Hydrogen uptake in titanium aluminides covered with oxide layers
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INTRODUCTION
TWO-phase titanium aluminides (Ti3Al(a2) and TiAl(g)), as well as other ordered intermetallic compounds, have been reported to exhibit poorer ductility in air than in vacuum at room temperature.[1,2] This environmental embrittlement in air is thought to involve the reaction of titanium and aluminum atoms with moisture in air, which results in the generation of atomic hydrogen that penetrates into crack tips and then reduces ductility of the alloys.[1] Formation of several hydrides in titanium aluminides has been also observed at elevated temperatures in a hydrogen gas environment[3–7] and it can also be expected to lead significant loss in ductility. It is considered that preventing hydrogen penetration by suitable coatings, including oxide films, seems to be the most practical means of avoiding the adverse effects of hydrogen.[8] Nelson and Murray[9] showed that a stainless steel sputter-coated with TiO2 (oxide thickness of 50 and 500 nm) prior to charging with hydrogen was more ductile than the sample without coating. Kuruvilla and Stoloff[10] reported that a thermally grown oxide layer (Al2O3) on an Ni3Al alloy (with boron addition) reduced the loss of tensile properties and showed no change in fracture mode compared with the sample without an oxide layer. It is considered from their results that TiO2 and Al2O3 oxide layers on the alloy surface could be effective barriers to hydrogen penetration. The oxidation kinetics of titanium aluminides have been AKITO TAKASAKI, Lecturer, is with the Department of Mechanical Engineering, Faculty of Engineering, Shibaura Institute of Technology, Saitama 330-0003, Japan. YOSHIO FURUYA, Associate Professor, is with the Department of Technology, Faculty of Education, Nagasaki University, Nagasaki 852-8131, Japan. YOUJI TANEDA, Professor, is with the Department of Mathematics and Physics, National Defense Academy, Kanagawa 239-0811, Japan. Manuscript submitted January 30, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
well characterized. It is known that TiO2 (rutile) and Al2O3 (corundum) are produced on binary titanium aluminides at elevated temperatures. Our previous study on the oxidation of titanium aluminides indicated that TiO2 oxide initially formed on the surface and then Al2O3 oxide formed at a higher temperature during heating from room temperature to an elevated temperature,[11] and the formation of TiO2 oxide was accelerated by a long cathodic charging with hydrogen because of the formation of surface damage.[12] The outermost layer is, generally, only TiO2, and the intermediate layer consists of a mixture of TiO2 and Al2O3. The mixture of the oxides, whose proportion is dependent on the alloy composition, is heterogeneous, which results in porous layers in the oxide scale.[13] Though alloy surfaces are generally modified (coated) by means of electroplating or physical or chemical vapor deposition, it is convenient for the discussion of the fundamental effect of oxide coating on titanium aluminides to use thermally grown oxide layers on them, bec
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