New kink-band formation mechanism discovered in Zn-Al alloy with mille-feuille microstructure

The discovery of kink-band strengthening in some Mg-alloys has focused attention on the development of mille-feuille-structured alloys composed of alternating stacks of plate-like soft and hard phases. In this study, we focused on the Zn-6.0mass%Al alloy that is composed of hcp-Zn and fcc-Al, and is used as a coating material on hot-dip galvanized steel sheet for automobile. The deformation behavior was examined by using directionally solidified crystal. As expected, kink bands formed in alloys but two different kink-band formation mechanisms were identified. One is a classical mechanism which involves the formation of an array of dislocations perpendicularly aligned with their slip planes. The other is a new mechanism which has never been reported in any materials before, and a prior {11(2) over bar2} pyramidal slip in the Zn matrix phase acts as the source, with dissociation of the <11(2) over bar (3)over bar> dislocation supplying the <11 (2) over bar 0> dislocation for kink-band formation. The latter newly found mechanism introduces fine kink bands uniformly into the specimen, which effectively causes kink-band strengthening during subsequent deformation compared to kink bands introduced by conventional formation mechanism. This study provides a new strategy for enhancing kink-band strengthening in alloys by the control of the morphology of kink bands.