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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.