Monotonic and cyclic compressive behavior of superelastic TiNi foams processed by sintering using magnesium space holder technique

TiNi foams with porosity contents varying in the range of 39-58 vol% were processed by sintering with space holder technique. Magnesium was employed as the space holder material because of its relatively low boiling temperature as well as high oxygen affinity. Processed foams were characterized in terms of both microstructural and mechanical aspects. Scanning electron microscope (SEM), X-ray diffractometer (XRD) and transmission electron microscope (TEM) were employed for microstructure and phase analyses while mechanical characterization of the foams was conducted via monotonic compression and superelasticity cycles both conducted at room temperature. It was observed that homogeneity of the microstructural features as well as high oxygen affinity of the magnesium, which had prevented the oxidation of TiNi during sintering, led to outstanding mechanical properties especially considering the porosity contents achieved as high as 58 vol%. A deformation model for TiNi foams under compression was proposed encompassing the variation in their compression response at different porosity contents. Furthermore, superelastic behavior of processed TiNi foams were examined in detail to reveal the deformation mechanism that is active at each stage of deformation, and effect of porosity and previous training at lower stress levels was also investigated seperately.