Plastic deformation and microstructural evolution during the shock consolidation of ultrafine copper powders

Abstract

Shock consolidation of ultrafine copper powders at room temperature for bulk nano/ultrafine structured materials is achieved in a gas gun system. The stress states in the powders during the shock consolidation process are analyzed using the finite element method associated with the dynamic densification model (P-alpha model). The mechanical properties of the shock-consolidated copper are evaluated in terms of hardness, static tensile and compressive strengths, and dynamic compressive strength. The microstructures are characterized using an optical microscope, a scanning electron microscope, and X-ray diffraction (XRD). The XRD patterns are quantitatively analyzed in order to estimate the crystallite sizes and dislocation densities using the Convolution Multiple Whole Profile method. The shock-consolidated specimens were highly densified over 98% of relative density with uniform spatial distributions of high hardness. However, insufficient consolidation due to the tensile stress wave induced by the interactions between shock waves in the powders and due to the ultrafine particles requiring high pressure for good bonding has resulted in several defects in the consolidated specimens. These defects cause tension-compression asymmetry in the shock-consolidated materials. Compared with the tensile results, where fractures occurred at low stresses without plastic deformation due to weak interparticle bonding, the high compressive yield stresses of 600 and 900 MPa with large plastic strains are achieved in the static and dynamic compression results, respectively. These high compressive flow stresses are attributed to the extremely high dislocation density and the refinement of the crystallite size via the shock deformations. A microstructure model is proposed for the extremely high dislocation density, where dislocations are generated not only by shock waves but also by plastic flow during the void collapses. The strengths of the shock-consolidated specimen are slightly decreased during the post-shock deformations due to decreases in the excess dislocations despite further refinement of the crystallite size. (C) 2014 Elsevier B.V. All rights reserved.