Abstract
We report the discovery of a rigorous nucleation mechanism for {101¯2} twins in hexagonal close-packed (hcp) magnesium through reversible hcp-tetragonal-hcp martensitic phase transformations with a metastable tetragonal phase as the intermediate state. Specifically, the parent hcp phase first transforms to a metastable tetragonal phase, which subsequently transforms to a twinned hcp phase. The evanescent nature of the tetragonal phase severely hinders its direct observation, while our carefully designed molecular dynamics simulations rigorously reveal the critical role of this metastable phase in the nucleation of {101¯2} twins in magnesium. Moreover, we prove that the reversible hcp-tetragonal-hcp phase transformations involved in the twinning process follow strict orientation relations between the parent hcp, intermediate tetragonal, and twin hcp phases. This phase transformation-mediated twinning mechanism is naturally compatible with the ultrafast twin growth speed. This work will be important for a better understanding of the twinning mechanism and thus the development of novel strategies for enhancing the ductility of magnesium alloys.
Highlights
The application of hexagonal close-packed magnesium (Mg) alloys as lightweight structural components in automotive and aerospace industries has been severely limited by their inferior ductility [1,2], which necessitates a rigorous understanding of the deformation mechanisms of Mg
We report the discovery of the nucleation of {101̄2} twins in Mg through reversible martensitic phase transformations, which is naturally compatible with the ultrafast twin growth speed
We have demonstrated the complete twinning process observed in our molecular dynamics (MD) simulations, which clearly reveals the critical role of the intermediate metastable tetragonal phase
Summary
The application of hexagonal close-packed (hcp) magnesium (Mg) alloys as lightweight structural components in automotive and aerospace industries has been severely limited by their inferior ductility [1,2], which necessitates a rigorous understanding of the deformation mechanisms of Mg.It is well understood that the brittleness of Mg arises from its largely anisotropic critical resolved shear stress between basal slip and non-basal slip, and restricted number of activated slip systems. Extensive efforts have been devoted to exploring the activation of deformation twinning in Mg [1,3,4,5,6,7,8,9,10,11,12], which is an important category of deformation mode to meet the von Mises’. The nucleation mechanism of the {101̄2} twin [9,10,13,14,15,16,17,18,19,20,21,22], which is the predominant twinning mode in Mg, still remains elusive. The earliest nucleation model describes the homogeneous nucleation of twins due to high stress concentration [5,16,17,23]. Subsequent studies have focused on heterogeneous nucleation through the gliding of disconnections generated by the dissociation of pre-exiting defects, such as various types of dislocations [18,19,20,24], dislocation pile-ups [14], grain boundary defects [13], and prismatic/basal interfaces [22,25]
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