NiTiHf Alloy Research Articles

NiTiHf Actuation Fatigue: How Much Do We Really Know?

Abstract The NiTiHf alloy is one of the most extensively studied high-temperature shape memory alloys. This article reviews the key factors affecting the actuation fatigue performance of NiTiHf as well as the current state of research in this area, and it identifies some factors that require further study.

A comparative study of the microstructure and shape memory behaviour of NiTi, NiTiHf, and Pt-added NiTiHf alloys

The microstructure, substructure, transformation temperature, and recovery ratio of a novel Ni45Pt 5 Ti30Hf20 alloy are reported in the current work. The properties of binary TiNi and ternary TiNiHf alloys are also characterized to establish a comparative basis for shape memory behaviour. The microstructure of the cast structure of Ni45Pt5Ti30Hf20 alloy showed a dendritic matrix and interdendritic eutectic-like mixture, and that of the homogenized structure consisted of martensitic matrix and secondary phase of dark contrast having the composition (Ti+Hf)2(Ni+Pt). Pt substitution of Ni in the ternary alloy with 20 at.% Hf by 5 at.% leads to a decrease in the Af, As, Ms, and Mf . Ms and Mf represent the temperatures at which austenite to martensite transformation starts and completes, respectively, while As and Af represent the temperatures at which martensite to austenite transformation starts and completes, respectively. The XRD studies showed that the structure of martensite is B19′ and TEM studies showed that the substructure of martensite in Pt modified NiTiHf alloy is similar to that of ternary NiTiHf alloy. The shape memory recovery of the Pt-modified alloy is similar to the ternary alloy at higher strength levels. The recovery ratio determined using Vickers indentation is compared with that determined using compression tests to provide an efficacy of small volume tests for screening of these alloys.

Simulating martensitic transformation in NiTi-Hf – effects of alloy composition and aging treatment

In the NiTi-Hf alloy system, coherent H-phase (orthorhombic) nanoprecipitates have been shown to have strong impact on the B2 (cubic) to B19’ (monoclinic) martensitic transformation (MT) and the corresponding stress-strain behavior. Two typical martensite-precipitate microstructures have been observed: intra-martensite precipitates (IMP) and inter-precipitate martensites (IPM). In the former, high density small H-phase nanoprecipitates are embedded in individual domains of different variants of the B19’ martensite that form a herringbone structure, while in the latter, finer twinning B19’ martensites are confined in between larger H-phase nanoprecipitates. In this study we investigate the mechanisms underlying the formation of these two distinct microstructures through computer simulations using the phase field method. In particular, we study temperature- and stress-induced MTs in the presence of H-phase nanoprecipitates, concentration variations and stress field in the B2 matrix created by the precipitation reaction, and the corresponding thermal and stress hystereses and superelasticity for different alloy compositions and aging treatments. The simulation results indicate that densely populated nanoprecipitates 7∼20 nm in length and 3∼8 nm in width lack the strength to impede the growth, coalescence and coarsening of martensitic domains, resulting in the formation of IMP. In contrast, plate-like nanoprecipitates with diameters of ∼100 nm and thickness of ∼20 nm effectively hinder the growth, coalescence and coarsening of martensitic domains, giving rise to IPM. The IPM microstructure is accompanied by an almost linear volume fraction vs. temperature curve with less than 1 °C hysteresis during thermal cycling and a slim stress-strain hysteresis as compared to that of the IMP microstructure. Therefore, thoughtful choices in alloy composition and aging treatment enable the attainment of distinctly varied precipitate and martensitic microstructures, each characterized by unique pseudoelastic or superelastic properties.

Hot deformation behavior of NiTiHf alloy under compression: Effect of deformation heating on flow softening

High-temperature shape memory alloys are suitable materials for substituting hydraulic elements in the aerospace industry due to light weight and high reactive stresses developing during thermoelastic martensitic transformation and acceptable recoverable strain. These properties may be improved by plastic deformation; however, the plastic deformation of NiTiHf alloys along with structure investigation has yet to be studied systematically. Here, the hot deformation behavior of a NiTiHf alloy with hafnium content >20 at.% has been investigated, and the processing map has been elaborated according to the dynamic material model (DMM) based on the Prasad instability criterion. The deformation heating effect has been shown to affect the flow curves and accounts for profound softening during deformation due to quasi-adiabatic deformation mode at high strain rates. NiTiHf alloys have a much higher activation energy of plastic flow, and the instability region of plastic flow is much larger compared to binary NiTi alloys. Microstructural observations have revealed that in the instability regions, oxide bands and crack nucleation occur, whereas outside this region, such defects are not observed. Electron backscatter diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the necklace-type structure, while at high strain rates, this phenomenon has not been observed. Based on the obtained results, the preferable deformation conditions are established to be 850–1000 °C and 0.003–0.05 s−1.

Microstructures, Mechanical Properties and Transformation Behavior in Ni49.6Ti35.4Hf15 Alloy Produced with High-Pressure Torsion

High-pressure torsion (HPT) was applied for the Ni49.6Ti35.4Hf15 (at.%) alloy up to 1/4, 2 and 16 turns under a pressure of 4.0 GPa. The samples were examined using X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness measurements. The results indicate that the mixture of an amorphous and nanocrystalline microstructure developed in the investigated NiTiHf alloy as the number of HPT turns was increased to two. The average hardness of the samples increased from 330 Hv to 500 Hv after 16 turns of HPT. Very fine martensite developed when the HPT-processed samples were annealed at 550 °C and the finer microstructures were attained with higher HPT turns. Differential scanning calorimetry (DSC) tests were performed in the post-HPT annealing samples to clarify the transformation behavior after severe plastic deformation in HPT and subsequent annealing, so as to provide an experimental basis for the application of the shape memory alloy. The transformation temperature of the alloy decreased remarkably when the number of turns of HPT reached 16. It is suggested that the deformation extent and annealing temperatures should be considered in order to maintain a high transformation temperature while utilizing the strengthening effect of HPT in the NiTiHf alloy.

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.

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.

Development and testing of a Ni50.5Ti27.2Hf22.3 high temperature shape memory alloy

A NiTiHf alloy with a composition of Ni50.5Ti27.2Hf22.3 (at%) was heat treated at various temperatures between 400 and 550 °C at times ranging from 1 to 51 h to determine the effect of aging on its thermomechanical properties. Uniaxial constant-force thermal cycling was performed to determine the transformation temperature, transformation strain, and dimensional stability. Through appropriate heat treatment, the initial austenite finish temperature of 126 °C could be increased to over 200 °C, and the dimensional stability improved to near 0% residual strain through precipitation strengthening, without a significant loss in transformation strain.

Actuation fatigue performance of NiTiZr and comparison to NiTiHf high temperature shape memory alloys

The actuation fatigue performance of a Ni-rich Ni50.3Ti29.7Zr20 (at.%) high temperature shape memory alloy (HTSMA) was evaluated and compared to previous results for Ni50.3Ti29.7Hf20 HTSMA. The shape memory properties of these alloys are known to improve significantly after proper aging heat treatments. Yet, their widespread implementation in high temperature applications requires a thorough characterization and a detailed understanding of their actuation fatigue performance. Consequently, actuation fatigue tests were performed by thermal cycling the NiTiZr alloy under constant loads from 200 to 500 MPa until failure. The results revealed that NiTiZr exhibits a shorter fatigue life with lower recoverable strain levels than a comparable NiTiHf alloy at all applied stress levels and the difference in fatigue life between the two alloys became more distinct with increasing stress. Building upon the extensive research on the microstructure, mechanical and functional behaviors of NiTiHf and NiTiZr HTSMAs during the last decade, this work is the first fundamental study that directly compares actuation fatigue behaviors of Ni-rich NiTiZr and NiTiHf alloys.

A study on the cold workability and shape memory effect of NiTiHf-Nb eutectic high-temperature shape memory alloy

In this work, we report a (Ni50.1Ti34.9Hf15)85Nb15 alloy that shows excellent cold workability and good high-temperature shape memory effect. The as-cast sample shows eutectic microstructure consisting of β-Nb and the NiTiHf(Nb) matrix. The homogenized (Ni50.1Ti34.9Hf15)85Nb15 alloy shows high compressive fracture strain of 40% and excellent cold-rolling reduction ratio of 80%. It is suggested that the formation of (semi-) coherent β-Nb is the main reason on improving the ductility of NiTiHf alloy. After solution treatment, the material shows a high martensitic transformation start temperature of 146 °C, and a maximum shape memory effect strain of 2.3%. This work demonstrates that (Ni50.1Ti34.9Hf15)85Nb15 alloy with good cold workability and high-temperature shape memory effect may provide new opportunities for applications.

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.

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.

Comparison of the reciprocating sliding wear of 58Ni39Ti-3Hf alloy and baseline 60NiTi

Hardened 60NiTi, due to combination of high compressive strength, high Rockwell hardness, low stiffness and large elastic compressive strain, is a desirable alloy to be employed in different load bearing applications. However, a more complex ternary NiTi-Hf alloy (~58 wt% Ni-~39 wt% Ti-~3 wt% Hf) designated as 58Ni39Ti-3Hf, shows improved properties over baseline 60NiTi and is recently receiving recognition as an alternative for this intermetallic. Nevertheless, it is not yet understood how the sliding wear properties of 58Ni39Ti-3Hf differ from those of baseline 60NiTi. In this research, a series of ball (WC)-on-plate reciprocating sliding wear tests are conducted under moderate and extreme sliding induced stress conditions to illustrate how 58Ni39Ti-3Hf responds as compared to 60NiTi. In addition, to fully understand the reasons causing divergence in the wear response of both materials, mechanical and micro-tribological properties of these two alloys were investigated and compared through indentation, hardness and scratch tests. Obtained results, other than confirming the possibility of employing 58Ni39Ti-3Hf as an alternative for 60NiTi, emphasize the role of good lubrication and stress design in sliding applications where 58Ni39Ti-3Hf and 60NiTi are exploited as wear resistant alloys. Fatigue wear, adhesion and abrasion are different wear mechanisms causing damage in these NiTi alloys. 58Ni39Ti-3Hf, due to its improved fatigue properties, shows a slightly improved unlubricated sliding wear behavior than 60NiTi. However, 58Ni39Ti-3Hf appears to be slightly more prone to cracking and brittle fracture than 60NiTi in the single pass scratch tests showing its inferiority to resist wear under abrasive modes.

Rapid Characterization of Local Shape Memory Properties through Indentation

AbstarctShape memory alloys (SMAs) have the ability to show large recoverable shape changes upon temperature, stress or magnetic field cycling. Their shape memory, material and magnetic properties (e.g. transformation temperatures, strain, saturation magnetization and strength) determine their prospects for applications from small-scale microelectromechanical systems to large scale aerospace and biomedical systems. It should be noted that properties of SMAs are highly temperature dependent. Generally, the conventional mechanical characterization methods (e.g, tension, compression, and torsion) are used on bulk samples of SMAs to determine those properties. In this article, it will be shown that indentation technique can be used as an alternative rapid method to determine some of the important shape memory properties of SMAs. Indentation response of a high-temperature NiTiHf alloy was determined as a function of temperature. A clear relationship between the work recoverable ratio and transformation temperatures, superelastic and plastic behavior was observed. This work shows that indentation response can be used to measure local superelasticity response, determine phase transformation temperatures and reveal the temperature intervals of the deformation mechanisms of shape memory alloys.

High-temperature functional behavior of single crystal Ni51.2Ti23.4Hf25.4 shape memory alloy

In this work the functional behavior of the new Ni51.2Ti23.4Hf25.4 high temperature shape memory alloy is investigated along three crystal orientations in compression and compared with the polycrystal behavior. Transformation temperatures were measured following specific aging treatments with the aim to optimize the shape memory and pseudoelastic behaviors. Two aging treatments were then selected in order to fulfill the optimum pseudoelastic behavior (500°C/4 h), and the maximum transformation temperatures (550°C/10 h) without compromising the alloy functionality. High temperature X-ray diffraction was utilized to determine the crystal orientations in the austenite phase for the single crystal specimens. The martensite structure was found to be the B19 orthorhombic, in contrast with monoclinic structure for lower Hf contents. Digital image correlation was successively used during isobaric strain-temperature experiments on the single crystal and polycrystal specimens. Large strain heterogeneities were found for the single crystals which pair with X-ray diffraction data that show un-transformed austenite below the martensite finish temperature. This reveals the heterogeneity of the austenite–to–martensite transformation for the present NiTiHf alloy which explains the divergence between the theoretical and experimental transformation strains. In addition, using digital image correlation it is possible to capture the local strain fields associated with fully transformed specimen regions. The transformation strains calculated locally are in close agreement with the theoretical transformation strains calculated with lattice deformation theory for the cubic austenite to the orthorhombic martensite phase transformation.

Microstructural characterization and shape memory characteristics of the Ni50.3Ti34.7Hf15 shape memory alloy

The effect of precipitation on the microstructure and shape memory characteristics of the Ni50.3Ti34.7Hf15 shape memory alloy has been investigated via transmission electron microscopy, differential scanning calorimetry and load-biased thermal cycling tests in tension. A one-stage martensitic transformation from B2 austenite to B19′ martensite was observed in all the aged samples but the transformation temperatures followed a more complicated trend depending on specific aging conditions. The transformation temperatures decreased below room temperature when the precipitate size, and thus the interparticle distance, was below ∼20nm, as occurred after short aging times at low temperatures. On the other hand, the transformation temperatures can be increased over a wide temperature range by increasing the precipitate size and volume fraction through aging for long durations or at higher temperatures. The alloy demonstrated excellent dimensional stability under stress levels as high as 300MPa as a consequence of precipitation hardening, with a maximum fully recoverable strain of 3.3% after aging at 450°C for 10h. The transformation thermal hysteresis also decreased in the aged samples due to reduced defect generation in the precipitation-strengthened samples. The precipitate crystal structure was identified as the H-phase that was recently reported in Ni-rich NiTiHf and NiTiZr alloys, and not the Ni4Ti3-type structure as reported in a few earlier studies. The H-phase present in the Ni-rich NiTiHf alloy does not change the twinning relations in the B19′ martensite phase compared to (Ti+Hf)-rich NiTiHf alloys, being a mixture of (001) compound twins and (011) Type I twins. On the other hand, the precipitates have a significant effect on martensite morphology and load-biased thermal cycling response, both of which can be manipulated by controlling the precipitate size.

Fabrication of (Ti,Hf)-rich NiTiHf Alloy Using Graphitic Mold and Crucible

In this research, fabrication of a (Ti,Hf)-rich NiTiHf alloy by using vacuum induction melting (VIM) process and a graphitic crucible was investigated. For this purpose, casts with the nominal composition of Ni 49 Ti 36 Hf 15 were prepared in graphitic crucible and mold. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) tests were employed to characterize the samples. Results demonstrated that microstructure of the first cast was composted of a B2 austenite phase as well as a great amount of two differently formed (Ti,Hf)C carbides. Moreover, no austenite ↔ martensite transformation peak was detected in the DSC curve of this sample, indicating a drastic decline in the transformation temperatures. In the succeeding cast, however, owing to the formation of carbide layers on the inner surfaces of the graphitic crucible and mold during the initial casting process, the amounts of carbides decreased remarkably. This cast exhibited transformation temperatures above 100 °C, while XRD pattern denoted the presence of B19′ monoclinic martensite phase at room temperature. All in all, results confirmed that VIM process using graphitic mold and crucible can be considered as an appropriate method for the fabrication of (Ti,Hf)-rich NiTiHf high temperature shape memory alloys.

Effects of nanoprecipitation on the shape memory and material properties of an Ni-rich NiTiHf high temperature shape memory alloy

Shape memory properties of a Ni50.3Ti29.7Hf20 (at.%) polycrystalline alloy were characterized after selected heat treatments. The effects of heat treatment temperature and time on the transformation temperatures (TTs) and temperature hysteresis were determined by differential scanning calorimetry. Thermal cycling under constant compressive stress was carried out to reveal the changes in transformation strain, temperature hysteresis, and TT as a function of stress. Isothermal stress cycling experiments were conducted to reveal the critical stresses, transformation strain, and stress hysteresis as a function of temperature. The crystal structure and lattice parameters of the transforming phases were determined by X-ray diffraction at selected temperatures. Precipitate characteristics and martensite morphology were revealed by transmission electron microscopy. Precipitation was found to alter the martensite morphology and significantly improve the shape memory properties of the Ni-rich NiTiHf alloy. For the peak aged condition shape memory strains of up to 3.6%, the lowest hysteresis, and a fully reversible superelastic response were observed at temperatures up to 240°C. In general, the nickel-rich NiTiHf polycrystalline alloy exhibited a higher work output (≈16.5Jcm−3) than other NiTi-based high temperature alloys.

Age-induced multi-stage transformation in a Ni-rich NiTiHf alloy

Abnormal multi-stage transformations have already been studied in binary Ni-rich NiTi alloys. In this research, this kind of transformation was investigated in a low supersaturated Ni-rich ternary NiTiHf high-temperature shape memory alloy by aging at intermediate temperatures for various durations. Meticulous examinations of the results of differential scanning calorimetry tests demonstrated the heterogeneous precipitation of (Ti,Hf)3Ni4 particles and the three-stage transformation (one-stage R and two-stage B19′) in the aged alloy. Aging provided a significant rise in transformation temperatures (TTs) until they reached their equilibrium states, corresponding to the equilibrium Ni content at each aging temperature. Equilibrium TTs were higher when aging was performed at a lower temperature. The remarkable increase in TTs was compared with those in aged Ni-rich NiTi alloys, and discussed based on the variation in valence electron concentration. A model was also proposed for the microstructural evolution during aging. Furthermore, aging provided enhanced hardness and strain recovery for the alloy. In particular, aging at a lower temperature resulted in a considerable improvement in hardness and shape recovery, which was discussed based on the microstructural changes in the aged alloy. Equilibrium Ni content at each aging temperature proved to be a crucial parameter in controlling the alloy properties, even in heterogeneous precipitations.

Microstructural Response During Isothermal and Isobaric Loading of a Precipitation-Strengthened Ni-29.7Ti-20Hf High-Temperature Shape Memory Alloy

A stable Ni-rich Ni-29.7Ti-20Hf (at. pct) shape memory alloy, with relatively high transformation temperatures, was shown to exhibit promising properties at lower raw material cost when compared to typical NiTi-X (X = Pt, Pd, Au) high-temperature shape memory alloys (HTSMAs). The excellent dimensional stability and high work output for this alloy were attributed to a coherent, nanometer size precipitate phase observed using transmission electron microscopy. To establish an understanding of the role of these precipitates on the microstructure and ensuing stability of the NiTiHf alloy, a detailed study of the micromechanical and microstructural behaviors was performed. In-situ neutron diffraction at stress and temperature was used to obtain quantitative information on phase-specific internal strain, texture, and phase volume fractions during both isothermal and isobaric testing of the alloy. During isothermal testing, the alloy exhibited low isothermal strains due to limited detwinning, consistent with direct measurements of the bulk texture through neutron diffraction. This limited detwinning was attributed to the pinning of twin and variant boundaries by the dispersion of fine precipitates. During isobaric thermal cycling at 400 MPa, the high work output and near-perfect dimensional stability was attributed to the presence of the precipitates that act as homogeneous sources for the nucleation of martensite throughout the material, while providing resistance to irrecoverable processes such as plastic deformation.