Design Of Hierarchical Twin Structure And Strengthening Mechanism In Mg Alloys

Magnesium（Mg）alloys,as promising lightweight and weight-saving metal structural material,have attracted increasing attention for extensive application in electronic communications,transportation and aerospace industries.However,high-strength Mg alloys often exhibit poor ductility.The strength-ductility dilemma seriously restricts the application of Mg alloys.The limited number of independent slip systems,and the difficulty in activating pyramidal-<c+a>slips due to the higher critical resolved shear stress（CRSS）lead to difficulties in accommodating the strain along c-axes of hexagonal close-packed（HCP）Mg alloys.In addition to dislocation slips,mechanical twinning plays a crucial role in accommodating the strain along c-axes of Mg alloys.Therefore,it may be feasible to enhance the ductility of Mg alloys by manipulating the twinning behavior,on the premise of ensuring strength.For Mg alloys,although a few twinning modes,such as{10（?）2}{（?）011},{（?）112}{2（?）（?）3}and{1（?）01}{（?）102},have been predicted in theory and confirmed in experiments in the literature at present,only{10（?）2}extension twins and{1（?）01}contraction twins have been widely observed experimentally.These two types of twins can accommodate tensile and compressive strains along the c-axis,respectively,significantly affecting the mechanical properties of HCP metals and alloys,such as strength,plasticity,and fracture toughness.In the plastic deformation of Mg alloys,the{10（?）2}extension twins will dominant when the c-axis of the grain is subjected to tensile stress,owing to their relatively low critical resolved shear stress.As a consequence,dislocation plasticity becomes unfavorable upon further compression,which leads to reduced ductility and premature failure.However,one extension twinning mode cannot accommodate complex deformation with c-axis extension theoretically.This inherent dilemma that only one extension twinning mode cannot accommodate complex strains along c-axes has propelled numerous studies of new twinning modes,such as{11（?）1}{（?）（?）26}secondary twins,as well as twin-like{33（?）4}tilt boundaries.However,there are no direct experimental measurements of their twinning paths and accommodated strains.Moreover,unfavorable detwinning usually occurs during multi-directional loading process,which competes with secondary/tertiary twinning and inhibits the formation of hierarchical twin boundaries.To resolve the critical issues in this research area,a low-alloyed Mg-Al-Gd alloy was designed in this paper.Preliminary exploration of a novel multistage twinning mode,i.e.,the dominant twinning mode changes from single{10（?）2}twins to the co-existence of{10（?）2}and{11（?）6}twins.By using quasi-in-situ EBSD characterizations,crystallographic analyses,topological defect theory and phase field simulations,the formation mechanisms of multistage twinning mode have been investigated systematically,and formation condition of a new type of{11（?）6}twinning has been identified.Our work not only suggests a new avenue to produce hierarchical twins through multi-directional loading in Mg alloys,but also sheds light on understanding the intrinsic correlations between the evolution of topological defects and hierarchical twinning induced by multi-directional loading.These findings provide new perspectives for designing and preparing high-performance Mg alloys by tailoring multiple twinning modes.Our major findings include:（1）A novel multistage twinning mode has been identified in a Mg-Al-Gd alloy subjected to compressive deformation at room temperature.The dominant twinning mode changes from single{10（?）2}twins to the co-existence of{10（?）2}and {11（?）6}twins.{11（?）6}twins have been identified in an Mg-Al-Gd alloy using advanced microstructure characterizations.Moreover,multiple slip systems are activated within{11（?）6}twins,including low Schmid Factor pyramidal-<c+a> and non-basal-<a>dislocations.The activated multiple slip systems within{11（?）6}twins can coordinate the complex plastic deformation along both the a and c-axes.Simultaneously,the mean free paths of dislocations are confined by the{11（?）6}twin boundaries,leading to a strong tendency for dislocation entanglement inside the twin as well as long-range back stresses at twin boundaries.These factors contribute to sustained strain hardening and enhanced ductility.（2）By investigating the symmetry of the deformation space through deformation path graph analysis,the formation and evolution of multi-stage twin modes during the plastic deformation of Mg alloys were systematically studied.We demonstrate that the{11（?）6}twin originates from the inter-connection of correlated deformation paths,which leads to a new classification of deformation twinning,i.e.,primary twinning mode（e.g.,{10（?）2}twins）,secondary twinning modes（e.g.,{11（?）6} and{11（?）2}twins）,and so on.Theoretical calculations show that{11（?）6}twins,as a novel twinning mode,can provide a tensile strain of～5.2%along c-axis and a compressive strain of～4.5%along{11（?）0}.（3）Through crystallographic analyses,experimental characterizations and phase field simulations,three different formation mechanisms of{11（?）6}twins have been demonstrated,i.e.,direct formation,double twinning and twin-twin intersection through reactions with disclinations.The formation condition of{11（?）6}twinning has been identified,which depends strongly on grain orientation and loading condition（e.g.,tension/compression direction,strain ratio）.The{11（?）6} secondary twinning is favored by compressive stress close to{11（?）0}direction,with a tilt angle ranging from～5°to～30°.（4）A hierarchical twinning structure Mg alloy has been developed by utilizing multi-stage twinning mode coupled with multi-directional loading,i.e.,consisting of a microscale twins and nanoscale twins.The present alloy exhibits excellent strength-plasticity combination at room temperature.Furthermore,the higher value ofσ_UCS-σ_YSimplies notably heightened work hardening capacity and exceptional deformation resistance in comparison with other low-alloyed Mg alloys.（5）Through experimental characterizations and topological defect theory,the formation mechanisms of hierarchical twins have been analyzed systematically.We have identified the formation mechanisms of new{11（?）6}twinning modes during multi-directional loading.Specifically,new twin modes are generated by twin-twin reactions mediated by the formation of disclinations,which interact with grain boundaries and suppress detwinning.Moreover,two major formation mechanisms of disclination-mediated secondary twins during multi-directional loading have been identified,i.e.,intragranular disclinations and grain boundary disclinations.Slip trace and Schmid Factor analyses suggest that the activation secondary twins can facilitate the activation of low Schmid Factor basal-<a>slips during multi-directional loading.The slip system activated within the secondary twins can coordinate complex plastic deformation along the a and c axes,effectively promoting isotropic deformation and preventing premature grain fracture during multi-directional loading.In summary,our work not only suggests a new avenue to produce hierarchical twins through multi-directional loading in Mg alloys,but also sheds light on understanding the intrinsic correlations between topological defects and hierarchical twinning induced by multi-directional loading.