Ni-rich NiTiHf Research Articles

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.

Atomistic Simulation of the Effect of H-Phase Precipitate on the Transformation Temperatures and Stress-Induced Phase Transformation in Ni-Rich NiTiHf

Precipitation hardening is considered the most feasible method for strengthening NiTiHf alloys. In order to design the optimum aging treatment to form precipitates, it is crucial to understand the effect of precipitates on the thermomechanical behavior of these alloys. In this research, the effect of H-phase precipitates was studied on the martensitic and superelastic behavior of Ni-rich NiTiHf. Using atomistic simulations, two scenarios for formation of precipitates, resembling the short and long aging time of the alloy, were considered. In the first case a single and large precipitate was embedded into the center of NiTiHf matrix, and in the second case eight fine precipitates were inserted into the model. Upon the calculation of the transformation temperatures, the models with precipitates showed higher austenite start and finish temperatures. Moreover, by simulating the stress-induced phase transformation, it was found that the presence of fine precipitates inhibits the formation of different martensite variants leading to smaller transformation strains.

Order of magnitude increase in actuation fatigue lifetime through partial austenitic transformation of NiTiHf high-temperature shape memory alloys

The higher transformation temperatures, strength, and thermomechanical stability of NiTiHf high-temperature shape memory alloys (HTSMAs) are attractive for use in solid-state actuation. Maximizing the actuation fatigue lifetime whilst preventing deterioration of actuation stroke during service are the two main challenges facing the widespread application of HTSMAs. Much research has focused on optimizing composition and processing to enhance performance; however, this has proven challenging to implement on a large scale. Simpler solutions involve optimizing the actuation environment to enhance performance. One of the most effective methods to both extend the lifetime and stabilize actuation strain is through partial thermal cycling. This method is more representative of how actuators are employed, as very rarely are they repeatedly cycled to their maximum output. Previous studies have shown partial transformation extends fatigue lifetime in low-temperature NiTi and NiTiCu SMAs, but it has yet to be conclusively demonstrated in the NiTiHf system. The present work is an extensive analysis of the effects of heating limited partial cycling compared to full cycling actuation fatigue of Ni50.3Ti29.7Hf20 HTSMA. The results confirm an order of magnitude increase in actuation fatigue lifetime with less transformation per cycle in partially heated Ni-rich NiTiHf samples. More importantly, keeping the actuation work output almost the same, controlling the actuation strain level is shown to be more effective in increasing the fatigue life than reducing the actuation stress level, yielding 5 to 10 times relative improvement in the fatigue life. The underlying microstructural evolution resulting in the observed enhanced response is provided.

The effect of cooling on machining and phase transformation responses of Ni-rich NiTiHf high-temperature shape memory alloy

A Ni-rich Ni50.3Ti29.7Hf20 (at. %) high-temperature shape memory alloy was machined under different cooling conditions, specifically cryogenic cooling and flood cooling, and results were compared with those from dry machining. The effects of cooling on machinability measures such as tool wear, cutting forces and surface quality were evaluated for various cutting speeds. Microhardness, phase transformation temperatures and latent heat of transformation corresponding to the machined specimens were determined using hardness and differential scanning calorimetry tests. Cryogenic cooling was found to notably improve the machinability of the Ni-rich NiTiHf alloy by reducing tool wear, cutting force and surface roughness. This study provides clear indication that the machining process alters the phase transformation temperature of the Ni-rich NiTiHf alloy. After the machining process, austenite finish temperature of specimens increased by 5–10% depending on the machining conditions. The smallest amount of latent heat required for transformation (ΔH) was obtained for each of the two specimens: 2.33 J/g (exothermic) for the cryogenically machined specimen and 3.38 J/g (exothermic) for the specimen machined under flood cooling.

Multi-objective Optimization of Cutting Parameters for Machining Process of Ni-Rich NiTiHf High-Temperature Shape Memory Alloy Using Genetic Algorithm

Multi-objective Optimization of Cutting Parameters for Machining Process of Ni-Rich NiTiHf High-Temperature Shape Memory Alloy Using Genetic Algorithm

Effect of Nickel Content on Processing of Ni-Rich NiTiHf High-Temperature Shape Memory Alloys

Effect of Nickel Content on Processing of Ni-Rich NiTiHf High-Temperature Shape Memory Alloys

Rolling contact fatigue deformation mechanisms of nickel-rich nickel-titanium-hafnium alloys

The deformation mechanisms that dictate the tribological performances of heat treated Ni55Ti45, Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys are revealed through rolling contact fatigue (RCF) testing and transmission electron microscopy (TEM) analyses. Analysis of worn samples that passed a 5×108 cycle RCF runout condition shows that damage is primarily confined to deformation bands that propagate several hundred nanometers – to – several microns beneath the surface. These bands nucleate via localization of dislocation slip within the B2 austenite phase of the alloys. For the Ni55Ti45 and Ni54Ti45Hf1 samples, further damage and eventual spall failures occur by shearing and dissolution of the strengthening Ni4Ti3 nanoprecipitates within the deformation bands, followed by nanocrystallization that sometimes includes stress-induced nucleation of B19′ nanocrystals. Eventually, complete amorphization occurs prior to fracture. The relative 15 – 25% increase in RCF contact stress performance of the Ni56Ti36Hf8 alloy correlates with a more limited depth of damaged material beneath the wear surface; heavily damaged material beneath spall failure sites only extends 1.5 µm into the sample, compared to > 6 µm for Ni55Ti45 and Ni54Ti45Hf1 alloys. This superior RCF damage resistance of the Ni56Ti36Hf8 alloy results from a lower fraction of B2 matrix phase (≤ 13 %) that is highly confined by both cubic Ni-rich NiTiHf and H-phase nanoprecipitates. Specifically, this microstructure limits the widths of deformation bands within the B2 austenite phase to less than the size of the strengthening nanoprecipitates, which in turn inhibits precipitate shearing and dissolution processes that precede nanocrystallization and amorphization.

Laser Powder Bed Fusion of NiTiHf High-Temperature Shape Memory Alloy: Effect of Process Parameters on the Thermomechanical Behavior

Laser powder bed fusion has been widely investigated for shape memory alloys, primarily NiTi alloys, with the goal of tailoring microstructures and producing complex geometries. However, processing high temperature shape memory alloys (HTSMAs) remains unknown. In our previous study, we showed that it is possible to manufacture NiTiHf HTSMA, as one of the most viable alloys in the aerospace industry, using SLM and investigated the effect of parameters on defect formation. The current study elucidates the effect of process parameters (PPs) on the functionality of this alloy. Shape memory properties and the microstructure of additively manufactured Ni-rich NiTiHf alloys were characterized across a wide range of PPs (laser power, scanning speed, and hatch spacing) and correlated with energy density. The optimum laser parameters for defect-free and functional samples were found to be in the range of approximately 60–100 J/mm3. Below an energy density of 60 J/mm3, porosity formation due to lack-of-fusion is the limiting factor. Samples fabricated with energy densities of 60–100 J/mm3 showed comparable thermomechanical behavior in comparison with the starting as-cast material, and samples fabricated with higher energy densities (>100 J/mm3) showed very high transformation temperatures but poor thermomechanical behavior. Poor properties for samples with higher energies were mainly attributed to the excessive Ni loss and resultant change in the chemical composition of the matrix, as well as the formation of cracks and porosities. Although energy density was found to be an important factor, the outcome of this study suggests that each of the PPs should be selected carefully. A maximum actuation strain of 1.67% at 400 MPa was obtained for the sample with power, scan speed, and hatch space of 100 W, 400 mm/s, and 140 µm, respectively, while 1.5% actuation strain was obtained for the starting as-cast ingot. These results can serve as a guideline for future studies on optimizing PPs for fabricating functional HTSMAs.

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.

Effect of Hf solute addition on the phase transformation behavior and hardness of a Ni-rich NiTi alloy

Hf substituted NiTi alloys are crucial for high temperature shape memory applications owing to their martensitic phase transformation temperatures around 100°C. For further high temperature applications above this limit, a balanced combination of high phase transformation temperatures, high strength and low thermal hysteresis is the foremost criterion. Here, we show that substitution of an array of Hf concentrations (5–25 at.%) to Ni50.3Ti49.7 alloy leads to a wide range of phase transformation temperatures from sub-zero to 280°C. Our studies establish complete solid solution solubility of Hf in the Ni50.3Ti49.7 alloy up to 25 at.% Hf. As a result, the hardness Ni50.3Ti49.7 alloy increases linearly with Hf concentration on account of solid solution strengthening effect. We further show that the Ni50.3Ti29.7Hf20 alloy exhibits least thermal hysteresis combined with high phase transformation temperatures and hardness amongst the compositions investigated. Our study paves the way for designing Ni-rich NiTiHf high temperature shape memory alloys, with operating temperatures up to 250°C and thermal hysteresis as low as 30°C.

Microstructural and Thermomechanical Comparison of Ni-Rich and Ni-Lean NiTi-20 at.% Hf High Temperature Shape Memory Alloy Wires

The processability of Ni-rich Ni50.8Ti29.2Hf20 and Ti–rich Ni49.8Ti30.2Hf20 (at.%) high temperature shape memory alloy wires was investigated. Hot-extruded NiTiHf rods with an initial diameter of 6.35 mm, were hot-rolled (HR) at 750, 775, 800, or 825 °C to a square cross section with ~ 1.9 mm side lengths, over a series of 33 non-consecutive hot passes. The thermomechanical behavior of HR, solutionized, and aged NiTiHf wires was examined with thermal cycling using differential scanning calorimetry and tensile testing to assess the viability of the hot-rolling process. The ideal rolling temperature for Ni-rich NiTiHf wires lies just over the H-phase dissolution temperature (near 800 °C), to avoid wire embrittlement caused by H-phase precipitates and to limit compositional instability at the wire surface caused by HfO2 internal oxidation. The characteristic oxide layers formed during the rolling process and subsequent heat treatments were compared across rolling temperatures and compositions to address several challenges associated with NiTiHf wire processing.

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.

Role of microstructure on the actuation fatigue performance of Ni-Rich NiTiHf high temperature shape memory alloys

The focus of the present study was to systematically investigate the influence of microstructure on the actuation fatigue performance of a Ni-rich NiTiHf high temperature shape memory alloy (HTSMA). Different aging heat treatments led to the formation of H-phase nano-precipitates with different sizes and morphology within the matrix, enhancing thermo-mechanical stability and enabling control of transformation temperatures. The actuation fatigue testing of specimens was performed until failure through thermally-induced reversible martensitic transformation under a constant stress (300 MPa) with two distinct upper cycle temperatures (UCT) of 300 °C and 350 °C. Consequently, the specimens heated to 700 °C and furnace cooled to 100 °C in 48 h, with relatively large precipitates, failed at average fatigue life of 15,500 cycles and exhibited 1.0% average actuation strain in the 300 °C UCT experiments, while those aged at 550 °C for 3 h, with the precipitate sizes less than 20 nm, attained an average fatigue life of 10,800 cycles and an actuation strain of 2.5%. Samples with intermediate precipitate sizes after aging at 600 °C for 10 h failed at the shortest average fatigue life of 8,200 cycles with an intermediate average actuation strain of 2.0%. Furthermore, increase in UCT decreased the fatigue life and resulted in larger average actuation strains for all samples. Overall, the current findings constitute the first systematic results demonstrating the microstructure dependence of the evolution of actuation strain, irrecoverable strain, and transformation temperatures during actuation fatigue, and actuation fatigue life of the Ni-rich Ni50.3Ti29.7Hf20 HTSMA, demonstrating the importance of controlling the H-phase precipitate size.

Machinability of Ni-rich NiTiHf high temperature shape memory alloy

Machining of Ni-rich Ni50.3Ti29.7Hf20 (at%) high temperature shape memory alloys was evaluated under flood cooling conditions. Effects of various cutting speeds (between 20 and 120 m min−1), depth of cuts (between 0.4 and 1 mm), and the associated cutting forces on the work-piece surface quality and tool wear of this alloy are presented. Experimental data demonstrates that abrasion and adhesion were the predominant wear mechanisms leading to extreme wear in a short machining time. Thermal softening mechanism dominated the trend of cutting forces until 95 m min−1 cutting speed. Beyond this speed, force components sharply increased resulting from extreme tool wear. Chip breakability of this alloy is much better when compared to NiTi shape memory alloy.

Viable low temperature shape memory alloys based on Ni-Ti-Hf formulations

Ni-rich NiTiHf alloys with Hf content from 1 to 12.5 at.% were evaluated for their potential as low temperature shape memory alloys. At 51 at.% Ni, a minimum in transformation temperature was observed at 8 at.% Hf with a martensite finish temperature of −140 °C. Beyond 8 at.% Hf, transformation temperatures started to rise. Increasing Ni content from 50.3 to 52 at.% resulted in a resilient alloy with remarkable dimensional stability and transformation strain in the range of 3% in compression. Work outputs above 30 J/cm3 in some of these alloys attest to their suitability for low temperature actuator applications.

High-energy synchrotron radiation X-ray diffraction measurements during in situ aging of a NiTi-15 at. % Hf high temperature shape memory alloy

NiTiHf high temperature shape memory alloys (HTSMAs) have begun to show considerable progress in their ability to be manufactured and are now on the cusp of becoming viable alloys for use in the aerospace industry, mainly as actuation devices. By aging the alloys at intermediate temperatures between 450 and up to 700 °C, Ni-rich NiTiHf HTSMAs form the H-phase, a Ni-rich nanoscale precipitate which can be used to strengthen the alloy and improve the shape memory response; however, precipitate phase evolution and growth has not yet been studied in-situ nor have the effects of previous thermo-mechanical processing on their formation. In this study, H-phase precipitate formation is observed experimentally in situ during aging of a Ni-rich NiTiHf alloy using high energy synchrotron radiation X-ray diffraction (SR-XRD) measurements. The effects of prior thermo-mechanical treatments, i.e. hot and cold rolling, on its formation rate and coarsening rate are compared. It is shown that growth of the H-phase can be observed in situ using SR-XRD and the H-phase formation rate seems to exhibit linear growth when aging at 550 °C, while increasing the temperature to 650 °C shows a logarithmic trend that plateaus after approximately 1 h of aging. Other properties such as incubation time, austenite lattice strains, texture evolution, and overall comparison between hot and cold rolling are discussed. New potential peak age conditions are suggested based on the diffraction data and evaluated using three-point bending and TEM imaging.

Laser welding of precipitation strengthened Ni-rich NiTiHf high temperature shape memory alloys: Microstructure and mechanical properties

High temperature shape memory alloys are currently attracting significant attention by the aerospace industry due to the potential use of shape memory and superelastic properties at temperatures above 100 °C. Virtually any advanced engineering material must, at some point, be joined either to itself, to create complex shaped structures, or to other materials to increase its potential applications. In this work, laser welding of a precipitation strengthened Ni-rich NiTiHf high temperature shape memory alloy is reported for the first time. Starting with a base material aged at 500 °C for 3 h and air cooled, defect-free joints with a conduction weld mode were obtained. Microstructural characterization, facilitated via microscopy and synchrotron X-ray diffraction, revealed that the fusion zone contained a single-phase martensitic structure at room temperature, compared to a mixture of martensite and H-phase precipitates in the base material. Isothermal loading in both the martensite (at 30 °C) and austenite (at 200 °C) phases revealed equivalent strength and near-perfect superelasticity in the welded and un-welded reference material.

H-Phase Precipitation and Martensitic Transformation in Ni-rich Ni–Ti–Hf and Ni–Ti-Zr High-Temperature Shape Memory Alloys

The distributions of H-phase precipitates in Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys formed by aging treatments at 500 and 550 °C or slow furnace cooling and their effects on the thermal martensitic transformation have been investigated by TEM and calorimetry. The comparative study clearly reveals faster precipitate-coarsening kinetics in the NiTiZr alloy than in NiTiHf. For precipitates of a similar size of 10–20 nm in both alloys, the martensite plates in Ni50.3Ti29.7Zr20 have larger widths and span a higher number of precipitates compared with the Ni50.3Ti29.7Hf20 alloy. However, for large H-phase particles with hundreds of nm in length, no significant differences in the martensitic microstructures of both alloy systems have been observed. The martensitic transformation temperatures of Ni50.3Ti29.7Hf20 are ~ 80–90 °C higher than those of Ni50.3Ti29.7Zr20 in the precipitate-free state and in the presence of large particles of hundreds on nm in length, but this difference is reduced to only 10–20 °C in samples with small H-phase precipitates. The changes in the transformation temperatures are consistent with the differences in the precipitate distributions between the two alloy systems observed by TEM.

Relationship between crystallographic compatibility and thermal hysteresis in Ni-rich NiTiHf and NiTiZr high temperature shape memory alloys

The relationship between the crystallographic compatibility of austenite and martensite phases and the transformation thermal hysteresis (ΔT) of Ni-rich Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys undergoing B2–B19′ martensitic transformation was studied as a function of microstructure, via differential scanning calorimetry, transmission electron microscopy, and X-ray diffraction. An experimental linear relationship of ΔT vs λ2 (the second eigenvalue of the transformation stretch matrix) was observed for these NiTi(Hf/Zr) alloys, but with a shallower slope as compared to the universal behavior followed by alloys showing B2–B19 martensitic phase transformation. Several ternary NiTiCu and binary NiTi alloys undergoing the B2–B19′ transformation were also found to deviate from the universal behavior attributed to alloys that undergo a B2–B19 transformation, and instead, follow the trend revealed for the present alloy systems. Aged NiTi(Hf/Zr) samples, which consist of very fine nano-precipitates, followed the newly established ΔT vs λ2 linear relationship, due to only minor changes in the microstructure. In contrast, samples with large precipitates, exhibited a large deviation from this relationship due to much more drastic changes in microstructure. Finally, despite the poor crystallographic compatibility of the austenite and martensite lattices observed in the present alloys, rationalized by large deviation of λ2 values from 1, relatively low ΔT values were measured. This behavior is actually consistent with the newly established relationship for ΔT vs λ2 for B2–B19′ transforming alloys, which is much less sensitive to compatibility (shallower slope). It is concluded that such a difference in the ΔT vs λ2 slope must be a consequence of the crystallography of monoclinic martensite formation in NiTi-based alloys as long as other factors such as plasticity or major constraints to the martensitic transformation do not intervene.

Effects of Ni content on the shape memory properties and microstructure of Ni-rich NiTi-20Hf alloys

Shape memory properties and microstructure of four Ni-rich NiTiHf alloys (Ni50.3Ti29.7Hf20, Ni50.7Ti29.3Hf20, Ni51.2Ti28.8Hf20, and Ni52Ti28Hf20 (at.%)) were systematically characterized in the furnace cooled condition. H-phase precipitates were formed during furnace cooling in compositions with greater than 50.3Ni and the driving force for nucleation increased with Ni content. Alloy strength increased while recoverable strain decreased with increasing Ni content due to changes in precipitate characteristics. When the precipitates were small (∼5–15 nm), they were readily absorbed by martensite plates, which resulted in maximum recoverable strain of 2% in Ni50.7Ti29.3Hf20. With increasing Ni content, the size (>100 nm) and volume fraction of precipitates increased and the growth of martensite plates was constrained between the precipitates when the Ni concentration was greater than 50.7 at.%. Near perfect dimensional stability with negligible irrecoverable strain was observed at stress levels as high as 2 GPa in the Ni52Ti28Hf20 alloy, though the recoverable strain was rather small. In general, strong local stress fields were created at precipitate/matrix interphases, which lead to high stored elastic energy during the martensitic transformation.