High Temperature Shape Memory Alloys Research Articles

Precipitation and coarsening kinetics of H-phase in NiTiHf high temperature shape memory alloy

Precipitate hardening is the most easiest and effective way to enhance strain recovery properties in NiTiHf high-temperature shape memory alloys. This paper discusses the precipitation, coarsening and age hardening of H-phase precipitates in Ni50Ti30Hf20 alloy during isothermal aging at temperatures between 450 °C and 650 °C for time to 75 h. The H-phase mean size and volume fraction were determined using transmission electron microscopy. Precipitation kinetics was analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a Time-Temperature-Transformation diagram was constructed. The evolution of H-phase size suggests classical matrix diffusion limited Lifshitz-Slyozov-Wagner coarsening for all considered temperatures. The coarsening rate constants of H-phase precipitation have been determined using a modified coarsening rate equation for non-dilute solutions. Critical size of H-phase precipitates for breaking down the precipitate/matrix interface coherency was estimated through a combination of age hardening and precipitate size evolution data. Moreover, time-temperature-hardness diagram was constructed from the precipitation and coarsening kinetics and age hardening of H-phase precipitates in Ni50Ti30Hf20 alloy.

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

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

Effect of Oxidation on the Transformation Temperatures and Microstructure of Ti-Ta-Al High Temperature Shape Memory Alloys

In this study, the effect of oxidation on the transformation temperatures and microstructure of Ti-Ta20-Al5 high temperature shape memory alloys (Ti-Ta20-Al5 HTSMAs) is studied. The alloy was studied under two conditions, as cast and after cold rolling and recrystallizing (CR +RE). The microstructure, phase composition, and phase transformation temperature of Ti-Ta20-Al5 HTSMAs before oxidation were studied. Thermogravimetric analysis (TGA) combination with microstructural investigations were performed after oxidation at temperatures range from 25°C up to 1000°C. For both conditions, the alloy consisted mostly of β – BCC and martensite ʺα – orthorhombic phase before oxidation. Ti-Ta20-Al5HTSMAs as (CR +RE) shows martensitic transformation temperature higher than 400°C. The oxidation behaviour was linear until 650°C, while parabolic at elevated temperatures. For both conditions, the alloy consisted of β – BCC phase (Ta – rich) and α-hexagonal phase (Ti – rich) with not strong precipitation of ω – phase after oxidation. The results show that the formation of multi – layer oxide, which consists of Al2O3, TiO2, and Ta2O5. For both conditions, oxidation can suppress martensitic phase transformation by precipitation of α-hexagonal phase (Ti – rich), Al2O3, TiO2, and Ta2O5, and ω phase, would result in degradation of the martensitic phase transformation.

Structure and substructure characterization of solution-treated Ni50.3Ti29.7Hf20 high-temperature shape memory alloy

We investigate the structure and substructure of the solution-treated martensitic Ni50.3Ti29.7Hf20 high-temperature shape memory alloy (SMA) by transmission electron microscopy and precession electron diffraction (PED). Most martensite plates contain high-density internal (0 0 1) compound twins. Four martensite variants (i.e., A, B, C, and D) were observed using PED. The orientation relationships were determined to be (1 1 0.64) Type II twins between A and B, and (1 1 1) Type I twins between C and D. B and C are divided by junction planes, in which (1 1 1) in C is parallel to (1 1 0.64) in B. These orientation relationships are rarely observed in other NiTi or NiTiHf SMAs, which offers new insight to understand the phase transformation behavior in Ni50.3Ti29.7Hf20 SMAs. Several low-angle grain boundaries were observed near the variant interfaces, which may explain the difficulty of martensite reorientation and detwinning in NiTiHf SMAs.

Atomistic simulations of AuTi high-temperature shape memory alloys

Herein, the atomic-scale phase transformation and deformation processes of AuTi shape memory alloys for high-temperature applications were investigated using molecular dynamics simulations based on a newly developed interatomic potential for the Au-Ti binary system. The developed second nearest-neighbor modified embedded-atom method potential exhibited good accuracy and transferability to reproduce various physical properties of the target alloy system, particularly in properties related to the reversible phase transformation of AuTi high-temperature shape memory alloys. As a conceivable application of the interatomic potential, the temperature-induced phase transformation of AuTi single-crystal shape memory alloys and the effect of off-stoichiometry on the phase transformation behavior were examined. Molecular dynamics simulations revealed that the transformation temperature and thermal hysteresis decreased with increasing Ti content, consistent with the reported experimental trend, suggesting that the reduced volume difference between the austenite and martensite phases was closely related to the reduced thermal hysteresis. We further examined the mechanical response and resultant superelasticity of nanocrystalline AuTi alloys to demonstrate that the cyclic deformation of nanocrystalline alloys exhibited a transformation ratcheting behavior caused by the plastic deformation of amorphous-like grain boundary region.

Mechanical response and actuation fatigue behavior of Nb-nanowire enhanced NiTiHf high temperature shape memory alloy wires

This study elucidates the influence of Nb addition on the mechanical and actuation response of the (Ni49.5Ti35.5Hf15)90Nb10 high temperature shape memory alloy (HTSMA) wires and presents the microstructure evolution of Nb with the drawing process. The change in morphology of β-Nb from network structure to nanowires greatly increases the yield strength of the NiTiHfNb alloy followed by an ascending stress plateau. The fatigue life of the (Ni49.5Ti35.5Hf15)90Nb10 wire is 7754 cycles with remarkably stable actuation strain of 2.3% under 418 MPa. The concept of in-situ nanowire composite is applied to HTSMAs for the first time and our finding implies the promising practical applications of NiTiHfNb wires in high temperature actuators.

The role of temperature in the microstructural evolution of HPT-processed NiTiHf high-temperature shape memory alloy

Ni50.3Ti29.7Hf20 alloy was subjected to high-pressure torsion (HPT) process at room temperature (RT) and 300 °C. The mix of amorphous phase and nanocrystalline debris was found by TEM observation in sample processed at RT. Two types of structure were clearly distinguished after HPT at 300 °C: the first one is martensite-like plates oriented in single direction; the second is nanocrystalline structure with narrow amorphous bands. The fraction of amorphous phase is significantly larger after RT deformation.

An experimental analysis on the influence of CO2 laser machining parameters on a copper-based shape memory alloy

The term 'shape memory alloys' (SMAs) refer to a group of metallic materials that have the ability to return to a previously defined shape when subjected to the appropriate thermal or loading cycles. They are now being employed in different real-life applications. Cu-Al-Fe is a High Temperature Shape Memory Alloy (HTSMA) and could replace Ni-Ti SMAs. Conventional machining technologies are not efficient enough in machining SMAs, and thus the properties of the SMAs are affected. One of the most successful technologies for processing these alloys is Laser Beam Machining (LBM). This work shows an investigation of the effect of process variables in laser machining on SMA. Differential scanning calorimetry, X-Ray Diffraction, Optical Microscopy, Scanning Electron Microscopy, and Hardness tests were used to analyze the laser-machined material. It has been found that power was the highest influencing variable which affects Material Removal Rate (MRR) and Surface Roughness (Ra), and the second most influencing parameter was cutting speed. DSC thermograms confirm that the Shape Memory Effect (SME) and the transition temperatures of the SMA have not been much affected after machining. The hardness of the machined surface slightly increased after machining, owing to the formation of a re-solidified layer.

Giant high-temperature superelasticity in a Ni-Fe-Mn-Sn shape memory microwire

High-performance high-temperature shape memory alloys (HTSMAs) are greatly needed for applications as intelligent components in aerospace, automotive, manufacturing and energy exploration industries. Here we developed a high-performance Ni-Fe-Mn-Sn HTSMA microwire, which exhibits giant high-temperature superelasticity with a maximum recoverable strain as high as ∼20% in the broad temperature range from 503 K up to 793 K. Incorporating the advantages of low cost of constituent elements and easy fabrication, this microwire shows great potential for applications in miniature high-temperature intelligent devices. This work is instructive for the development of small-scale high-performance HTSMAs.

Effects of microstructure and composition on constitutive response of high temperature shape memory alloys: Micromechanical modeling using 3-D reconstructions with experimental validation

NiTiHf alloys are high temperature Shape Memory Alloys (SMAs) suitable for actuation applications in a wide range of temperatures. Aging heat treatments in NiTiHf SMAs cause the formation of nano-sized, non-transforming and elastic precipitates, which modify phase transformation behavior. Experimental observations of the aged alloys showed a counterintuitive trend of increasing phase transformation strain with precipitation. In order to predict such microstructure effects on constitutive response of precipitated NiTiHf SMAs, in this work a finite element based micromechanical modeling framework is developed. Two types of representative volume elements are considered: (i) exact 3-D reconstructions of precipitates using transmission electron microscopy, and (ii) less expensive ellipsoidal representation of precipitates. The composition changes in the material are modeled, and their effects are incorporated by considering composition dependency of phase transformation properties in the matrix. The resulting model is used to predict the constitutive responses for different aging conditions in Ni50.3Ti29.7Hf20 and Ni50.3Ti34.7Hf15 SMAs. The model predictions are in good agreement with the experimental measurements, and experimental and modeling results are presented with confidence intervals from sample-to-sample variations. The present work constitutes the first micromechanical modeling of high temperature SMAs with consideration of real microstructures.

Low-cost (ZrCu)50-xTax high temperature shape memory alloys showing excellent shape memory effect

ZrCu-based alloys, as one of the most potential high-temperature shape memory alloys, show quite high martensitic transformation temperature, relatively low price for the raw materials, and good casting fluidity. In this work, the martensitic transformation (MT) and shape memory effect of (ZrCu)50-xTax high-temperature shape memory alloys were systematically investigated. Both X ray diffraction and SEM backscattered electron image demonstrated the coexistence of the main phase monoclinic martensite phase and the second phase Ta2Cu. SEM results also revealed that increasing Ta content resulted in a larger volume fraction of Ta2Cu phase. Differential scanning calorimetry results showed that the MT temperatures remarkably increased with Ta addition. The ductility of the (ZrCu)50-xTax alloys was reduced to some degree with increasing Ta content. The shape memory effect was improved significantly, in which (ZrCu)98Ta2 alloy showed the 6.19% maximum recovery strain during 8% pre-strain under compression at 25 ​°C.

Research on the Transmission Performance of a High-Temperature Magnetorheological Fluid and Shape Memory Alloy Composite

To address the fact that the performance of magnetorheological fluid decreases with increasing temperature, a high-temperature magnetorheological fluid and shape memory alloy spring friction composite transmission method is proposed, and its transmission performance is shown to essentially maintain stability under high temperatures. We introduce a composite transmission method, performed a magnetic field finite element analysis, and present the equation of torque transmission of the composite. The results show that the amount of torque transferred by the magnetorheological fluid reached its maximum value at magnetic saturation, but decreased with increasing temperature, down to 33.41%, whereas the frictional torque generated by the shape memory alloy spring increased with increasing temperature. When the temperature reached 100 °C, the frictional torque effectively compensated for the decrease in the magnetorheological fluid’s performance.

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 considered the cause of (001)B19′ compound twinning in Ti–(Zr, Hf)–Ni alloys, and then selected 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 this alloy-design concept through presenting a candidate composition range of Ti–(15–20)Zr–49.7Pd and surrounding Ni-containing alloys for new high temperature shape memory alloys which readily undergo detwinning.

Ferroelastic oligocrystalline microwire with unprecedented high-temperature superelastic and shape memory effects

A compelling demand exists for high-performance high-temperature shape memory alloys (HTSMAs) that can be applied as intelligent components in the rapidly developing aerospace, robotics, manufacturing, and energy exploration industries. However, existing HTSMAs are handicapped by their high cost and unsatisfactory functional properties, which impede their practical application. Here, by using the strategy of creating an oligocrystalline structure, we have developed a high-performance, cost-effective high-temperature shape memory microwire exhibiting an exceptional combination of superb superelasticity with a large recoverable strain of up to 15%, an outstanding one-way shape memory effect with a maximum recoverable strain as high as 13% and a remarkable two-way shape memory effect with a large recoverable strain of 6.3%. These unparalleled comprehensive properties provide this microwire with a high potential for use in high-temperature actuation, sensing, and energy conversion applications, especially in miniature intelligent devices, such as high-temperature microelectromechanical systems. The present strategy may be universally applicable to other brittle phase-transforming alloys for achieving outstanding functional properties at high temperatures.

Crystal-plasticity modeling of phase transformation–viscoplasticity coupling in high-temperature shape memory alloys

A coupling between phase transformation and viscoplasticity is observed in HTSMAs during actuation. The dislocations generated and retained phases accumulated are responsible for the coupling. It results in functional fatigue, which is reflected as an alteration in the functional properties of the alloy, and an increase in the irrecoverable deformations. The objective of the present study was to develop a theoretical framework to account for the interactions between viscoplasticity and phase transformation to simulate the alteration in functional properties and generation of irrecoverable deformations. A crystal-plasticity based multi-scale approach was followed to develop the framework. This approach accounts for the mechanisms of: phase transformation, transformation induced plasticity, accumulation of retained martensite, plasticity (in a rate-independent manner), and viscoplasticity (in a rate-dependent manner). The coupling was accounted by a direct effect of the dislocation densities produced by the irrecoverable mechanisms onto the transformation resistance and retained martensite. This resulted in an indirect effect on the functional properties of phase transformation. On simulating the response of single crystals and polycrystals, anisotropic responses are captured at the grain scale (from single crystals), and a nearly isotropic response is captured at the macro scale (from polycrystals). The results show consistency between the functional property trends (such as TT and hysteresis) and those observed experimentally at the macroscale. The contributions of this study are presenting an effect of: (i) texture, (ii) thermal cycling rate and (iii) functional fatigue, through the response of several randomly oriented single crystals and a polycrystal of Ni–Ti–Hf HTSMA.

Compositional and microstructural sensitivity of the actuation fatigue response in NiTiHf high temperature shape memory alloys

Martensitic transformation temperatures and other transformation characteristics are extremely sensitive to composition and microstructural features in shape memory alloys (SMAs), particularly in well-known NiTi based SMAs. Because of this sensitivity, it is challenging to synthesize NiTi based SMAs with the same transformation characteristics and thermomechanical properties repeatedly; making qualification and certification of these materials difficult. The present study analyses variations in actuation fatigue properties of four batches of Ni50.3Ti29.7Hf20 (at.%) high temperature SMA (HTSMA), with similar fabrication methods, through microstructural observations, thermal and compositional studies, thermomechanical testing and tensile actuation fatigue experiments. Small differences in the Ni content and volume fraction of non-metallic inclusions of different batches are found to alter the transformation temperatures and actuation fatigue properties significantly. In general, it is observed that higher Ni content and volume fraction of non-metallic inclusions shorten the fatigue life. The results presented in this study draw attention to the difficulties associated with the fabrication of a target NiTiHf HTSMA composition with desired mechanical and shape memory properties, while also presenting a methodology to screen different batches of the alloy for superior actuation fatigue response using conventional microstructural, thermal and mechanical characterization.

The effect of cooling on machining and phase transformation responses of Ni-rich NiTiHf high-temperature shape memory alloy

A Ni-rich Ni50.3Ti29.7Hf20 (at. %) high-temperature shape memory alloy was machined under different cooling conditions, specifically cryogenic cooling and flood cooling, and results were compared with those from dry machining. The effects of cooling on machinability measures such as tool wear, cutting forces and surface quality were evaluated for various cutting speeds. Microhardness, phase transformation temperatures and latent heat of transformation corresponding to the machined specimens were determined using hardness and differential scanning calorimetry tests. Cryogenic cooling was found to notably improve the machinability of the Ni-rich NiTiHf alloy by reducing tool wear, cutting force and surface roughness. This study provides clear indication that the machining process alters the phase transformation temperature of the Ni-rich NiTiHf alloy. After the machining process, austenite finish temperature of specimens increased by 5–10% depending on the machining conditions. The smallest amount of latent heat required for transformation (ΔH) was obtained for each of the two specimens: 2.33 J/g (exothermic) for the cryogenically machined specimen and 3.38 J/g (exothermic) for the specimen machined under flood cooling.

Influence of Er-doping on the microstructure and properties of β-Ti high temperature shape memory alloy

The influence of Er-doping on the microstructure evolution and corresponding properties of Ti82-xNb16Al2Erx high temperature shape memory alloys were systematically investigated in the present study. A small number of Er-rich phases were observed except for the α″ martensite phase when Er content was above 0.3 at.%. The Er-rich phase accrued and agglomerated with the increasing of Er doping. The Er-rich phase would affect the nucleation and growth of martensite plates. The martensite morphology was featured with the fine martensite plates at the nearby of Er-rich phase. The martensitic transformation temperatures of Ti82-xNb16Al2Erx alloys decreased firstly and then increased as the Er contents increased, which was mainly ascribed to the variation of the matrix composition, grain refinement and the formation of the non-transformation Er-rich phase. It was noteworthy that the yield strength and fracture strength raised firstly and then dropped sharply. The yield strength and fracture strength up to 780Mpa and 902 MPa could be obtained in the Ti81Nb16Al2Er1 alloy. The shape memory properties could be optimized by doping the proper Er content. The maximum recoverable strain of 3.1% could be gained with 4% pre-strain.

Effects of cooling rate on martensitic transformation behavior and shape memory property for Ni-rich NiTiHf-Nb high-temperature shape memory alloy

The NiTiHf-Nb alloy exhibits good workability as well as high-temperature shape memory property. In this work, effects of cooling rate on martensitic transformation behavior and shape memory property for Ni-rich (Ni 50.1 Ti 34.9 Hf 15 ) 85 Nb 15 alloy were investigated. For comparison, the martensitic transformation behavior and shape memory property of Ni 50.1 Ti 34.9 Hf 15 alloy were also investigated. As the cooling rate decreased from water cooling to furnace cooling after solution treatment, the martensitic transformation temperatures (TTs) of (Ni 50.1 Ti 34.9 Hf 15 ) 85 Nb 15 alloy decreased gradually, whereas the decrease was not obvious for Ni 50.1 Ti 34.9 Hf 15 alloy. The increased valence electron concentration (c v ) should account for the decrease in TTs of (Ni 50.1 Ti 34.9 Hf 15 ) 85 Nb 15 alloy. The shape memory effect strain increased with the decrease in the cooling rate for both two alloys, and the largest shape memory effect strain of 2.6% can be obtained in furnace-cooled (Ni 50.1 Ti 34.9 Hf 15 ) 85 Nb 15 alloy. This enhancement in the shape memory property should be ascribed to the precipitation of H-phase in furnace-cooled (Ni 50.1 Ti 34.9 Hf 15 ) 85 Nb 15 alloy. • Higher shape memory effect strain could be obtained by slow cooling after solution treatment. • Higher martensitic transformation temperatures could be obtained by fast cooling after solution treatment. • The martensitic start transformation temperatures were c v dependent. • Precipitation of H-phase could be improved by addition of Nb.

Investigations of Deformation Behavior and Microstructure of Al Tailored Ti–Mo High Temperature Shape Memory Alloys during Isothermal Holding at 393 K

To utilize β-Ti based high temperature (HT) shape memory alloys (SMAs), a high Al concentration of 14 mol% was designed for sufficient suppressing the undesired ω-phase. HTSMA exhibits shape memory effect (SME) above 373 K, and thus the operating temperature is over 373 K. However, the SME and the mechanical properties of most β-Ti SMAs deteriorate after holding at elevated temperatures due to the ω-embrittlement. The Ti-4.5Mo-14Al alloy (mol%) and the Ti-6Mo-7Al alloy as a comparison, both of which possess the identical reverse martensitic transformation start temperature of 407 K, were isothermally held at 393 K for up to 360 ks, and deformation behaviors and microstructures were investigated. It was found that after the isothermal holding, the deformation behavior of the Ti-6Mo-7Al alloy altered significantly; on the other hand, that of the Ti-4.5Mo-14Al alloy remained almost intact. Transmission electron microscopy observations revealed that the isothermal ω-phase (ωiso) was successfully suppressed in the Ti-4.5Mo-14Al alloy, while the ωiso phase grew in Ti-6Mo-7Al alloy. Moreover, the isothermal α″-phase coexisted in the Ti-4.5Mo-14Al alloy. It is concluded that a high Al concentration is a crucial prerequisite in the practical β-Ti HTSMAs. The presented design could be a useful guideline for developing Ti SMAs with comparable Mo- and Al-equivalents.