Effect of Microstructure on Fracture Characteristics of Ti-6Al- 2Sn-2Zr-2Mo-2Cr-Si MITSUO NIINOMI, KEI-ICHI FUKUNAGA, GUNAWARMAN, GENZO TONO, JUNICHI KOIKE, DANIEL EYLON, and SHIRO FUJISHIRO The effects of intermetallic compounds of Ti 3 Al ( 2 ) and silicide separately on fracture characteristics of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-62222S) alloy were investigated in this study. The alloys with only Ti 3 Al and only silicide precipitated were established by aging treatments at temperatures of 913 K followed by air cooling and 1088 K followed by water quenching, respectively. X-ray diffraction analysis results showed that the volume fraction of either Ti 3 Al or silicide increases with increasing aging time. Tensile properties, namely, yield stress (0.2 pct proof stress), ultimate tensile strength, and elongation of as-received alloy are much better than those of the aged alloys. The strength of the alloy with only Ti 3 Al is better than that of the alloy with only silicide, while elongation of the alloy with only silicide is better than that of the alloy with only Ti 3 Al. Fracture toughness, J IC , of the alloy with only silicide is better than that of the alloy with only Ti 3 Al. The intergranular fracture appears in the alloy with only Ti 3 Al. Coarsening of Widmansta ¨tten  structure and increasing ductility of  phase during aging are considered to be effective for increasing fracture toughness. I. INTRODUCTION II. EXPERIMENTAL PROCEDURES ALUMINUM alloys, titanium alloys, composite materi- A. Material and Aging Processes als, etc. are used as structural materials for aircrafts. Among Materials used in this study were rolled plates of Ti- them, titanium alloys are attracting attention due to their 62222S made by RMI Titanium Inc., Niles, OH, with a excellent specific strength as well as excellent corrosion thickness of approximately 38 mm. The alloy contained (by resistance and fatigue characteristics. In the field of devel- mass) 5.44 pct Al, 1.99 pct Sn, 1.99 pct Zr, 2.16 pct Mo, oping titanium alloys for aircrafts, Ti-6Al-2Sn-2Zr-2Mo- 2.06 pct Cr, 0.16 pct Si, 0.09 pct Fe, 0.11 pct O, 0.006 pct 2Cr-Si (Ti-62222S) was developed in the RMI Titanium N, 62 ppm H, and the balance Ti. The  transus of this alloy Inc., Niles, OH, in the beginning of the 1970s as a titanium is approximately 1250 K. The materials were subjected to alloy combining the characteristics of the -type alloy, which heat treatment of three stages:  solution treatment, - has excellent high-temperature strength and creep resistance, stabilizing treatment, and aging process (1261 K–1 hour, and the  + -type alloy, which has high toughness and fan cool + 1200 K–1 hour, fan cool + 811 K–8 hours, air high strength. [1,2,3] Ti-62222S has greater strength, greater cool), as shown in Figure 1(a) by RMI Titanium Inc., Niles, elastic modulus, greater toughness, and much better damage OH, (hereinafter referred to as the as-received material). The tolerant characteristics than Ti-6Al-4V. [4–7] It has been as-received materials were subjected to two types of heat reported that the formation of both intermetallic compounds, treatment in this study. Schematic diagrams of the heat treat- i.e., Ti 3 Al and silicide precipitates, lowers the fracture tough- ment processes are shown in Figures 1(b) and (c), respec- ness and strength of the alloy. [8] However, it is not well tively. The heat treatment shown in Figure 1(b) was carried understood which intermetallic compound affects the tough- out in order to precipitate Ti 3 Al only. In this heat treatment ness and strength more strongly, and how the changes in process, materials were aged at 913 K for 4, 8, and 16 hours, the precipitation volume of such compounds affect the same respectively, and then cooled in air (hereinafter referred to characteristics is not yet examined. as the A913 process). On the other hand, the heat treatment In this study, therefore, aging processes [9] for various process shown in Figure 1(c) was carried out to precipitate lengths of time to precipitate intermetallic compounds of silicide only. In this process, materials were aged at 1088 Ti 3 Al and silicide separately were applied to Ti-62222S. K for 2, 4, and 8 hours, respectively, and then quenched Static fracture toughness tests and tensile tests were then into water (hereinafter referred to as the A1088 process). carried out on the aged Ti-62222S, and the effects of micro- structure on fracture characteristics were examined. B. Microstructural Observations MITSUO NIINOMI, Professor, KEI-ICHI FUKUNAGA, Research Small specimens with a size of 10  10  10 mm 3 were Associate, and GUNAWARMAN, Graduate Student, are with the Depart- cut from the as-received material and aged materials that ment of Production Systems Engineering, Toyohashi University of Technol- have been subjected to the aging processes shown in Figures ogy, Toyohashi 441-8580, Japan. GENZO TONO, Engineer, is with 1(b) and (c), respectively. They were buff polished into Interproject Corp., Tokyo 163-0828, Japan. JUNICHI KOIKE, Associate Professor, is with the Department of Materials Science, Faculty of Engi- mirror finishing using alumina powder with a size of 0.3 neering, Tohoku University, Sendai 980-8579, Japan. DANIEL EYLON, m after polishing with emery paper and then etched with Professor and Director, is with Graduate Materials Engineering, The Univer- 3 pct HF + 10 pct HNO 3 solution. The microstructure of the sity of Dayton, Dayton, OH 45469-0240. SHIRO FUJISHIRO is former specimen was then examined using an optical microscope. Director, Asian Office of CSAFOSR, Tokyo, 106-0032, Japan. Manuscript submitted January 29, 2001. On each material that was subjected to either process METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 32A, NOVEMBER 2001—2795