Environmental Effects on the Room Temperature Ductility of Polysynthetically Twinned (PST) Crystals of Binary and some T
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ENVIRONMENTAL EFFECTS ON THE ROOM TEMPERATURE DUCTILITY OF POLYSYNTHETICALLY TWINNED (PST) CRYSTALS OF BINARY AND SOME TERNARY TiAI COMPOUNDS M. H. OH, H. INUI, M. MISAKI, M. KOBAYASHI AND M. YAMAGUCHI Department of Metal Science and Technology, Kyoto University, Sakyo-ku, Kyoto 606, Japan. ABSTRACT Environmental effects on the room temperature ductility of polysynthetically twinned (PST) crystals of binary and some ternary TiAl compounds have been investigated through tensile tests conducted in four different atmospheres. The tensile ductility of TiAI PST crystals is sensitive to test environment. It is higher when tested in vacuum or in dry air than in air or in hydrogen gas. The environmental loss in ductility of PST crystals decreases with increasing strain rate. The environmental embrittlement of binary TiAI PST crystals can be interpreted in terms of hydrogen embrittlement. The ternary TiAl PST crystals containing Cr, Mo or Mn exhibit higher ductility than the binary TiAl PST crystals when tested in air, while the ternary ones show lower ductility than the binary ones in vacuum. Additions of alloying elements such as Cr, Mo and Mn seem to be effective in reducing the environmental loss in ductility of TiAl PST crystals. INTRODUCTION Ti-rich TiAI compounds with the two-phase TiA,/Ti3Al lamellar structure have been of considerable interest in the last few years as a new class of high-temperature structural materials [1-3]. One of the major limitations for the practical use of the compounds is the poor ductility at ambient temperature. Resent researches on environmental effects on the ductility of intermetallic compounds such as Ni3Ai and FeA! have shown that environmental embrittlement can be a possible reason for their limited ductility at ambient temperature [4]. There is some evidence to indicate that TiAl-based compounds are susceptible to environmental embrittlement [5,6]. However, the two-phase TiAI compounds exhibit only a limited ductility in polycrystalline form and the difference in the tensile elongation between in air and in vacuum is rather small to characterize their environmental embrittlement behavior [7-9]. With the use of so-called polysynthetically twinned (PST) crystals of TiAI in which only a single grain with the TiAI/ Ti3AI lamellar structure is contained, we have made a systematic study on the deformation behavior as a function of the angle (0) between the lamellar boundaries and loading axis [10-12] and demonstrated that a tensile elongation as large as 20% can be obtained in air at room temperature for 0=31" where the easy type of deformation (shear deformation parallel to the lamellar boundaries) occurs [12]. This is far larger than any other values of room temperature ductility ever reported on two-phase TiAI compounds. We have thus decided to investigate environmental effects on the ductility of two-phase TiAI compounds using PST crystals to characterize the environmental embrittlement behavior of the two-phase TiAI compounds. In this paper, we report the results of deformation expe
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