2020 Volume 84 Issue 6 Pages 200-207
Titanium alloys have been used not only in air frames for commercial aircrafts, but also in its jet engine components such as fans and compressor disks, which have a function at relatively low temperatures up to 673 K. Near β-type Ti-5Al-2Sn-2Zr-4Cr-4Mo (Ti-17) exhibits greater strength, crack propagation resistance, and creep resistance at intermediate temperatures compared with those of α+β-type Ti-6Al-4V. In particular, it is important to estimate the fatigue life of engine components made of Ti-17. To solve this problem, the quantitative relationship between fatigue properties and microstructural factors of Ti-17, therefore, in this study, the fatigue properties including tensile properties and microstructures of Ti-17 samples fabricated by hot-forging hot-forged at various temperatures followed by high and low temperature solution treatment (ST) and same aging treatment were investigated to define the quantitative relationship between the fatigue properties and the microstructural factors.
The microstructures of all forged Ti-17 samples exhibit elongated prior β grains composed of two different microstructural feature regions: mainly acicular α and fine equiaxed α phase regions. The volume fraction of acicular α regions decreases with in the increasing ST temperature. The Vickers hardness, 0.2％ proof stress and tensile strength increases with increasing ST temperature. On the other hand, the elongation and reduction of area exhibit a reverse trend to that of Vickers hardness, 0.2％ proof stress and tensile strength. The Ti-17 samples forged at 1173 K followed by solution treatment at 1073 K and aging treatment exhibits the highest fatigue limit of 975 MPa. The fatigue strength of the forged Ti-17 samples is considered to be strongly related with the microstructural factor such as the volume fraction of the equiaxed α phase region, which is one of crack initiation sites in the forged Ti-17 samples subjected to ST at low temperature and aging, and the difference in strength between the acicular α phase and the fine α+β phase region, which leads to the crack initiation in the forged Ti-17 sample subjected to ST at high temperature and aging.
