Microstructural Effect on Environmental Embrittlement of Isothermally Forged TiAl-Based Intermetallic Alloys
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Microstructural Effect on Environmental Embrittlement of Isothermally Forged TiAl-Based Intermetallic Alloys T. Takasugi, T. Tsuyumu, Y. Kaneno and H. Inoue Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan ABSTRACT The TiAl-based (Ti-46Al-7Nb-1.5Cr (at%)) intermetallic alloy was tensile tested in vacuum and air as a function of temperature to investigate microstructural effect on the moisture-induced embrittlement. The reduction in tensile strength (or elongation) due to testing in air diminishes as testing temperature increases. From the fracture strength (or elongation)-temperature curves, it was found that the near gamma grain microstructure was most resistant, and the dual-phase microstructure most susceptible to moisture-induced embrittlement. Also, the moisture-induced embrittlement of the TiAl-based intermetallic alloy with fully lamellar microstructure depends on the lamellar spacing, and reduced with decreasing lamellar spacing. INTRODUCTION A number of investigations have focused on the influence of hydrogen on the mechanical properties of intermetallic alloys based on gamma (γ) TiAl [1-5]. In these studies, hydrogen is introduced by cathodic precharging or thermal precharging, resulting in reduced elongation or fracture strength. On the other hand, the so-called environmental embrittlement (see review articles [6-8]) occurs at room temperature in intermetallic alloys based on gamma (γ) TiAl [9-12]. In these cases, hydrogen is introduced from test atmospheres, 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 [13] or for PST (poly-synthetically twinned) TiAl crystals [14,15]. For an ally design towards improved environmental resistance, the mechanism that leads to moisture-induced failure has to be known in detail. In this study, using an isothermally forged TiAl-based intermetallic alloy, various kinds of microstructures are prepared by heat treatment. The TiAl-based intermetallic alloy is tensile tested in vacuum and air at temperatures from room temperature to 473K. It will be shown that the microstructure greatly affects the moisture-induced embrittlement. EXPERIMENTAL PROCEDURE A TiAl-based intermetallic alloy with a composition of Ti-46Al-7Nb-1.5Cr was used in this study. The alloy was fabricated by induction scull-melting→annealing at 1473K for 5h→isothermal forging at 1308K to one third reduction→HIP at 1473K for 2h under 200MPa. Tensile specimens with gauge dimensions of 10x2x1 (or 10x3x1) mm3 were prepared by electro-discharge machining. To obtain various microstructures, the tensile specimens were then annealed in vacuum at high temperatures and held for prescheduled time, followed by cooling to room temperature at a controlled cooling rate. Tensile tests were conducted at a fixed strain rate of 1.67x10-5 s-1 in air and
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