DSC-investigation of ultra thin sheet of NiTi Shape Memory Alloy

The objective of this work was to develop and investigate the 3d-transition metal NiTi shape memory alloy for its transformation temperatures. The alloy was prepared by using chips of metallic Ni and Ti in the vacuum arc melting furnace. Melting was carried out under the protective atmosphere of Argon gas. An ultra-thin sheet (0.3mm) was prepared by cold-rolling the alloy. An intensive cold rolled sheet of NiTi was inspected for structural, microstructure, elemental composition and thermal cyclic behavior using XRD, FE-SEM, EDX and DSC studies respectively. The alloy sheet sample was annealed in the temperature range of (300-900) ˚ C for 20 minutes. DSC scan was carried out in the temperature range between (200 C→ -60 C→ 200 C for as rolled, annealed and for thermal cycling. In the early stage of annealing, highly diminished martensitic and austenitic peaks in the DSC-thermo gram evidenced that the alloy microstructure structure was under high stress. An optical micrograph revealed the rolled structure of the alloy sheet. DSC examinations further revealed that the cyclic and phase-transformation behaviour of the alloy was significantly influenced by heat treatment. As a result of annealing, the transformation temperatures of austenite and martensite have shifted from 37.922 C to 61.344 C and from 53.453 C to 72.322 C respectively. The hysteresis between martensite and austenite decreases by 10.978 C as the annealing temperatures increase. Stable behaviour of transformation temperature is observed after annealing at 650 C and above. No change in the transformation temperatures was observed during thermal cycling.  This study concludes that the variation in the transformation temperatures is the result of stress relief of martensitic plates during annealing treatment. The stable behaviour of transformations temperatures during annealing as well as during thermal cycling is due to dislocation tangling and precipitation.