Evolution of microstructure and mechanical property during long-term agingin Udimet 720Li

Thermal stabilities of microstructure and mechanical property have been inv estigated on superalloy U720Li, which is of great interest of application f or jet engine and land-based turbine disc. The results showed that, the pri mary and secondary gamma' particles maintain good thermal stability at 650 and 700 degreesC with aging time up to 3000 h, while the tertiary gamma' is apparently dependent on aging temperature and time. The tertiary gamma' pa rticles undergo a procedure of coarsening, dissolution and eventually compl ete disappearance with the increasing of aging time and temperature. They e xhibit unusual high sensibility upon aging temperature, which is attributed to the lattice misfit between the gamma' precipitates and the matrix in th e alloy. The grain boundary phase M23C6 remains stable without forming of s igma phase even with aging time up to 3000 h at 700 degreesC. Microhardness decreases apparently with increasing aging time and aging temperature. The oretical analysis based on dislocation mechanism indicates that the change of microhardness should be attributed to the evolution of the tertiary gamm a' during aging.