Microstructural tailoring in reverse gradient-structured copper sheet using single-roll angular-rolling and subsequent annealing

Abstract

This paper reports on mechanical properties dependent on microstructural heterogeneity in a reverse gradient-structured Cu sheet fabricated using a new continuous severe plastic deformation process called single-roll angular-rolling (SRAR). The material consists of coarse-grained surface and fine-grained core regions of which the spatial arrangement is the opposite of conventional gradient structures with fine-grained surface and coarse-grained core regions. The microstructure was diversified by manipulating the subsequent annealing time after the 6th pass of SRAR, by which three kinds of heterogeneous structures were manufactured. These consisted of a recrystallized fine-grained core, recrystallized coarse-grained surface, and non-recrystallized region. All these materials commonly exhibited outstanding combinations of strength and ductility exceeding that of the initial material. The additional strengthening and strain hardening in each material related to the microstructural heterogeneity, were discerned by comparison using the rule-of-mixture. Consequently, a briefly annealed material with the largest area fraction of non-recrystallized region was most beneficial to the extra strengthening for the yield strength, whereas a fully recrystallized reverse gradient structure gained superiority for flow stress from further strain hardening. The mechanical uniqueness of each material was primarily governed by the back-stress evolution originating from its microstructural heterogeneity. Ultimately, this study has laid a cornerstone for microstructural tailoring in reverse gradient-structured materials using the SRAR process.