Precipitate evolution and strengthening behavior during aging process in a 2.5 GPa grade maraging steel

Possible nucleation mechanism as well as the following evolution of precipitation, microstructure and mechanical properties have been studied in a 2.5 GPa grade maraging steel during aging at 480 ℃. An initial hardening reaction within 15 min is reported, and this strengthening is caused by the formation of complex multi-component atomic co-clusters containing primarily Ni and Ti. First principle calculations showed that Ni and Ti were preferred to segregate together for their lowest interaction energy. With the extension of aging time, Ni-Ti clusters grew to Ni3Ti, and regardless of Co or Fe occupied position the formation energy of Ni-Ti clusters was higher than pure Ni3Ti. Therefore, Co and other atoms were eventually expelled from Ni3Ti. Meantime, owing to difficult Mo diffusion, Mo-rich phase adjacent to Ni3Tiphase grew up and formed a core-shell structure with Ni3Ti phase together. Finally, three modified theoretical prediction models were introduced to describe the yield strength as a function of microstructure and precipitates characteristics of the experimental steel. When both the volume fraction and dimension of precipitates were considered, the modified equation of Orowan relationship showed more reasonable prediction compared with experimental results of the yield strength.