Materials Transactions Online

Materials Transactions, Vol.61 No.10 (2020) pp.2017-2024
© 2020 The Japan Institute of Metals and Materials

Relationship between Microstructure and Fatigue Properties of Forged Ti-5Al-2Sn-2Zr-4Mo-4Cr for Aircraft Applications

Saki Tanaka1, Toshikazu Akahori2, Mitsuo Niinomi2, 3, 4, 5, 6 and Masaaki Nakai7

1Graduate School of Science and Technology, Meijo University, Nagoya 468-8502, Japan
2Faculty of Science and Technology, Meijo University, Nagoya 468-8502, Japan
3Institute for Materials Research, Tohoku University, Sendai 980-5377, Japan
4Department of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
5Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8601, Japan
6Faculty of Chemistry, Materials and Bioengineering, Kansai University, Osaka 564-860, Japan
7Faculty of Science and Technology, Kinki University, Higashiosaka 577-8502, Japan

Titanium alloys have applications in air frames for commercial aircraft, and jet engine components such as fans and compressor disks, which function at low temperatures (up to 673 K). Near β-type Ti–5Al–2Sn–2Zr–4C–4Mo (Ti-17) exhibits greater strength, crack propagation resistance, and creep resistance at intermediate temperatures compared to the (α + β)-type Ti–6Al–4V. It is important to estimate the fatigue life of engine components made of Ti-17. This requires problem quantitative relationship between the fatigue properties and microstructural factors of Ti-17. Therefore, the fatigue properties including tensile properties and microstructures of Ti-17 samples fabricated by hot-forging at various temperatures, followed by high- and low-temperature solution treatment (ST), and same aging treatment were investigated to define a quantitative relationship between the fatigue properties and the microstructures.

The microstructures of all forged Ti-17 samples exhibit elongated prior β-grains composed of two microstructural feature regions: acicular α and fine equiaxed α-phase regions. The volume fraction of the acicular α region decreases with increasing ST temperature. The Vickers hardness, 0.2% proof stress and tensile strength increases with increasing ST temperature. However, the elongation and reduction of area exhibit a reverse trend. The Ti-17 samples forged at 1173 K followed by solution treatment at 1073 K and aging treatment exhibits the highest fatigue limit of around 975 MPa. The fatigue strength of the forged Ti-17 samples is strongly related to the microstructural factor such as the volume fraction of the equiaxed α-phase region, which is one of the crack initiation sites in the forged Ti-17 samples subjected to low temperature ST and aging, and the strength difference between the acicular α-phase and the fine (α + β)-phase, which leads to the crack initiation in the forged Ti-17 sample subjected to high temperature ST and aging.

[doi:10.2320/matertrans.MT-M2020184]

(Received 2020/06/10; Accepted 2020/07/14; Published 2020/09/25)

Keywords: aircraft materials, near β type titanium alloy, microstructure, tensile properties, fatigue properties

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