The nature of yielding and anelasticity in metals

Abstract

Recent reports in the literature identified pre-yield stress-strain nonlinearity and hysteresis (anelasticity) as occurring in metals at virtually all stresses. These observations raise foundational questions about metal deformation. Does a critical stress exist below which dislocations are immobile? Is there a critical stress for which permanent deformation occurs? Is anelasticity distinct from elasticity and plasticity? To answer such questions, special tensile tests with loading-unloading cycles after various prestrains were performed for 11 commercial sheet alloys: 9 AHSS (advanced high strength steels) and two Mg alloys. A dissipative dislocation bow-out model of anelasticity was derived that closely reproduces the experimental results and is consistent with the evolving experimental picture of anelasticity. Following prestrain, a finite yield stress was found to exist, below which no permanent deformation or hardening occurs. Anelasticity is distinct from elasticity and plasticity: it is recoverable and dissipative; mechanically reversible and thermodynamically irreversible. Corresponding tests of initial loading suggest radically different conclusions. Without prestrain, i.e. without a developed internal stress pattern, plastic deformation occurs near zero stress. A postulate of local and nonlocal interactions accounting for elastic, plastic and anelastic deformation was proposed.