Effects of the γ''-Ni3Nb phase on mechanical properties of Inconel 718 superalloys with different heat treatments

Abstract

The effects of the "γNi3Nb phase on the mechanical properties of Inconel 718 superalloys, with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing+ direct aging, were characterized by morphological observation, X-ray diffraction, and tensile experiments. The results of the morphological observation revealed that many fine γ'' precipitates of ∼26.49 ± 1.82 nm in mean size were formed in all samples. However, the relatively coarser γ'' precipitates formed in the grain boundaries were only observed in the sample treated with hot isostatic pressing + direct aging. The yield strengths of the hot isostatic pressing + direct aging sample at room temperature and at 650°C both exhibited the maximum values about 993 ± 5.7 and 811 ± 12.6 MPa, respectively. The γ'' precipitate was considered to be the dominant strengthening phase in the sample according to the lattice misfits E of γ/γ''. The strengthening mechanism of the samples can be explained as the coherency strain strengthening of fine γ'' precipitates. Moreover, due to the coarserγ'' precipitates in the grain boundaries, dislocation-cut ordered particle strengthening also occurred in the sample after hot isostatic pressing + direct aging treatment.