Hardening behavior of three metallic alloys under combined stresses at elevated temperature

Initial and subsequent yield loci for type 316 stainless steel, Haynes 188, and Inconel 718 are determined experimentally in the axial-shear stress plane at 650°C. Each of these materials has a face centered cubic (fee) crystal structure, but entirely different chemical compositions and strengthening mechanisms. Material hardening behavior is described along three cyclic strain paths having a maximum equivalent strain of 0.015 m/m: fully reversed axial (Path I), fully reversed shear (Path II), and a nonproportional hourglass shaped path (Path III). Yielding is defined by a Mises-type equivalent offset strain definition having a target value of 30 μm/m. These types of yield surface determination tests are difficult, but fairly common in lower temperature ranges where precision strain gages can be used. Due to the high temperature in this study, axial and shear strains were measured by an extensometer. These results, which describe material hardening by the evolution of size, shape, and position of the current yield surface can be used in support of constitutive models for high temperature metallic materials. Hardening behavior of all three alloys is well described by a mixed hardening rule comprised of isotropic, kinematic, and distortional components. Relative to published work at lower temperatures, the isotropic hardening component is larger and the distortional hardening component is smaller than anticipated.