Effects of Dy on the microstructure and hardness of NiAl alloys

Intermetallic compound NiAl is considered as a potential candidate material for bond coat inthermal barrier coating (TBC) system due to its capability of forming a continuous and uniformalumina scale at temperatures even high than 1200 °C, but its cyclic oxidation is rather poor.Previous study has found that cyclic oxidation resistance of NiAl alloys and coatings can bedrastically improved by minor Dy doping as reactive element. In this paper, NiAlDy alloys wereproduced by vacuum arc-melting and the effects of various Dy contents on the microstructure andhardness of NiAl alloys were investigated. The results suggest that Dy tends to segregate at grainboundaries 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 reducedfrom ~1 mm to ~300 μm, with increasing the content of Dy to 0.5 at.%. The addition of ~0.1 at.%Dy caused a 10% improvement in both the microhardness and macrohardness due to Dysolid-solution and grain-boundary segregation, but the alloy revealed decreased microhardness andmacrohardness with further increasing the content of Dy to 0.5 at.% as the formation of Dy-richphase.