Dispersoids and γ′ precipitates in an ultrafine grained René 88DT - 5vol.%Y2O3 alloy with outstanding thermal stability

We report on an ultrafine-grained (UFG) René 88DT-5vol.%Y2O3 alloy with a high thermal stability fabricated through high energy mechanical milling and thermomechanical powder consolidation. The as-extruded sample consists of an ultrafine grained γ matrix with an average size of 122 nm with Y4Al2O9 nanoparticles (average size: 12.6 nm) and TiN particles (size about 150 nm) being dispersed in the matrix. The UFG microstructure of the alloy showed an outstanding thermal stability with the average grain size only increasing slightly from 122 to 145 nm after holding at 1120 °C (0.85Tm, where Tm is the solidus temperature of the alloy in Kelvin scale) for 2 h and subsequently holding at 760 °C for 16 h. Due to the consumption of γ′ forming elements, Ti and Al, by the formation of the Y4Al2O9 and TiN dispersoids, the content of γ′ precipitates in the microstructure of the heat treated sample is significantly reduced, as confirmed by experimentation and calculation using the JMatPro software. Additionally, the size of γ′ precipitates also sharply decreased to 20-30 nm in the heat treated UFG René 88DT-5vol.%Y2O3 alloy, in contrast to a size of 100-150 nm in the Y2O3-free base alloy under the same heat treatment condition.