TiNi Shape Memory Alloys with High Sintered Densities and Well-Defined Martensitic Transformation Behavior

摘要

This study demonstrates that a high density and a high transformation heat can be attained for PM TiNi. With the use of fine elemental powders, a composition of Ti₅₁Ni₄₉, two-step heating, and persistent liquid-phase sintering at 1553 K (1280 °C), a 95.3 pct sintered density was attained for compacts with a green density of 66 pct. A transformation heat, ΔH, of 31.9 J/g was also achieved, which is much higher than reported previously for sintered TiNi and is approaching the highest ΔH reported to date, 35 J/g, for wrought TiNi with low C, O, and N contents. The main reason for having these properties in powder metal TiNi with higher amounts of C, O, and N is that the extra Ti, that over the equiatomic portion in the Ti-rich Ti₅₁Ni₄₉, forms Ti₂Ni compound, which traps most of the C, O, and N. This results in low interstitial contents and a high Ti/Ni ratio of 50.5/49.5 in the TiNi matrix. The tensile strength, elongation, and shape recovery rate after five training cycles were 638 MPa, 14.6, and 99.1 pct, respectively, despite the presence of Ti₂Ni compounds at grain boundaries.