The Effect of Cooling Rate from Solution Treatment Temperature on Phase Constitution and Tensile Properties of Ti-4.3Fe-7.1Cr-3.0Al Alloy

摘要

Titanium and its alloys are one of very attractive metallic materials for health-care and welfare goods, because these alloys have high specific strength and high biocompatibility. However, high cost of Ti alloys is disadvantage in application to the health-care and welfare goods. To overcome high cost barrier of Ti alloys, Ti-4.3Fe-7.1Cr-3.0Al alloy was developed. This alloy has good tensile properties, i.e. about 1 GPa as tensile strength, about 20 percent as elongation and about 50 percent as reduction in area, in solution treated state. It is very important that effect of cooling rate from solution treatment temperature on tensile properties is investigated, because diffusion coefficients of Fe and Cr in beta phase are higher than other beta stabilizers, e.g. V and Mo. When a beta stabilizer with higher diffusion coefficient is contained in beta phase, isothermal w precipitation that makes the alloy brittle becomes fast. To suppress isothermal m precipitation, it is necessary to set the cooling rate higher than an appropriate value. In this study, the effect of cooling rate from solution treatment temperature on phase constitution and tensile properties was investigated by electrical resistivity and Vickers hardness measurement and tensile test in Ti-4.3Fe-7.1Cr-3.0Al alloy. In Ti-4.3Fe-7.1Cr-3.0Al alloy cooled by a cooling rate of 0.46 Ks~(-1) or more, only beta phase was identified by X-ray diffraction, while beta and alpha phases were identified in the alloy cooled by furnace cooling, i.e. 0.024 Ks~(-1). Resistivity ratio remained almost constant between 0.46Ks~(-1) and 34.7Ks~(-1). In specimen cooled by 0.024 Ks~(-1), resistivity ratio significantly decreased and HV drastically increased because of a phase precipitation. Tensile strength remained about 1020 MPa between 0.46 Ks~(-1) and 34.7 Ks~(-1). In specimen cooled by 0.024 Ks~(-1), tensile strength slightly increased. Elongation remained almost constant between 0.85 Ks~(-1) and 34.7 Ks~(-1), and then decreased with decrease in cooling rate below 0.46 Ks~(-1).