Improved functional stability of a coarse-grained Ti-50.8 at.% Ni shape memory alloy achieved by precipitation on dislocation networks

Miyazaki S.

The present author has been studying shape memory alloys including Cu–Al–Ni, Ti–Ni-based, and Ni-free Ti-based alloys since 1979. This paper reviews the present author’s research results for the latter two materials since 1981. The topics on the Ti–Ni-based alloys include the achievement of superelasticity in Ti–Ni alloys through understanding of the role of microstructures consisting of dislocations and precipitates, followed by the contribution to the development of application market of shape memory effect and superelasticity, characterization of the R-phase and monoclinic martensitic transformations, clarification of the basic characteristics of fatigue properties, development of sputter-deposited shape memory thin films and fabrication of prototypes of microactuators utilizing thin films, development of high temperature shape memory alloys, and so on. The topics of Ni-free Ti-based shape memory alloys include the characterization of the orthorhombic phase martensitic transformation and related shape memory effect and superelasticity, the effects of texture, omega phase and adding elements on the martensitic transformation and shape memory properties, clarification of the unique effects of oxygen addition to induce non-linear large elasticity, Invar effect and heating-induced martensitic transformation, and so on.

Paranjape H.M., Bowers M.L., Mills M.J., Anderson P.M.

Compression of a single crystal, superelastic NiTi shape memory alloy (SMA) micro-pillar and the stress-field around an ellipsoidal twinned martensite (M) plate embedded in an austenite (A) matrix were simulated using a coupled phase transformation and crystal plasticity model. Post-mortem transmission electron microscopy (TEM) analysis of the dislocation structures in a foil extracted from a compressed NiTi micro-pillar was also performed. Based on these modeling and experimental data, we propose mechanisms for phase-transformation-induced defect generation in superelastically stressed NiTi SMA. The geometry of the simulated slip bands shows that dislocations nucleate and grow in the austenite phase adjacent to a growing or receding martensite plate to accommodate local strain gradients. The simulated resolved shear stress on individual slip systems, and Burgers vector analysis of dislocations in the TEM data show that the slip system and amount of slip activity depend on the magnitude of the strain gradients, which are controlled by the martensite crystallography, the dynamics of martensite plate growth, and scale of the twinned structure and A-M interface. In addition to a[0 1 0] (1 0 1 ¯ ) and a [0 0 1] ( 1 ¯ 1 0) slip systems observed in prior experiments, we report the activation of a third slip system: a[0 0 1](1 1 0). We show that the three slip systems are likely to be active at different locations around a martensite plate. The modeling component in this work complements ex-situ TEM characterization by furnishing the resolved shear stress and slip activity on austenite slip systems throughout the cyclic loading.

Chowdhury P., Sehitoglu H.

Performance degradation in shape memory alloys (SMA) arises due to a gradual loss of strain recoverability attributable to slip mediated plasticity. The slip-induced changes in SMAs can be profound creating accumulation of permanent strains, altering the critical stress and hysteresis in an adverse manner. Slip nucleation in ordered SMA lattices can often be triggered due to energetically favorable dissociation reactions. Partial slip can dominate over full slip, generating planar defects (e.g. anti-phase boundary, superlattice or complex stacking faults) as evidenced through electron microscopy. Considerable advances are made lately on physically rationalizing the observed plastic micromechanism(s) benefitting from quantum mechanical models. In-depth analyses of crystal variables (e.g. lattice ordering, atomic stacking and stable/metastable fault structures) subjected to intrinsic solid-state effects have unequivocally established the genesis of empirical slipping propensity in terms of atomic fault energetics. This article systematically revisits the empirical physical evidence of slip in important SMAs from the literature, presents the pertinent experimental findings, and then embarks on reviewing the investigations of atomistic studies as exemplified by the authors’ group. In closing, we discourse on the potential use of lattice scale theories in devising other important structure-property relationships such as role of precipitates, cracking resistance, constitutive modeling.

Gao Y., Casalena L., Bowers M.L., Noebe R.D., Mills M.J., Wang Y.

Functional fatigue (FF) during thermal and mechanical cycling, which leads to the generation of macroscopic irrecoverable strain and the loss of dimensional stability, is a critical issue that limits the service life of shape memory alloys (SMAs). Although it has been demonstrated experimentally that such a phenomenon is related to microstructural changes, a fundamental understanding of the physical origin of FF is still lacking, especially from a crystallographic point of view. In this study, we show that in addition to the normal martensitic phase transformation pathway (PTP), there is a symmetry-dictated non-phase-transformation pathway (SDNPTP) during phase transformation cycling, whose activation could play a key role in leading to FF. By investigating crystal symmetry changes along both the PTPs and SDNPTPs, the characteristic types of defects (e.g., dislocations and grain boundaries) generated during transformation cycling can be predicted systematically, and agree well with those observed experimentally in NiTi. By analyzing key materials parameters that could suppress the SDNPTPs, strategies to develop high performance SMAs with much improved FF resistance through crystallographic design and transformation pathway engineering are suggested.

Wang X., Verlinden B., Kustov S.

Precipitation hardening is an effective way to improve the functional stability of NiTi shape memory alloys. The precipitates, mainly Ni4Ti3, could be introduced by aging treatment in Ni-rich NiTi alloys. However, the presence of Ni4Ti3 precipitates could disturb the transformation behavior, resulting in the multi-stage martensitic transformation (MMT). With the presence of MMT, it is difficult to control the transformation behavior, and thus limits the applicability of NiTi alloys. In this work, previous efforts on explaining the observed MMT are summarized. The difficulties in developing a unified explanation are discussed, and a possible way to avoid the MMT is proposed.

Elahinia M., Shayesteh Moghaddam N., Taheri Andani M., Amerinatanzi A., Bimber B.A., Hamilton R.F.

Nickel-titanium (NiTi) is an attractive alloy due to its unique functional properties (i.e., shape memory effect and superelasticity behaviors), low stiffness, biocompatibility, damping characteristics, and corrosion behavior. It is however a hard task to fabricate NiTi parts because of the high reactivity and high ductility of the alloy which results in difficulties in the processing and machining. These challenges altogether have limited the starting form of NiTi devices to simple geometries including rod, wire, bar, tube, sheet, and strip. In recent years, additive manufacturing (AM) techniques have been implemented for the direct production of complex NiTi such as lattice-based and hollow structures with the potential use in aerospace and medical applications. It worth noting that due to the relatively higher cost, AM is considered a supplement technique for the existing. This paper provides a comprehensive review of the publications related to the AM techniques of NiTi while highlighting current challenges and methods of solving them. To this end, the properties of conventionally fabricated NiTi are compared with those of AM fabricated alloys. The critical steps toward a successful manufacturing such as powder preparation, optimum laser parameters, and fabrication chamber conditions are explained. The microstructural characteristics and structural defects, the influencing factors on the transformation temperatures, and functional properties of NiTi are highlighted to provide and overview of the influencing factors and possible controlling methods. The mechanical properties such as hardness and wear resistance, compressive behaviors, fatigue characteristics, damping and shock absorption properties are also reported. A case study in the form of using AM as a promising technique to fabricate engineered porous NiTi for the purpose of creating a building block for medical applications is introduced. The paper concludes with a section that summarizes the main findings from the literature and outlines the trend for future research in the AM processing of NiTi.

Sedmák P., Šittner P., Pilch J., Curfs C.

Motivated by an assumption that the instability of the cyclic tensile superelastic behavior of NiTi polycrystal is linked to its fatigue performance (number of cycles till failure), the instability was investigated by high resolution in situ synchrotron X-ray diffraction method. NiTi wires were cyclically deformed in tension at room temperature while X-ray diffraction patterns were recorded in three preselected states along the superelastic stress–strain curve, analyzed and interpreted in terms of the gradual evolution of microstructural state during cycling. It is found that the cyclic instability is due to the gradual redistribution of internal stresses originating from the accumulation of incremental plastic strains accompanying the stress induced martensitic transformation in constrained polycrystalline environment. The degree of cyclic instability increases with the increasing involvement of slip in the hybrid slip/transformation process, which depends on initial microstructure (grain size, defects, precipitates), martensitic transformation (crystallographic incompatibility between transforming phases), temperature and parameters of the cyclic loading (strain rate, amplitude, stress state, type of loading etc.).

Wang X., Verlinden B., Van Humbeeck J.

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni4Ti3 precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni4Ti3 precipitates.

Bowers M.L., Gao Y., Yang L., Gaydosh D.J., De Graef M., Noebe R.D., Wang Y., Mills M.J.

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.

Ahadi A., Sun Q.

In situ X-ray diffraction during loading and unloading is used to investigate the effects of grain size (GS) on the stress-induced nanoscale phase transition (PT) mechanism in polycrystalline superelastic NiTi. The average GS studied (10–1500 nm) spans the range in which significant changes of macroscopic thermomechanical properties (due to GS reduction) have been observed. It is shown that when the GS ⩾ 68 nm, the evolution of the diffraction profiles (DPs) during loading and unloading exhibits well-defined distinct diffraction peaks with significant changes in their diffracted intensities corresponding to the nucleation and growth mechanism of B19′ martensite (high strain) phase. However, when GS 

Wang X., Kustov S., Li K., Schryvers D., Verlinden B., Van Humbeeck J.

In order to take advantage of both grain refinement and precipitation hardening effects, nanoscaled Ni 4 Ti 3 precipitates are introduced in a Ti–50.8 at.% Ni alloy with micron-sized grains (average grain size of 1.7 μm). Calorimetry, electrical resistance studies and thermomechanical tests were employed to study the transformation behavior and functional properties in relation to the obtained microstructure. A significant suppression of martensite transformation by the obtained microstructure is observed. The thermomechanical tests show that the advantageous properties of both grain refinement and precipitation hardening are combined in the developed materials, resulting in superior shape memory characteristics and stability of pseudoelasticity. It is concluded that introducing nanoscaled Ni 4 Ti 3 precipitates into small grains is a new approach to improve the functional properties of NiTi shape memory alloys.

Ahadi A., Sun Q.

We investigated the effects of grain size on the rate-dependent thermomechanical responses of polycrystalline superelastic NiTi (with an average grain size from 10 to 90 nm) under both monotonic and cyclic tensile loading–unloading. Measurements of stress–strain curves, hysteresis loop area, and temperature fields are synchronized using in situ infrared thermography in the strain rate range from ε ̇ = 4 × 10 −5 s −1 to ε ̇ = 1 × 10 −1 s −1 . It is found that with the grain size reduction to the nanoscale, the rate dependence of the transformation stress and the hysteresis loop area gradually weakens and finally tends to vanish for a grain size of 10 nm. Under cyclic loading, the non-isothermal cyclic stability of the polycrystal is significantly improved as manifested by 64% reduction in heat accumulation and 91% reduction in stress variations when the grain size is reduced from 90 to 27 nm. It is shown that such significant improvements in the cyclic stability and decrease of the rate sensitivity (while preserving large (≈5%) recoverable strain) originate from the rapid decrease of internal heat sources (the latent heat and the hysteresis heat) and the rapid decrease of the temperature dependence of the transformation stress with the grain size. This work strongly implies that the heat accumulation in cyclic loading of superelastic NiTi SMAs, as one of the sources of poor fatigue response, can be reduced by extreme grain refinement.

Bowers M.L., Chen X., De Graef M., Anderson P.M., Mills M.J.

[1 1 0]-Oriented micropillars of 50.7 at.% NiTi were compressed to stress-induce a single martensite plate. Scanning transmission electron microscopy identified two distinct slip systems in the prior transformation zone with dislocation loops of alternating Burgers vectors and a doubly periodic dislocation substructure. Micromechanics-based modeling predicts these systems to be dominant for a specific martensite plate type and correlates these observations with the alternating stress state near twinned martensite plates. This combined experiment–modeling approach provides new mechanistic insight into defect generation during pseudoelastic cycling.

Karbakhsh Ravari B., Farjami S., Nishida M.

The effects of Ni concentration and aging conditions on the multistage martensitic transformation (MMT) in aged Ni-rich Ti–Ni alloys have been investigated by differential scanning calorimetry and in situ scanning electron microscopy. The effect of Ni concentration was evaluated using Ti–50.6, 50.8 and 51.0 at.% Ni alloys. These alloys were heat treated at 1223 K for 3.6 ks and then aged at 773 K for 3.6 ks. Although the triple-stage transformation appeared in the Ti–50.6 and 51 at.% Ni alloys during cooling, the transformation sequence of the two alloys was completely different. Quadruple-stage transformation was observed in the Ti–50.8 at.% Ni alloy. The characteristic microstructure responsible for the MMT in each of the aged alloys strongly depended on the degree of Ni supersaturation and the aging temperature and time. We have proposed and experimentally verified a general rule that explains the effects of Ni concentration and aging conditions on the microstructural changes and thus the MMT sequences. This will allow the MMT in Ni-rich Ti–Ni alloys to be controlled by selecting appropriate aging conditions.

Wang X., Li K., Schryvers D., Verlinden B., Van Humbeeck J.

A cold-drawn Ti–50.8 at.% Ni wire was annealed at 600 °C for 30 min, followed by aging at 250 °C for different times. A microstructure with small grains and nanoscaled precipitates was obtained. The thermally induced martensite transformation is suppressed in the samples aged for 4 h or longer, leaving a one-stage R-phase transition between −150 and +150 °C. The transformation behavior, work output and recovery stress associated with the R-phase transition are presented.

Chen J., Liu F., Fang G., Ramamurty U.

Zhao H.C., Liu J.C., Chen F., Chen F.H., Li L., Tong Y.X.

Li Y., Zhang P., Sheng C., Wu Z., Wang T.

Chu H., Zhao Z., Fang J., Xiao Y., Lin J., Min J.

Xu B., Xiao X., Zhang Q., Yu C., Song D., Kan Q., Wang C., Wang Q., Kang G.

Jiang M., Jiang H., Xi R., Ren D., Ji H., Lei J., Wang X.

Xu B., Yu C., Xiong J., Hu J., Kan Q., Wang C., Wang Q., Kang G.

Wang X., Pu Z., Van Humbeeck J.

Vashishtha H., Collins D.M.

Xu Z., Zhang Y.

Dang P., Li C., Yang Y., Zhou Y., Xu Y., Ding X., Sun J., Xue D.

AbstractElastic materials that store and release elastic energy play pivotal roles in both macro and micro mechanical systems. Uniting high elastic energy density and efficiency is crucial for emerging technologies such as artificial muscles, hopping robots, and unmanned aerial vehicle catapults, yet it remains a significant challenge. Here, a nanocrystalline structure embedded with elliptical martensite nanodomains in ferroelastic alloys was utilized to enable high yield strength, large recoverable strain, and low energy dissipation simultaneously. As a result, the designed Ti–Ni–V alloys demonstrate ultrahigh energy density (>40 MJ m−3) with ultrahigh efficiency (>93%) and exceptional durability. This concept, which combines nano‐sized embryos to minimize energy dissipation of psuedo‐elasticity and employs a fine‐grained structure to enhance yield strength, can be applied to other ferroelastic materials. Furthermore, it holds promise for the development of phase transformation‐involved functionalities such as high‐performance dielectric energy storage, ultralow‐hysteresis magnetostrain, and high‐efficiency solid‐state caloric cooling.

Yang Y., Zhang Y.Q., Lu H.Z., Luo Y., Long T.H., Tong W.T., Zhang Y., Yu X., Yang C.

Although the simulation results had demonstrated that the strain field introduced by Ni4Ti3 nano-precipitates in NiTi shape memory alloys (SMAs) was related with their superelasticity inherently, the corresponding experimental result was rarely documented heretofore, especially in additive manufactured NiTi SMAs. In this work, we tailor the morphologies and resultant strain field of Ni4Ti3 nano-precipitates by heat treatment of a NiTi SMA subjected to laser powder bed fusion (LPBF), and further authenticate relationship between the superelasticity and the strain field in the LPBF NiTi samples. When holding times were 1 h, 3 h, and 5 h at aging temperature of 350 °C after solution treatment, the Ni4Ti3 nano-precipitates in the LPBF NiTi samples exhibit spherical, ellipsoidal, and lenticular morphologies, respectively. Accordingly, the strain field around Ni4Ti3 nano-precipitates in B2 matrix decrease from 0.15 % to 0.13 % and 0.10 %, respectively. The LPBF and aged NiTi samples present large superelasticity, which exceeds 6 % recovery strain together with high recovery rate of ˃99 % during 10-times cyclic compression loading. Interestingly, the LPBF and aged sample with the spherical Ni4Ti3 and highest strain field displays the worst superelasticity stability, while the one with the lenticular Ni4Ti3 and smallest strain field exhibits the relatively stable and biggest superelasticity of 6.36 %. Basically, this is attributed to different mechanisms between the Ni4Ti3 nano-precipitates and dislocations generated during cyclic loading, which is induced by different interfaces between the Ni4Ti3 and B2 matrix in the three types of the NiTi samples. For the sample with the highest strain field, its spherical Ni4Ti3 was cut through by generated dislocations due to coherent interface between the spherical Ni4Ti3 and B2 matrix. In contrast, the one with the smallest strain field, its lenticular Ni4Ti3 can impede effectively generated dislocations because of semi-coherent or non-coherent interface between the lenticular Ni4Ti3 and B2 matrix. Therefore, these results can provide meaningful insights into tailoring the nano-precipitates and thereby obtaining excellent superelasticity of NiTi SMAs by LPBF.

Hou R., Xiao F., Zuo S., Cai X., Zhou Y., Porta M., Planes A., Jin X.

The stress-induced transformation from austenite phase to R phase in NiTi and TiNi-based shape memory alloys has a small stress hysteresis and a high stability to cycling across the transformation due to a small lattice distortion. However, the elastocaloric effect is limited due to the intrinsic small transformation strain and this significantly hinders its application in solid-state refrigeration. Here, we report a macroscopically inhomogeneous stress-induced R-phase transformation in Ti50Ni48.5Fe1.5 wire with a stress plateau in the stress-strain curve, showing difference from the conventional macroscopically homogeneous transformation with an associated continuous increase of the stress. This novel transformation behavior permits reaching a temperature decrease of 5.8 K with a transformation strain of only 0.4 %, which brings a near 100 % increase in the elastocaloric effect of R-phase transformation compared to Ni50.8Ti49.2 and an excellent cycling stability. Infrared tensile tests and in-situ X-ray diffraction measurements prove the existence of transformation bands, confirming the localized deformation of this transformation behavior. Transmission electron microscopy results show that the microstructure condition for inducing such a macroscopically inhomogeneous R-phase transformation in Ti50Ni48.5Fe1.5 is the limited hindrance effect from the local stress induced by doped Fe atoms. The stress-induced R-phase transformation with an enhanced cooling capacity and an excellent fatigue property makes Ti50Ni50-xFex alloys a new alternative material for solid-state refrigeration.

Kim J., Rehman A., Ryu H., Oh I., Sim G.

In this study, we investigated the mechanical behavior of sputter-deposited Ni-rich NiTi thin films with various chemical compositions. Freestanding thin films were successfully produced by employing magnetron sputtering and microfabrication techniques. Notably, films containing Ni within the range of 52.6–57.6 at. % exhibited distinct stress-induced martensitic transformations. Among these, 53.3 at. % and 54.2 at. % Ni–Ti films demonstrated a remarkable combination of strength exceeding 1.5 GPa and elastic strain approaching 5%. Conversely, films with the highest Ni content displayed nearly linear elastic behavior, showcasing an exceptional tensile strength of 2.2 GPa. As the element composition deviated from the equiatomic ratio, the grain size of the deposited films underwent a transition from several micrometers to nano-scale. In addition, the critical stresses showed lower sensitivity to chemical composition compared to bulk alloys, indicating that superelastic behavior persists over a wider composition range in Ni-rich NiTi thin films. By comparing the NiTi matrix and precipitate morphology between bulk alloys and thin films, we discussed the variations in the mechanical behavior of thin films.

Xue D., Zuo Q., Pan Y., Zhang G.

Shape memory alloys (SMAs) are the key components of actuators and sensors due to their shape memory effect and superelasticity. However, the thermal hysteresis associated with martensitic phase transformation limits their use in the long-duration precise control. We report a strategy that obtains SMAs with low hysteresis by constructing a composition-temperature pseudo phase diagram. This strategy is inspired by the physically parallel ferroelectric system where the hysteresis is minimized at the multiple phase coexistence point, due to the absence of energy barrier across different phases. Following this, an alloy in the phase diagram with a low hysteresis of about 5 K is synthesized. In contrast, those alloys compositionally different from the optimal one have large hysteresis. Microstructures characterization and diffraction analysis are employed to identify the multiple phase coexistence. The proposed strategy should be general and can shed light on the rational design of SMAs with low hysteresis.