Influence of aging treatment on microstructure and properties of a novel spray formed powder metallurgy superalloy FGH100L

A novel nickel-based superalloy, designated as FGH100L, was prepared by spray forming, hot isostatic pressing and isothermal forging. Following this, the billets were subjected to different heat treatments including sub-solvus and super-solvus solution heat treatments. The microstructure stability and mechanical properties of the alloy during the long-term aging treatment at 760 °C were studied. During the aging treatment, the grain size did not change significantly in both of the sub-solvus and super-solvus alloys, but the recrystallization around the coarse grain boundaries occurred in the former. When the sub-solvus alloy was aged for 500h, some secondary γ′ precipitates coarsened and split at a critical size into doublet or octet of cubes and there were two sizes of tertiary γ′ precipitates. With the aging time extending from 1000h to 2000h, secondary γ′ precipitates continued to coarsen and split, but tertiary γ′ precipitates grew uniformly. When the super-solvus alloy was aged from 500h to 2000h, only tertiary γ′ precipitates with uniform size were seen and they tended to coarsen at approximately the same rate. After aging for 2000h, the carbides in sub-solvus and super-solvus alloys are mainly M23C6, which can improve grain boundary strength. For the sub-solvus alloy, with the extension of aging time, both the room temperature and high temperature tensile strength originally decreased, then increased, and finally decreased again. Meanwhile the room temperature plasticity decreased gradually, while the high temperature tensile plasticity increased first and then decreased. Compared with the super-solvus alloy, the sub-solvus alloy after long term aging exhibited superior high temperature tensile properties. Cutting γ′ precipitates by stacking faults and forming a continuous stacking fault by increasing the width of the stacking faults is another deformation mechanism of the sub-solvus alloy. In the super-solvus alloy there are still a large number of dislocations accumulating between γ′ precipitates, but only a small amount of superdislocations cutting γ′ precipitates.
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