Comparative tensile behavior of freestanding γ-γ′ and β-(Ni,Pt)Al bond coats and effect on tensile properties of coated superalloy

Abstract

The tensile behavior of freestanding 40μm thick γ-γ′ and 50μm thick β-(Ni,Pt)Al coatings was evaluated at room temperature (RT) and 870°C by microtensile testing method. Simultaneously, the effect of coatings on the tensile properties of the directionally solidified (DS) CM-247LC superalloy was also examined. Both the freestanding coatings had lower strength than that of the superalloy and the yield as well as the ultimate tensile strength (YS and UTS) of the superalloy substrate was lowered by the application of either coating. At RT, the freestanding γ-γ′ coating was significantly stronger (YS=660MPa) and ductile than that of the brittle β-(Ni,Pt)Al coating which fractured at ~300MPa with negligible ductility. Comparatively, the β-(Ni,Pt)Al coating caused more deterioration in substrate tensile properties (especially ductility) than that of the γ-γ′ coating at RT. On the contrary, at 870°C, albeit their similar YS, the γ-γ′ coating exhibited brittle intergranular fracture and limited ductility whereas the β-(Ni,Pt)Al coating showed ductile failure. Numerous sharp cracks formed by the de-cohesion of the γ and γ′ phase boundaries within the γ-γ′ coating and their penetration into the substrate aggravated degradation in the substrate ductility than in case of the ductile β-(Ni,Pt)Al coating.