Effect of microstructure on fracture characteristics of Ti-6Al-2Sn-2Zr-2Mo-2Cr-Si
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TRODUCTION
ALUMINUM alloys, titanium alloys, composite materials, etc. are used as structural materials for aircrafts. Among them, titanium alloys are attracting attention due to their excellent specific strength as well as excellent corrosion resistance and fatigue characteristics. In the field of developing titanium alloys for aircrafts, Ti-6Al-2Sn-2Zr-2Mo2Cr-Si (Ti-62222S) was developed in the RMI Titanium Inc., Niles, OH, in the beginning of the 1970s as a titanium alloy combining the characteristics of the ␣-type alloy, which has excellent high-temperature strength and creep resistance, and the ␣ ⫹ -type alloy, which has high toughness and high strength.[1,2,3] Ti-62222S has greater strength, greater elastic modulus, greater toughness, and much better damage tolerant characteristics than Ti-6Al-4V.[4–7] It has been reported that the formation of both intermetallic compounds, i.e., Ti3Al and silicide precipitates, lowers the fracture toughness and strength of the alloy.[8] However, it is not well understood which intermetallic compound affects the toughness and strength more strongly, and how the changes in the precipitation volume of such compounds affect the same characteristics is not yet examined. In this study, therefore, aging processes[9] for various lengths of time to precipitate intermetallic compounds of Ti3Al and silicide separately were applied to Ti-62222S. Static fracture toughness tests and tensile tests were then carried out on the aged Ti-62222S, and the effects of microstructure on fracture characteristics were examined.
II. EXPERIMENTAL PROCEDURES A. Material and Aging Processes Materials used in this study were rolled plates of Ti62222S made by RMI Titanium Inc., Niles, OH, with a thickness of approximately 38 mm. The alloy contained (by mass) 5.44 pct Al, 1.99 pct Sn, 1.99 pct Zr, 2.16 pct Mo, 2.06 pct Cr, 0.16 pct Si, 0.09 pct Fe, 0.11 pct O, 0.006 pct N, 62 ppm H, and the balance Ti. The  transus of this alloy is approximately 1250 K. The materials were subjected to heat treatment of three stages:  solution treatment, ␣- stabilizing treatment, and aging process (1261 K–1 hour, fan cool ⫹ 1200 K–1 hour, fan cool ⫹ 811 K–8 hours, air cool), as shown in Figure 1(a) by RMI Titanium Inc., Niles, OH, (hereinafter referred to as the as-received material). The as-received materials were subjected to two types of heat treatment in this study. Schematic diagrams of the heat treatment processes are shown in Figures 1(b) and (c), respectively. The heat treatment shown in Figure 1(b) was carried out in order to precipitate Ti3Al only. In this heat treatment process, materials were aged at 913 K for 4, 8, and 16 hours, respectively, and then cooled in air (hereinafter referred to as the A913 process). On the other hand, the heat treatment process shown in Figure 1(c) was carried out to precipitate silicide only. In this process, materials were aged at 1088 K for 2, 4, and 8 hours, respectively, and then quenched into water (hereinafter referred to as the A1088 process). B. Microstructural Observati
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