Effects of the gamma ''-Ni3Nb Phase on Mechanical Properties of Inconel 718 Superalloys with Different Heat Treatments

The effects of the gamma ''-Ni3Nb 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 gamma '' precipitates of similar to 26.49 +/- 1.82 nm in mean size were formed in all samples. However, the relatively coarser gamma '' 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 degrees C both exhibited the maximum values about 993 +/- 5.7 and 811 +/- 12.6 MPa, respectively. The gamma '' precipitate was considered to be the dominant strengthening phase in the sample according to the lattice misfits (epsilon) of gamma/gamma ''. The strengthening mechanism of the samples can be explained as the coherency strain strengthening of fine gamma '' precipitates. Moreover, due to the coarser gamma '' precipitates in the grain boundaries, dislocation-cut ordered particle strengthening also occurred in the sample after hot isostatic pressing + direct aging treatment.