High Temperature Shape Memory Alloys Research Articles

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-back scatted diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the neckless 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.

Effect of Cooling Rate on Microstructure, Shape Memory Effect and Mechanical Properties of Cu–13Al–5Fe High-Temperature Shape Memory Alloy Fabricated by Laser Powder Bed Fusion

Effect of Cooling Rate on Microstructure, Shape Memory Effect and Mechanical Properties of Cu–13Al–5Fe High-Temperature Shape Memory Alloy Fabricated by Laser Powder Bed Fusion

Stress-induced detwinning/reorientation of hierarchically twinned martensite and deformation micro-mechanism in Ni52Mn27Ga17Co4 shape memory alloys: Experimental and phase-field studies

Mechanical training via cyclic loading and unloading was widely used in NiMnGa shape memory alloys (SMAs) to create single or double variant states for reducing the resistance to variant reorientation and thus achieving a maximum shape memory strain. In this work, an effort was made to reveal the deformation mechanism of a polycrystalline Ni52Mn27Ga17Co4 high-temperature SMAs consisting of non-modulated (NM) martensite with uniform orientation and surrounding γ precipitates. The NM martensite microstructure was characterized as a self-accommodated hierarchical twinned-structure prior to deformation. Based on the interrupted in-situ EBSD measurements and phase-field simulations, it was demonstrated that the compressive loading resulted in the thickening of the stress-favored variants with the <001>NM direction perpendicular to compression axis at different dimensional scales. The path consisted of two subsequent processes–first detwinning of the nano-lamellae within one micro-variant through inter-lamellae boundary motion, and then reorientation of micro-variants through inter-plate boundary motion. The Schmid factor and preferred orientation of nano-lamellae (or strain accommodation) dominated the entire detwinning/reorientation process of NM martensite. As a result, the hierarchically twinned microstructure almost evolved into a single-variant state with the disappearance of packet boundaries. The ductile γ phase with network structure possessed more excellent strain-accommodated ability to the macroscopic deformation, hence significantly enhancing the mechanical properties including the ultimate compressive strength and elongation of NiMnGa SMAs.

Modeling and analysis of process parameters in EDM of Ni35Ti35Zr15Cu10Sn5 high-temperature high entropy shape memory alloy by RSM Approach

The Electrical Discharge Machining (EDM) technique demonstrates proficiency in fabricating precise and intricate geometries, especially in challenging-to-machine materials like high-entropy shape memory alloys. Analyzing and optimizing machining parameters are crucial for their direct impact on mechanical properties and overall product efficiency. The main responses chosen to evaluate the processes are material removal rate (MRR), electrode wear rate (EWR), and surface roughness (Ra). At the same time, the associated machining conditions were discharge current (Ip), pulse-on time (Ton), and pulse-off time (Toff). EDM is a multi-response process; therefore, the method of Response Surface Methodology (RSM) is utilized to assess the influence of machining parameters on Ni35Ti35Zr15Cu10Sn5 (at%) high-temperature high entropy shape memory alloy (HT-HE-SMA) using a copper electrode. Based on a center composite design (CCD), experiments were analyzed using Minitab19 software. To identify the most influential parameters, a thorough analysis of variance (ANOVA) at various significance levels (5%) was performed, checking the sufficiency of all fitted second-order regression models. Discharge current, pulse-on time, and pulse-off time were identified as significant factors that affect output (MRR, EWR, and Ra). The model adequacy of the current experimental investigation is perfect, with determination coefficients (R2) of 97.82% for MRR, 95.36% for EWR, and 99.53% for Ra.

Property and microstructure of Ni50.3Ti29.7Hf20 high-temperature shape memory alloys with different aging conditions

NiTiHf is a class of promising high-temperature shape memory alloys (SMAs) that find many applications. However, their complex martensitic microstructure and attendant thermomechanical properties are not well understood. In this work, we used solution-treated (precipitate-free) and aged (precipitate-bearing) Ni50.3Ti29.7Hf20 (at.%) SMAs as a model system. We observed that the presence of precipitates refines the martensite plates, reduces the number of martensite variants, and changes the orientation relationship between the martensite plates compared with the solution-treated counterpart. Furthermore, the aged samples exhibited higher transformation temperatures, narrower phase transformation temperature windows, improved thermal stability, and retained or even improved actuation strain. The improved thermomechanical properties observed in the aged samples are attributed in part to the reduction of the number of martensite variants and the change in martensite and twin interface characteristics, both of which are induced by the presence of precipitates. The findings of this study offer new information on the processing-property-microstructure relationship in NiTiHf-based SMAs. These insights can guide future materials design efforts, facilitating the development of advanced SMAs tailored for specific high-temperature applications.

Effect of Post-treatments on the Thermomechanical Behavior of NiTiHf High-Temperature Shape Memory Alloy Fabricated with Laser Powder Bed Fusion

Effect of Post-treatments on the Thermomechanical Behavior of NiTiHf High-Temperature Shape Memory Alloy Fabricated with Laser Powder Bed Fusion

Compositional effects on strain-controlled actuation fatigue of NiTiHf high temperature shape memory alloys

Nickel-rich NiTiHf high-temperature shape memory alloys continue to garner significant interest for actuator applications. One issue impeding their widespread use is the question of actuation fatigue lifetime under high loads. This paper describes how these fatigue challenges can be overcome by partial thermomechanical cycling. By limiting actuation strain to less than the maximum value that is attainable with full transformation, actuation life cycles can be significantly extended. Strain-controlled cycling is achieved by limiting the upper cycle temperature during heating and stopping each cycle short of the austenite finish temperature. Four compositions of NiTiHf were tested to three levels of transformation. Strain-controlled cycling increased the actuation fatigue lifetimes of all compositions. However, specimens resistant to dislocation plasticity attained longer lives during full transformation cycling, but during partial cycling, they failed earlier than samples with higher resistance to plasticity. This highlights the importance of the competition between functional and structural fatigue.

Hot deformation behavior of Ni50.4Ti34.6Hf15 high-temperature shape memory alloy

Due to their extreme requirements, conventional Ni–Ti shape memory alloys (SMAs) are no longer suitable for aerospace applications. Adding Hf to Ni–Ti SMAs can significantly enhance their suitability for aerospace applications, particularly in high-temperature environments. In this regard, the hot/thermal deformation behavior of Ni50.4Ti34.6Hf15 high-temperature shape memory alloys (HTSMAs) was systematically investigated by using a Gleeble-3800 thermocompression simulation machine. The results prove that the Ni50.4Ti34.6Hf15 alloys have more strain sensitivity than the temperature during the hot/thermal deformation process. In detail, the flow stress increases with the increase in strain rate and decreases with the increase in deformation temperature. From the microstructure evolution point of view, the recovery effect will increase with the increase in deformation temperature, and the recrystallization effect will increase with the increase in strain rate. Following the obtained stress-strain curves, the flow behavior of Ni50.4Ti34.6Hf15 alloys during hot/thermal deformation was established. A modified constitutive equations model was proposed using careful consideration of deformation temperature and strain rate. It was found that the calculated strain activation energy was 363.945 kJ/mol in the range of 700∼800 °C and 512.489 kJ/mol in the range of 900∼1000 °C. Based on the constitutive equation, the calculated values were consistent with the experimental values, which indicated reliable analysis with high prediction accuracy. Furthermore, the thermal processing maps under different conditions were constructed, and it was determined that the optimum hot/thermal deformation processing parameters of Ni50.4Ti34.6Hf15 alloys were 940∼1000 °C and 0.01–0.04 s−1.

A Safety Device Based on a High Temperature Shape Memory Alloy

A Safety Device Based on a High Temperature Shape Memory Alloy

Thermal stability of Ag-doped Ni-Mn-Ga high-temperature shape memory alloy

In the present study, the microstructure features and martensitic transformation behaviour of a Ni-Mn-Ga alloy were tailored by adjusting the Ag content. In addition, the effect of Ag content on the thermal stability was also explored. The results revealed that the non-modulated martensite phase and Ag particles coexisted in the Ni-Mn-Ga-Ag alloys. Different martensite variants were in the {112} type I twinning relationship. With increasing Ag content, not only all transformation temperatures but also the thermal hysteresis decreased continuously in a quasilinear manner. This was attributed to the introduction of small and soft Ag particles, which were applied to release the elastic energy during the martensitic transformation. The decreasing trend of elastic energy was highly nonlinear. The alloy with 4 at% Ag addition was the percolation threshold. In addition, the relationship of energy dissipation against Ag is not monotonic. While the energy dissipation of Ni48Mn25Ga19Ag8 is almost zero, the others were much higher. Furthermore, unlike others, Ni48Mn25Ga19Ag8 featured good martensitic transformation stability upon thermal cycling.

Effect of laser power on the microstructure, shape memory effect and mechanical properties of Cu–13Al–5Fe high temperature shape memory alloy by laser powder bed fusion

Effect of laser power on the microstructure, shape memory effect and mechanical properties of Cu–13Al–5Fe high temperature shape memory alloy by laser powder bed fusion

A Safety Device Based on a High Temperature Shape Memory Alloy

A Safety Device Based on a High Temperature Shape Memory Alloy

Effect of 5.31Ti (at.%) content used as ternary in quaternary Cu-rich high-temperature shape memory alloy cast via arc melting

Effect of 5.31Ti (at.%) content used as ternary in quaternary Cu-rich high-temperature shape memory alloy cast via arc melting

Development of Ni–Ti–Zr–Hf–(Nb, Ta) Multi-Principal Element High-Temperature Shape Memory Alloys with High Cold Workability

Development of Ni–Ti–Zr–Hf–(Nb, Ta) Multi-Principal Element High-Temperature Shape Memory Alloys with High Cold Workability

Order of magnitude increase in actuation fatigue lifetime through partial austenitic transformation of NiTiHf high-temperature shape memory alloys

The higher transformation temperatures, strength, and thermomechanical stability of NiTiHf high-temperature shape memory alloys (HTSMAs) are attractive for use in solid-state actuation. Maximizing the actuation fatigue lifetime whilst preventing deterioration of actuation stroke during service are the two main challenges facing the widespread application of HTSMAs. Much research has focused on optimizing composition and processing to enhance performance; however, this has proven challenging to implement on a large scale. Simpler solutions involve optimizing the actuation environment to enhance performance. One of the most effective methods to both extend the lifetime and stabilize actuation strain is through partial thermal cycling. This method is more representative of how actuators are employed, as very rarely are they repeatedly cycled to their maximum output. Previous studies have shown partial transformation extends fatigue lifetime in low-temperature NiTi and NiTiCu SMAs, but it has yet to be conclusively demonstrated in the NiTiHf system. The present work is an extensive analysis of the effects of heating limited partial cycling compared to full cycling actuation fatigue of Ni50.3Ti29.7Hf20 HTSMA. The results confirm an order of magnitude increase in actuation fatigue lifetime with less transformation per cycle in partially heated Ni-rich NiTiHf samples. More importantly, keeping the actuation work output almost the same, controlling the actuation strain level is shown to be more effective in increasing the fatigue life than reducing the actuation stress level, yielding 5 to 10 times relative improvement in the fatigue life. The underlying microstructural evolution resulting in the observed enhanced response is provided.

Predicting the martensitic transition temperatures in ternary shape memory alloys Ni0.5Ti[formula omitted]Hf[formula omitted] from first principles

Martensitic transition temperatures (MTTs) can be tuned by alloying binaries with other elements to create multi-component shape memory alloys (SMAs). However, it is inefficient to use the trial-and-error approach to find compositions with desirable operating temperatures because of the large number of combinations of metals possible to form ternaries, quaternaries, etc. Thus it is crucial to develop the theoretical capability of accurately predicting MTTs as a function of composition, in order to provide experimentalists with necessary and reliable guidance. Previous work has focused on developing and applying first-principles methods to compute phase transitions and MTTs in binary SMAs such as NiTi (nitinol), but certain technical problems associated with the multi-component SMAs remain unsolved. In this work, we employed ab initio molecular dynamics (MD) and thermodynamics integration to study the NiTiHf-based high-temperature ternary SMAs. We overcome the technical challenges to accurately obtain the Gibbs free energy in cubic ternaries where the reference structures are unknown. Specifically, we examined the cubic, monoclinic and orthorhombic structures of Ni0.5Ti0.5−xHfx for x∈[0,0.5], and our results suggest that the cubic-to-monoclinic martensitic transition occurs when x<0.08, for x>0.17 the martensitic transition is between the cubic and orthorhombic phases, whereas in between our calculations cannot distinguish these two martensite structures near the MTT. The computed MTTs vs Hf content x are in good agreement with measured data. Thus our current work paves the way for computational design of multi-component SMAs with desired properties.