Transformation Behaviour Of NiTi Research Articles

Deformation induced martensite stabilization of NiTi in constrained composite systems

This study investigated the effect of tensile deformation on the transformation behaviour of NiTi within the constrained environment of NiTi–Nb composites. Particular attention is given to the deformation-induced stabilisation of the stress-induced martensite manifested as the increase of the reverse transformation temperature. Two different forms of the NiTi–Nb composites were used, including nanowires and microparticles, in comparison with the single-component binary NiTi alloy. This stabilisation effect has been reported for NiTi in the literature. The effect is attributed to the changes in the internal elastic and plastic states of the martensite variant structure caused by the deformation. The NiTi–Nb nanocomposites present different internal mechanics environments. It was found that the presence of a second phase within the NiTi matrix hinders the lattice distortion martensitic transformation, enlarging its hysteresis in the order of the binary NiTi, the NiTi–Nb microparticle composite and the NiTi–Nb nanowire composite. After deformation, the NiTi–Nb microparticle composite exhibited the highest stabilisation effect, followed by the NiTi–Nb nanowire composite and then the binary NiTi alloy. These contrasting observations are attributed to dual effects of the Nb inclusions within the NiTi matrix, to obstruct the lattice distortions of the martensitic transformation, and to retain elastic strains and thus stresses that may assist and resist a transformation depending on their relative directions.

Characterization of NiTi Shape Memory Alloy Sintered at Different Temperatures in Reducing Environment

NiTi has received significant interest as medical implant materials due to its shape memory effect behavior apart from its good biocompatibility and mechanical properties. The formidable challenge of obtaining single phase NiTi from elemental powders via solid state is due to oxidation problem of elemental powders and the oxygen atoms dissolve in NiTi matrix as interstitial impurities forming stable oxygen-rich TiNiOx. This may deterioriate the shape memory behavior of NiTi. This research investigates the use of MgH2 in combination with CaH2 as in-situ reducing agent to eliminate oxidation of the specimen during sintering both at lower and higher sintering temperatures. Here, the effect of sintering temperature on phase formation and transformation behavior of NiTi in reducing environment was studied. The phase formation was characterized by using x-ray diffraction (XRD) where the morphology and elemental analysis were characterized by using the scanning electron microscope (SEM) equipped with EDS. The martensitic transformation behavior was analyzed using differential scanning calorimeter (DSC). The use of MgH2 and CaH2 as reducing agent has a significant influence on the phase formation of NiTi synthesized via solid state especially at 930 °C, where almost single phase NiTi was formed with good transformation behavior. This reducing agent creates a conducive environment for the production of single phase NiTi.

Comparison Studies on Phase Formation and Transformation Behavior between Ni-Ti and Ni-TiH2 Synthesized via Solid State Sintering

Solid state synthesis is an alternative processing technique that has been attempted for producing novel-structured NiTi such as porous NiTi and other novel forms. Whilst shape memory effect is a unique advantage of NiTi shape memory alloy like, no successful attempt so far to produce single phase NiTi with good martensitic transformation behavior. As oxidation of Ti is the main problem that may retard the formation of single phase NiTi, the preliminary study of the effect of powder precursor was done. In this research, a systematic comparative investigation was performed on phase formation and transformation behavior of Ni-Ti and Ni-TiH2 sintered specimen. It revealed that no martensitic transformation was observed for Ni-Ti specimen, attributed to minor NiTi phase formation. In addition, the XRD result of powder analysis indicates that part of Ti powder was oxidized during specimen preparation which creates imbalance composition for Ni-Ti reaction. In contrast, Ni-TiH2 specimen displayed martensitic transformation behaviour attributed to high amount of NiTi phase formation. This reflects that the use of TiH2 to replace Ti as powder precursor has a significant effect towards NiTi phase formation and its transformation behaviour.

Effect of Titanium Depletion of High Temperature Treated NiTi Shape Memory Alloy Wire

Shape memory behaviour of NiTi is very sensitive to the presence of inclusion in the matrix and oxide and the surface. This work studied the influence of high temperature treatment towards thermal martensitic transformation behaviour of NiTi. The alloy was subjected to heat treatment at 1223 K for various times in argon and air environment. It was found that treatment in air severely destroyed the shape memory property of the alloy due to the formation of multilayer of oxides. Treatment in argon also reduced the volume percentage of martensite phase transformation, even though no oxide layer was observed at the surface. This reduction is caused by the depletion of Ti element in the matrix due to the outward diffusion of Ti element. The outward diffusion of Ti is however stabilized at 15 hours exposure time of 1223 K heating temperature.

Partial thermal cycling of NiTi

Effect of partial thermal cycling through incomplete B2 → B19′ martensitic transformation on the transformation behaviour of NiTi was investigated. This investigation was carried to assess the property stability of NiTi during operation in applications involving repeated transformation cycles. Partial thermal or mechanical cycling has been considered a remedy for more stable operation in such applications. It was observed that on equal transformation volume basis partial transformation cycling was less damaging compared to full transformation cycling. This observation is indicative that less structural damages are created during the early stages of the B2 → B19′ martensitic transformation and that incomplete transformation cycling has certain advantage in minimising the shift of shape memory properties of NiTi.

Effect of incomplete thermal cycling on the transformation behaviour of NiTi

The effect of partial thermal cycling through incomplete transformation cycles on the transformation behaviour of NiTi was investigated. This investigation was carried out with two purposes: (1) to establish an understanding of the evolution of internal plastic deformation during the process of thermally induced B2 ↔ B19′ martensitic transformation and (2) to provide experimental data for the application of NiTi alloys involving repeated transformation cycles. It was observed that on equal transformation volume basis partial reverse transformation caused the least changes to transformation behaviour, full transformation cycling caused the most severe changes whereas partial forward transformation cycling was in between. These experimental observations indicate that more structural damages are created towards the end of the forward transformation than at the early stages of the transformation and that more structural damages are created during the forward transformation than during the reverse transformation.

Effect of aluminum addition on the transformation of NiTi alloy

The transformation behavior of ternary NiTi(Al) alloy was investigated with a differential scanning calorimeter in order to clarify the effect of aluminum substitution in NiTi, showing the shape memory due to the thermoelastic martensitic transformation between a high temperature phase and a low temperature phase. An intermediate phase appears due to aluminum substitution and its temperature region is broadening with increasing number of thermal cycles. By replacing titanium by aluminum, a unique behavior is observed in the transformation from the intermediate phase to the low temperature phase. The transformation behavior of NiTi(Al) depends significantly on the concentration of the substituted aluminum. The temperature region of the intermediate phase is controlled by the substituted aluminum. The shape memory effect also occurred in the transformation between the high temperature and the intermediate phases. Therefore, the substitution of aluminum is useful for various applications of the transformation to the intermediate phase.

Effect of deformation by stress-induced martensitic transformation on the transformation behaviour of NiTi

It has been reported that martensite in near-equiatomic NiTi is thermally stabilised after a moderate deformation via martensite reorientation. This work continues the study by investigating the effect of deformation via stress-induced martensitic transformation on the transformation behaviour of the alloy. It was observed that the stress-induced martensite was also stabilised relative to the thermal martensite formed on cooling, as indicated by an increase in the critical temperature for the reverse transformation. Associated with the stabilisation, the heat effect, as determined by differential scanning calorimetry, and the temperature interval of the reverse transformation, were measured and found to decrease with increasing level of deformation. The experimental results also demonstrated that the stress-induced martensitic transformation was microscopically localised, as expected for a first-order phase transformation.

Transformation behaviour of NiTi in relation to thermal cycling and deformation

The transformation behaviour of an equiatomic NiTi alloy was investigated by measurement of the electrical resistivity and by differential scanning calorimetry. The temperature region of transformation depends on the number of thermal cycles and the intermediate phase appears progressively during cooling with increasing thermal cycles, resulting in a two-stage transformation, while the reserse transformation during heating remains a one-stage transformation. The temperature region of transformation is broadened by plastic deformation, and with subsequent annealing the intermediate phase is enhanced, depending on the annealing temperature. Effects of thermal cycling and annealing after deformation on the transformation of NiTi are attributable to transformation-induced defects as dislocations, and it is concluded that the defects make it possible to stabilize the intermediate phase.