CREEP RUPTURE PROPERTIES OF BARE AND COATED POLYCRYSTALLINE NICKEL-BASED SUPERALLOY RENE®80

Creep deformation is one of the life time limiting reasons for gas turbine parts subjected to stresses at elevated temperatures. In this study, creep rupture behavior of uncoated and platinum-aluminide coated Rene (R) 80 has been determined at 760 degrees C/657 MPa, 871 degrees C/343 MPa and 982 degrees C/190 Mpa in air. For this purpose, an initial layer of platinum with a thickness of 6 mu m was applied on the creep specimens. Subsequently, the aluminizing coatings were formed in the conventional pack cementation method via the Low Temperature-High Activity (LTHA) and High Temperature-Low Activity (HTLA) processes. Results of creep-rupture tests showed a decrease in resistance to creep rupture of coated specimen, compared to the uncoated ones. The reductions in rupture lives in LTHA and HTLA methods at 760 degrees C/657 MPa, 871 degrees C/343 MPa and 982 degrees C/190 MPa were almost (26% and 41.8%), (27.6% and 38.5%) and (22.4% and 40.3%), respectively, compared to the uncoated ones. However, the HTLA aluminizing method showed an intense reduction in creep life. Results of fractographic studies on coated and uncoated specimens indicated a combination of ductile and brittle failure mechanisms for all samples. Although the base failure mode in substrate was grain boundary voids, cracks initiated from coating at 760 degrees C/657MPa and 871 degrees C/343. No cracking in the coating was observed at 982 degrees C/190MPa.