A Methodology for Determining Rate-Dependent Flow Surfaces for Inconel 718

A detailed set of experiments has been conducted in an attempt to characterize the inelastic flow behavior of materials used in aeronautics applications. These experiments have shown that aged (precipitate-hardened) Inconel 718 exhibits an initial nonlinear elastic behavior as well as a strength differential in tension and compression at room temperature. This nonlinear elastic behavior correlates reasonably well with a second order stress-strain law that was developed to account for interactions between dislocations and interstitial solute atoms. Flow loci in the axial-shear stress plane at 25 and 650°C have been determined from yield locus data using both inelastic power and equivalent inelastic strain definitions of flow. Flow loci are more theoretically meaningful than yield loci for describing the time-dependent material response. These flow loci are especially important if they are proportional to the dissipation potential since the normality rule is associated with dissipation and not necessarily with a particular yield or flow definition employed in an experiment. A threshold function that depends only on the second deviatoric stress invariant, J₂, is inadequate for predicting the threshold surface (initial yield locus) for Inconel 718 due to the difference in flow behavior in tension and compression. Threshold functions including all three stress invariants (I₁, J₂, and J₃) with each term having units of stress raised to the first or third power were equally successful in fitting the initial flow locus. Addition-ally, both F = al₁ + bJ^1/2₂ - 1 and F = b³J^3/2₂ + c³J₃ - 1 are shown to represent the data very well. The outward normals for these two representations of the threshold surface are consistent with experimentally determined directions of the inelastic strain rate vectors.