Determination of Stress Vs. Temperature Phase Diagram Of Niti Shape Memory Alloys With Different Porosity Levels
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Having many superior properties compared to conventional materials, shape memory alloys (SMAs) have been studied and used in many engineering applications over the last four decades. As one of the commonly used SMAs, NiTi alloys also distinguish from Cu-based and Fe-based SMAs with their large recoverable strains, biocompatibility, and thermo-mechanical performance which make them promising materials in aerospace, biomedical and other commercial applications. Beside these properties, porous NiTi alloys also provide lower density with high stiffness, higher gas permeability and better biomechanical compatibility due to its Young's Modulus which is similar to bones. In spite of these advanced properties, thermo-mechanical behavior of porous NiTi alloys has not been well-known yet. In this study, stress vs temperature phase diagram and super-elasticity behavior of porous Ni-rich NiTi alloys with different porosity levels including 10%, 20%, 30% and 40% by volume were determined by microstructural, thermal and mechanical characterizations tests. Porous specimens with 10 mm diameter were produced from NiTi and Mg powders with hydraulic pressing and conventional sintering. The porosity in these specimens was formed by space holder technique with magnesium powder particles. The porosity levels and porosity architecture of specimens were validated via using Optical Microscope (OM). The thermal characterization of the porous alloys was done by DSC analysis to determine the phase transformation temperatures (Ms, Mf, As and Af) of the samples. The superelasticity experiments with the application of compressive force to these specimens were conducted for the determination of the Md point. Md point is defined as the highest temperature at which stress induced martensite can be obtained by loading. Uniaxial compression tests performed at different temperatures starting from Ms+15°C by loading up to 3% constant strain value was achieved and unloading to 0 MPa level. The temperature of the specimen was increased by 30°C in each cycle till the reverse transformation was vanished. After these procedures, the phase diagrams of Ni-rich NiTi alloys with different porosity levels were obtained in order to identify temperature window at which superelasticity was observed for novel actuator applications.