Microstructure and resistance to cracking of modified Al-Si and Al-Cr diffusion coatings on ZS6K nickel-based superalloys

A superalloy with the following chemical composition, in wt.%: 0.14 carbon, 10.2 Cr, 5.3 Al, 2.8 Ti, 3.7 Mo, 5.2 tungsten, 4.7 cobalt, 1.5 Fe and the balance Ni was investigated. A pack cementation technique of coating was applied. Acoustic emission signals were recorded as a function of specimen elongation ( Sigma N- epsilon ). The values of elongation ( epsilon sub 0 ) at the moment of the first crack appearance and at the beginning of rapid fracture were measured. The crack resistances of the aluminide, silicoaluminide and chromoaluminide diffusion coatings were the main subject of the studies performed. If the epsilon sub 0 parameter is taken as a crack resistance criterion it was found that, when compared with the aluminide coatings, Al--Si and Al--Cr coatings are less resistant. The crack morphologies of Al and Al--Cr coatings were similar in character. Among the typical features were cracks that did not spread through the entire layer thickness and discontinuous cracks with branches. In the depleted zone of the coatings, cracks developed along the NiAl phase grain boundaries.