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. Additionally, both F=aI1+bJ21/2-1 and F=b3J23/2+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.

An experimental program has been implemented to determine small offset yield loci for the nickel-base superalloy In conel 718 (IN718) under axial-torsional loading at elevated temperatures. Initial and subsequent yield loci were determined for solutioned IN718 at 23 and 454°C and for aged (precipitation-hardened) IN718 at 23 and 649°C. The shape of the initial yield loci for solutioned and aged IN718 agrees well with the von Mises prediction. However, in general, the centers of initial yield loci are eccentric to the origin due to a strength-differential (S-D) effect that increases with temperature. Subsequent yield loci exhibit kinematic and distortional hardening, and solutioned IN718 shows a weak cross effect. This work demonstrates that it is possible to determine yield surfaces for metallic materials at temperatures up to at least 649°C using multiple probes of a single specimen. The experimental data are first of their kind for a superalloy at these temperatures and will enable a better understanding of multiaxial material response.