Strengthening effects and thermal stability of the ultrafine grained microstructure of a nickel base superalloy at room and elevated temperatures

Highlights: • UFG Alloy 718-5 vol.%Y₂O₃ alloy is fabricated from alloy machining chips. • The kind of oxide nanoparticles is identified in Nb-rich superalloys. • High thermal stable nanoparticles and UFG microstructure is achieved. • The GB strengthening effects at 25 °C and 650 °C of UFG alloy are calculated. - Abstract: An ultrafine grained (UFG) nickel base superalloy doped with 5 vol.%Y₂O₃ nanoparticles, Alloy 718-5 vol.%Y₂O₃, was fabricated by a powder metallurgy route which combines high energy mechanical milling of Alloy 718 machining chips, spark plasma sintering, hot extrusion and heat treatment, and its microstructure and mechanical properties at room temperature and 650 °C were studied. The study showed that the Y₂O₃ nanoparticles reacted with Al from the base alloy and transformed to Y₄Al₂O₉ nanoparticles (average diameter: 12.5 nm) which were stable during heat treatment at 970 °C (0.78T_m, where T_m is the solidus temperature of the alloy in K). As a result of the high thermal stability of Y₄Al₂O₉ nanoparticles and their effective pinning of the grain boundaries, the UFG microstructure (average grain size: 179 nm) of the alloy was stable during heat treatment. Grain boundary and nanoparticle strengthening rendered the heat treated Alloy 718-5 vol.%Y₂O₃ alloy with a notable room temperature tensile yield strength of 1870 MPa. It was demonstrated that the grain boundary strengthening effect associated with the UFG microstructure was still significant at 650 °C, but clearly decreased from its level at room temperature. The grain boundary strengthening and nanoparticle strengthening effects which are likely to be independent of test temperature sustain a reasonably high tensile yield strength of 800 MPa at 650 °C, despite the absence of γ′ and γ″ precipitates in the UFG microstructure.