R-phase Transformation Research Articles

Effect of Thermal Cycling on Multistage Martensitic Transformation in Aged Ti–50.8 at% Ni Alloy

The effect of thermal cycling on multistage martensitic transformation (MMT) in aged Ti­50.8at% Ni alloy was investigated. The specimens were solution-treated at 1273K for 3.6ks and then aged at 773K for 3.6ks in vacuum without atmosphere regulation. Upon cooling, the aged specimens clearly showed quadruple-stage transformation denoted as B2 ¼ R ¼ M1 ¼ M2 ¼ M3. The peak temperatures of exothermic reactions R*, M1*, and M2* (corresponding to the R-phase, M1 and M2 transformations, respectively) in the differential scanning calorimetry (DSC) cooling curve were rather stable up to 100 thermal cycles, since there were fine Ti3Ni4 precipitates with high distribution density in the intermediate and grain boundary regions. These precipitates prevented the formation of dislocations during thermal cycling. On the other hand, the peak temperature of exothermic reaction M3* (corresponding to the M3 transformation) drastically decreased with increasing the number of thermal cycles since there were large Ti3Ni4 precipitates with low distribution density. Many dislocations were observed in the central regions of grains after 100 thermal cycles. [doi:10.2320/matertrans.M2013309]

Study on the compressive behavior of Ti–53.22 at%Ni alloy sheets after pre-compression and aging treatments

Three factors and three levels of orthogonal experiments were conducted to study the effects of pre-compression, aging temperature and aging time on the compressive behavior of Ti–53.22at%Ni alloy sheets. Phase transformation temperatures and compressive behavior at room temperature were investigated. Range analysis of differential scanning calorimeter (DSC) curves shows that R-phase transformations are dependent on aging temperature, and there is no R-phase transformation after aging at 523K, the R-phase transformation reliably occurs after aging at 673K. A saw tooth shape on the stress–strain curves is observed and indicates the occurrence of martensitic transformation. Two kinds of martensitic transformation exist under the compression conditions. Higher compressibility can be achieved by pre-compression of 0.2%, and aging at 523K results in better recovery.

A Methodology for the Development, Production, and Validation of R-Phase Actuators

The R-phase transformation has interesting features with potential for applications that need a small temperature hysteresis and good dynamic behavior, such as thermostatic valves. The aim of this article is to show the development, production, and validation process of different R-phase shape memory alloy (SMA) actuators, starting with a semi-finished wire and concluding with a finalized R-phase spring actuator. This study focuses mainly on the calculation, the thermomechanical treatment, and experimental validation of the designed actuators. The first section of this article presents a mathematical dimensioning tool for different R-phase actuators, especially for extension SMA springs. The second part shows specific parameters on the R-phase transformation during thermomechanical treatment. The parameters Ni-content and annealing temperature are being varied to achieve different transformation behavior of the R-phase. The third section relates to the production process of calculated SMA spring actuators based on the R-phase transformation. In the fourth and last section of the article, the performance of selected actuators will be characterized in functional tests, and the results will be compared with the calculated results of the mathematical model.

Effect of proportional control treatment on transformation behavior of Ti–50.9at.% Ni shape memory alloys

► We designed TGA furnace for gradient annealing. ► R-phase and martensitic transformation temperature were changed in the specimen. ► The dɛ / dT of TGA specimens was lower than that of isochronously annealed specimens. ► TGA treatment was most effective at 773 K in case of R-phase and martensitic transformation. ► TGA effect increased with increasing applied stress in case of R-phase and martensitic transformation. In this study, the shape memory behavior of Ti–50.9 at.% Ni alloys with time gradient annealing (TGA) was investigated via differential scanning calorimetry (DSC) and thermal cycling tests under constant load. The ingot was prepared using a high-frequency induction vacuum furnace. The as-cast ingot was hot forged and extruded, followed by cold-drawing and intermediate annealing to produce wires of 1.0 mm in diameter, with a final cold-drawing of 30% reduction in cross-section. For the TGA treatment, a new type of radiant furnace was designed to maintain a constant temperature and create a time gradient along the length of the specimen. According to DSC measurements, a 34 K variation in the R-phase transformation interval (i.e., R s − R f ) was obtained along the length of the specimen (80 mm) that was time-gradient annealed from 3 min to 20 min at 773 K. The results of thermal cycling tests under constant load revealed that the temperature dependence of transformation elongation ( dɛ / dT ) of the TGA specimen is smaller than that of the isochronously annealed specimen at every heat-treatment temperature (673–773 K). The lowest dɛ / dT of R-phase transformation (0.0031%/K) was obtained for the specimen that was TGA-treated at 673 K. The difference in dɛ / dT of R-phase and the martensitic transformation of the TGA-treated and isochronously annealed specimen was largest at 773 K and increased with increasing stress. Such behavior provides superior controllability for actuation applications.

Review on the temperature memory effect in shape memory alloys

Shape memory alloys (SMAs) are well known for their unique shape memory effect (SME) and superelasticity (SE) behavior. The SME and SE have been extensively investigated in past decades due to their potential use in many applications, especially for smart materials. The unique effects of the SME and SE originate from martensitic transformation and its reverse transformation. Apart from the SME and SE, SMAs also exhibit a unique property of memorizing the point of interruption of martensite to parent phase transformation. If a reverse transformation of a SMA is arrested at a temperature between reverse transformation start temperature (A s) and reverse transformation finish temperature (A f), a kinetic stop will appear in the next complete transformation cycle. The kinetic stop temperature is a ‘memory’ of the previous arrested temperature. This unique phenomenon in SMAs is called temperature memory effect (TME). The TME can be wiped out by heating the SMAs to a temperature higher than A f. The TME is a specific characteristic of the SMAs, which can be observed in TiNi-based and Cu-based alloys. TME can also occur in the R-phase transformation. However, the TME in the R-phase transformation is much weaker than that in the martensite to parent transformation. The decrease of elastic energy after incomplete cycle on heating procedure and the motion of domain walls have significant contributions to the TME. In this paper, the TME in the TiNi-based and Cu-based alloys including wires, slabs and films is characterized by electronic-resistance, elongation and DSC methods. The mechanism of the TME is discussed.

The effect of aging on the B2-R transformation behaviors in Ti-51at%Ni alloy

Aging is an effective way to control the transformation behaviors of Ti–Ni shape memory alloys. In the present study, the transformation behaviors of Ti-51at%Ni polycrystals after aging at various temperatures (250 °C–500 °C) for different time were investigated by DSC measurement. It is found that B2-R transformation temperature (RS) of the aged samples is uniquely determined by the aging temperature for aging above 400 °C, being almost independent on aging time. Higher aging temperature causes a lowering of RS temperature. At lower aging temperatures (e.g., 250 °C, 300 °C), RS gradually increases with aging time and finally saturates. The saturated RS value is also a unique function of the aging temperature. These facts can be explained by the phase equilibrium between Ti–Ni matrix (B2) and Ti3Ni4 precipitates. The Ti3Ni4 precipitates formed at different aging temperatures alter the Ni concentration in B2 matrix and consequently cause an aging temperature dependence of the R-phase transformation of B2 matrix. By considering the kinetics of precipitation reaction, the “time-independence” of RS for high temperature aging and the “time-dependence” of RS for low temperature aging can also be reasonably understood. Besides, the R–B19′ transformation temperature (MS) gradually increases with aging time as well, which can also be understood by considering the growth of precipitates during aging.

Advanced TEM and SEM Methods Applied to 3D Nano- and Microstructural Investigations of Ni<sub>4</sub>Ti<sub>3</sub> Precipitates in Ni-Ti (SMA)

Two different kinds of experimental approaches yielding three-dimensional structural information on metastable semi-coherent precipitates are demonstrated. By combining high-resolution images from two independent viewing directions a full description of the strain field surrounding a nano-sized Ni4Ti3 precipitate in Ni-Ti can be obtained. The principal axes and strains correlate well with the transformation strain of the observed R-phase transformation close to the precipitate. Using a slice-and-view procedure in a FIB/SEM dual-beam instrument, a three-dimensional voxel dataset is produced from which morphological and distributional information on the same precipitates can be obtained yielding new insight into the particular transformation paths of these alloys, relevant for their functional behaviour.

A comparative study of Ni–Ti and Ni–Ti–Cu shape memory alloy processed by plasma melting and injection molding

Shape memory alloys (SMA) are smart materials that present potential applications in such diverse areas as aeronautics, automotive, electronics, biomedicine and others. This work aimed at comparing some physical and functional properties of a Ni–Ti–Cu and equiatomic Ni–Ti SMA. Therefore, Ni–50Ti and Ni–50Ti–5Cu (at.%) were manufactured using plasma melting followed by injection in metallic mold, named Plasma Skull Push–Pull (PSPP) process. Afterwards, samples of both Ni–Ti based SMA were annealed at 1113 K during 2400 s and water quenched. The obtained specimens were analyzed by optical microscopy, microhardness, differential scanning calorimetry, electrical resistance as a function of temperature, and force generation tests. The results showed that Ni–Ti alloy presented higher levels of hardness and lower generated recover forces during heating when compared to the Ni–Ti–Cu SMA. Moreover, the Ni–Ti alloy holds hysteresis larger than the Ni–Ti–Cu SMA as a result of the presence of the R-phase transformation. There was also a better stability under thermal cycling of NiTiCu SMA compared with the equiatomic NiTi.

PHASE FIELD SIMULATION OF EFFECTS OF PORES ON B2-R PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY

Recently, porous NiTi shape memory alloys (SMAs) have drawn great interest in vari- ous engineering fields, in particular for biomedical applications as one of the promising biomaterials for hard-tissue replacements and orthopedic implants. It is well known that the porous NiTi SMAs exhibit three transformations, B2-B19 � , B2-R and R-B19 � . Among these phase transformations B2-B19 � and R-B19 � involve high lattice distortion and large transformation hysteresis. Consequently, these distortion and transformations usually introduce structural defects which may result in degradation of mechanical stability for the functional application. On the contrary, B2-R transformation is governed by small lattice distortion which indicates less damage to the microstructure and lower sensitivity to the defects, and thus generates higher reversibility and mechanical stability. Due to these unique virtues, it is of great importance to study the R-phase transformation behavior in porous NiTi SMAs, since the pore has a significant influence on B19 � martensitic transformation as well as the R-phase transforma- tion. In this paper, a three-dimensional phase field model aiming at accounting for the pore effect on phase transformation in NiTi SMAs was developed to study the B2-R phase transformation behavior in porous NiTi SMAs. The model was applied to characterize the microstructure evolution of B2-R phase

Frequency Dependence of the Internal Friction for Ti50Ni48.5Fe1.5 Shape Memory Alloy

The objective of this work is to investigate the influence of measuring frequency on the internal friction of Ti50Ni48.5Fe1.5 shape memory alloy. The DSC, DC electrical resistivity and internal friction measurements were employed to characterize the sample in this study. Comparing the DSC result, the two internal friction peaks were confirmed which are corresponding to the R-phase transformation and martensitic transformation of Ti50Ni48.5Fe1.5 alloy during the cooling process, respectively. The height of internal friction peak corresponding to martensitic transformation increased with decreasing of measuring frequency. But, the height of the internal friction peak corresponding to R-phase transformation exhibited little measuring frequency dependence. Meanwhile, the value of the relative modulus peaks of the specimen increased with measuring frequency increasing. Furthermore, there was an interesting phenomenon that the martensitic transformation finish temperature shifted to higher temperature side with increasing of measuring frequency. It could be associated with the variation of applied vibration energy.

Phase Transformation in Ni-Ti Shape Memory and Superelastic Alloys Subjected to High Pressure Torsion

A systematic study on the phase transformation of Ni-Ti shape memory and superelastic alloys subjected to Severe Plastic Deformation (SPD) – High Pressure Torsion (HPT) technique has been carried out. Ni-Ti alloys of three compositions were chosen for the study. Specimens of these alloys in as-received (AR) condition and after HPT have been subjected to Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD) analyses. In this study, while comparing the results of DSC thermograms and XRD spectra for the same sample conditions, some differences were observed. In the case of NiTi-H alloy after HPT, there appeared one stage phase transformation with DSC both while heating and cooling suggesting Martensite↔Austenite transformation but, with respect to XRD spectra while cooling, at the intermediate temperature of 55°C, the R-phase peaks corresponding to (1 1 2)R and (3 0 0)R planes appeared. In the thermogram obtained for the NiTi-B alloy subjected to HPT, it is observed that, while cooling, the Austenite to R-phase transformation is merged with R-phase to Martensite transformation. The results of the XRD obtained at -180°C show the presence of R-phase along with M-phase. The DSC curve of the NiTi-S alloy subjected to HPT corresponds to one stage phase transformation both while heating and cooling but, the diffractogram of the sample obtained at -180°C corresponds to the presence of both R-phase and M-phase.

Thermomechanical behaviors of Ni–Ti shape memory alloy ribbons and their numerical modeling

The thermomechanical behaviors of Ni–Ti shape memory alloy (SMA) ribbons, associated with stress and temperature-induced transformations, are experimentally and numerically investigated. A 2-D incremental formulation of an SMA model is proposed and implemented in an ABAQUS finite element program through a user-defined material (UMAT) subroutine for the purpose of predicting the thermomechanical characteristics of SMA films. The evolution of stress–strain curves and phase transformation temperatures is examined with various annealing temperatures. The rhombohedral phase (R-phase) transformation appears in the specimen of an Ni–Ti SMA ribbon. The uncommon behaviors of permanent elongation and partial pseudoelastic behavior are observed. The 2-D SMA model with a new transformation function shows a good prediction for the hysteresis of the strain–stress curve and the recovery stress. For application of the developed numerical model of 2-D SMAs, the feasibility of the gripper actuator with the SMA ribbon is numerically demonstrated.

Evidence of two-length-scale kinetics of R-phase transformation by high-energy X-ray diffraction

The cubic-to-rhombohedral (C-R) phase transition in an AuCd shape memory alloy was investigated by in situ high-energy X-ray diffraction. We present the first direct experimental evidence of two-length-scale phase transition kinetics in this alloy system. It was further found that aging in the R-phase leads to two-way memory “loss”, characterized by the reselection of new variants. We attributed this two-way memory “loss” to the internal stresses caused by the defects formed in the off-equilibrium state of the studied AuCd system.

Phase transformation and structural study on the severely plastic deformed Ni–Ti alloys

In an effort to obtain nanostructures in bulk Nickel-Titanium (Ni–Ti) shape memory alloys (SMAs) and to carryout their characterisation, samples of the alloys with different compositions have been chosen and subjected to severe plastic deformation (SPD). The selected Ti-rich alloy in the as-received (AR) condition has martensite finish temperature (Mf) above room temperature (RT) and Ni-rich has austenite finish temperature (Af) below RT. SPD method is found to be one of the efficient methods to attain nanostructures. SPD of Ni–Ti alloys (Ti-rich and Ni-rich) have been performed by High Pressure Torsion (HPT) at RT. The changes in the phase transformation temperatures before and after SPD are analyzed by studying the Differential Scanning Calorimeter (DSC) plots. Further, the structural evolution of the samples subjected to SPD in the phase transformation temperature region is studied using in situ X-ray diffraction (XRD) from −180 to +180∘C. Heat treated Ti-rich NiTi samples, previously subjected to HPT, show the same transformation characteristics (transformation temperatures and transformation sequence) as those of the as-received ones. But, Ni-rich alloy shows distinct intermediate R-phase transformation, both while heating and cooling, after subjecting it to HPT followed by heat treatments.

Texture Development and Phase Transformation Behavior of Sputtered Ni-Ti Films

It is essential to identify and control the preferential orientation of Ni-Ti shape memory alloy (SMA) films since it is a crucial factor in determining the shape memory behavior. In the present work, in situ studies by synchrotron radiation scattering enabled to identify the different steps of the structural evolution of Ni-Ti films during co-sputtering deposition. For micro-electromechanical systems (MEMS) integration, there is a need for an electrically and thermally insulating or sacrificial layer. Widening the scope of previous experiments concerning the influence of the deposition parameters on the structure of the Ni-Ti films, the incorporation of a TiN buffer layer has been tested. Here, it is established a relationship between the TiN substrates and Ni-Ti texture development (B2 phase). Ni-Ti films mainly containing grains with (110) or (211) planes of the B2 phase parallel to the film surface could be produced using TiN buffer layers with distinct thickness values. The electrical resistivity measurements performed during temperature cycling have shown that the crystallographic orientations of the Ni-Ti films influence their phase transformation characteristics. The resistivity increase during R-phase transformation, especially visible on cooling, is higher for Ni-Ti films with a higher fraction of grains of the B2 phase with (211) parallel to the film surface.

R-phase formation in Ti 39Ni 45Cu 16 shape memory thin films and bulk alloys discovered by combinatorial methods

The combinatorial fabrication and high-throughput characterization of a Ti–Ni–Cu shape memory thin film composition spread led to the discovery of the shape memory alloy Ti 39Ni 45Cu 16, which exhibits a single B2 → R-phase transformation above 25 °C with a thermal hysteresis width <1 K. Here we show that the thin film results correctly predict the phase transformation behavior of bulk material upon cooling from the high temperature phase. For both thin film and bulk, a two-step B2 → R-phase → B19′ transformation was found. The B2 → R-phase transformation can be exploited independently, due to a significant temperature separation of the two transformation steps. The shape memory effect in both thin film and bulk samples is limited due to the two-phase microstructure. Transmission electron microscopy investigations revealed the existence of Ti(Ni,Cu) 2 precipitates within the TiNi(Cu) matrix, which are concluded to be responsible for the R-phase formation and separation of the transformation steps.

Development of sputtered Shape Memory Alloy (SMA) Ni–Ti films for actuation in ice cooled environments

Due to the high sensitivity of Ni–Ti films to environmental changes, e.g. thermal, and/or to stress, they are ideal materials for applications on micro-sensors. It was aimed to obtain Ni–Ti films exhibiting the beginning of the B2 ⇔ R-phase transformation between room temperature (RT) and 0 °C. Thus, films with a slightly Ni-rich composition were prepared by sputtering, without intentional heating of the substrate. The Ni–Ti films were deposited on an Si 3N 4 intermediate layer previously deposited on naturally oxidized Si(100). The crystallization behaviour of the samples (at a constant temperature of 430 °C) was studied by X-ray diffraction in grazing incidence geometry off-plane (GIXD) at a synchrotron-radiation beamline. The GIXD patterns obtained during the annealing process of the Ni–Ti polycrystalline films revealed mainly an austenitic structure (B2 phase) and the precipitation of Ni 4Ti 3. The results have also shown that the presence of an intermediate layer of Si 3N 4 enhances the crystallization process of the Ni–Ti sputtered films when compared to the films deposited directly on single-crystal Si (with native oxide). The phase transformation behaviour of the Ni–Ti film on Si 3N 4 was evaluated by XRD in off-plane Bragg–Brentano geometry during cooling (RT → −40 °C) and heating (−40 °C → RT). It has been observed that a high fraction of the Ni–Ti film is already transformed to R-phase at 9 °C (austenitic at RT), as well as a very small temperature hysteresis for the B2 ⇔ R-phase transformation. After the characterization described above, the film was removed from the substrate. The free-standing film showed a pronounced “two-way” shape memory effect (SME). In the austenitic state the film presents a flat shape. During cooling, by reducing its distance from ice cubes (i.e., decreasing the surrounding temperature), the film starts bending exhibiting a final curled shape (yet without touching the ice). On heating it recovers its flat shape. The authors attribute the nature of this “two-way” SME to the Ni 4Ti 3 precipitates that formed during the heat treatment.

The effect of Mo additions to high damping Ti–Ni–Nb shape memory alloys

The effect of Mo addition on the microstructure, phase transformation, mechanical behavior and damping capacity has been investigated. It is found that the Mo addition depresses the solubility of Nb in the TiNi matrix and results in abundant dispersive Nb-rich precipitation. The R-phase transformation is induced in Ti–Ni–9.0 at.%Nb alloys by the substitution of Mo for Ni. The yield strength and rupture strength increase with increasing Mo content due to the precipitation strengthening of Nb-rich particles and the solution strengthening of the Mo, while the elongation remains at a high level. With the increment of Mo content, the damping in the martensitic state and the damping during the phase transformation are enhanced. The increment of the martensite volume fraction and the presence of abundant Nb-rich particles are responsible for the high damping in the martensitic state. The improvement of the damping properties during the phase transformation is related to the volume fraction of the transformed martensite phase per unit time and the contribution of the R-phase transformation.

On the transformation behaviour of NiTi particulate reinforced AA2124 composites

The transformation behaviour of AA2124 alloys reinforced with NiTi particulates of 10 vol pct and 20 vol pct has been studied by differential scanning calorimetry (DSC), thermoelectric power (TEP) and internal friction (IF). Addition of NiTi particulates increases the damping capacity as well as the precipitation kinetics of AA2124 composites with respect to the base AA2124. Heat treatments performed to change the matrix condition do not alter the transformation significantly, other than homogeneising the Al level in the particulates, which is present due to the preceding extrusion treatment. The presence of Al in NiTi stabilizes the R-phase transformation but the M s remains stable.

EFFECT OF ANNEALING ON SHAPE MEMORY CHARACTERISTICS OF Ti-50.85at.%Ni ALLOY

In order to clarify the effect of annealing on the shape memory behavior of Ti -50.85at.% Ni alloy, the deformation and transformation behavior were investigated using tensile tests and differential scanning calorimeter (DSC). The martensitic transformation temperature increases with increasing annealing temperature until it reach as a maximum, and then decreases with further increasing annealing temperature. This can be rationalized by interaction between the distribution of Ti 3 Ni 4 precipitates and recovery of cold-worked structure. The R-phase transformation temperature increases with increasing annealing temperature until reaching a maximum, and then decreases with a further increase of annealing temperature. This is attributed to the change of Ni content in the matrix caused by precipitation of Ti 3 Ni 4. The critical stress for slip decreases rapidly with increasing annealing temperature, influenced by interaction between the distribution of Ti 3 Ni 4 precipitates and recovery of cold-worked structure.