13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effects of Short-Time Duplex Heat Treatment on Microstructure and Fatigue Strength of Ti-6Al-4V Alloy Satoshi Tanaka1,*, Tatsuro Morita1, Kosuke Shinoda1 1 Department of Mechanical and System Engineering, Graduate School, Kyoto Institute of Technology, Kyoto 606-8585, Japan * Corresponding author: satoshi.tanaka.10011@gmail.com Abstract This study was conducted to investigate the effects of short-time duplex heat treatment on the microstructure, mechanical properties and fatigue strength of α+β titanium alloy Ti-6Al-4V. The duplex heat treatment was composed of short-time solution treatment (1203 K, 60 s) and short-time aging (753-903 K, 40 s). The first heat treatment transformed a part of prior β phase to acicular α' martensite phase. The second heat treatment precipitated fine α phase in metastable β phase. These change in microstructure increased the hardness of prior β phase. At the same time, the tensile strength and fatigue strength greatly improved without reduction in ductility and their maximum improvement rates reached 29 % and 22 %, respectively. Keywords Ti-6Al-4V Alloy, Short-Time Duplex Heat Treatment, Microstructure, Mechanical Properties, Fatigue Strength 1. Introduction Ti-6Al-4V alloy is a typical α+β titanium alloy possessing high specific strength (tensile strength/density) and excellent corrosion resistance, and has been widely used in the aerospace industry. For example, this titanium alloy is used to fix CFRP components in the airplanes recently developed because of its high electric corrosion resistance as well as high specific strength [1]. Ti-6Al-4V alloy occupies more than 50 % of the titanium market in the USA [2]. Accordingly, further improvement in the strength of this titanium alloy will have a great impact. The strength of Ti-6Al-4V alloy is improved by controlling microstructure through heat treatment. Conventional heat treatment for titanium alloys is solution treatment and aging [3, 4]; however, it needs relatively long hours. In the previous study, one of the authors showed short-time solution treatment (1173-1263 K, 60 s) improves the tensile and fatigue strengths of Ti-6Al-4V alloy. Moreover, the above heat treatment was combined with short-time aging (753-953 K, 40 s). This duplex heat treatment further improved the tensile strength without reduction in ductility [5-8]. Although fatigue strength of metals is usually related to tensile strength, the fatigue strength of Ti-6Al-4V alloy markedly depends on the microstructure [9]. For the newly developed heat treatment, therefore, a detailed study was needed to investigate its effect on the fatigue strength. From the above back ground, this study comprehensively investigated the effects of the short-time duplex heat treatment on the microstructure, mechanical properties and fatigue strength of Ti-6Al-4V alloy. To accumulate data, we conducted the metallographic examinations such as the optical observation of microstructure, TEM (transmission electron microscopy) observation and electron diffraction. Furthermore, the hardness measurement, tensile test and fatigue test were performed. 2. Materials and Experimental Procedures Table 1 shows the chemical composition of Ti-6Al-4V used in this study. The material was supplied as round bars (diameter: 14 mm) and machined to the three specimen shapes shown in Fig. 1. For the short-time solution treatment, the specimens were kept at 1203 K for 60 s and quenched.
RkJQdWJsaXNoZXIy MjM0NDE=