R-phase Transformation Temperatures Research Articles

Effect of Fe Composition and Heat Treatment Temperature on the Transformation Temperature of R-Phase of Ti-Ni-Fe Alloy

R-phase transformations in TiNi shape memory alloys (Nitinol) have various applications because of their small thermal hysteresis and low fatigue resistance. Adding a third element such as Fe or Al is one of the useful ways to induce R-phase transformation. Controlling the R-phase transformation temperature is crucial for industrial and bio-field applications. However, the effects of adding Fe have rarely been reported. In this study, the effects of Fe addition and heat treatment temperature on the R-phase transformation temperature of TiNiFe shape memory alloys were systematically investigated. Results showed that increasing Fe composition decreases the R-phase and martensitic transformation temperatures of 19K/1at%Fe and 51 K/1at%Fe, respectively. Additionally, in the Ti–49Ni–1Fe alloy, rather than a martensite transformation, the R phase transformation temperature was constant irrespective of heat treatment temperature and increasing number of thermal cycles. This means that the R-phase transformation temperature is not affected by dislocation density resulting from the heat cycling or cold working processes. This allows practical applications of R-phase transformation to be easily realized. Furthermore, this means that the R-phase transformation temperature is only affected by the Ni content of the matrix.

Two-step constrained aging treatment enabled superior two-way shape memory effect and elevated R-phase transformation temperatures in a rapidly solidified Ni51Ti49 alloy

A novel two-step constrained aging (CA) treatment, in the form of CA firstly at 500 °C for 1 h and then CA at 300 °C for 39 h, is proposed to obtain enhanced two-way shape memory effect (TWSME) and elevated R-phase transformation temperatures of the Ni51Ti49 alloy strip fabricated by rapid solidification technique. The two-step constrained aged Ni51Ti49 alloy strip possesses a much increased recovery ratio of 96.4% in the fully martensitic state, and elevated R-phase transformation start and finish temperatures of 60.2 °C and 40.9 °C respectively, compared with these of the alloy subjected to the one-step CA treatment. Higher recovery ratio and elevated R-phase transformation temperatures are attributed to preferentially oriented lenticular coherent Ni4Ti3 precipitates, with an average length of 149.1 ± 14.3 nm, dispersed homogeneously in the NiTi matrix. The first-step CA with a relatively high aging temperature triggers the fast growth of Ni4Ti3 precipitates, while the second-step CA with a low aging temperature leads to decrease of the solid solubility of Ni atoms and promotes further precipitation of Ni atoms and thus induces formation and growth of Ni4Ti3 precipitates.

Uniaxial Pre-strain and Free Recovery (UPFR) as a Flexible Technique for Nitinol Characterization

The measurement of phase transformation temperatures of superelastic (SE) and shape memory (SM) NiTi alloy products and components was studied in this work. The transformation temperatures of a set of twenty different 300 μm NiTi superelastic wires were measured by two well-established and standardized techniques, namely differential scanning calorimetry (DSC) and bend and free recovery (BFR) and then compared with the results from the Uniaxial Pre-Strain and Free Recovery (UPFR) test. UPFR is a tension-based test, whose aim is to overcome the limitations associated with BFR testing. Within this work, a test procedure has been set up and validated. UPFR is found to be the only method showing a very strong correlation with the mechanical properties measured using the standard uniaxial tensile test method for superelastic NiTi alloy. Further, UPFR has been validated as a robust technique for measuring the R-phase and austenitic transformation temperatures in specimens of various sizes, composition, and of different geometries. This technique overcomes the limitations of BFR and DSC which cannot be used for testing products such as 25 μm SM wire, a 50 μm SE strip, and different springs and microsprings for actuation.

Fundamental Development on Utilizing the R-phase Transformation in NiTi Shape Memory Alloys

In near equiatomic NiTi alloys, the reversible thermoelastic transformation between B2-structured austenite phase and the R-phase is attracting increasing interest for practical applications. However, the following two issues limit the widespread utilization of the R-phase transformation: (1) there is no effective approach to control the R-phase transformation temperatures; (2) it is not easy to largely separate the temperature domain of the R-phase and the B19′ martensite phase transformation, especially in the presence of an external force. This article reviews concisely the work of the present authors on solving the above two problems. The effect of grain size on the aging microstructure and related transformation behavior is first discussed. Inspired by these findings, an approach to solve the above two problems has been developed by introducing nanoscaled Ni4Ti3 precipitates in the samples with micron-sized grains. The performance of alloys associated with the R-phase transformation, which shows controllable transformation temperatures, is summarized.

Effect of post-deformation annealing on the R-phase transformation temperatures in NiTi shape memory alloys

In NiTi shape memory alloys, both the annihilation of dislocations and the formation of Ni4Ti3 precipitates may occur during post-deformation annealing. Different responses of the R-phase transformation temperatures to the annealing conditions have been reported. In order to find out the main factor(s) affecting the R-phase transformation temperatures during post-deformation annealing, a Ti-49.8 at% Ni and a Ti-50.8 at% Ni alloy were subjected to various post-deformation annealing and thermal cycling treatments. The results show that the R-phase transformation temperatures are very stable in the Ti-49.8 at% Ni alloy, while a significant variation is observed in the Ti-50.8 at% Ni alloy with respect to the annealing and thermal cycling conditions. These findings suggest that the R-phase transformation temperatures are not susceptible to the change of dislocation density and depends mainly on the Ni concentration of the matrix, which can be modified by the formation of Ni4Ti3 precipitates.

R-phase transformation in NiTi alloys

In near equiatomic NiTi alloys, the thermoelastic transformation between austenite and the R-phase shows unique properties, which make the R-phase transformation very promising for applications. In the present paper, the fundamental issues related to the R-phase transformation, especially the effects of different thermomechanical treatments, are reviewed. Inspired by the literature review, recent work on controlling the R-phase transformation temperature by low temperature aging treatments is summarised.

Thermomechanical properties of Ni–Ti shape memory wires containing nanoscale precipitates induced by stress-assisted ageing

This paper systematically examines the thermomechanical properties and phase transformation behaviour of slightly Ni-rich Ni–Ti biomedical shape memory wires containing homogeneously distributed nanoscale precipitates induced by stress-assisted ageing. In contrast to previous studies, particular attention is paid to the role of precipitates in impeding twin boundary movement (TBM) and its underlying mechanisms. The size and volume fraction of precipitates are altered by changing the ageing time. The martensitic transformation temperatures increase with prolonged ageing time, whereas the R-phase transformation temperature remains relatively unchanged. The stress–strain behaviour in different phase regions during both cooling and heating is comprehensively examined, and the underlying mechanisms for the temperature- and thermal-history-dependent behaviour are elucidated with the help of the established stress–temperature phase diagram. The effect of precipitates on TBM is explored by mechanical testing at 133K. It is revealed that the critical stress for TBM (σcr) increases with increasing ageing time. There is a considerable increase of 104MPa in σcr in the sample aged at 773K for 120min under 70MPa compared with the solution-treated sample, owing to the presence of precipitates. The Orowan strengthening model of twinning dislocations is insufficient to account for this increase in σcr. The back stress generation is the predominant mechanism for the interactions between precipitates and twin boundaries during TBM that give rise to the increase in σcr. Such results provide new insights into the thermomechanical properties of precipitate containing Ni–Ti biomedical shape memory wires, which are instructive for developing high-performance biomedical shape memory alloys.

Structure and Properties of Ni-Rich Shape Memory Alloy Subjected to Severe Deformation and Annealing

Ni-rich alloy (50.8at.%Ni) was plastically deformed by compression combined with reversion oscillating torsion. The compression rate was 0.05 mm/s and the torsion frequency and angle were 1,5Hz and ±3°. After the deformation process the specimens were annealed at 350°C/1h (623K/1h) and 400°C/1h (673K/1h). Their structure and transformation characteristic were studied with the use of TEM, X-ray phase analysis and DSC measurements. TEM observation of specimens after annealing at 350°C/1h (623K/1h) indicate presence of highly deformed B2 phase. In the specimen annealed at 400°C/1h (673K/1h) additionally Ni4Ti3 precipitates and the R-phase in the B2 matrix was found. DSC measurements show that the transformation sequence changes for the deformed and then annealed samples from single step B2  B19’ transformation to the B2  R  B19’ on cooling. Also increase of R-phase transformation temperature TR with the decrease of annealing temperature was observed.

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.

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.

Effect of mechanical cycling on stabilizing the transformation behaviour of NiTi shape memory alloys

The influence of mechanical cycling on the behaviour of NiTi shape memory alloy (SMA) wire in the martensitic and R-phase transformation range is analyzed as a function of the cycling temperature in order to obtain the best cycling conditions: that is to say, the material stabilizes after few cycles and the post-cycling mechanical properties are good. Mechanical cycling, irrespectively of temperature, tends to lower the transformation stresses and move the transformation lines on the stress–temperature plane in the direction of low stress and high temperature as cyclic deformation progresses. The elastic modulus also changes significantly with cycling but the trend is different at lower temperatures than at higher ones. If the mechanical cycling is performed at temperatures well above or below the R-phase transformation temperatures, this phase tends to vanish or diminish with the mechanical cycling. The main constitutive parameters that describe the SMA mechanical behaviour depend significantly on the temperature at which the wire is cycled and it is best to obtain the parameters at the lowest temperature at which the material is to be used. The sample cycled at the lowest temperature presents the best mechanical properties, which tend to be stable for at least 100 mechanical cycles.

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature ( 600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.

Effects of proton irradiation on transformation behavior of TiNi shape memory alloy thin films

In this work, Ti–49.8 at.% Ni shape memory alloy (SMA) thin films were irradiated by 60-keV protons with two types of doses: 5×1016 and 1×1017 H+/cm2. The influence of proton irradiation on the transformation behavior of TiNi shape memory alloy films was investigated by differential scanning calorimetry (DSC) and glancing-incidence X-ray diffraction (GIXRD). The martensite transformation temperatures and austenite transformation temperatures shifted to lower value after proton irradiation, whereas proton irradiation has little effect on the R-phase transformation temperatures. This observation can be attributed to the formation of hydrides after proton irradiation.

Effects of Composition and Annealing on Shape Memory Behavior of Ti-Rich Ti-Ni Thin Films Formed by Sputtering

Ti-rich Ti-Ni thin films of Ti-45.2, 46.1, 47.0, 47.9, 48.5 at%Ni were prepared by sputtering. The sputter-deposited thin films were annealed at 773, 823 and 873 K for I h. Transmission electron microscopy revealed that Ti-45.2 at%Ni thin films contain randomly oriented Ti 2 Ni particles, while the other films contain Ti 2 Ni precipitates with the same orientation as that of the TiNi matrix. In addition to these Ti 2 Ni precipitates, GP zones were also observed in Ti-47.9 and 48.5 at%Ni thin films annealed at 773 K for 1 h. The shape memory behavior of these Ti-Ni thin films was investigated with a thermomechanical tester. The following results were obtained with respect to the martensitic transformation. (1) The transformation temperature decreases with increasing Ti content and decreasing annealing temperature. However, thin films containing GP zones show low transformation temperatures despite of the low Ti contents. (2) The critical stress for plastic deformation associated with the transformation increases with increasing Ti content and decreasing annealing temperature. GP zones seem to increase the critical stresses of Ti-47.9 and 48.5 at%Ni thin films annealed at 773 K for 1 h. (3) The recoverable strain increases with decreasing Ti content and decreasing annealing temperature. Especially, the thin films containing GP zones show large recoverable strains, On the other hand, the R-phase transformation showed the following features. (1) The transformation temperature is insensitive to film composition and heat treatment, being almost 335 K, though thin films containing GP zones show relatively low R-phase transformation temperatures. (2) No residual strain is detected for any specimen after a thermal cycle. These characteristics suggest that the shape memory effect due to the R-phase transformation is suitable for practical use.

The Effect of Cold Work, Heat Treatment, and Composition on the Austenite to R-Phase Transformation Temperature of Ni-Ti Shape Memory Alloys

The influence of cold work (CW) and heat treatment (HT) on the austenite to R-Phase (A→R) transformation temperature (TT) of a near equiatomic and three other Ti rich NiTi SMA is examined. For the SMA having a near equiatomic composition, the A→R TT increases with increasing CW at low HT temperatures. For the SMA having the maximum possible Ti content, the A→R TT decreases with increasing CW at low HT temperatures. For all compositions, the A→R TT is not sensitive to CW at high HT temperatures. At a Ti content slightly below the maximum possible, the A→R TT is relatively insensitive to CW and HT temperature. For all of the SMA investigated, the A→R TT increases with increasing Ti content for a specific CW and HT temperature, and this effect is greatest at low CW and at high HT temperatures.

Evolution of the R-Phase Transformation Temperature of Ti50Ni50-xFex Shape Memory Alloys with Iron Concentration

AbstractThe evolution of the R-phase transformation temperature with iron concentration is studied by Mössbauer spectroscopy (MS), magnetic susceptibility and calorimetric measurements in a series of three Ti50Ni50-xFex, samples, with x = 1, 2 and 3. The changes in the Mössbauer spectra, from a single line at room temperature to a quadrupole doublet when the temperature is lowered below 280 K, are related to structural transformations occurring in the system. The temperature changes in the magnetic susceptibility are interpreted in terms of a gap opening characteristic of charge density waves (CDW) formation when the R-phase transformation takes place. The calorimetric measurements show specific heat fluctuations near room temperature in two of the samples (x = 1 and 3) whose peaks shift during the cooling and heating cycles.