Mechanisms of grain boundary engineering

This paper reports new data for the analysis of grain boundary misorientation populations and microstructure characteristics in grain boundary engineering (GBE) copper, accompanied by parallel measurements in a non-GBE control specimen of the same material. The GBE processing resulted in a doubling the number fraction of 13 boundaries to 41%, accompanied by a 12 times increase in Sigma 9. The Sigma 3 population has a convoluted morphology, indicating a range of boundary planes and associated mobilities, and moreover indicating that such Sigma 3s arise from Sigma 3(n) interactions. Many Sigma 3s become incorporated into the grain boundary network. Hence, the grain boundary network is modified directly via partial replacement by Sigma 3s. This is termed the 'Sigma 3 regeneration mechanism'. The data from the GBE copper were compared with data previously acquired from GBE brass. The brass has approximately the same number fraction of Sigma 3s (38%), but a lower number fraction of Sigma 9s and Sigma 27s and a markedly different microstructure where twins are not incorporated into the grain boundary network and instead the network is modified indirectly via the modification to the boundary crystallography that results from twinning. This is termed the 'new twinning' mechanism. It is Suggested that these two mechanisms are general for all GBE metals where the GBE is based on annealing twinning. After the condition of a minimum number fraction (approximate to 20%) of Sigma 3s is met, the proportion of each mechanism that operates will be governed by processing variables, e.g., cold deformation level and annealing time/temperature. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.