Plastic flow and dislocation structure at small strains in OFHC copper deformed in tension, torsion and combined tension-torsion

Abstract

OFHC copper specimens of 39 μm grain size were deformed to small strains (up to 8%) in tension, torsion and combined tension-torsion at 300 K and the resulting dislocation structures, distributions and densities were determined using transmission electron microscopy. Employing the von Mises yield criterion and the plastic-work hypothesis good agreement was obtained for the three testing conditions for (i) equivalent stress \\ ̄ gs vs equivalent strain \\ ̄ g3 p curves, (ii) the dislocation structure, distribution and density ρ as a function of \\ ̄ g3 p, and (iii) \\ ̄ gs as a function of ρ 1 2 . Furthermore, upon comparing the \\ ̄ gs vs ρ 1 2 curve for polycrystalline copper with the τ RSS vs ρ 1 2 curve for single crystals, an average Taylor factor M= (σ/τ RSS) of approximately 3.2 was obtained, which is in good accord with that predicted theoretically for FCC metals. Almost equally good correlations for the stressstrain curves and for the dislocation density were obtained on the basis of maximum shear stress τ max and maximum shear strain γ p max as on the basis of \\ ̄ gs and \\ ̄ g3 P. Therefore, the present results do not permit a positive decision on the question whether the dislocation density correlates better with \\ ̄ gs and \\ ̄ g3 P or with τ max and γ P max. A single test in which the direction of straining in torsion was reversed yielded a density and distribution of dislocations (and a corresponding value of \\ ̄ gs) equivalent to those that developed at a smaller strain in unidirectional straining.