Near-equiatomic NiTi Shape Memory Alloys Research Articles

Reveling the orientation preference along with localized Lüders-type deformation in polycrystalline NiTi SMA by in-situ synchrotron-based high energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) in near-equiatomic NiTi shape memory alloys (SMAs) predominantly occur through localized, inhomogeneous, and intense Lüders-type mechanisms, which significantly influence the recoverable strain and mechanical response of the material. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this study, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial tensile loading, the austenite with the favored orientation of ⟨110⟩A//loading direction (LD) is consumed faster compared to other orientations, resulting in residual austenite with an orientation of ⟨431⟩//LD within the Lüders banding area. In contrast, the high-strain residual austenite with few favored orientations transforms fairly slowly and remains well beyond the transformational plateau. Our work provides valuable new insights into the microstructural nature of the Lüders-type deformation mechanism of polycrystalline NiTi, and the enhanced understanding of these complex interactions holds promise for optimizing the performance and design of SMAs in practical applications.

Effect of Severe Torsion Deformation on Structure and Properties of Titanium–Nickel Shape Memory Alloy

In the present work, the possibility of applying severe torsion deformation (STD) to a bulk near-equiatomic NiTi shape memory alloy in order to accumulate super-high strain and improve mechanical and functional properties was studied. STD was performed using the multidirectional test system “BÄHR MDS-830” at a temperature of 500 °C (the upper border temperature for the development of dynamic polygonization) in 14 and 30 turns with accumulated true strain values of 4.3 and 9.1, respectively. Structural phase state and properties were studied using differential scanning calorimetry, X-ray diffractometry, transmission electron microscopy, hardness measurements, and thermomechanical bending tests. STD at 500 °C allowed for the accumulation of high strain without failure. As a result of STD in 30 turns, a submicrocrystalline structure with an average grain/subgrain size of about 500 nm was formed. This structure ensured the achievement of high maximum completely recoverable strain values of 6.1–6.8%. The results obtained show the prospects of applying severe torsion straining deformation to titanium nickelide in terms of forming an ultrafine-grained structure and high properties.

Phase transition, twinning, and spall damage of NiTi shape memory alloys under shock loading

Shape memory alloys are emerging functional materials in various fields due to their unique mechanical properties. In this work, plate-impact experiments have been performed to study the dynamic behavior of near-equiatomic NiTi shape memory alloys with different initial phases, i.e., the single austenite (B2) and the mixed phase of austenite and martensite (B19′). A Doppler pin system and soft-recovery cylinder are employed to in-situ probe the macroscopic response and catch the shocked sample for postmortem characterizations, respectively. The observed bilinear behavior of the Hugoniot data, combined with the X-ray diffraction analysis of recovered samples, suggests that the B2 austenite undergoes a martensitic transformation at ∼5 GPa. Such a phase transition affects not only the spall damage but also the deformation mechanism of NiTi alloys. Specifically, it is identified that, in association with the B2 to B19’ transformation, the spall strength is reduced by ∼7% and the austenite twinning disappears in the recovered NiTi sample. These results thus indicate that the deformation and fracture behaviors of NiTi shape memory alloys under shock compression are strongly coupled with the martensitic transformation.

On the Lüders band formation and propagation in NiTi shape memory alloys

• Both the B19′ martensite and B2 austenite can form Lüders band. • Lüders band initiation strain rate is two orders of magnitude of the applied strain rate. • Lüders band forms with a finite minimum width. • Lüders band propagates with a transition zone half of the band minimum width. • Transition zone local strain rate is more than one order of magnitude of the applied strain rate. Near-equiatomic NiTi shape memory alloys are known to exhibit Lüders-type deformation associated with a stress-induced transformation. Many studies have been conducted in the past mainly focusing on the macroscopic characteristics of the phenomenon and some theories have been proposed in the literature to explain its mechanisms, but some aspects of this phenomenon are still unclear, particularly at the microscopic scale. This study investigated the local strain evolution during the initiation and propagation of Lüders band in a pseudoelastic NiTi alloy during tensile deformation by means of digital image correlation (DIC) analysis. Based on the evidence collected, distinct stages of Lüders band formation and propagation are defined and the corresponding local strain rates are obtained. These local strain rates are much higher than the global strain rate of the testing, giving insight to the mechanism of this phenomenon.

Evolution of Transformation Strain During Partial Transformation Cycling of an NiTi Shape Memory Alloy

This paper deals with how the magnitude of transformation strain changes on partial transformation cycling of an NiTi shape memory alloy. A near-equiatomic NiTi shape memory alloy was allowed to undergo partial thermal cycling keeping the stress constant at 100 MPa for various upper cycle temperatures (between austenite start and austenite finish), using a custom-built thermomechanical cycling test setup. The displacement and the temperature of the sample during cycling were measured using a LASER extensometer and an optical pyrometer, respectively. The test results show that the recovery strain and thermal hysteresis width decrease with increasing number of cycles during partial cycling. In addition, martensite start and martensite finish temperatures increase during the initial cycles, whereas austenite start and austenite finish temperatures decrease during the initial cycles, followed by their saturation.

Effect of Mode of Heating on Cyclic Temperature Range during Partial Thermal Cycling under Constant Stress of a Near-Equiatomic Ni-Ti Shape Memory Alloy

Effect of Mode of Heating on Cyclic Temperature Range during Partial Thermal Cycling under Constant Stress of a Near-Equiatomic Ni-Ti Shape Memory Alloy

High-temperature martensitic transformation of CuNiHfTiZr high- entropy alloys

One of the major challenges of near-equiatomic NiTi shape memory alloys is their limitation for high-temperature applications. To overcome this barrier, researchers have tried to enhance the transformation temperatures by addition of alloying elements or even by introducing the concept of high-entropy alloys (HEAs). In this study, the CuNiHfTiZr HEAs were developed for high-temperature shape memory effect. Based on their solubility and electron configurations, the alloying elements are divided into two groups, (CuNi)50 and (HfTiZr)50. The content of Cu in (CuNi)50 is modulated to investigate the influences of Cu on martensitic transformation of the HEAs by studying structural evolution and transformation behavior. The results of x-ray diffraction and thermal expansion tests revealed that Cu15Ni35Hf16.67Ti16.67Zr16.67 possesses high transformation temperature, narrow hysteresis temperature loops, and good dimensional stability within this HEA system.

Constitutive Modeling of Near-Equiatomic NiTi Shape Memory Alloys Considering Composition and Heat Treatment

A predictive microscale-informed model that takes into account the precipitate–shape memory performance relations and allows for the evaluation of the effective thermomechanical response of precipitated Ni-rich NiTi shape memory alloys on the basis of composition and heat treatment is presented. The model considers the structural effect of the precipitates (coherency stresses due to the lattice mismatch between the precipitates and the matrix material and precipitate volume fraction), as well as the effect of the Ni-concentration gradient in the matrix, resulting from Ni-depletion during precipitate growth. The predictive capability of the model is tested against experimental data obtained fromNi50.7Ti (at. %) that has been heat treated under different conditions and good agreement is shown.

The effect of tensile deformation on the damping capacity of NiTi shape memory alloy

The vibration damping capacity of a near-equiatomic NiTi shape memory alloy was characterized as a function of tensile deformation by utilizing dynamic mechanical analysis. The intrinsic internal friction (IF) values of the martensite phase (Qint,m−1) decreased with increasing deformation levels up to 10%, attributed to a decrease in the density of martensite variant and twin boundaries. Deformation increased the temperature of the transformation peak during the first reverse transformation of thermal cycling. A two-stage behavior was observed in terms of the IF values of the transformation peak. IF reached a peak value at 4%, dropped at 5% and monotonically increased again up to 10% deformation. Thermal cycling partially restored Qint,m−1 levels and shifted the transformation peak close to its undeformed IF and temperature values during the second reverse transformation.

The Effect of Aging Under Loading on the Phase Transformation Behavior of Near-Equiatomic NiTi Shape Memory Alloy

The phase transformation temperature of nickel titanium (NiTi) shape memory alloy (SMA), which is commonly used in biomaterial fields, is strongly influenced by aging heat treatments. In this study, we apply a new aging heat treatment under loading conditions to near-equiatomic NiTi SMA wires to investigate its effect on phase transformation temperatures. We determine changes in the phase transformation temperatures via differential scanning calorimetry measurements. We analyze transformation temperatures, hysteresis properties, and enthalpy changes and discuss significant results with the help of micrographs. Moreover, we evaluate our results using two-way analysis of variance, axiomatic design methods, and customary analysis in order to make reliable inferences. We observe a significant difference between first and second heating-cooling cycle results for NiTi SMA samples on the basis of austenitic transformation temperatures. Consequently, we are able to theorize correlations between design parameters and functional requirements.

Austenite grain refinement during load-biased thermal cycling of a Ni49.9Ti50.1 shape memory alloy

A near-equiatomic NiTi shape memory alloy was subjected to a variety of thermomechanical treatments including pure thermal cycling and load-biased thermal cycling to investigate microstructural evolution of the material under actuating conditions. In situ and post mortem scanning transmission electron microscopy (STEM) was used to study the effects of stress on the development of defect substructures during cycling through the martensitic transformation. High temperature observations of the austenite phase show rapid accumulation of dislocations and moderate deformation twinning upon thermomechanical cycling. Additionally, TEM-based orientation mapping suggests the emergence of fine crystallites from the original coarse austenite grain structure. A possible mechanism is proposed for the observed grain refinement based on the crystallographic theory of martensite transformation.

Fine-Grained Bulk NiTi Shape Memory Alloy Fabricated by Rapid Solidification Process and Its Mechanical Properties and Damping Performance

A near-equiatomic NiTi shape memory alloy was fabricated by rapid solidification process through vacuum arc melting followed by vacuum suction casting in water-cooled thick copper mold. The rapidly solidified (or suction cast) NiTi alloy shows much finer grains and homogenous microstructure, in particular a uniform distribution of various fine precipitates, compared to the conventional cast one. The resultant alloy also exhibits the homogenous Ni distribution in the matrix of the alloy, allowing the martensitic transformation to take place throughout the NiTi alloy matrix simultaneously and resulting in sharper transformation peaks compared to the conventional cast alloy. Moreover, the suction cast NiTi alloy shows a significant improvement over the conventional cast one, in terms of possessing higher deformation recovery rates and displaying the increased compressive strength and damping capacity by 4% and 20%, respectively.

INFLUENCES OF HEAT TREATMENT ON TRANSFORMATION BEHAVIORS OF NEAR-EQUIATOMIC POROUS NITI SHAPE MEMORY ALLOY

The pore characteristics and pore size distribution of porous near-equiatomic NiTi shape memory alloy fabricated by self-propagating high-temperature synthesis (SHS) are described in detail. The effects of different heat treatments on the transformation of porous NiTi alloy were investigated by differential scanning calorimetry (DSC), x-ray diffraction (XRD), and scanning electron microscopy (SEM). The results indicate that heat treatment had strong influences on the transformation temperatures and latent heats of transformation. When the porous alloy was annealed at 648K and 748K for 3.6ks, two steps transformation including R transformation occurred during cooling and heating and the R transformation temperatures are lower than B 2↔ B 19' transformation temperatures. However, no transformation was detected within the experimental temperature range if the porous alloy was solution treated at 1133K for 2.4ks. This novel phenomenon was the results of extensive Ti2Ni intermetallic compound precipitation. The transformation temperatures of porous NiTi alloy after annealing at 1323K for 3.6ks were much lower than those of the untreated alloy.

Gradient anneal of functionally graded NiTi

This paper reports on a novel heat treatment method for the creation of functionallygraded near-equiatomic NiTi shape memory alloys. The method is an annealwithin a temperature gradient after cold work, thus creating a structural gradientwithin the matrix of the alloy. It is based on the principle that the transformationbehaviour and the thermomechanical properties of NiTi are sensitively dependenton the annealing temperature. For the Ti–50.2 at.% Ni alloy used, it was foundthat the gradient-anneal resulted in varying thermal transformation behaviouralong its length and a unique Lüders-type deformation behaviour with a positivestress gradient. Such behaviour provides improved controllability for actuationapplications.

Design of functionally graded NiTi by heat treatment

A novel design of the functionally graded near-equiatomic NiTi shape memory alloys that exhibit expanded ranges of transformation temperature and stress is developed. The approach utilises the sensitivity of the alloys' thermomechanical properties with respect to heat treatment conditions. The functionally graded properties are achieved by anneal within a temperature gradient after cold work, thus to create a continuous structural variation. The optimum annealing temperature range for the Ti–50.5 at.% Ni alloy is determined to be 630–800 K. Gradient-anneal of the alloy wire in this temperature range resulted in varying thermal transformation behaviour along its length and unique Lüders-type deformation behaviour with a positive stress gradient for both the forward and the reverse transformations. Such behaviour enhances the functionality of the alloy for actuation applications.

Thermodynamic analysis of ageing-induced multiple-stage transformation behaviour of NiTi

Near-equiatomic NiTi shape memory alloys normally exhibit three martensitic transformations among three phases: the B2 phase, the monoclinic (M) phase and the rhombohedral (R) phase. Some recent work, however, has revealed complex transformation behaviour involving multiple-stage martensitic transformations and multiple-stage R-phase transformations. This paper presents an analysis of these complex transformation behaviours based on thermodynamic concepts of reversible and irreversible energies associated with the transformations. The analysis is successful in identifying all observed transformations and in defining relative positions of various stages of transformations on a temperature scale. It also defines positions of thermodynamically prohibited transformations as well as permitted transformations that have not been experimentally measured. Such identifications enable the determination of actual transformation hystereses that are not directly measured experimentally. Based on the thermodynamic principles adopted, the analysis also renders it possible to identify the possible causes that contribute to the complex multiple-stage transformation behaviour.

Synchrotron X-Ray Study of Texture in Cold-Worked Shape-Memory NiTi-Wires

AbstractA series of martensitic, near-equiatomic NiTi shape-memory alloy wires was deformed to strains ranging from 1 to 40% up to stresses of 920 MPa. After deformation, the wires were exposed to a monochromatic, parallel beam of high energy x-rays oriented perpendicular to the wire axis. The transmitted low index diffraction rings show that martensitic texture is increasing with prestrain up to ε=15% after twinning is complete. Further prestraining in the plastic range lowers the texture again indicating that twinning- and plasticity-textures cancel partially each other. Also, deformed NiTi-wires were heated and cooled from 20°C to 200°C under a small constant stress of 6 MPa. The strain change due to the Two-Way Shape-Memory Effect was measured and correlated to the diffraction results.

Lüders-like deformation associated with martensite reorientation in NiTi

It is known that near-equiatomic NiTi shape memory alloys may exhibit a Lueders-like deformation behavior under a variety of testing conditions. These conditions include the tensile deformation associated with the stress-induced martensitic transformation from the austenite, the reverse transformation of the stress-induced martensite to austenite in pseudoelasticity, and the deformation in martensitic state via a martensite variant reorientation process. The Lueders-like deformation behavior is characterized by a stress plateau and a stress-drop at the beginning of the process for the forward transformation upon loading and a stress minimum for the reverse transformation on unloading. Based largely on this observation in pseudoelasticity, it has been suggested that the stress peaks at the beginning of the stress plateau are associated with the nucleation of the product phase for the corresponding transformations and that the stress plateau corresponds to the process of the transformations. This common belief, however, is challenged by a number of experimental observations. This seems to suggest that the occurrence of the stress plateau on a stress-strain curve is mechanical in nature instead of being determined by the transformation. This uncertainty, however, needs to be clarified.

Apparent modulus of elasticity of near-equiatomic NiTi

Stress-to-strain differential values were measured in tensile testing at various stages of deformation in both austenitic and martensitic states of a near-equiatomic NiTi shape memory alloy as a measurement of the moduli of elasticity for the two phases. It was found that the measurements varied with testing conditions, including strain and temperature. This observation indicates that the mechanical measurements are only the apparent modulus of elasticity, which is affected by contributions to the apparent elastic deformation from other deformation mechanisms. These additional contributions to deformation are attributed to the stress-induced martensitic transformation and martensite reorientation processes.

Criteria for pseudoelasticity in near-equiatomic NiTi shape memory alloys

It was observed that the reverse transformation of stress-induced martensite occurred at a temperature some 20 K higher than that of thermal martensite. The increase in temperature for the reverse transformation was indicative of a stabilisation effect. This stabilisation effect was attributed to the change in the accommodation morphology of martensite variants from a self-accommodating state for the thermal martensite to an orientated state for the stress-induced martensite. This observation led to the reconsideration of the criteria for pseudoelasticity. Quantitative expressions of criteria on both testing temperature and austenite yield strength were derived, which showed satisfactory agreement with experimental observations.