Influence of heat treatment on Inconel 625 superalloy sheet: carbides, γ'', δ phase precipitation and tensile deformation behavior

In this study, commercial Inconel 625 superalloy was subjected to solution and longer-term aged treatment. The effects of the temperature and time of aged treatment on the secondary-phase evolutions, tensile properties, and strengthening mechanism of Inconel 625 superalloy were systematically investigated. The results showed that the evolution of M23C6 carbides mainly included two paths: MC+ γ→M23C6 at 650 °C and MC+ γ→M23C6 + δ at 750 °C. The disk-like γ'' phase with three mutually orthogonal variants dispersedly precipitated within the grains after aged treatment at 650 °C for 200 h. After aged treatment at 750 °C for 100 h, the needle-like δ phase parallelly precipitated around the grain boundaries. As the aged time reached 200 h and 300 h, δ phases precipitated within the grains at the expense of the metastable γ" phase. Both γ'' and δ phases strengthened the alloy during tensile deformation at room temperature (RT). However, the γ'' phase showed a better strengthening effect than the δ phase. Coarsened δ phases would cause severe stress concentration around δ phase/matrix interfaces during the non-uniform deformation stage. With the increase of tensile strain, many microvoids appeared around the δ phases/matrix interfaces, which weakened the strengthening effects of the δ phases.