Ti-Fe-Cr-Al 4 元系合金のミクロ組織と機械的性質に及ぼす鉄添加量と冷却速度の影響

Titanium and its alloys first found applications in the fields of aerospace and chemical engineering. Recently, titanium alloys have been used in consumer products due to their high specific strengths and corrosion resistances. They have also been employed in medical applications because of their excellent biocompatibility. Unfortunately, the principal alloying elements of titanium alloys are very expensive due to their low crustal abundances; this is especially true of the beta-stabilizing elements (e.g., V, Mo, Nb, and Ta). Although titanium is considered to be a ubiquitous element since it has the tenth highest Clarke number of all the elements, it is actually classified as a rare metal because its current refinement process is more environmentally damaging than the processes used to refine iron or aluminum. By contrast, iron and chromium are very attractive as beta stabilizers for titanium alloys since iron is highly ubiquitous and chromium is relatively inexpensive and abundant, while aluminum is very attractive as an alpha stabilizer. It is thus very important to investigate the properties of Ti-Fe-Cr-Al system alloys. In this study, the effects of iron content and the cooling rate after solution treatment on the tensile properties of Ti-3.2 to 5.1 mass%Fe-7.1 mass%Cr-3.0 mass%Al alloys were investigated. This investigation was done by measuring their electrical resistivities and Vickers hardnesses, by performing X-ray diffraction and optical microstructure investigations, and by conducting tensile testing. It was found that the beta phase could only be identified in solution-treated and quenched (STQed) alloys. For alloys in the STQed state, the resistivity ratio decreased with an increase in iron content, whereas the Vickers hardness exhibited the opposite trend. From the results of isochronal heat treatment, the beta transus was estimated to be 1093 K in 3.2 and 3.5Fe alloys and 1073 K in 3.9Fe to 5.1Fe alloys. Alloys in the STQed state have almost the same tensile strength, elongation, and area reduction. The tensile strength, elongation, and area reduction in all the alloys cooled in air were almost the same as those of the STQed alloys. Employing furnace cooling increased the tensile strength of the alloys, but reduced their elongation and area reduction. For the air-cooled alloys, the tensile properties of the Ti-3.2 to 5.1 mass%Fe-7.1Cr-3.0Al alloys were almost the same as those of the STQed alloys.