Mechanism of twinning-induced grain boundary engineering in low stacking-fault energy materials

The current status of "grain boundary engineering" is overviewed, i.e. the deliberate manipulation of grain boundary crystallography in polycrystals in order to produce a material containing grain boundaries which have superior properties compared to average boundaries. A particular focus of attention is annealing twinning in low stacking-fault energy (SFE) materials as a means to achieve this aim, since the role of such twinning in improving the grain boundary network has not hitherto been satisfactorily explained. The twinning is discussed from the viewpoint of strain retention, imposition of crystallographic constraints at grain junctions, "relative specialness" rather than "absolute specialness" and proposal of a new "Sigma 3 regeneration model" to explain how rains can enhance the Sigma 3 boundary fraction in the network. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.