Microstructural evaluation of Ti-6-22-22 alloy
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I.
INTRODUCTION
Ti-6-22-22 (6 pct Al - 2 pct Cr - 2 pct Mo - 2 pct Sn 2 pct Zr) is an a/b Ti alloy, which is being considered for use in fracture critical applications in the aerospace industry. Investigations have been performed to determine the best method of processing this alloy to produce an optimum combination of mechanical properties, including high strength, ductility, fracture toughness, and fatigue crack growth resistance.[1] In one such study, Chakrabarti et al.[2] applied different combinations of preform forgings, forgings, and solution treatments to this alloy, and the resulting samples were tested to meet specific mechanical properties. Their investigations determined that a transformed b microstructure was required to produce material with a sufficiently high requisite fracture toughness. They concluded the best combination of properties could be achieved by following an a/b preform forging and a/b finish forging with a three-step heat treatment consisting of a b solution treatment, an a/b stabilization treatment, and an aging treatment. However, even with this general heat treatment, different properties can arise with different aging treatments. A standard treatment consisting of a b solution treatment at the b transus (bT) 1 28 7C for 30 minutes, an a/b stabilization treatment for 1 hour at bT 247 7C, and an aging treatment of 8 hours at 538 7C (designated ‘‘Triplex’’ heat treatment) has proven to produce samples with a good combination of mechanical properties.[3] Variations from this heat treatment have shown reductions in fracture toughness without obvious changes in microstructure. Wood and Phelps[4] studied the relationship between the Si content and the corresponding mechanical properties of Ti-6-22-22S as a function of aging treatment. The Triplex heat treatment gave the highest fracture toughness of the three treatments they tested, but all showed significant decreases in toughness with increasing Si content. While a high-temperature aging treatment was found to produce silicides with high Si content, an intermediate-temperature aging treatment applied to a high-Si sample was found to
A.W. WILSON, Graduate Research Assistant, and J.M. HOWE, Associate Professor, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted November 3, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
have a similarly low fracture toughness without the presence of silicides. The role of Si in Ti alloys has been shown to affect the presence of both silicides and a2. Work on the structure and properties of Ti-5331S revealed that the presence of Si affected the amount of ordering that occurred in the a phase.[5] In a study[6,7] on silicide and a2 precipitation in Ti6-22-22S, it was shown that silicides formed in the b phase while a2 remained in the a phase. Consequently, it was postulated that the two phases work independently toward reducing the fracture toughness of the alloy. The purpose of this research is (1) to determine the
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