準安定β型チタン合金の時効特性

Abstract

Aging characteristics of the beta metastable titanium alloys Ti-15Mo-5Zr and Ti-15Mo-5Zr-3Al were investigated with detailed aging treatments by means of the tensile test, hardness measurement, microstructure observation and the X-ray diffraction method.In the as-solution treated condition, the strength decreased with increasing solution temperature and the relationships between the yield strength (YS kg/mm2) and the beta grain diameter (d mm) were given as follows:(This article is not displayable. Please see full text pdf.) The optimum solution temperature for the aged material was beta transus for Ti-15Mo-5Zr and 50°C below the beta transus for Ti-15Mo-5Zr-3Al, as the finer beta grains gave the good tensile properties after heat treatment.In the Ti-15Mo-5Zr alloy solution treated at the beta transus and aged at 400°C, the maximum age hardening and the minimum value in ductility were observed for each aging time. It was found from the X-ray diffraction results that these phenomena were related to the omega phase precipitation. In this alloy, the alpha phase precipitation led to the decrease in tensile strength and the increase in ductility. In the Ti-15Mo-5Zr-3Al alloy the omega phase precipitation during aging was suppressed by aluminum addition and the age hardening was related to the amount of the alpha phase strengthened by aluminum. The largest age hardening was observed after heat treatment at 425 to 450°C. Therefore, the mechanical properties were controlled by the omega and the alpha precipitation in the Ti-15Mo-5Zr alloy and mainly by the alpha precipitation in the Ti-15Mo-5Zr-3Al alloy. The tensile properties obtained by the optimum heat treatment conditions are as follows:(This article is not displayable. Please see full text pdf.) The notched tensile test, the impact test and the fatigue test showed that the Ti-15Mo-5Zr-3Al alloy hardened by the alpha precipitation was superior to the Ti-15Mo-5Zr alloy hardened by the omega precipitation as a commercial alloy, with good combinations of strength, ductility, toughness and fatigue property.