Physical and Mechanical Characterization of Ti50Ni50-XFeX Shape Memory Alloy Fabricated by Powder Metallurgy Process

Abstract

Shape memory alloys (SMAs) shows crystal structure transformation at low temperature. Due to this reason, the metals change their shape without any microstructural effect. Also, SMAs exhibit superior properties such as shape memory effects (SMEs) and superelasticity (SE), along with higher biocompatibility, higher corrosion resistance, low thermal conductivity, higher abrasion resistance, better fatigue resistance, and better processability. NiTi-based alloys with shape memory effect are used in different areas such as electronics, medicine, aerospace, robotics, and structural applications. Various techniques are used to alter the properties of binary NiTi alloys by adding different alloying elements. Compared to the binary NiTi alloy, the ternary TiNiFe alloy generally shows low temperature hysteresis, higher toughness, good mechanical properties, higher corrosion and wear resistance, radiopacity, and lower martensitic phase transformation temperatures. TiNiFe alloys are mostly used in aeronautical (e.g., heat-shrinkable hydraulic couplings and sleeves), couplings in jet fighters/aircraft and other engineering applications (like hydrogen storage materials and pipe couplings etc.) and actuator applications. In the present study, ternary Ti50Ni(50−X)FeX alloy (where X: 0, 2, 4, 6, 8, and 10 in at.%) has been prepared by the powder metallurgy method (mixing and with or without mechanical milling). To study the effect of the variation of Fe percentage, sintering temperature and various mixing methods on the microstructure and mechanical properties of TiNiFe alloy are investigated. The characterization of milled powders is carried out by High Temperature Differential Scanning Calorimetry (HTDSC), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) for thermal, phase and morphology analysis. Then the milled and unmilled powders were cold compacted at 600MPa pressure and sintered by conventional pressure-less sintering using Ar gas at 1050-1200 °C for 4 hours in a tubular furnace. Also, unmilled mixed powder was sintered in a SPS unit at 1000ºC for 5 min holding time and 50 MPa applied pressure at a heating rate of 50 ºC/min. The phase evolution, microstructure, shape memory effect, and physical and mechanical properties of the prepared alloy have been carried out using XRD, SEM, EDS, density, porosity, hardness, compression, indentation technique (shape memory effect), nanoindentation, and wear test. In investigation through milled routes, the 4 at.% Fe sample sintered at 1200ºC shows higher density, lower porosity, and a higher hardness value but higher compressive and yield strength values of the 4 at.% Fe sample sintered at 1150ºC were observed compared to other compositions and other sintered samples. Both 6 at.% Fe sample sintered at 1050ºC and 1200ºC shows a higher shape memory effect of 3.51% and 3.37%, respectively, than other compositions and other sintered samples. After heat treatment process, the 4 at.% Fe heat-treated sample shows higher relative density, lower porosity, higher hardness, and higher elastic modulus due to the presence of more amount secondary phases compared to other samples. But, the 10 at.% Fe heat-treated sample shows a higher shape memory effect, higher elastic recovery and higher COF value because of the more amount hard NiTi (B2) phase present than other samples. The abrasive and adhesive wear mechanisms are seen on the worn surface of the wear samples. Here, the observed NiTi phase and densification favoured the shape memory behaviours of the sample. In unmilled routes investigation, the 8 at.% Fe sample sintered at 1150ºC shows higher relative density, lower porosity, higher hardness, higher elastic modulus, and higher wear resistance due to the presence of more amount secondary phases. Still, the 2 at.%Fe sample sintered at 1050ºC shows a higher shape memory effect and elastic recovery value compared to other compositions and other sintering temperature samples because of more amount of NiTi (B19’) phase present. The overall wear mechanism of the FESEM analysis of the worn surfaces sample is mainly abrasive with minor adhesive in nature. Also, the delamination wear mechanism is seen on the worn surface of the wear samples. Again, the microstructural and phase analysis of the SPS sample includes the NiTiFe phase along with βTi, Fe2Ti and Ni-rich phases. The 8 at.%Fe sample shows higher relative density, lower porosity, higher hardness and higher wear resistance due to the presence of more amount secondary phases compared to other samples. The 8 at.%Fe sample shows a higher shape memory effect because more amounts of NiTi (B19’) martensitic phase are present compared to other samples.