Behaviour and effect of Ti2Ni phase during processing of NiTi shape memory alloy wire from cast ingot

Binary NiTi alloy is one of the commercially successful shape memory alloys (SMAs). Generally, the NiTi alloy composition used for thermal actuator application is slightly Ti-rich. In the present study, vacuum arc melted alloy of 50.2Ti–Ni (at.%) composition was prepared and characterized using optical, scanning and transmission electron microcopy. Formation of second phase particles (SPPs) in the cast alloy and their influence on development of microstructure during processing of the alloy into wire form has been investigated. Results showed that the present alloy contained Ti2Ni type SPPs in the matrix. In the cast alloy, the Ti2Ni particles form in varying sizes (1–10 lm) and shapes. During subsequent thermo- mechanical processing, these SPPs get sheared/fragmented into smaller particles with low aspect ratio. The presence of SPPs plays a significant role in refinement of the microstructure during processing of the alloy. During deformation of the alloy, the matrix phase around the SPPs experiences conditions sim- ilar to that observed in severe plastic deformation of metallic materials, leading to localized amorphisa- tion of the matrix phase

Influence of stored elastic strain energy on fatigue behaviour of NiTi shape memory alloy thermal actuator wire

Influence of stored elastic strain energy, Eelse, on thermo-mechanical fatigue behaviour of NiTi shape memory alloy (SMA)thermal actuator wire was investigated. Two near equi-atomic NiTi SMA wires obtained from different sources were evaluated for quasi-static and functional fatigue properties. Study showed that the wires had similar chemical composition, transformation temperatures and static mechanical properties. However, the functional fatigue behaviour of the wires upon thermo-mechanical cycling (TMC) was found to be significantly different. Under avariable TMC stress in the range 150–450 MPa and 4% recovery strain, one of the wires showed better stability, and substantially higher fatigue life(~30000cycles)than the other (~3500 cycles). Thermodynamic and microstructural analyses indicated that the wide variation in fatigue response of the wires was due to difference in magnitude of Eelse in the material. It is observed that at a given temperature above austenite start temperature (As), the wire with higher stored Eelse, generated about 70–100MP a higher recovery stress than that of the wire with lower stored Eelse. As a consequence,the maximum temperature, Tmax, necessary for generation of preset peak stress during reverse(martensite-austenite)transformation, was always less in the former wire than that of the latter. This in turn was responsible for wide variations in thermo-mechanical fatigue behaviour of the two wires upon TMC

Microstructure and functional behaviour of shape memory annealed Ni 24.7 Ti 50.3 Pd 25 and Ni 24.7 Ti 49.3 Pd 25 Sc 1 alloys

Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 high-temperature shape memory alloys were thermo-mechanically processed and characterized in the present study. The microstructural characteristics of cold-worked alloys showed the presence of shear/deformation bands and defects. TEM observation of cold-worked and annealed (400 to 500 °C) microstructures revealed the emergence of small fraction of newly formed martensite twins at 400 °C. With the increase in the annealing temperature from 400 to 500 °C, the fraction of the martensite twins in the microstructure was found to increase. The Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 alloys showed martensite finish temperature of ~180 °C and ~136 °C with an hysteresis of ~10 °C and ~9 °C, respectively. Ni24.7Ti50.3Pd25 and Ni24.7Ti49.3Pd25Sc1 alloys showed a recovery strain of 1.5% and 1.7% at a stress of 175 MPa. During cycling under load, the residual strain for the Ni24.7Ti49.3Pd25Sc1 alloy was found to be negligible as compared to ~0.2% in the Ni24.7Ti50.3Pd25 alloy. The results of the study indicate that these alloys have potential for use as high-temperature thermal actuator materials

A new approach to control and optimize the laser surface heat treatment of materials

Laser heating is often used to perform the surface treatment by modifying local microstructural and mechanical properties of components having complex geometries. In this study, the laser surface heat treatment of a rotating cylindrical work-piece was investigated using both experimental and numerical modeling approaches, with an aim to correlate and predict the temperature distribution during the process. The depth of the laser affected zone was predicted by solving the transient heat transfer with a moving laser heat source, using finite element analysis. The temperatures derived from the microstructural examination of the experimental specimen were found to closely agree with the predicted results from the numerical simulations. The numerical and experimental results have also led to a new observation, indicating a linear variation of the absorptivity with the laser scan speed. The prediction of the cooling curves from simulation suggested the beta -> alpha '' phase transformation and the recovery of the 3 phase, and the existence of new phases were confirmed through electron microscopy. The rapid cooling during the laser surface treatment was found to induce a flake-structure that consisted of both martensite (alpha '') and regained bcc (beta) phase. A new polynomial input power function has been proposed to achieve uniform distribution of the heat penetration along the cylinder axis, saving about 10% of the material wastage

TEM Studies on the Microstructural Changes during Thermomechanical Cycling of NiTi Shape Memory Alloy Wire

NiTi based shape memory alloys (SMAs) have been identified as potential candidates for sensors and actuators in various industrial applications. During service life of actuators, SMAs are subjected to large number of stress/strain cycles through the complete transformation range, known as thermomechanical cycling (TMC), which introduces various defects in the material. In the present study, an attempt has been made to understand the microstructural changes taking place during thermomechanical cycling of NiTi wires using Transmission Electron Microscope (TEM). These microstructural observations have been discussed in conjunction with the cyclic shape memory behavior of the wires