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

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 (0.78Tm, where Tm 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.