Influence of Heat-Treatment on Structure and Properties of Nickel Titanium Alloy

Abstract

In this present investigation, the low temperature annealing heat-treatment was carried out at four different temperatures between 300oC and 450oC. The observation of the microstructure has been carried out using transmission electron microscopy as per ASTM F86. An EDAX analysis has been carried out as per ASTM F1375 - 92(2012) to ascertain the chemical composition. An x-ray diffraction has been carried out as per ASTM F2024 - 10(2016) to ascertain the phases present in the alloy. The DSC has been carried out as per ASTM D3418 on the alloy to analyze the transformation temperatures to confirm the superelastic nature of the material. The material has been subjected to mechanical testing by performing the tensile test as per ASTM E8 and Vickers hardness test as per ASTM E92 – 17. The tribological characteristics of the material has been analyzed as per ASTM G-132a by conducting abrasive wear test at room temperature. The superelasticity test has been performed as per ASTM F2516-18 at room temperature by varying the magnitude of strain. An electrochemical corrosion test has been conducted as per ASTM F-2129 on NiTi alloy with a prepared solution to study the corrosion resistance of the same. The salient results of the systematic investigation carried out within the scope of the investigation indicate that the chemical composition of the constituents present in the alloy assessed by EDAX analysis indicate that the Ni-Ti alloy used in this investigation is a 50:50 Ti-Ni alloy which is well within the tolerance limit for 50:50 TiNi alloy as per ASTM F1375 - 92(2012). The alloy is seen to be slightly on the Titanium-rich side. The TEM of the as-received NiTi alloy indicates the presence of martensite which appears as a needle-like region and also a shaded region indicating presence of dislocation network. Whereas the 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment at 350oC for one-hour duration indicates the presence of martensite, dislocation network and formation of NiTi alloy grains. The extent of dislocation network has relatively reduced and the grain size of NiTi alloy has relatively increased. vi The XRD of as-received NiTi alloy indicates that the presence of martensitic and austenitic phase. Whereas, the X-ray diffractogram of the 50% Nickel – 50% Titanium alloy subjected to optimum low temperature heat-treatment at 350oC for one-hour duration indicates that the presence of martensitic phase, austenitic phase and an additional Rhombohedral phase. The DSC thermogram of the as-received NiTi alloy indicates that there are no significant peaks seen in the heating as well as the cooling curves meaning that there are no distinct phase transformations of either Austenite-Martensite or Martensite- Austenite present in the material. Whereas, the DSC thermogram of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature heat-treatment at 350oC for onehour duration indicates that the material shows a single-stage transformation from austenite-martensite phase in the cooling cycle and a two-stage transformation from martensite-rhombohedral phase and rhombohedral-austenite phase in the heating cycle. The tensile test carried out for the as-received material in this investigation indicates that the material is super-elastic by nature. The comparison of the ultimate tensile strength of as-received 50% Nickel - 50% Titanium alloy and the alloy sample subjected to an optimum low temperature annealing heat-treatment of 350oC for a duration of 1 hour indicates that there is an improvement in ultimate tensile strength of 350oC heat-treated sample by 44.40% as compared to that of as-received 50% Nickel - 50% Titanium alloy. The Vickers Pyramid Number of as-received material is 421 VPN. The comparison of hardness of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment at 350oC for one-hour duration with the hardness of as-received material indicates that the hardness has increased by 14.5% as compared to hardness of as-received 50% Nickel - 50% Titanium alloy. The abrasive wear test indicates that when the load is increased, the wear mass loss rate is relatively at a higher rate upto 15N and with further increase in the load, the mass loss rate is relatively at a slower rate. The comparison of the abrasive wear between the as-received 50% Nickel - 50% Titanium alloy and 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing treatment at 350oC for one-hour duration indicates that the mass loss of low temperature annealed 50% Nickel - 50% Titanium alloy is lesser by 37.17% to 47.58% than the mass loss of as -received 50% vii Nickel - 50% Titanium alloy. This trend of reduction in the mass loss or in other words the improvement in wear resistance has been found true at all the axial loads investigated within the scope of this investigation. The variation of strain for different levels of stress during loading and after release of load at different pre-determined strain values indicate that the material even after loading upto stress level of 700 MPa does not break but returns back to the original stress value after release of the load indicating that the unloading curve had followed a hysteresis path compared to loading curve by returning back to the same point which means that the material is exhibiting superelastic behavior. The extent of improvement in Superelasticity of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment of 450oC for one-hour duration is in the range of 54.5% to 95.2 % as compared to superelasticity of as-received material. The electrochemical corrosion study carried out for as-received NiTi alloy indicates that the electrochemical corrosion rate for the material was found to be 0.0613 mm/year. The corrosion rate of 50% Nickel - 50% Titanium alloy subjected to low temperature annealing heat-treatment at different temperatures is less than the corrosion rate of asreceived 50% Nickel - 50% Titanium alloy. The extent of improvement in the corrosion resistance of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment at 350oC for one-hour duration is 35.72% as compared to corrosion resistance of as-received 50% Nickel - 50% Titanium alloy.