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Iterative processing, involving sequential deformation and annealing, has been carried out on
copper specimens with the aim of grain boundary engineering (GBE) them. The data have provided
some interesting insights into the mechanisms of GBE. The results have demonstrated that development
of a high proportion of Σ3s is beneficial to properties, as shown by improved strain$to$failure for the
same strength. The proportion of Σ3s saturates at approximately 60% length fraction. Analysis of the
data indicates that iterative processing is not always necessary for the development of beneficial
properties, and it is further suggested that the condition of the starting specimen has a large influence on
the subsequent microstructural development. The present, new data are also compared with previous
research on copper where all five parameters of the grain boundary network population have been
measured.
It is now established that thermomechanical manipulation of grain boundary crystallography improves
material properties such as corrosion resistance, intergranular cracking or ductility. This practice has
become know as ‘grain boundary engineering’ (GBE). GBE is linked to prolific twinning in low
stacking$fault energy metals and alloys. Recently GBE in this class of materials has been reviewed and
discussed in detail [1,2]. Areas where GBE research is ongoing include the role of grain boundary
planes [3], interface connectivity [4] and modelling and predictions of intergranular failure [5].
Application of cold deformation followed by annealing schedules is at the heart of GBE
processing. Usually iterative processing regimes are employed. A typical GBE processing schedule, as
quoted in the original patent for GBE [6], would be several iterations (e.g. 3$7) of cold work (e.g. 20%$
40% reduction) and annealing at a temperature which is approximately 0.6$0.8 of the absolute melting
temperature for a few minutes which results in a fine grain size and certain property improvements such
as improved intergranular corrosion resistance. For example, for Inconel 600 the requirements for GBE
were 3$7 iterations of 5%$30% cold work followed by annealing at 900°C$1050°C for 2$10 minutes [6].
Such a schedule claimed to ensure that the grain size remained small, <30m, and that the fraction of
coincidence site lattice (CSL) boundaries increased to 50%$70%.
A variant on the original GBE processing specification is strain$anneal processing. This
processing involves low levels of cold work, up to 6% strain, followed by anneals of up to many hours
at temperatures typically lower than those used for recrystallisation GBE. A similar improvement in
properties and/or attributes occurs when strain$annealing is used compared to recrystallisation
annealing, although the former may result in larger a grain size [1]. The ‘strain$recrystallisation’
description of GBE processing is probably a misnomer and more accurately it should be termed ‘strain$
recovery’, since TEM evidence has indicated that insufficient strain is localised at grain boundaries to
instigate nucleation of recrystallisation during the short annealing cycles [7].
Materials Science Forum Vol. 550 (2007) pp 35-44 Online: 2007-07-15
© (2007) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.550.35
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.ttp.net. (ID: 137.44.1.174, Swansea University, Swansea, United Kingdom-13/11/15,10:48:51)