Development of New Fe-based Shape Memory Alloy (Fe-Ni-Co-Ti Alloy)

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1. The properly ausaged Fe-Ni-Co-Ti alloys produce a thin plate martensite which is completely twinned. The thermal hysteresis of thin martensite is widely changed by the adjustment of chemical composition and the ausaged condition, and these thin plate martensites cause a perfect shape memory effect irrespective of the extent of the thermal hysteresis. For example, when Fe-33Ni-10Co-4Ti alloy is ausaged at 973K for 18ks, the thermoelastic martensite whose Ms and Af temperature are below room temperature is formed and the complete shape memory effect occurs during heating up to room temperature after deformation at 77 K. In the case of Fe-31Ni-10Co-3Ti alloy ausaged at 873K for 3.6ks, thin plate martensite with a large thermal hysteresis (i.e., Ms 200K, As 350K, Af 500K) is formed. This specimen which is fully austenitic at room temperature shows a complete shapememory effect when specimen is heated up above Af after the deformation at temperatures near or below Ms. The as-ausaged spec … More imen hardly exhibits the shape memory effect when deformed at room temperature. However, when the specimen is once subzero cooled to liquid nitrogen to produce a large amount of martensite, the almost complete shape memory effect occurs even by the deformation at room temperature. It has been found to be characteristic that the shape memory effect in Fe-Ni-Co-Ti alloys occurs over the wide temperature range from low temperature to high temperature by the change in the extent of thermal hysteresis.2. The ductility of austenite after ausaging is improved by the addition of B or Al to Fe-Ni-Co-Ti alloys. This is due to the suppression of precipitation of phase along austenite grain boundaries.3. When the thermoelastic martensite in Fe-33Ni-10Co-4Ti alloy is thermally cycled between 273K and 77K, the thermal hysteresis becomes markedly large, i.e., the Ms temperature is slightly decreased and the Af is markedly increased. After several thermal cycles, the Af temperature is raised above room temperature and the some amount of martensite remains even after the heating up to room temperature. Therefore, the degree of shape recovery is decreased from 100% to about 60% by the transformation. Less