Microstructural effects on moisture-induced embrittlement of isothermally forged TiAl-based intermetallic alloys
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NTRODUCTION
GAMMA (␥ )–TiAl alloys are of interest as a potentially important aerospace and vehicle structural material because of their light weight, good high-temperature mechanical properties, and oxidation resistance.[1] However, they exhibit low ductility and poor fracture toughness at ambient temperature. With alloy modification and microstructural control, their drawbacks have been improved significantly during the last decades,[2–6] and, consequently, these intermetallic alloys have begun to be utilized. As one of the benefits of intermetallic alloys based on ␥ -TiAl, various kinds of microstructures, such as a ␥ -grain microstructure, a duplex microstructure consisting of ␥ and ␥ /␣2, a dual-phase microstructure consisting of ␥ and ␣2, and a fully lamellar microstructure consisting of ␥ /␣2 could be attained by alloy modification and microstructural control. Depending on the desired properties and applications, the most appropriate microstructure could be chosen from these microstructures. Here, ␥ -TiAl phase has an L10 ordered tetragonal structure and ␣2-Ti3Al phase has a D019 ordered hexagonal structure. A number of investigations have focused on the influence of hydrogen on the mechanical properties of intermetallic alloys based on ␥ -TiAl.[7–11] In these studies, hydrogen is introduced by cathodic precharging or thermal precharging, resulting in reduced elongation or fracture strength. On the other hand, it has been reported that the so-called environmental embrittlement occurs in intermetallic alloys based on ␥ -TiAl.[12–15] The reduction of elongation or fracture strength in air at room temperature has also been observed for a number of intermetallic alloys.[16,17,18] In these cases, hydrogen is introduced from test atmospheres such as in a T. TSUYUMU, Graduate Student, Y. KANENO, Research Associate, H. INOUE, Associate Professor, and T. TAKASUGI, Professor, are with the Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan. Contact e-mail: [email protected] Manuscript submitted June 25, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
flowing hydrogen gas or in air, the moisture in which is suggested to be able to react with the alloy and to generate atomic hydrogen, resulting in reduced tensile elongation in air. Also, similar results have been obtained under fatigue loading[19] or for polysynthetic textured (PST) TiAl crystals.[20,21] Fatigue-crack growth rates were enhanced by moisture in the test environment.[19] When PST crystals with a soft orientation are deformed at room temperature at slow strain-rates, the tensile elongation was greatly dependent on the testing atmosphere and was lower in air than in vacuum.[20,21] However, it is not yet known how the moistureinduced embrittlement is affected by microstructure or by constituent phases in intermetallic alloys based on ␥ -TiAl. In order to obtain adequate microstructures of intermetallic alloys based on ␥ -TiAl (which is highly resistant to moistur
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