Materials Transactions Online

Materials Transactions, Vol.60 No.09 (2019) pp.1740-1748
© 2019 The Japan Institute of Metals and Materials

Quantitative and Qualitative Relationship between Microstructural Factors and Fatigue Lives under Load- and Strain-Controlled Conditions of Ti-5Al-2Sn-2Zr-4Cr-4Mo (Ti-17) Fabricated Using a 1500-ton Forging Simulator

M. Niinomi1, 2, 3, T. Akahori1, M. Nakai4, Y. Koizumi2, A. Chiba3, T. Nakano2, T. Kakeshita5, Y. Yamabe-Mitarai6, S. Kuroda6, N. Motohashi6, Y. Itsumi7 and T. Choda7

1Department of Materials Science and Engineering, Faculty of Science and Technology, Meijo University, Nagoya 468-8502, Japan
2Department of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan
3Institute for Materials Research, Tohoku University, Sendai 980-5377, Japan
4Department of Mechanical Engineering, Faculty of Science and Engineering, Kindai University, Osaka 577-8502, Japan
5Faculty of Engineering, Fukui University of Technology, Fukui 910-8505, Japan
6National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan
7Kobe Steel, LTD., Kobe 675-0023, Japan

The fatigue lives of forged Ti-17 using a 1500-ton forging simulator subjected to different solution treatments and a common aging treatment were evaluated under both load- and strain-controlled conditions: high and low cycle fatigue lives, respectively. Then, the tensile properties and microstructures were also examined. Finally, the relationships among fatigue lives and the microstructural factors and tensile properties were examined.The microstructure after solution treatment at 1203 K, which is more than the β transus temperature, and aging treatment exhibits equiaxed prior β grains composed of fine acicular α. On the other hand, the microstructures after solution treatment at temperatures of 1063, 1123, and 1143 K, which are less than the β transus temperature, and aging treatment exhibit elongated prior β grains composed of two different microstructural feature regions, which are acicular α and fine spheroidal α phase regions. The 0.2% proof stress, σ, and tensile strength, σ, increase with increasing solution treatment temperature up to 1143 K within the (α + β) region, but decrease with further increasing solution treatment temperature to 1203 K within the β region. The elongation (EL) and reduction of area (RA) decrease with increasing solution treatment temperature, and it becomes nearly 0% corresponding to a solution treatment temperature of 1203 K. The high cycle fatigue limit increases with increasing solution treatment temperature up to 1143 K, corresponding to the (α + β) region. However, it decreases with further increase in the solution treatment temperature to 1203 K in the β region. The fatigue ratio in high cycle fatigue life region is increasing with decreasing solution treatment temperature, namely increasing the volume fraction of the primary α phase, and it relates well qualitatively with the volume fraction of the primary α phase when the solution treatment temperature is less than the β transus temperature. The low cycle fatigue life increases with decreasing solution treatment temperature, namely increasing the volume fraction of the primary α phase. The low cycle fatigue life relates well quantitatively with the tensile true strain at breaking of the specimen and the volume fraction of the primary α phase for each total strain range of low cycle fatigue testing.The number of cycles to failure was obtained by fatigue tests under strain-controlled conditions carried out at representatively selected total strain ranges of 1.05, 1.1, and 1.2, respectively, and the true strain at breaking of specimen was obtained by tensile test on forged Ti-17 at various temperatures followed by solution treatments at different temperatures and aging treatment at a constant temperature.

[doi:10.2320/matertrans.ME201904]

(Received 2018/12/25; Accepted 2019/02/19; Published 2019/08/25)

Keywords: Ti-17, low-cycle fatigue, high-cycle fatigue, microstructure, primary α phase, solution treatment temperature

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