Ni-rich NiTi Shape Memory Alloy Research Articles

Experimental study on Response Parameters of Ni-rich NiTi Shape Memory Alloy during Wire Electric Discharge Machining

Nowadays NiTi SMAs are gaining more prominence due to their unique properties such as superelasticity, shape memory effect, high fatigue strength and many other enriched physical and mechanical properties. The current studies explore the effect of machining parameters namely, peak current (Ip), pulse off time (TOFF), and pulse on time (TON) on wire wear ratio (WWR), and dimensional deviation (DD) in WEDM. It was found that high discharge energy was mainly ascribed to high WWR and DD. The WWR and DD increased with the increase in pulse on time and peak current whereas high pulse off time was favourable for low WWR and DD.

Deep Drawing of a Ni-Ti Shape Memory Alloy Sheet

This study is aimed to investigate the properties of Ni-rich Ni-Ti shape memory alloy sheet parts during and after deep drawing process at room temperature, in which the metal is under plane tension-compression stress state. After annealing at different elevated temperature, shape memory alloy sheet is formed at room temperature by a circular cylindrical punch in different forming depths. Thereafter the deep drawn sheet part is subjected to aging for shape memory treatment. A free shape recovery test is further conducted onto the deep drawn shape memory alloy sheet part. As a result, the sheet metal has austenitic phase at room temperature, which shows superelasticity. Too small forming amount causes severe springback and the form of the deep drawn sheet part is very hard to detect with the naked eye. Only if a forming amount greater than or equal to 4 mm is applied, a form can be achieved on to the sheet part. The subsequent aging shape memory treatment can induce further springback of the deep drawn sheet part. The higher the temperature of the previous annealing, the less the springback by aging. The free shape recovery test shows that the deep drawn sheet part only undergoing annealing without shape memory treatment still possesses shape memory effect. Nevertheless, the deep drawn sheet part aged for shape memory can show a better shape recovery.

Study on Effect of Machining Parameters on Performance Characteristics of Ni-rich NiTi Shape Memory Alloy during Wire Electric Discharge Machining

Nowadays Shape memory alloys (SMAs) are most widely used in different sectors of society such as biomedical, aerospace & robotics due to their unique properties. The current studies explore the effect of machining parameters, namely, pulse on time, pulse off time and peak current on material removal rate (MRR) and surface roughness (SR) for Ni-rich NiTi SMAs during WEDM. Both MRR and SR show the similar trend, both increasing on increasing pulse on time and peak current while decreasing with pulse off time.

Correlating microstructure and superelasticity of directed energy deposition additive manufactured Ni-rich NiTi alloys

Laser-based directed energy deposition (LDED) additive manufacturing of Ni-rich NiTi shape memory alloys was shown to produce inhomogeneous precipitate morphologies and characteristic grain structures consisting of columnar grains coexisting with equiaxed and subgrain structures. Post-processing solutionizing and aging heat treatments impacted microstructure and martensitic phase transformation (MT) responses underpinning superelastic shape memory responses. A solution treatment of 950 °C for 24 h was found to produce a uniform composition of the B2 austenite parent phase without affecting the coexistence of columnar and equiaxed substructures. Aging the solution treated material brought about a spatially uniform Ni4Ti3 precipitate morphology. Due to the uniform morphology, an underlying austenite-martensite interface motion accompanies the compressive stress-induced MT (SIMT). Reversible interface motion underpinned the compressive superelastic response for the solutionized and aged condition. On the other hand, strain concentrations existed at different spatial locations in the as built condition as well as when the as built material was aged. The stark contrasts in the SIMT exposed precipitate morphology as a controlling factor in tailoring the superelastic response of Ni-rich NiTi SMAs fabricated by LDED.

A Two-Way Shape Memory Study on Ni-Rich NiTi Shape Memory Alloy by Combination of the All-Round Treatment and the R-Phase Transformation

Ni4Ti3 precipitates are formed in Ni-rich NiTi shape memory alloys (SMAs) after a certain heat treatment. Such a treatment then results in the changed phase transformation behavior of the alloy switching from the one-step reversible phase change B2 ↔ B19′ to the two-step, B2 ↔ R ↔ B19′, phase change. The present study reports a two-way shape memory (TWSM) due to an all-round treatment followed by an R-phase constraint aging at room temperature. The enhanced TWSM behavior was observed upon temperature cycling between 273 K (R-phase) and 77 K (B19′ phase). The effect of various constraint strains in R-phase aging by employing different diameters of the constraint ring was studied. However, the TWSM effect due to constraining the R-phase will be eliminated after a temperature rise of specimen to the fully parent phase (373 K).

On the Influence of the Heat Treatment Temperature on the Superelastic Compressive Behavior of the Ni-Rich NiTi Shape Memory Alloy

In this paper, the relationship between the heat treatment temperature and the thermomechanical behavior of the Ni-rich NiTi shape memory alloy was experimentally investigated. Thermal analysis was performed through a differential scanning calorimetry experiments, and the NiTi superelastic effect was exhibited through loading/unloading compression cycle. It was found that the transformation temperatures and the compressive mechanical response are strongly affected by the heat treatment temperatures. The stress-induced phase transformation was characterized through the relationship between the heat treatment temperature and transformation stresses, elastic moduli and the deformation level upon loading. Subsequently, the evolution of the mechanical hysteresis loops and the mechanical dissipated energy was discussed. The obtained findings are supported by hardness measurements. To better understand the global tendency of the various phase transformation stages, the critical stress-temperature diagram was provided for different proposed heat treatment temperatures.

Transformation entropy change and precursor phenomena in Ni-rich Ti–Ni shape memory alloys

Abstract

Improved Functional Properties and Efficiencies of Nitinol Wires Under High-Performance Shape Memory Effect (HP-SME)

Martensitic Ti-rich NiTi intermetallics are broadly used in various cyclic applications as actuators, which exploit the shape memory effect (SME). Recently, a new approach for exploiting austenitic Ni-rich NiTi shape memory alloys as actuators was proposed and named high-performance shape memory effect (HP-SME). HP-SME is based on thermal recovery of de-twinned martensite produced by mechanical loading of the parent phase. The aim of the manuscript consists in evaluating and comparing the fatigue and actuation properties of austenitic HP-SME wires and conventional martensitic SME wires. The effect of the thermomechanical cycling on the actuation response and the changes in the electrical resistivity of both shape memory materials were studied by performing the actuation tests at different stages of the fatigue life. Finally, the changes in the transition temperatures before and after cycling were also investigated by differential calorimetric tests.

Novel B19′ strain glass with large recoverable strain

We report a strain glass state (B19\ensuremath{'} strain glass) in a Ni-rich TiNi shape memory alloy produced by cold rolling. As compared to previously reported strain glasses, this strain glass state has outstanding properties including quasilinear superelasticity with a large recoverable strain (\ensuremath{\sim}4%), and slim hysteresis and high strength (\ensuremath{\sim}1.0 GPa) over a wide temperature range (\ensuremath{\sim}200 K). The existence of the B19\ensuremath{'} strain glass state is confirmed by (i) frequency dispersion of storage modulus, (ii) continuous decrease of electrical resistivity, and (iii) continuous growth of B19\ensuremath{'} nanodomains upon cooling. This study proves that the effect of defect strength on the creation of a strain glass state is in parallel to the effect of cooling rate on the creation of a structural glass, e.g., any strain crystal (i.e., martensite) can be turned into a strain glass if strong enough defects could be engineered.

Multi-stage martensitic transformation in Ni-rich NiTi shape memory alloys

Precipitation hardening is an effective way to improve the functional stability of NiTi shape memory alloys. The precipitates, mainly Ni4Ti3, could be introduced by aging treatment in Ni-rich NiTi alloys. However, the presence of Ni4Ti3 precipitates could disturb the transformation behavior, resulting in the multi-stage martensitic transformation (MMT). With the presence of MMT, it is difficult to control the transformation behavior, and thus limits the applicability of NiTi alloys. In this work, previous efforts on explaining the observed MMT are summarized. The difficulties in developing a unified explanation are discussed, and a possible way to avoid the MMT is proposed.

An inverse design framework for prescribing precipitation heat treatments from a target microstructure

The computer-aided materials design process is highly iterative in nature and as such requires flexible tools that have the ability to link processing, properties, and performance not only in the usual forward direction but also in the inverse direction more associated with a goal-oriented/design framework of Integrated Computational Materials Engineering (ICME). While many computational techniques exist that relate properties to performance in both forward/inverse directions, tools that prescribe a process when given a desired microstructure have not been developed in detail. This research fills that gap by coupling physics-based precipitation models with “mesh adaptive direct search” optimization techniques as a strategy to develop (inverse) microstructure-processing relations. This framework is demonstrated by prescribing heat treatments in Ni-rich NiTi shape memory alloys that will result in a desired size distribution of Ni4Ti3 precipitates. This prescriptive technique provides a rigorous strategy for the identification of materials processing schedules—provided the forward models connecting processing and microstructure are available—that yield specific microstructural features and that can significantly reduce the experimental search space that needs to be explored, accelerating the materials development process.

Constitutive Modeling of Near-Equiatomic NiTi Shape Memory Alloys Considering Composition and Heat Treatment

A predictive microscale-informed model that takes into account the precipitate–shape memory performance relations and allows for the evaluation of the effective thermomechanical response of precipitated Ni-rich NiTi shape memory alloys on the basis of composition and heat treatment is presented. The model considers the structural effect of the precipitates (coherency stresses due to the lattice mismatch between the precipitates and the matrix material and precipitate volume fraction), as well as the effect of the Ni-concentration gradient in the matrix, resulting from Ni-depletion during precipitate growth. The predictive capability of the model is tested against experimental data obtained fromNi50.7Ti (at. %) that has been heat treated under different conditions and good agreement is shown.

Multiple plastic deformation mechanisms of NiTi shape memory alloy based on local canning compression at various temperatures

A Ni-rich NiTi shape memory alloy (SMA), which was in its austenitic state at ambient temperature, was subjected to plastic deformation by means of local canning compression at various temperatures ranging from room temperature to 800 °C. Depending on temperatures, NiTi SMA exhibited multiple plastic deformation mechanisms, such as dislocation slip, deformation twinning, grain boundary slide, grain rotation, dislocation climb and grain boundary migration. Amorphization, dynamic recovery and dynamic recrystallization of NiTi SMA were also observed at various temperatures. Mechanism of localized amorphization, in particular, was investigated based on dislocation slip and deformation twinning. Statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) were found to play an important role in the amorphization of the current NiTi SMA. There appeared a critical dislocation density below which NiTi SMA was unable to amorphize. Accordingly, at a fixed deformation strain, there should be a critical temperature above which amorphous phase would not occur in the NiTi SMA matrix. Furthermore, when NiTi SMA experienced plastic deformation at the critical temperature, amorphization and crystallization would occur simultaneously and compete with each other.

Thermal cycling induced room temperature aging effect in Ni-rich NiTi shape memory alloy

This work shows that the characteristics of B19′ martensite transformation in Ni-rich NiTi alloys are rather stable during aging at room temperature after quenching from 1273K. However, if transformation cycles between austenite and martensite are performed, the B19′ martensite transformation can be strongly suppressed following aging at room temperature. As the NiTi elements normally experience cyclic transformation in applications, this phenomenon has to be taken into account as a factor for the functional degradation of NiTi alloys. Possible mechanisms of the uncovered aging effect are discussed.

Cold Bending a Ni-Ti Shape Memory Alloy Wire

Because of being in possession of shape memory effect and superelasticity, Ni-Ti shape memory alloys have earned more intense gaze on the next generation applications. Conventionally, Ni-Ti shape memory alloys are manufactured by hot forming and constraint aging, which need a capital-intensive investment. To have a cost benefit getting rid of plenty of die sets, this study is aimed to form Ni-Ti shape memory alloys at room temperature and to age them at elevated temperature without any die sets. In this study, starting with solution treatments at various temperatures, which served as annealing process, Ni-rich Ni-Ti shape memory alloy wires were bent by V-shaped punches in different curvatures at room temperature. Subsequently, the wires were aged at different temperatures to have shape memory effect. As a result, springback was found after withdrawing the bending punch and further after the aging treatment as well. A higher solution treatment temperature or a smaller bending radius leads to a smaller springback, while a higher aging treatment temperature made a larger springback. This springback may be compensated by bending the wires in further larger curvatures to keep the shape accuracy as designed. To explore the shape memory effect, a reverse bending test was performed. It shows that all bent wires after aging had a shape recovery rate above 96.3% on average.

Effect of Aging on Phase Transition Behavior of Ti<sub>50</sub>Ni<sub>15</sub>Pd<sub>25</sub>Cu<sub>10</sub> High Temperature Shape Memory Alloys

Formation of Ni4Ti3 precipitates during aging of Ni-rich binary NiTi shape memory alloys and its effect on transition behavior during transformation from austenite to martensite phase has been studied extensively. However for equi-atomic NiTi-based quaternary high temperature shape memory alloy, two-stage martensitic transformation was detected for the first time. The Ti50Ni15Pd25Cu10 high temperature shape memory alloys were investigated for the hardness and transformation temperatures at aging temperature of 550°C. Aging at 550°C for 6 h resulted in remarkable increase in the hardness, whereas the phase transformation temperatures decreased significantly. During forward transformation from austenite to martensite, two-stage martensitic transformation; B2 (cubic) → R-phase and R-phase → B19 (orthorhombic) was observed.

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti50−xNi50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti48.4Ni51.6) to ∼2.1 eV (Ti50Ni50), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.

Gradient deformation behavior of NiTi alloy by ageing treatment

The applicability of ageing treatment in creating gradient thermomechanical behavior in a Ni-rich NiTi shape memory alloy is investigated in this work. Based on the deformation behavior of isothermal ageing, the NiTi wire was designed to have a near-perfect gradient pseudoelastic behavior by means of gradient ageing treatment. The linear slope on both forward and reverse transformations was achieved by ageing a wire specimen in a temperature gradient of 400–460°C. The gradient-aged wire exhibited martensitic transformation over a linear slope, producing 130MPa of transformation stress interval on both forward and reverse. The slope illustrates a gradient thermomechanical property of the wire specimen, and corresponds to the effect of gradient ageing temperature. Such gradient pseudoelastic behavior provides an activation stress of 130MPa for progressive motion of NiTi apparatuses, especially in control and actuation applications.

Evaluation of kinetic equations describing the martensite–austenite phase transformation in NiTi shape memory alloys

Analysis and comparison of existing phenomenological equations for transformation kinetics has been performed for the evolution of the austenite fraction as a function of temperature during the athermal reverse martensite→austenite transformation in NiTi shape memory alloys (SMA) of different compositions. The evaluation has been carried out using experimental data presented in the literature and experimental data obtained in this work, using differential scanning calorimetry, for thermally-cycled equiatomic and Ni-rich NiTi SMAs. Large discrepancies exist between the experimental kinetics and the kinetics, as described by the previously proposed phenomenological equations. A new model has been presented which adequately describes the experimental data, in particular as sensitively expressed by the transformation rate as a function of the degree of transformation.

Microstructure, Phase Transformations, and Properties of Hot-Extruded Ni-Rich NiTi Shape Memory Alloy

Processing of NiTi shape memory alloys strongly influences their microstructure, phase transformations, mechanical, and shape memory properties. Hot forging, hot swaging, or hot rolling are efficient techniques for obtaining the desired shape, but during multiple operations the material must be heated and worked in the temperature range from 700 to 900 °C. During these processes, intense oxidation takes place. In order to reduce it, the hot-pack working is applied. The hot extrusion is more effective for reduction of ingot, billet, and rod diameters than hot forging, hot swaging, or hot rolling. Also, during hot extrusion the material surface undergoes considerably less oxidation. In the present work, results of the characterization by differential scanning calorimetry, low-temperature x-ray powder diffraction, and three-point bending and free recovery ASTM F2082-06 tests of the samples after hot direct extrusion and heat treatment are presented. The obtained alloy after hot direct extrusion exhibits desired shape memory effect. The phase transformations during cooling and heating cycle occur with the presence of the R phase. The range of the characteristic temperatures for the obtained material gives possibility for further medical applications. After annealing at 400 and 500 °C, the characteristic temperatures shift to higher values.