Effect of surface treatment and crystal orientation on microstructural changes in aluminized Ni-based single-crystal superalloy

The effect of pre-surface treatment and crystal orientation on microstructural changes in the aluminized 4th generation Ni-based single- crystal superalloy TMS-138 was investigated. The substrate superalloy was cut along the {100} and {110} planes on which three kinds of surface finishing, such as grit blasting, mechanical polishing and electro-polishing were conducted prior to the conventional high-activity aluminizing process. A thermal cycling test at 1373 K revealed that heavy deformation of the substrate surface by grit blasting gave rise to the formation of a secondary reaction zone (SRZ) in the vicinity of the interdiffusion zone/substrate interfaces. When the surfaces were finished by electropolishing, voids were formed in the vicinity of the interdiffusion zone/substrate interfaces. It was also found that accelerated formation of SRZ and voids was observed along the h110i directions rather than the h100i directions during the thermal cycling test. The difference in morphological changes of substrates can be related to the residual stress introduced by the surface finishing.