INTERGRANULAR AND INTERPHASE BOUNDARIES IN MATERIALS Grain boundary reorientation in copper V. Randle Æ Y. Hu Æ M. Coleman Received: 23 May 2007 / Accepted: 23 August 2007 / Published online: 6 March 2008 Ó Springer Science+Business Media, LLC 2008 Abstract The present route to grain boundary engineering (GBE) is usually based on multiple annealing twinning which can only be applied to a certain subset of materials, namely those that twin prolifically. A more general approach has been highlighted recently, following experimental evidence that certain boundary planes in iron bicrystals are ‘special’, and that this classification is not based on misorientation. It was suggested that, under suitable conditions, individual interfaces could reorient the most energetically advantageous orienta- tions. This approach concurs with a similar concept of ‘grain boundary plane engineering’, proposed previously. In the present article we explore this concept and report the effect of long duration, low temperature annealing on the distribution of boundary misorientation and planes in copper. The new findings give support to the possibility of grain boundary structure optimisation via controlled annealing. To have established that grain boundary plane reorientation is feasible opens up new avenues and challenges in the field of grain boundary research. This could have significant impact both scientifically in terms of understanding grain boundary structure and technologically in the field of GBE. Introduction The most important component of polycrystalline materials is the internal interface (grain boundary) network. The properties of grain boundaries are different from those of the lattice. For example, grain boundary diffusion is much more rapid than its counterpart in the lattice. Grain boundaries are not uniform with respect to their properties; properties such as energy or mobility can vary enormously depending on grain boundary structure. Whereas this fact has been known for a long time, it is only in recent years that attempts have been made to understand in detail the relationship between boundary structure and properties, and hence to exploit it to improve material performance. This endeavour has come to be known as ‘grain boundary engineering’, GBE [1, 2]. Most attempts to understand the relationship between grain boundary structure and properties have focussed on face-centred cubic materials which form annealing twins readily. There are several illustrative examples associated with the GBE initiative, where characterisation of bound- aries has remained very largely misorientation-based [e.g., 3]. This means that of the five parameters required to characterise an interface, only three are used. Although this practice has the advantage of experimental simplicity, it ignores the orientation of the grain boundary plane (the remaining two parameters). Yet a growing body of evi- dence is indicating that ‘special’ (i.e., good) boundary properties depend crucially on the orientation of the boundary plane. For example, a recent molecular dynamics simulation has shown that in aluminium grain boundary mobility increases as the plane deviates from {111} [4]. This was shown to have particular relevance to R3 boundaries (found to be either very high or very low mobility) and R7 boundaries (found to have rather low mobility), where the R notation refers to a misorientation characterisation in the coincidence site lattice (CSL) notation. Other work, based on both computer simulation and high-resolution electron microscopy, has also firmly established the role of the boundary plane. It is shown that the grain boundary energy depends on the tilt and twist V. Randle (&) Á Y. Hu Á M. Coleman Materials Research Centre, School of Engineering, University of Wales Swansea, Swansea SA2 8PP, UK e-mail: v.randle@swansea.ac.uk 123 J Mater Sci (2008) 43:3782–3791 DOI 10.1007/s10853-007-2128-2