Evolution of γ' precipitates in GH720Li alloy during long-term aging

A standard heat treatment(1105℃/4 h/(oil cooling) + 650℃/24 h/(air cooling) + 760℃/16 h/(air cooling)) and a subsequent long term exposure at temperatures of 720℃ for 0-5000 h and 800℃ for 0-1000 h were proposed to investigate the microstructural evolution of γ' precipitates in GH720 Li alloy. The size,morphology and quantity of the primary γ' phase have no significant variation at 720℃. However,at the same temperature,obvious grain coarsening and the grain size inhomogeneity of secondary γ'particles appear with the aging time up to 200 h and 500 h,respectively. The descending slope of hardness decreases first and when the aging time exceeds 500 h it flattens. At 800℃,the primary γ' phase coarsens after 500 h of aging,while both obvious grain coarsening and the grain size inhomogeneity of secondary γ' particles happen after 100 h of long term exposure. The descending slope of hardness decreases first,and unlike the results at 720℃,it is still downward when the aging time exceeds 500 h. The coarsening behavior is controlled by lattice diffusion. There shows a linear relationship between the average radius of the γ' phase and the cube root of aging time,which accords with the L-S-W(Lifshiz-Slyozov-Wanger) theory.