NiTiHf Shape Memory Research Articles

Enhanced thermal cycle stability and shape memory effect in NiTiHf shape memory alloys fabricated by laser powder bed fusion

We report on the fabrication of NiTiHf high temperature shape memory alloys (SMAs) via laser powder bed fusion (LPBF), which are almost free of micro-pores and display (Ti, Hf)2Ni nanoprecipitates with variable size and volume fraction. The effect of variable laser powers of 50 W, 60 W and 70 W on the martensite variant microstructures and mechanical properties of the LPBFed SMAs was investigated. Interestingly, all LPBFed NiTiHf samples display enhanced thermal cycle stability of transformation temperatures with a temperature eviation of −3°C, which is attributed to the lack of dislocation slip originated from the habit plane configuration characteristic with multidomain (001) martensite compound twins. Meanwhile, all LPBFed NiTiHf samples exhibit trained one-way and two-way shape memory effects due to the dislocation slip upon compression deformation at room temperature.

Functional Properties of a Ni-rich Ni–Ti–Hf Shape Memory Alloy Fabricated via Laser Beam Powder Bed Fusion—Impact of Porosity and Precipitation Characteristics on the Thermal Hysteresis

Additive manufacturing (AM) is very promising for the fabrication of complex parts made from shape memory alloys (SMAs). In the present study, a Ni–Ti–Hf shape memory alloy has been processed by laser beam powder bed fusion of metals (PBF-LB/M). Employing different sets of processing parameters, i.e., a variation of scanning speed, specimens characterized by various microstructures and porosities were obtained. Microstructural analysis revealed that processing of Ni–Ti–Hf SMAs at a low energy level promotes the formation of a fine-grained microstructure with numerous lack of fusion defects. Transmission electron microscopy (TEM) studies revealed that H-phase precipitates are present in any case, leading to an increase in the Ms-temperature and, thus, a phase transformation at room temperature. The thermal stability of the phase transformation behavior in different Ni–Ti–Hf as-built conditions was studied using differential scanning calorimetry (DSC). DSC analysis showed that specimens with a higher defect density are characterized by smaller thermal hysteresis.

Fracture toughness and fatigue crack growth resistance of precipitate-free and precipitation hardened NiTiHf shape memory alloys

The present work investigates fracture toughness, and actuation and mechanical fatigue crack growth responses of Ni50.3Ti29.7Hf20 HTSMAs across martensitic transformation with two different microstructures, one with H-phase nanoprecipitates and one without. H-phase precipitation is known to stabilize the actuation cycling response of NiTiHf HTSMAs and notably impacts transformation-induced plasticity. The fracture toughness tests performed reveal that precipitate-free NiTiHf has a higher fracture toughness and undergoes significantly more inelastic deformation than the one with the precipitates resulting in toughness enhancement, i.e., stable crack advance during fracture toughness experiments, which is not observed in the precipitated NiTiHf for the crack configuration and loading conditions tested. Furthermore, the precipitate free NiTiHf has higher actuation and mechanical fatigue crack growth resistance than the precipitation-hardened microstructure. This is attributed to plasticity buildup, which exacerbates the manifestation of retained martensite upon repeated transformations. The fatigue crack growth rates obtained from both actuation and mechanical fatigue experiments align to a single Paris Law Curve for the precipitation-hardened NiTiHf. This work aims to determine if unified Paris Law curves can be generated from mechanical and actuation fatigue experiments, irrespective of composition and microstructure, to estimate actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.

Stress-assisted atomic diffusion in NiTiHf shape memory alloys

Stress-assisted atomic diffusion behavior in NiTiHf shape memory alloys was investigated by measuring the internal friction at temperature range of 300–1070 K. A thermally activated internal friction peak was recorded in the B2-ordered phase in the temperature range of about 850–950 K (at 1 Hz, depending on alloy composition), with activation energy of 1.79 ± 0.05 and 2.00 ± 0.10 eV and relaxation time of 2.7−2+1∙10−12 and 1.7−5+1∙10−13 s for Ni50Ti35Hf15 and Ni50Ti30Hf20 alloys, respectively. It was proposed that the short-range diffusion of Ni-atoms under the stress is the main relaxation mechanism in the studied alloys. The diffusion coefficients of Ni in the B2-ordered phase were estimated to be 3.72·10−8 and 5.90·10−7 m2s−1 for Ni50Ti35Hf15 and Ni50Ti30Hf20 alloys, correspondingly.

Predicting Transformation Temperatures of Additively Manufactured NiTiHf Shape Memory Alloy Using Neural Network Modeling

Predicting Transformation Temperatures of Additively Manufactured NiTiHf Shape Memory Alloy Using Neural Network Modeling

Load-induced local phase transformation and modulus of shape memory alloys under spherical indentation by finite element method

Shape memory alloys are a unique class of materials that are capable of large reversible deformations under external stimuli such as stress or temperature. The present study examines the phase transformations and mechanical responses of NiTi and NiTiHf shape memory alloys under the loading of a spherical indenter by using a finite element model. It is found that the indentation unloading curves exhibit distinct changes in slopes due to the reversible phase transformations in the SMAs. The normalized contact stiffness (F/S2) of the SMAs varies with the indentation load (depth) as opposed to being constant for conventional single-phase materials. The load-induced phase transformation that occurred under the spherical indenter was simulated numerically. It is observed that the phase transformation phenomenon in the SMA induced by an indentation load is distinctly different from that induced by a uniaxial load. A pointed indenter produces a localized deformation, resulting in a stress (load) gradient in the specimen. As a result, the transformation of phases in SMAs induced by an indenter can only be partially completed. The overall modulus of the SMAs varies continuously with the indentation load (depth) as the average volumetric fraction of the martensite phase varies. For NiTi (Ea > Em), the modulus decreases with the depth, while for NiTiHf (Ea < Em), the modulus increases with the depth. The predicted young modules during indentation modeling agree well with experimental results. Finally, the phase transformation of the SMAs under the indenter is not affected by the post-yield behavior of the materials.

An interatomic potential for ternary NiTiHf shape memory alloys based on modified embedded atom method

NiTi-based high temperature shape memory alloys (HTSMAs) can operate in high temperature applications, where binary NiTi cannot function. NiTiHf is among the most popular HTSMAs due to its lower preparation cost and good thermal stability. Various aspects of material properties of NiTiHf and the effect of different factors, such as composition, heat treatment and precipitates, on its behavior is yet to be understood. Molecular dynamics (MD) simulations can provide insights to acquire such an understanding. However, MD simulations of NiTiHf have not been possible due to the absence of a reliable interatomic potential, which is a necessary tool for such simulations. In this study, density functional theory (DFT) simulations were employed and a ternary Second Nearest Neighbor Modified Embedded Atom Method (2NN MEAM) potential was calibrated for NiTiHf. To perform this study, the related unary and binary potentials were calibrated by fitting the values of their reproduced physical properties to the DFT results. The final ternary MEAM potential was checked for reliability and transferability by performing MD simulations. The results showed that the developed potential can appropriately capture the temperature-induced and stress-induced martensitic phase transformations in NiTiHf.

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.

Strain glass transition in Nb nanowire toughened NiTiHf shape memory alloy composite wires

In this work, we successfully prepared Nb nanowire toughened NiTiHf composite wires with different diameters. The strain glass transition was found in the as-drawn 0.1 mm diameter wire characterized by the Vogel-Fulcher type frequency-dependent modulus anomaly. However, the strain glass transition was not detected in 0.3 mm diameter wires. Microstructural characterization reveals that densely distributed Nb nanowires with small diameter and discontinuous orientation impose both physical confinement and heterogeneous stress fields on the matrix, which interrupt the long-range ordered martensitic transformation, leading to a continuous, high-order like strain glass transition. This work provides a new design strategy for searching new strain glass systems.

Machine learning guided alloy design of high-temperature NiTiHf shape memory alloys

With the increasing use of CubeSats in space exploration, the demand for reliable high-temperature shape memory alloys (HTSMA) continues to grow. A wide range of HTSMAs has been investigated over the past decade but finding suitable alloys by means of trial-and-error experiments is cumbersome and time-consuming. The present work uses a data-driven approach to identify NiTiHf alloys suitable for actuator applications in space. Seven machine learning (ML) models were evaluated, and the best fit model was selected to identify new alloy compositions with targeted transformation temperature (Ms), thermal hysteresis, and work output. Of the studied models, the K-nearest neighbouring ML model offers more reliable and accurate prediction in developing NiTiHf alloys with balanced functional properties and aids our existing understanding on compositional dependence of transformation temperature, thermal hysteresis and work output. For instance, the transformation temperature of NiTiHf alloys is more sensitive to Ni variation with increasing Hf content. A maximum Ms reduction rate of 6.12 °C per 0.01 at.% Ni is attained at 30 at.% Hf, and with a Ni content between 50 and 51 at.%.Graphical abstract

Neural Network Modeling of NiTiHf Shape Memory Alloy Transformation Temperatures

Data-driven techniques are used to predict the transformation temperatures (TTs) of NiTiHf shape memory alloy. A machine learning (ML) approach is used to overcome the high-dimensional dependency of NiTiHf TTs on numerous factors, as well as the lack of fully known governing physics. The elemental composition, thermal treatments, and post-processing steps that are commonly used to process NiTiHf and have an impact on the material phase transitions are used as input parameters of the neural network model (NN) to design the TTs. Such a feature selection led to the use of most of the accessible information in the literature on NiTiHf TTs, as all processing features required to be fed into the NN model. Considering most of the regular NiTiHf processing factors also enables the option of tuning additional characteristics of NiTiHf in addition to the TTs. The work is unique as all the four main TTs and their associated peak transformation temperatures are predicted to have complete control over the material phase change thresholds. Since 1995, extensive experimental research has been conducted to design NiTiHf TTs with a large temperature range of around 800 °C, paving the path for the current work’s ML algorithms to be fed. A thorough data collection is created using both unpublished data and available literature and then analyzed to select twenty input parameters to feed the NN model. To forecast the NiTiHf TTs, a total of 173 data points were gathered, verified, and selected. The model's overall determination factor (R2) was 0.96, suggesting the viability of the proposed NN model in demonstrating the link between material composition and processing factors, as well as identifying the TTs of NiTiHf alloy. The effort additionally validates the generated results against existing data in the literature. The validation confirms the significance of the proposed model.

Design of a NiTiHf shape memory alloy with an austenite finish temperature beyond 400 °C utilizing artificial intelligence

This paper details the design process of a ternary NiTiHf shape memory alloy (SMA) with an austenite finish temperature (Af) beyond 400 °C. Specifically, available experimental data on the ternary NiTiHf SMA system was utilized to construct a database, which was employed to train and test a machine learning (ML) algorithm to predict the ideal NiTiHf SMA composition to exhibit an Af beyond 400 °C and a relatively smaller hysteresis. For this purpose, a multi-layer feedforward neural network (MLFFNN) model was proposed, trained, and tested. Consequently, the Ni49.7Ti26.6Hf23.7 and Ni50Ti27Hf23 alloys predicted by this ML algorithm were selected for validation experiments to assess the accuracy of the ML model’s predictions. As a result, the Ni49.7Ti26.6Hf23.7 alloy with an Af temperature of 403.5 °C and remarkable cyclic stability was established as a new NiTiHf SMA composition, which can be utilized in applications demanding reversible austenite-to-martensite phase transformation beyond 400 °C.

NiTiHf shape memory alloys as phase change thermal storage materials

Motivated by the recent advancements demonstrating the effectiveness of NiTi shape memory alloys (SMAs) as high figure of merit (FOM) phase change materials (PCMs) for thermal management and storage, NiTiHf SMAs were explored as candidate solid-solid PCMs with high temperature capability. Differential scanning calorimetry and Archimedes’ method were used to determine the transformation temperatures, thermal hysteresis, enthalpy of transformation, and density of several different NiTiHf SMAs with varying compositions. Ni50.3Ti29.7Hf20 (at. %) demonstrated a high transformation enthalpy of 32.5 J/g, a relatively low thermal hysteresis of 31°C and a thermal conductivity of 11.19 Wm−1K−1 in the austenite phase. In general, NiTiHf SMAs exhibited FOM values an order of magnitude higher than traditional PCMs and up to about 120 % higher than the FOM value measured in binary NiTi SMAs. The clear trends relating transformation temperatures, transformation enthalpy, and thermal hysteresis to composition presented here provide for tunability of NiTiHf SMAs to specific thermal energy storage applications through composition control. High FOM values combined with transformation temperatures surpassing 500°C allows NiTiHf alloys to populate previously empty regions of FOM vs. transformation temperature property space for high FOM PCMs.

Machined helical springs from NiTiHf shape memory alloy

Machined helical springs were fabricated from rod stock using a Ni-rich Ni50.3Ti29.7Hf20 (at. %) high-temperature shape memory alloy. Springs with three different spring rates of 35, 87.5, and 175.1 N mm−1 were designed, and the rectangular cross-section spring formulas were used to approximate the deflection and shear stresses. Springs were then subjected to isothermal testing at room temperature and constant-force thermal cycling experiments at varying loads. Actuation displacements on the order of 19 mm were obtained for starting flexures of 20 mm in length, resulting in nearly 95% total actuation extension. Thermal cycling under loads ranging from 0 to ∼500 N (stresses of 0 to ∼500 MPa) showed higher actuation displacements for springs with the lowest spring rates compared to the stiffer, higher spring rate counterpart. With increasing applied load, actuation displacements were still increasing even at stresses near 500 MPa (no maxima reached), indicating more actuation potential at higher loads. Two-way shape memory effect was assessed at 0 load and found to be very minimal (less than 3 mm) due to lack of training.

Tuning the thermal cyclic stability of martensitic transformation in Ni50.3Ti29.7Hf20 high temperature shape memory alloy

Thermal stimulus-responsive NiTi-based shape memory alloys, which undergo reversible, diffusion-less martensitic phase transformation, have emerged as important functional materials. Amongst them, Ni-rich NiTiHf high temperature shape memory alloys exhibit martensitic phase transformation temperatures well above 100 °C, making them suitable for high temperature applications. However, during repeated thermal cycles their transformation temperatures show instabilities which limit their practical applications. Here, we perform thermal cycling experiments at fixed as well as variable heating/cooling rates on the Ni50.3Ti29.7Hf20 alloy which is heat-treated at different temperatures. We successfully minimized the variations in phase transformation temperatures to an acceptable range by aging the alloy below 500 °C for 3 h. We further show that this minimization correlates well with high activation energy required for the phase transformation elucidating the physical reason behind this observation. Our results pave the way forward for designing Ni-rich NiTiHf shape memory alloys with a stable functional response.

Fabrication and Characterization of Freestanding NiTi Based Thin Film Materials for Shape Memory Micro-actuator Applications

Thin film shape memory actuators fabricated by micro-system technology processes are of particular interest in micro-actuator applications due to their high work output, large displacements, and a comparably easy and compact system setup. Micro-system technology processes allow to deposit and structure ultra-precise parts from a wide range of alloys, among them alloys that are difficult to process with conventional fabrication techniques. However, sputtered NiTi based thin films are seldom deposited with thicknesses > 10 µm, which limits their range of application as well as makes the handling of freestanding films difficult. In this work, freestanding NiTi, NiTiCu, and NiTiHf shape memory structures were fabricated by means of sputtering, lithography, and wet etching, with a process that allows for thicknesses up to 80 µm. Their high cycle actuation behavior and microstructure were characterized. NiTiCu actuators show an excellent cyclic stability, resulting in high fatigue lives of > 108 cycles at 1.5% strain, even for stresses as high as 550 MPa. A distinct martensite/austenite interface is observed during transformation, in contrast to NiTi exhibiting a rather homogeneous transformation during heating and cooling throughout the sample volume. Sputtered NiTiHf actuators tested in air can reach fatigue lives as high as 1.5 × 106 cycles at 1% strain.

Effects of thermo-mechanical processing on precipitate evolution in Ni-rich high temperature shape memory alloys

NiTiHf-based high temperature shape memory alloys have attracted considerable attention for their high temperature (>115 °C) reversible phase transformations. Additionally, the shape memory response of the material can be tailored based on microstructure. In the case of Ni-rich NiTiHf shape memory alloys, this response can be tuned with the formation of a nano-phase that can increase the transformation temperature as well as improve mechanical properties. However, NiTiHf-based shape memory alloys are not without their drawbacks, such as difficulty in processing and controlling of nano-precipitation. In this study, the thermo-mechanical processability of a Ni-rich NiTiHf high temperature shape memory alloy, Ni50.5Ti34.5Hf15, is presented. Samples are hot-rolled to 25% and 50% reductions and then further heat treated at 550 and 750 °C to examine the formation of H-phase nano-precipitates. The size and morphology of the nano-precipitates is examined using synchrotron radiation X-ray diffraction during in situ heat treatments and compared with high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) results from ex situ heat treated samples. Changes in transformation temperatures due to the thermo-mechanical processing are determined from differential scanning calorimetry measurements. Results from this study show that SR-XRD can be used to estimate the size and observe the growth and coarsening behavior of H-phase nano-precipitates.

Microstructure Analysis and Thermal Characteristics of NiTiHf Shape Memory Alloy with Different Composition

Improving shape memory alloys in term of microstructural analysis is important for some related applications. In this study, three different NiTiHf shape memory alloys were produced from high pure elements and by utilizing arc-melting device. The morphologies of different precipitations and microstructures were analyzed by optical microscope, mapping, and EDX compositional analysis. The DSC measurements showed that the phase transformation temperatures is increased by adding more hafnium instead of nickel content in NiTiHf alloy, however, the enthalpy change for both endo/exothermic process were decreased. Also, XRD analysis has been carried out for all alloys to find different phases, and to obtain grain size as a function of different composition of Ni and Hf elements. The oxidation shows that titanium dioxide (TiO2) is spread out over the surface, but the microstructures are completely different for each case, e.g. needle-like shapes (nanorods) has extended over the surface of Alloy A with 3 at% Hf, but plate-like Ti-rich microstructures is dominated on the surface of Alloy B that has twice as more Hf as Alloy A; on the other hand, Alloy C with 9 at% Hf has some precipitates with TiO phase.

Scale-up of NiTiHf shape memory alloy tubes with high torque capability

High-torque, NiTiHf high-temperature shape memory alloy tubes were examined by means of constant-torque thermal cycling experiments. The alloy, with a Ni-rich composition of Ni50.3Ti29.7Hf20 (at%), was hot worked via extrusion from large ingots to produce final rods with an outer diameter of 25.4 mm. After aging at 550 °C for 3 h and air-cooling, the manufactured torque tubes were subjected to shear stresses from 0 to 500 MPa (torques of 0–1400 N m) and thermally cycled 20 times under stress, and twice after stress removal to evaluate the two-way shape memory effect. A transformation shear strain of 6% was obtained, and a maximum work output of 27.5 J cm−3. Two-way shear strains approached 3% strain without any extended training or cycling. The strain values obtained for the first time in this large-diameter, high-torque tube consistently matched smaller diameter tubes previously reported. This highlights the successful production and scale-up of both the material and the component level in torque tubes without degrading the actuation response, while providing much higher torque capability.

The effects of wide range of compositional changes on the martensitic transformation characteristics of NiTiHf shape memory alloys

The effects of composition on the martensitic transformation characteristics of NiTiHf shape memory alloys are systematically explored. Transformation temperatures (TTs) demonstrate a non-monotonic dependence on Ni and Hf concentrations. For Ni > 50 at%, TTs decrease with small Hf additions, attain a minimum or are completely suppressed, depending on Ni content, followed by a significant increase with Hf content. This initial drop in TTs becomes more pronounced with higher Ni contents and is attributed to the increase in nickel anti-site defects along with bulky substitutional Hf contributing to the increase in local lattice distortions along with the re-distribution of Ti and Hf.