High Temperature Shape Memory Alloys Research Articles

Optimization of wire-EDM process parameters for Ni–Ti-Hf shape memory alloy through particle swarm optimization and CNN-based SEM-image classification

Shape memory alloys (SMAs) have the unique ability to regain their shape under specified conditions, making them extremely useful for a wide range of applications. However, non-traditional machining techniques could be more effective for high-temperature SMAs as they provide better shape recovery properties than conventional methods, necessitating this study's unconventional wire electric discharge machining procedure. The surface morphology of Ni–Ti-Hf-based alloys was examined using scanning electron microscopy. A convolutional neural network model was used to categorize SEM images based on the material removal rate, utilizing the pixel intensity histogram of the processed images. The study discovered that the material remelted as the discharge energy increased, leaving lumps and globules on the surface. The size of debris lumps, pores, and globules increased with increasing Ra values. Moreover, widening the inter-electrode gap by expanding the servo-voltage facilitated efficiently removing debris from the machined site. The TOPSIS method for particle swarm optimization was employed to find the optimal solutions, yielding TON = (124.239, 116.228), TOFF = (54.532, 40.781), Servo voltage = (36.216, 43.766), and Wire-Feed = (2.157, 8). These results were validated through experimentation, with minimal error percentages of 2.01%, 2.046% (MRR), and 3.86%, 1.611% (Ra) for both sets of input parameters, respectively.

Effect of aging on isothermal ω/α″ growth and resulting mechanical behaviors of the Ti–Mo–Al high temperature shape memory alloys

Effect of aging on isothermal ω/α″ growth and resulting mechanical behaviors of the Ti–Mo–Al high temperature shape memory alloys

Review on the β-Ti Based High Temperature Shape Memory Alloys

Review on the β-Ti Based High Temperature Shape Memory Alloys

Effect of Homogenization on the Transformation Temperatures and Mechanical Properties of Cu15Ni35Hf12.5Ti25Zr12.5 and Cu15Ni35Hf15Ti20Zr15 High-Entropy Shape Memory Alloys.

The major challenge of high-temperature shape memory alloys (SMAs) is the collocation of phase transition temperatures (TTs: Ms, Mf, As, Af) with the mechanical properties required for application. Previous research has shown that the addition of Hf and Zr into NiTi shape memory alloys (SMAs) increases TTs. Modulating the ratio of Hf and Zr can control the phase transformation temperature, and applying thermal treatments can also achieve the same goal. However, the influence of thermal treatments and precipitates on mechanical properties has not been widely discussed in previous studies. In this study, we prepared two different kinds of shape memory alloys and analyzed their phase transformation temperatures after homogenization. Homogenization successfully eliminated dendrites and inter-dendrites in the as-cast states, resulting in a reduction in the phase transformation temperatures. XRD patterns indicated the presence of B2 peaks in the as-homogenized states, demonstrating a decrease in phase transformation temperatures. Mechanical properties, such as elongation and hardness, were improved due to the uniform microstructures achieved after homogenization. Moreover, we discovered that different additions of Hf and Zr resulted in distinct properties. Alloys with lower Hf and Zr had lower phase transformation temperatures, followed by higher fracture stress and elongation.

Novel Quaternary CuAlZnMg High Temperature Shape Memory Alloy (HTSMA) Fabricated by Minor Batch of Zn and Mg Additions

Shape memory alloys (SMAs) constitute the second largest commercial smart material class after piezoelectric materials. Different SMA alloy systems or SMAs with miscellaneous functionalities and characteristic properties have been designed for using in different applications until today. High temperature shape memory alloys (HTSMAs) are also widely desired to be used in various smart materials applications. HTSMAs with different functional and characteristic properties are muchly demanded for different tasks to be done by these alloys or devices designed by these alloys. A common and practical way to fabricate SMAs or HTSMAs with different shape memory effect (SME) and other properties is to fabricate them with different alloying compositions and add different additive elements. In this work, a quaternary CuAlZnMg HTSMA with an unprecedented composition consisting minor amount of zinc and magnesium additives was produced by arc melting method. As a result of applying post-homogenization in high β–phase temperature region and immediate quenching, the microstructural mechanism of a SME property was formed in the produced alloy. After then, to examine SME characteristics of the CuAlZnMg alloy some differential thermal analysis (DTA), microstructural (XRD) and magnetization (VSM) characterization tests were carried out. The DTA results showed that the alloy is a HTSMA exhibiting reverse martensitic transformations at temperature range between 167 °C and 489 °C. The XRD pattern obtained at room temperature revealed the martensite phases formed in the alloy, which phases are the base mechanism of the reversible martensitic transformation (the SME property) of the alloy. The VSM test showed that the alloy exhibit a diamagnetic property with a weak ferromagnetic coercivity contribution.

Molecular dynamics simulation of phase transformation and wear behavior of Ni35Al30Co35 high temperature shape memory alloy

Ni–Al–Co alloys as one of the ferromagnetic high-temperature shape memory alloys (HTSMA) are attractive in aerospace and defense applications, due to their high phase transition temperature range (400–700K), high recoverable strain and good machinability. However, the underlying physics for the phase transformation behavior of β single-phase and β+γ two-phase Ni–Al–Co ternary alloys remain unclear. In this work, molecular dynamics simulation of phase transition, pseudoelasticity (PE) and wear behavior of Ni35Al30Co35 alloy were analyzed using the EAM potential. It shows that Ni35Al30Co35 alloy has a high martensite start temperature of 825K and an austenite start temperature of 631K, respectively. After five thermal cycles, martensite and austenite start and finish temperatures are stable around 500K and 700K. The initial structure of Ni35Al30Co35 alloy at 1000K is identified as BCC (β phase) and FCC (γ phase) structure, transforming to L10 structure through martensitic transformation. Furthermore, PE of Ni35Al30Co35 alloy exhibits about a 9.1% recoverable strain under <001> oriented uniaxial tension tests and about a 6.6% recoverable strain under <001> oriented uniaxial compression tests at a temperature of 1200K, showing the potential used in extreme environment. Moreover, spherical indenters with a radius of 15 Å were used to investigate the wear behavior of Ni35Al30Co35 alloy under different temperature. It was found that the PE effect affects the frictional behavior during nano-scratching. Combining the effect of high temperature softening and phase transition PE, the wear rate increases nearly by 5.3% when the temperature rises from 300K to 1000K. Furthermore, the friction force curve obtained is consistent with the friction force curve obtained by simulation at 300K.

A micromechanical constitutive model of high-temperature shape memory alloys

In this work, a new micromechanical constitutive model is constructed for high-temperature shape memory alloys. Different from the classical constitutive models, the microstructural patterns, heterogeneities of stress and strain fields at the microscopic scale are addressed. In the proposed model, three different inelastic deformation mechanisms, i.e., reversible martensitic transformation, irreversible transformation-induced plasticity, viscoplasticity, as well as their complex interactions, are considered. In addition to the well-known austenite and martensite phases, a new austenite-martensite interface phase is introduced to address the high local stress induced by the martensitic transformation. The modified Mori-Tanaka's homogenization scheme and Hill's interfacial operator are employed to determine the non-uniform stress fields of the three phases. To satisfy the thermodynamic compatibility of the proposed model, the driving forces and evolution equations of the three inelastic deformation mechanisms are derived based on the constructed Helmholtz free energy and the fundamental laws of irreversible thermodynamics. Inheritances of the plastic deformation induced by moving austenite-martensite (A-M) interface during repeated martensitic transformation and its reverse are incorporated. Temperature evolution caused by internal heat production and heat exchange with the ambient medium is also addressed. To validate the capability of the new constructed model, the predicted results of creep behavior and stress-assisted two-way shape memory effect of NiTiPd high-temperature shape memory alloys are compared with the experimental ones. The proposed model provides a theoretical tool for simultaneously analyzing the deformation behaviors at the macroscopic scale and underlying mechanisms at the microscopic scale.

The role of H-phase in thermal hysteresis and shape memory properties in Ni50Ti30Hf20alloy

Ni50Ti30Hf20 high temperature shape memory alloy was aged at 550 °C for 10 h and 450 °C for 50 h which corresponds to the peak hardening state with equal gain in strength caused by different size/volume fraction ratio of nano-precipitation of H-phase. In order to determine the effect of such structures on the martensitic transformation, thermal cycling and shape-memory properties were measured. It is shown that finer precipitates with higher volume fraction provide superior dimensional stability, good thermal cycling stability and small transformation hysteresis.

Influence of Deformation Constraint Conditions on the Structural-Phase State and Thermomechanical Characteristics of the Properties of an Ni49.5Ti48Hf2.5 High-Temperature Shape Memory Alloy

Influence of Deformation Constraint Conditions on the Structural-Phase State and Thermomechanical Characteristics of the Properties of an Ni49.5Ti48Hf2.5 High-Temperature Shape Memory Alloy

Effects of Hf and Zr Additions on Microstructure, Phase Components and Martensitic Transformation Temperatures of Ni3Ta High-Temperature Shape Memory Alloys

Effects of Hf and Zr Additions on Microstructure, Phase Components and Martensitic Transformation Temperatures of Ni3Ta High-Temperature Shape Memory Alloys

Order degree and self-accommodation of γ phases in Ni-Mn-Ga alloys

High temperature shape memory alloys Ni-Mn-Ga have high martensitic transformation temperature, good thermal stability and moderate shape memory performance. The second phase γ significantly affects the alloys’ macroscopic performances, but its atomic structure and microstructural feature still remain almost unknown. Microstructure and degree of atomic ordering of γ precipitate are uncovered in Ni54Mn30+xGa16–x (x = 0, 2, 5, 9, 11) and Ni57+yMn25Ga18–y (y = 0, 1, 2, 3, 4) alloys. For as-cast Mn-rich alloys, a hierarchical “nano-lamellae within micro-lamellae” microstructure is found. Nano-lamellae are paired to form a variant, two variants constitute a micro-lamella, a pair of micro-lamellae makes up single group, and there are six groups in a grain. Single nano-lamella is a partially ordered phase with tetragonal crystal structure. All nano-lamellae are of nearly identical crystallographic orientation. This arrangement of the 24 variants is self-accommodated. In annealed state, the lamellae in large γ precipitates still exist due to the constraint from martensites, but in small grains partly or completely disappear and further evolve into nanotwins. For as-cast Ni-rich alloys, the paired nano-lamellae are disordered and another partially ordered phases, respectively. After annealing, they become single crystal with ordered structure. Models describing atomic positions of γ phases with different orderings are built. For Mn-rich alloys, the martensitic transformation temperature reaches about 600 °C, shape memory strain is up to 5%, and compressive stress and strain are higher than 2100 MPa, and 26%, respectively. Relative to Ni-rich samples, Mn-rich alloys have very high martensitic transformation temperature, moderately enhanced ductility and reasonably good shape memory strain. They are closely related to martensite’s electron concentration and γ’s atomic ordering, crystal structure, and lamellar microstructure.

Influence of chemical composition on the amount of second phases precipitates and transformation temperatures of TiNiPdCu shape memory alloys prepared through novel powder metallurgy route.

The chemical composition of TiNiPdCu high temperature shape memory alloys (HTSMAs) affects their microstructure and martensitic transformations. In TiNiPdC HTSMAs, second phase precipitates were found. Extremely high densities of nanoscale precipitates of TiPdCu and Ti2Pd of particle size 0.8-4.4 μm were found to be produced by the TiNiPdCu alloys. It was found that the TiPdCu-type precipitates were favoured by the preferential migration of Cu atoms towards the heterogeneous nucleation sites, which in turn favoured the fine Ti2Pd-type precipitates. Due primarily to their high-temperature stability, these precipitates significantly strengthened the resistance against transformation-induced plasticity and creep deformation, especially at high loads and high temperatures. High concentrations of these nanoscaled precipitates led to a significant rise in transformation temperatures by 12% and a small decrease in thermal hysteresis by 30%. It was anticipated that the current research findings will have a significant positive impact on the creation of HTSMAs that maintain their benefits of simplicity of manufacture. The HTSMAs developed in current research are beneficial for high temperature sensors and actuators at optimum cost.

Crystallographic variant mapping using precession electron diffraction data

In this work, we developed three methods to map crystallographic variants of samples at the nanoscale by analyzing precession electron diffraction data using a high-temperature shape memory alloy and a VO2 thin film on sapphire as the model systems. The three methods are (I) a user-selecting-reference pattern approach, (II) an algorithm-selecting-reference-pattern approach, and (III) a k-means approach. In the first two approaches, Euclidean distance, Cosine, and Structural Similarity (SSIM) algorithms were assessed for the diffraction pattern similarity quantification. We demonstrated that the Euclidean distance and SSIM methods outperform the Cosine algorithm. We further revealed that the random noise in the diffraction data can dramatically affect similarity quantification. Denoising processes could improve the crystallographic mapping quality. With the three methods mentioned above, we were able to map the crystallographic variants in different materials systems, thus enabling fast variant number quantification and clear variant distribution visualization. The advantages and disadvantages of each approach are also discussed. We expect these methods to benefit researchers who work on martensitic materials, in which the variant information is critical to understand their properties and functionalities.

Influence of Process Parameters on Surface Crack Density in Electrical Discharge Machining of Ni35Ti35Zr15Cu10Sn5 high-temperature high entropy shape memory alloy by Response Surface Methodology Approach

Influence of Process Parameters on Surface Crack Density in Electrical Discharge Machining of Ni35Ti35Zr15Cu10Sn5 high-temperature high entropy shape memory alloy by Response Surface Methodology Approach

The effect of systematic substitution of Nb with Ti on a potential high-temperature shape memory alloy: B2 NbRu

NbRu has been considered as a potential high-temperature shape memory alloy due to its high martensitic transformation temperature, however, it has poor superelasticity and other shape memory properties, which limits its applications. In this paper, the effect of systematic substitution of Nb with Ti on the structural, thermodynamic, and elastic properties of supercell 2x2x2 B2 Ru50Nb50-xTix spanning the entire composition range (x = 0, 6.25, 12.50, 18.75, 25.00, 31.25, 37.50, 43.75, 50 at. %) is investigated using ab initio calculations. Elasticity results showed that the mechanical stability of the B2 phase improved with increasing Ti content, which could be associated with a decrease in transformation temperature. However, a suitable composition range within which Cʹ~0, resembling that of well-known TiNi, is identified. This predicts Ti addition to be effective in improving shape memory properties of B2 NbRu alloy.

Self-consistent charge density functional tight-binding (SCC-DFTB) parameterization and validation for Ti50Pd50-XRuX alloys

Titanium-palladium based systems are in high demand currently, particularly for the development of high-temperature shape memory alloys with a unique application in the aerospace industry. In particular, the Ti50Pd50-XRuX is found to be one of the promising alloys. This is due to their shape memory effect and superelasticity. Previous studies revealed that these alloys are mechanically stable above 25 at. % Ru with a positive C′ at 0 K. So, an understanding of its transformation temperature behavior is required, which is currently not clear. The self-consistent-charge density functional tight-binding (SCC-DFTB) approach was used to develop parameters for Ti50Pd50-XRuX employing the parameterization technique. The structural, electronic and elastic properties were determined to check their relative stability on derived potentials using DFTB + code. As part of the validation for the developed set, the results were reproducible to within 5 % agreement as compared with other theoretical findings. Furthermore, the SCC sets of parameters were also used to determine the lattice expansions and check the transformation temperature of ternary Ti50Pd50-XRuX. It was observed that the addition of Ru reduces the transformation temperature. The findings imply that the SCC-DFTB approach was able to predict the stability and transformation behavior of Ti50Pd50-XRuX alloys.

Micro-actuation in bi-layered shape memory alloy structures

The use of shape memory alloys to generate actuation is analyzed in structures generated by attaching at high temperature shape memory alloy films to a substrate. In particular, the actuation of the free end of a cantilever is modelled for the case where the film that is deposited on a substrate is not a homogeneous one, but one that consists of two layers. The difference between the layers can be – in one case – a compositional one or – in the second case – can be one that is produced from the same target, but with one layer deposited using one set of conditions and the other one using another set of conditions. For the second case, the simplest analysis starts from using two different deposition temperatures. The current work details the results obtained by modelling the actuation of the free end of cantilever type architectures with shape memory alloy films deposited as bilayers. The results show that the slope of the actuation on heating and on cooling can be modeled, a result that enlarges the spectra of application for shape memory alloy micro-actuators.

Creep behavior of 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy under constant load

In the last three decades, aerospace industry has required shape memory alloys with higher phase transformation temperatures. In response to this technical challenge, High Temperature Shape Memory Alloys that can be actuated at higher temperatures were developed. However, high temperature actuation applications also lead to observe much higher dislocation motion and thus plastic deformation due to creep and phase transformation. In this study, 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy was used for the investigation of creep deformation under tensile loads corresponding to stress magnitudes between 200 MPa and 500 MPa. Deformation behavior of the alloy in austenitic phase was investigated by means of creep deformation experiments at 450 °C, 550 °C, and 600 °C. 50at%Ni 25at%Ti 25at%Hf alloy has similar creep behavior as NiTi despite higher loading conditions. Almost no noticeable creep strain values were observed under 200 MPa and 300 MPa at 550 °C and 600 °C. Creep parameters such as the stress exponent and the activation energy values were calculated using power law creep model and viscous glide creep was found to be the main mechanism in 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy.

Characterization of Martensitic Transformation, Microstructure and a Kinetic Study of Ti-based High Temperature Shape Memory Alloy

High temperature shape memory alloys (HTSMAs) are widely used in many fiels such as industry, biomedical, aerospace, etc. In order to expand the usage areas of these alloys, it is necessary to improve the materials, especially the martensitic transformation temperatures should be controlled. Third elements are often added to the material to control the martensitic transformation temperature. Ti-12V-8Al (wt. %) alloy, which is prepared for use in aircraft engines in the aviation industry, is a good choice due to its low density. In this study, Ti-12V-8Al (wt. %) alloy was prepared with the help of arc-melting technique. The martensite-austenite transformation temperatures, phase formations, microstructure of Ti-12V-8Al (wt. %) alloy were examined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscope (SEM) and optical microscope (OM) respectively. In the DSC test, it was determined that the martensitic transformation temperature reduced according as the heating rate of the alloy. In XRD and SEM measurements, it was observed that the alloy has α″ martensitic phases as well as some β austenite phases. Thermal activation energies of the alloy were founded by Kissinger and Ozawa techniques. It was concluded that the activation energy amounts computed by these two techniques are parallel to each other.

Structural changes of TiPt high-temperature shape memory alloys induced by high pressure torsion

The microstructure of a TiPt high temperature shape memory alloy induced by high pressure torsion (HPT) was studied using synchrotron radiation experiments and transmission electron microscopy. Upon the severe plastic deformation, the parent B19 martensite is strongly refined and highly strained. While neither strong chemical disorder nor amorphization are encountered, the occurrence of crystal reflections is observed that can be indexed in terms of the L10 phase and long range ordered stacking structures. The occurrence of the L10 phase might be triggered by the high hydrostatic stress as well as shear stress superimposed during the HPT. The austenite and martensite start and finish temperatures of TiPt are not affected by the HPT since upon heating, recrystallization occurs prior to the onset of the reverse martensite to austenite transformation.