High-temperature environmental embrittlement of thermomechanically processed TiAl-based intermetallic alloys
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I. INTRODUCTION
GAMMA () TiAl intermetallic alloys have been considered as a potentially important aerospace and vehicle structural material because of their light weight, good hightemperature mechanical properties, and oxidation resistance.[1] Various kinds of microstructures, such as a -grain microstructure, a duplex microstructure consisting of and /2, a dual-phase microstructure consisting of and 2, a fully lamellar microstructure consisting of /2, and a fully lamellar microstructure containing phase, could be obtained through compositional modification and microstructural control of intermetallic alloys based on gamma () TiAl. Here, TiAl, 2 Ti3Al, and phases have an ordered tetragonal structure L10, an ordered hexagonal structure D019, and a bcc structure, respectively. Depending on the desired properties and applications, the most appropriate microstructure could be chosen from among these microstructures. However, these microstructures generally exhibit low ductility and poor fracture toughness, up to an intermediate temperatures. With compositional modification and microstructural control, however, their drawbacks have been improved significantly during the last decades;[2–6] consequently, these intermetallic alloys have begun to be used. The influence of cathodically or thermally precharged hydrogen on the ambient mechanical properties of intermetallic alloys based on gamma () TiAl has been the focus of some researchers.[7–11] Also, it has been reported that the so-called environmental embrittlement occurs at ambient temperatures in intermetallic alloys based on gamma Y. HOTTA, Graduate Student, Y. KANENO, Research Associate, and T. TAKASUGI, Professor, are with the Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka, 599-8531, Japan. Contact e-mail: [email protected] T. TETSUI, Senior Researcher, is with Mitsubishi Heavy Industries Ltd., Nagasaki Research and Development Center, Nagasaki, 851-0392, Japan. Manuscript submitted April 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
() TiAl.[12–16] In these cases, hydrogen is introduced from test atmospheres, such as in 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. Furthermore, similar results have been obtained under fatigue loading[17] or for polysynthetic texture (PST) TiAl crystals.[18,19] Fatigue crack growth rates were enhanced by moisture in the test environment.[17] The tensile elongation of PST crystals with soft orientation deformed at room temperature was greatly dependent on the testing atmosphere, and was lower in air than in vacuum.[18,19] For the Ti-49 at. pct Al alloy with a dual-phase microstructure, it has been reported that the presence of hydrogen, whether inside or outside the material, resulted in a reduction in the tensile elongation at temperatures up to 573 K. [20,21] Also, a recent study showed that a Ti-48Al-2Cr
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