Microstructure Evolution Behavior Of GH4742 Superalloy During Long-time Aging

GH4742 alloy is a type of nickel-base precipitation strengthening superalloy which is used for making turbine disc of gas turbine and aero-engine. The turbine disc serves under medium and high temperature, as well as complex stress and gas corrosion environment for a long time, so structure stability is the important bases for selecting turbine disc materials and confirming the service temperature range of turbine disc.This paper observes the microstructure evolution behavior of GH4742 alloy when aging at 650℃ and 750℃ for a long time from 100 hours to 5000 hours with methods such as SEM and TEM. It was detimeined that the main precipitated phase of GH4742 alloy is γ’ phase, and there are also MC and M23C6 carbide by adopting selected aero electron diffraction, energy spectrum analysis or phase analysis. The average diameter and volume fraction of γ’ phase, as well as the hardness of GH4742 alloy after long-time aging were measured and detimeined.It was found by study that there exists three type γ’ phases in different morphologies and dimension ranges, that is large size irregular blocky type, petal type (similar to dendritic-like shape) and spherical type, which is a type of special structure formed in GH4742 large size discs in forging and air cooling process as well as air cooling and low temperature aging process after solution treatment. It was found by study that there exists phenomenon such as splitting, unstable grew out and self-decomposition of γ’ phase in forging and cooling process as well as in the cooling process after solution treatment, which are all unstable form of γ’ phase.The study shows that when aging for a long time between 100 hours to 5000 hours at 650℃, the γ’ phase and carbide in GH4742 are basically stable without coarsening and degradation phenomenon. After aging for 100 hours, the hardness increased to the highest point due to additional precipitation of γ’ phase and the hardness maintained in the subsequent aging process.The study shows that when aging at 750℃ after 100 hours, some spherical γ’ phase re-dissolved, and some spherical γ’ phase coarsened and evolved into square shape. After 1000 hours, almost all spherical phases re-dissolved into the matrix, and the square γ’ phase coarsened further. After 5000 hours, the carbide is close to degradation. After aging for 100 hours at 750℃, the spherical γ’ phase of GH4742 alloy grows to certain size, which has strengthening effect and increases the hardness, but in the subsequent aging process, the γ’ phase starts coarsening, splitting and self-decomposition, and the hardness decreased continuously.It was found by study that during long time aging at 750℃, the spherical type γ’ phase of GH4742 conforms to Ostwald ripening mechanism and the petal type(dendritic-like shape) γ’ phase has the shape transformation mechanism which reduces the total amount of interface energy and elastic interaction energy.