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The temperature-dependent elasticity of the B19′ NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19′ crystal, we present results of in-situ neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young's modulus (TDYM) of the B19′ crystal is strongly anisotropic. For different crystallographic orientations, the change in Young's modulus over the temperature range of 300–50 K ( Δ E ( h k l ) = E ( h k l ) 50 K − E ( h k l ) 300 K ), ranges from Δ E ( 10 2 ¯ ) = 2.8 ± 3.5 GPa (extremely weak dependence) to Δ E ( 103 ) = 59.6 ± 9.1 GPa (strong dependence). Moreover, it is found that the orientation-specific TDYM and thermal expansion (TE) of the B19′ crystal are correlated. The crystallographic orientations with weak and negative TE responses exhibit a weaker TDYM than the orientations with positive TE. The DFT-MD and QHA results capture qualitatively the above experimental observations and further show that there are orientations in a B19′ crystal exhibiting elastic softening ( Δ E ( h k l ) Δ E ( h k l ) = 0) with cooling. This is found to originate from the strong negative temperature dependence of c35 stiffness constant. The experimental results along with the first-principles calculations confirm that the Elinvar and Invar are two confluent properties in NiTi SMAs and can be tailored by texturing B19′ crystallographic orientations.

Superelastic shape memory alloys (SMAs) exhibit a reversible elastocaloric effect that originates from a release/absorption of latent heat associated with a stress-induced martensitic phase transformation. In typical SMAs, the conventional elastocaloric effect will vanish when the operating temperature falls below the temperature range in which martensitic phase transformation can be triggered by stress. We report emergence of an unprecedented elastocaloric effect with a decrease of temperature, well below the temperature range of martensitic phase transformation, in a model nanocrystalline NiTi that preserves slim-hysteresis superelasticity at ultra-low temperatures. The new elastocaloric effect emerges at a temperature of ∼ 90 K, exhibits an opposite sign than the conventional elastocaloric effect, and intensifies gradually with a decrease of temperature to 18 K. At 18 K, a large adiabatic temperature change ΔTad of +3.4 K is measured upon rapid release of tensile stress. The measured ΔTad are larger and extend over a wider temperature span than the existing electrocaloric, piezocaloric, and barocaloric cryo-refrigeration materials. We show that such low temperature elastocaloric effect originates from an entropic elasticity associated with large non-linear elastic deformations of the nanocrystalline microstructure at ultra-low temperatures. Our study suggests a new avenue to cool ultra-low temperature ambients.

This paper investigates the effect of minority W additions on the thermal stability of nanocrystalline NiTi thin films. The films were produced in an amorphous state, and the addition of W was found to increase the activation energy for crystallization and lead to finer grain sizes after crystallization. In the crystallized films, W was observed to both segregate to the NiTi grain boundaries and to phase separate into fine precipitates; together these effects contribute to the stability of the nanocrystalline state up to 1200 °C. Using in-situ transmission electron microscopy, grain growth was observed concomitantly with coarsening of W precipitates, indicating that a primary mechanism for stability is Zener pinning by the W precipitates. At the same temperature where coarsening begins to occur rapidly, grain boundaries were also observed to undergo a transformation to thick, amorphous complexions. Monte Carlo simulations showed that W segregation to grain boundaries increases with temperature, which contributes to an increased rate of coarsening and loss of stability against grain growth.

Shape memory alloys sustaining stable functional properties for millions of cycles have been reported in NiTiCuCo thin films. The mechanism behind such ultralow functional fatigue was proposed to originate from the epitaxy strains surrounding coherent Ti2Cu precipitates. Despite having a well-understood microstructural origin, such fatigue-resistant property has never been achieved in bulk NiTiCuCo counterparts. In this study, we first show that the semi-coherent Ti2Cu precipitates help satisfy closely the compatibility criteria under thermal phase transformation. Assisted by aging at 600 °C for 1 h, an unprecedented thermal stability is reported in bulk coarse-grained Ti54Ni31.7Cu12.3Co2, where the transformation temperatures migrate by ∼ 0.1 °C for 200 cycles. To achieve stable superelastic response, a thermomechanical processing route is proposed that results in a uniform nanocrystalline microstructure with embedded Ti2Cu precipitates. Such a microstructure exhibits much improved superelastic cyclic stability as evidenced by ∼ 22 MPa shakedown of plateau stress for 200 superelastic cycles.

Fatigue cracking in polycrystalline NiTi was investigated using a multiscale experimental framework for average grain sizes (GS) from 10 to 1500 nm for the first time. Macroscopic fatigue crack growth rates, measured by optical digital image correlation, were connected to microscopic crack opening and closing displacements, measured by scanning electron microscope DIC (SEM-DIC) using a high-precision external SEM scan controller. Among all grain sizes, the 1500 nm GS sample exhibited the slowest crack growth rate at the macroscale, and the largest crack opening level (stress intensity at first crack opening) and minimum crack opening displacements at the microscale. Smaller GS samples (10, 18, 42, and 80 nm) exhibited nonmonotonic trends in their fatigue performance, yet the correlation was strong between macroscale and microscale behaviors for each GS. The samples that exhibited the fastest crack growth rates (42 and 80 nm GS) showed a small crack opening level and the largest crack opening displacements. The irregular trends in fatigue performance across the nanocrystalline GS samples were consistent with nonmonotonic values in the elastic modulus reported previously, both of which may be related to the presence of residual martensite only evident in the small GS samples (10 and 18 nm).

We have investigated the physical origin of anomalous in-plane thermal expansion (TE) anisotropy leading to invar-like behavior and negative TE in nanostructured NiTi sheets manufactured via severe cold-rolling. The roles of grain size (GS), crystallographic texture, thermally-induced phase transformation, and intrinsic (lattice level) TE of austenite (B2) and martensite (B19′) phases on the macroscopic TE behavior are addressed. It is shown that by controlling the cold-rolling thickness reduction and heat-treatment temperature the coefficient of thermal expansion (CTE) can be controlled in a wide range from positive ( α ∼ 2.1 × 10 −5 K −1 ) to negative ( α ∼ −1.1 × 10 −5 K −1 ) via in-plane anisotropy of TE. A very small CTE of α ∼ −5.3 × 10 −7 K −1 (invar-like behavior) in a wide temperature window of 230 K (353–123 K) is obtained at an angle of 33.5° to the rolling direction (RD) of the severely cold-rolled sheet. TEM and XRD studies show that the microstructure underlying such anomalous TE behavior consists of a mixture of B2 nano-grains and retained/residual deformation-induced martensite and that the observed anomalous TE anisotropy is due to the intrinsic anisotropic TE of residual martensite. The invar-like behavior is the result of the cancellation of the positive TE of austenite phase with the negative TE of residual martensite along 33.5° to the RD. A simple rule of mixture model incorporating the intrinsic TE of B2 and B19′ lattices and the texture coefficients of the sample is proposed which successfully captures the anomalous in-plane TE anisotropy. The discovery of high dimensional stability over a wide temperature window along with temperature insensitive non-hysteretic linear superelasticity of the severely-deformed NiTi opens up a new route for designing stable SMAs for applications in ragged environments.

Selective laser melting has been applied as a production technique of nickel titanium (NiTi) parts. In this study, the scanning parameters and atmosphere control used during production were varied to assess the effects on the final component transformation criteria. Two production runs were completed: one in a high (~1800 ppm O2) and one in a low-oxygen (~220 ppm O2) environment. Further solution treatment was applied to analyze precipitation effects. It was found that the transformation temperature varies greatly even at identical energy densities highlighting the need for further in-depth investigations. In this respect, it was observed that oxidation was the dominating factor, increased with higher laser power adapted to higher scanning velocity. Once the atmospheric oxygen content was lowered from 1800 to about 220 ppm, a much smaller variation of transformation temperatures was obtained. In addition to oxidation, other contributing factors, such as nickel depletion (via evaporation during processing) as well as thermal stresses and textures, are further discussed and/or postulated. These results demonstrated the importance of processing and material conditions such as O2 content, powder composition, and laser scanning parameters. These parameters should be precisely controlled to reach desired transformation criteria for functional components made by SLM.

The transformation and retransformation paths realized at constant stress in wires of NiTi SMAs show a horizontal or “flat” behavior in the hysteretic cycle. After sinusoidal cycling at 0.01 Hz (i.e., training) with a maximal strain of 8 %, the thicker wires of NiTi SMAs, with a diameter of 2.46 mm, have the stress–strain cycles with one S-shaped behavior. A “similar” change appears by varying the grain sizes in the samples, i.e., from 80 to 20 nm. Furthermore, the local measurements of temperature suggest that cycling induces changes in the transformation mechanisms associated with the evolution from horizontal to S shape. For instance, the associated energy evolves from localized transformation to homogeneous heat production in S-shaped cycles. Thermodynamic analysis establishes a link between heat capacity and the slopes of the coexistence curve in f–x or in σ–ε via the (∂ε/∂σ)coex. The flat cycles were coherent with the classical latent heat, i.e., dissipation at constant stress. In the S-shaped cycles, the heat during the phase change seems redistributed as heat capacity against progressive stress. Preliminary direct measurements are coherent with the evolution of the (∂ε/∂σ)coex against strain. Fil: Torra, Vicenç. Personal Research Group; España Fil: Martorell, F.. Personal Research Group; España Fil: Sun, Q. P.. Hong Kong University of Science and Technology; República de China Fil: Ahadi, A.. Tsukuba University. National Institute For Materials Science; Japón. Wuhan University; China Fil: Lovey, Francisco Carlos. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina Fil: Sade Lichtmann, Marcos Leonel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentina

The grain size (GS) dependence of fracture toughness (KIC) and static crack-growth resistance (KR) of superelastic NiTi with average GS from 10 to 1500 nm are investigated. The measurements of strain and temperature fields at the crack-tip region are synchronized with the force–displacement curves under mode-I crack opening tests. It is found that with GS reduction down to nanoscale, the KIC and the size of crack-tip phase transformation zone monotonically decrease and the KR changes from a rising to a flat R-curve. The roles of intrinsic and extrinsic toughening mechanisms in the GS dependence of the fracture process are discussed.

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

Effect of severe plastic deformation on properties of TiNi shape memory alloy has been investigated for cold-drawn wire and cold-rolled sheet. Severe plastic deformation introduces solid-state amorphization and nanocrystallization. These microstructures lead to anomalous properties such as linear superelasticity and invar effect.

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 e = 4 × 10−5 s−1 to e = 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.

In this study, a near equiatomic NiTi shape memory alloy was hot rolled at 800uC using the thickness reductions of 30 and 50%. Optical and transmission electron microscopy, together with X-ray diffraction were used to demonstrate the microstructural changes associated with the hot rolling at different thickness reductions. Repeated transformation cycling was employed to investigate the evolution of R phase during cycling. Microstructural observations revealed the presence of deformation twins embedded in an elongated grain matrix in the hot rolled material. Moreover, it was found that with increasing degree of thickness reduction, the size and number of deformation twins increased throughout the grains. After transformation cycling, higher values of DH ARR were noticed in the 50% hot rolled material in comparison to the annealed condition. Transmission electron microscopy microstructure of the 50% hot rolled material transformation cycled up to 50 cycles showed that R phase packets are frequently observed near grain boundaries. The observation of R phase packets near the grain boundaries in the hot rolled material was attributed to the high stress concentrations at these regions.

In this study, the effects of stress-assisted heat treatment on the microstructure and phase transformation of a Ti-rich (Ti-49.52 at.% Ni) shape memory alloy were investigated. For this purpose, the alloy was heat treated at temperature of 500 °C for 10 h under applied stresses of 100 and 200 MPa. XRD, TEM, and repeated thermal cycling were employed to study the microstructure and transformation behavior of the heat-treated materials. Room temperature XRD diffractogram of the stress-free heat-treated material showed a weak reflection of austenite (B2), while that for the stress-assisted heat-treated materials had a high intensity implying the presence of residual austenite in the microstructure. TEM observations confirmed the presence of residual austenite and revealed mechanical twins as another constituent of the microstructure in the stress-assisted heat-treated materials. Moreover, with increasing the value of applied stress the size of mechanical twins was increased and a high density of structural defects was observed at the interfaces of the twins. DSC results demonstrated two-stage transformation in the initial cycles of transformation in the stress-assisted heat-treated material. After about eight cycles of transformation, the two-stage transformation has vanished, and a single-stage transformation remained up to 100 cycles. It was suggested that the accommodation of stresses at Ti2Ni/matrix interface provides a suitable condition for local transformation of B2 to B19′ that is manifested by a two-stage phase transformation. Introduction of structural defects during repeated thermal cycling may counteract the stress field at Ti2Ni/matrix interface leading to a single-stage transformation.

In this study, a set of constitutive equation corrected for deformation heating is proposed for a near equi-atomic NiTi shape memory alloy using isothermal hot compression tests in temperature range of 700 to 1000 °C and strain rate of 0.001 to 1 s−1. In order to determine the temperature rise due to deformation heating, Abaqus simulation was employed and varied thermal properties were considered in the simulation. The results of hot compression tests showed that at low pre-set temperatures and high strain rates the flow curves exhibit a softening, while after correction of deformation heating the softening is vanished. Using the corrected flow curves, the power-law constitutive equation of the alloy was established and the variation of constitutive constants with strain was determined. Moreover, it was found that deformation heating introduces an average relative error of about 9.5% at temperature of 800 °C and strain rate of 0.1 s−1. The very good agreement between the fitted flow stress (by constitutive equation) and the measured ones indicates the accuracy of the constitutive equation in analyzing the hot deformation behavior of equi-atomic NiTi alloy.

Stress hysteresis (H) and temperature dependence of phase transition stress (dσ/dT) are the two signatures of first-order phase transition in shape memory alloys. We studied the effects of grain size on these two properties in polycrystalline superelastic NiTi with the average grain size from 10 nm to 1500 nm. We identified a critical grain size (∼60 nm) below which both H and dσ/dT rapidly decrease, leading to vanishing hysteresis and breakdown of Clausius-Clapeyron equation. The physics behind such grain size effects are the dominance of interfacial energy in the energetics of the polycrystal and the lack of two-phase coexistence at nano-scales.