Creep behavior of conventional and Nb-modified as-cast MAR-M246 superalloy

Abstract Considering the lower cost of Nb and its chemical similarity to Ta, this work investigated the effect of complete replacement of Ta by Nb (in at.% basis) in MAR-M246 Ni-based superalloy for the automotive turbocharger applications. Conventional and Nb-modified polycrystalline MAR-M246 superalloys were prepared through vacuum induction melting and investment casting from commercial raw-materials. Creep tests were performed on the as-cast alloys in air, at 800 °C (475/500/550 MPa), 900 °C (300/350/400 MPa) and 1000 °C (150/200/225 MPa). Additional creep tests were carried out at 900 °C/350 MPa under high vacuum conditions to verify effects of oxidation on the creep behavior of the alloys. Both alloys presented similar creep behavior at 800 °C. At 900 °C and 1000 °C, the conventional alloy has shown better properties associated with less creep damages in the bulk and near the surface regions. Both alloys have a better creep life in high vacuum which revealed an important role of oxidation on the creep damage mechanisms of these alloys. The lower creep properties of Nb-modified alloy at T > 800 oC should be primarily associated with higher internal diffusivity in this alloy compared to the conventional one, leading to faster oxidation and microstructure degradation during the high temperature creep.