Microstructure Evolution of Boundaries with different Angles in Single Crystal Ni-base Superalloy during High Temperature Oxidation Process

The single crystal Ni-base superalloy samples with low angle boundaries and high angle boundaries were selected. The grain boundary angles of three samples were 5.4°, 12.3° and 25.7°, respectively. The microstructure evolution of samples with different boundary angles and after standard heat trestment was studied in 1 100 ℃ high temperature oxidation process. The results show that the surface of nickel single crystal superalloy with different grain boundary angles was divided into three areas: outer oxide film, γ′-free layer and γ′-reduced layer, after oxidation for 100 h and there was a little internal oxidation micropores in the γ′-free layer and γ′-reduce layer. The number of micropores for three samples all increasd after oxidation for 500 h. The size of micropores increased along the grain boundaries with the increase of the grain boundary angle and the micropores grew up and formed a long and narrow channel. After oxidation for 1 000 h, the size of micropores continued to increase along grain boundaries with the increase of the grain boundary angle. A consecutive internal oxidation micropore channel was formed in the sample with 25.7° angle and extend into the γ′-reduced layer and a internal oxidation layer was formed inside the single crystal.