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 4^th generation Nibased 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, mechanically 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 substrate surface by grit blasting gave rise to the formation of a secondary reaction zone (SRZ) in the vicinity of interdiffusion zone/substrate interfaces. While when the surfaces were finished by electro-polishing, voids were formed in the vicinity of interdiffusion zone/substrate interfaces. It is also found that accelerated formation of SRZ and voids was observed along <110> directions rather than <100> 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.