conditions, including laser power and scan speed. This comprehensive investigation enhances our understanding of the processability challenges and opportunities for NiTiHf HTSMAs in advanced engineering applications.

Hysteresis gap-shrinking and structural effects of minor Al and Ti modifications on binary CuAl-based high-temperature shape memory alloys

Cu-based shape memory alloys (SMAs), except for exhibiting shape recovery, superelasticity, and high damping, are desirable because these smart materials have higher electrical and thermal conductivity and much lower prices than NiTi SMAs. However, they also have some downsides in mechanical strength and brittleness (mostly stemming from their coarse grain structure) and thermal instability. Therefore, adding some grain refining elements to these SMAs to improve their shape memory effect (SME), and thermal, structural, and mechanical properties is a widespread and simple way that significantly affects their martensitic phase transitions, structure, and mechanical properties. One of these grain-refining elements is titanium. Its thermal conductivity is lower than those of Cu and Al elements and has a low solubility in Cu-matrix. Besides the effects of small Al variations, the use of minor amounts of titanium in binary CuAl-base alloys can show impressive effects on all characteristics of these shape memory alloys, such as shape memory effect properties, martensitic transformation kinetics parameters, and microstructural features. In this research work, CuAlTi ternary high-temperature shape memory alloys (HTSMAs) with new compositions were produced by the arc melting method without a complicating use of Mn or Ni components in usual ternary CuAlMn and CuAlNi shape memory alloys. Thermal analyses of the prepared samples of the alloys were investigated by using differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements. In contrast, x-ray diffraction (XRD) test results and optical micrographs were used for analyzing the structure of the alloy samples. The effect of different amounts of low soluble and grain refining Ti element on the binary CuAl alloy system was investigated.

Oxidization on phase transformations in Ti-Nb high temperature shape memory alloys

Properties of metastable β-Ti alloys can be optimized by tuning the rich phase transformations. Oxygen (O) introduction, inevitable during melting and processing, affects greatly on phase transformations and the associated properties in β-Ti. We investigated the effects of oxidation on phase transformations in Ti-Nb alloys. Based on microscopic characterizations, the as-quenched microstructure of oxidized Ti-20 Nb is revealed to contain 3 distinct layers with respect to the phase constitutions and subsequent transformations, i.e. oxide, oxygen influenced (OI), and center (C) layers. On subsequent heating, β→ωiso occurs in OI layer, and in C layer undergo successive two step α”→β and β→ωiso transformations. In C layer of low O-content, both the starting temperatures of α”→β (AsC) and β→ωiso (ωsC) transitions increase slightly with O-content. In OI layer of high O-content, since the β→α” transition is suppressed upon quenched to 273 K, the retained β starts to precipitate ωiso far below ωsC upon subsequent heating. In summary, with the increase of O-content, the As and ωs slightly increase at low O-content and then undergo a drastic decrease. The underlying mechanisms are further discussed. Our work may shed lights on the high temperature thermomechanical processing and designing high performance metastable β-Ti alloys.

Investigation of bio-corrosion behavior, structural and thermal properties of Ni20Ti50Sn30 high-temperature shape memory alloy

In this study, Ni20Ti50Sn30 (atomic %) sample was prepared using the arc melting method. Some thermodynamic parameters (phase transformation temperatures, PTT; enthalpy, ΔH; and phase transformation hysteresis, H) obtained from the phase transformations of the alloy were determined using differential scanning calorimetry (DSC). The sample exhibited high-temperature shape memory alloy (HTSMA) properties. X-ray (XRD) analysis was performed to determine the crystal structure properties at the alloy. The presence of B2, B19′, Ti2Ni, β-Ti, Ni3Sn4, TiNiSn and Ti3Sn phases was identified through X-ray analysis of the alloy. Scanning electron microscopy (SEM) was used to examine the pre- and post-corrosion surface morphologies of the sample, with EDX analysis being performed to reveal chemical structure determinations (matrix phases, precipitates and weathering zones). The presence of dendritic arms was observed, while martensite plates were absent in all SEM images. Particularly in the SEM image taken after corrosion, it was noted that the TiO2 layer placed on the surface of the matrix rich in Ti elements altered the surface morphology of the alloy. The presence of Ti and O elements in the post-corrosion EDX analysis confirmed the existence of this layer. Biocompatibility studies of the alloy were conducted using potentiodynamic corrosion tests and electrochemical impedance spectroscopy (EIS) methods. It was determined that the NiTiSn sample was in the excellent stability class according to corrosion standards.

Enhanced thermal cycle stability and shape memory effect in NiTiHf shape memory alloys fabricated by laser powder bed fusion

We report on the fabrication of NiTiHf high temperature shape memory alloys (SMAs) via laser powder bed fusion (LPBF), which are almost free of micro-pores and display (Ti, Hf)2Ni nanoprecipitates with variable size and volume fraction. The effect of variable laser powers of 50 W, 60 W and 70 W on the martensite variant microstructures and mechanical properties of the LPBFed SMAs was investigated. Interestingly, all LPBFed NiTiHf samples display enhanced thermal cycle stability of transformation temperatures with a temperature eviation of −3°C, which is attributed to the lack of dislocation slip originated from the habit plane configuration characteristic with multidomain (001) martensite compound twins. Meanwhile, all LPBFed NiTiHf samples exhibit trained one-way and two-way shape memory effects due to the dislocation slip upon compression deformation at room temperature.

Electron beam powder bed fusion of Ti-30Ta high-temperature shape memory alloy: microstructure and phase transformation behaviour

ABSTRACT The present study reports on additive manufacturing of a Ti-30Ta (at.%) high-temperature shape memory alloy (HT-SMA) using electron beam powder bed fusion (PBF-EB/M) technique. Detailed microstructure analysis was conducted to reveal the microstructural evolution along the entire process chain, i.e. from gas-atomised powder to post-processed material. PBF-EB/M processed structures with near full density and an isotropic, β-phase stabilised microstructure, i.e. equiaxed β-grains of around 20 µm in diameter with no preferred crystallographic orientation, are reported. As revealed by differential scanning calorimetry, post-process heat-treated Ti-Ta demonstrates a reversible martensitic phase transformation well above 100°C. Although partly unmolten Ta-particles after both gas atomisation and PBF-EB/M remain a challenge towards robust processing, PBF-EB/M appears to show significant potential for fabrication of Ti-Ta HT-SMAs, especially when functional metal parts and components with complex shapes are required, which are difficult to fabricate conventionally.

Hot deformation behavior of NiTiHf alloy under compression: Effect of deformation heating on flow softening

High-temperature shape memory alloys are suitable materials for substituting hydraulic elements in the aerospace industry due to light weight and high reactive stresses developing during thermoelastic martensitic transformation and acceptable recoverable strain. These properties may be improved by plastic deformation; however, the plastic deformation of NiTiHf alloys along with structure investigation has yet to be studied systematically. Here, the hot deformation behavior of a NiTiHf alloy with hafnium content >20 at.% has been investigated, and the processing map has been elaborated according to the dynamic material model (DMM) based on the Prasad instability criterion. The deformation heating effect has been shown to affect the flow curves and accounts for profound softening during deformation due to quasi-adiabatic deformation mode at high strain rates. NiTiHf alloys have a much higher activation energy of plastic flow, and the instability region of plastic flow is much larger compared to binary NiTi alloys. Microstructural observations have revealed that in the instability regions, oxide bands and crack nucleation occur, whereas outside this region, such defects are not observed. Electron backscatter diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the necklace-type structure, while at high strain rates, this phenomenon has not been observed. Based on the obtained results, the preferable deformation conditions are established to be 850–1000 °C and 0.003–0.05 s−1.

Outstanding shape memory effect and thermal stability of Cu–Al–Ni-based high temperature shape memory alloys achieved by Gd and Mn doping

In this paper, the effects of Mn element and rare earth element Gd on the microstructure, thermal stability, mechanical properties and shape memory effect of polycrystalline Cu-12.0Al-4.0Ni and Cu-12.0Al-4.0Ni-1.5Mn-xGd high-temperature shape memory alloy were investigated. The results show that the Cu-12.0Al-4.0Ni alloy exhibits millimeter-sized grain, dual-phase structure with coexistence of 18R and 2H martensite and poor thermal stability. When the Mn and Gd element was added to Cu-12.0Al-4.0Ni alloys, the grain size can be refined by an order of magnitude, and the 2H martensite disappeared completely, accompanied by the formation of the AlCu4Gd phase, meanwhile, thermal stability is significantly improved. The grain refinement improves the mechanical properties and shape memory effect of Cu-12.0Al-4.0Ni-1.5Mn-xGd alloys. When x = 0.3, the compressive fracture stress, compressive fracture strain and recoverable strain can reach 969 MPa, 15 % and 8 %, respectively.

Effect of short heat treatment on mechanical properties and shape memory properties of Cu–Al–Ni shape memory alloy

Abstract The transformation of shape-memory alloys to high-temperature shape-memory alloys can be achieved through either the addition of alloying elements or heat treatment. However, heat treatment is more effective in improving the properties of the alloys. This research paper explores the impact of annealing on the mechanical and shape memory properties of Cu–Al–Ni shape memory alloys. The alloys were first cast, homogenized, and then machined before being aged at temperatures of 250, 300, and 350°C, and finally air-cooled. The results showed an increase in transformation temperature and recovery strain, as well as shape memory effect, and a decrease in hardness. Moreover, there was an increase in yield stress and strain. In conclusion, aging was found to improve the shape memory properties and mechanical properties better than thermomechanical treatment and some alloying elements.

High thermal stability effect of vanadium on the binary CuAl base alloy for a novel CuAlV high-temperature shape memory alloy

Abstract In this study, two high-temperature shape memory alloys (HTSMAs) of CuAlV with unprecedented chemical compositions were fabricated using the arc melting technique, followed by traditional ice-brine water quenching after the melting process. To characterize the shape memory properties and structure of the alloys, a series of tests including differential calorimetry (DSC and DTA), EDS, optical microscopy, and XRD were conducted. The DSC tests, performed at different heating and cooling rates, demonstrated highly stable reversible martensitic phase transformation peaks at high temperatures, which were also confirmed by the results of DTA tests. Microstructural XRD and optical microscopy tests were conducted at room temperature, revealing the martensitic structure of the alloys in both cases. Based on all the results, the effects of different minor amounts of vanadium additives directly on the CuAlV alloy were discussed.

Effect of Ni doping on mechanical properties and phase transformation in Co–V–Ga high-temperature shape memory alloys

In the present work, the effects of Ni doping on the microstructure and mechanical properties of Co–V–Ga high-temperature shape memory alloy have been studied. It has been found that γ phase occurs in the form of precipitation or even dendrite microstructure when Ni content continuously increases in the alloy. The composition distribution showed the elements Co and Ni segregated in the γ phase, while elements V and Ga concentrated in the martensite phase. Moreover, the phase transition temperature increased by Ni-doping in Co–V–Ga alloys due to the increase in e/a of the alloy. However, the abundant presence of γ phase hindered the shear phase transformation and created a large number of grain boundaries to reduce Ms temperature when Ni content continuously increased. In addition, electron backscattered diffraction (EBSD) results verified that the presence of γ phase could hinder the expansion of cracks, and improve the strength and plasticity of the alloy. Furthermore, it could be found the shape memory effect could maintain a relatively high recovery rate when Ni content was within a certain extent. However, the shape memory effect of the alloy significantly decreased in the presence of γ phase with dendrite microstructure. The current research results not only clarify the influence of Ni doping on the microstructure and mechanical behavior of Co–V–Ga alloys, but also provide guidance for element doping to prepare high-temperature shape memory alloys (HTSMAs) with excellent performance.