| Research Abstract |
For practical applications of intermetallics, their stability in the environment has been concerned as well as their strength and ductility. High temperature oxidation and embrittlement induced by hydrogen and oxygen which come from the environment are the most important phenomena. In order to improve the high temperatrure oxidation resistance of TiAl which is expected to be utilized in quite near future, addition of minor alloying elements and surface treatment, or combination of both methods were studied. Addition of small amounts of Hf and Zr (0.2-0.5mass%) was found to increase the oxidation resistance up to 1300K.. Surface modification by a 30 mu m thickness coating of Co-30Cr-4Al with magnetron sputtering increased the oxidation resistance up to 1300k. Combination of alloying element addition (0.2mass%Hf) and surface treatment (Cr_2O_3/Cr pack) was found to improve drastically the oxidation resistance.
Suppression of hydrogen embrittlement of intermetallics by addition of alloying
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elements and its mechanism have been studied. Addition of small amount of B and C to Ni_3 (Si, Ti) was found to inhibit the embrittlement in the atmosphere and distillled water and also to suppress partly the embrittlement in hydrogen gas. Aggregation of hydrogen atoms at grain boundaries and decrease in hydrogen diffusivity in lattice were suggested to suppress grain boundary fracture. Addition of small amount of Cr, Mn and Fe to Ni_3 (Si, Ti) was found to inhibit the embrittlement. The embrittlement of Co_3Ti was suppressed by addition of Fe and Al, the role of which was explained rationally by suppression of water dissociation to hydrogen atoms on the surface. The role of hydrogen from the environment and residual hydrogen in the bulk was studied by using Ni_3 (Al, Ti) and Ni_3 (Si, Ti) single or poly crystals. Extremely small amount ( ppm level) of residual hydrogen caused the embrittlement, and hydrogen from the environment affeted on ductility loss even at the high vacuum condition of 10^<-6> torr. Less
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