Effects of Dy on the Microstructure and Hardness of NiAl Alloys

Intermetallic compound NiAl is considered as a potential candidate material for bond coat in thermal barrier coating (TBC) system due to its capability of forming a continuous and uniform alumina scale at temperatures even high than 1200 degrees C, but its cyclic oxidation is rather poor. Previous study has found that cyclic oxidation resistance of NiAl alloys and coatings can be drastically improved by minor Dy doping as reactive element. In this paper, NiAlDy alloys were produced by vacuum arc-melting and the effects of various Dy contents on the microstructure and hardness of NiAl alloys were investigated. The results suggest that Dy tends to segregate at grain boundaries and precipitate within grains as brittle DyNi2Al3 phase with little DyNiAl needles in it. The addition of minor Dy resulted in grain refinement. The grain size of NiAl alloy were reduced from similar to 1 mm to similar to 300 mu m, with increasing the content of Dy to 0.5 at.%. The addition of similar to 0.1 at.% Dy caused a 10 % improvement in both the microhardness and macrohardness due to Dy solid-solution and grain-boundary segregation, but the alloy revealed decreased microhardness and macrohardness with further increasing the content of Dy to 0.5 at.% as the formation of Dy-rich phase.