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• A Zn-containing alloy with improved crack tolerance and strength is exploited. • Trace Zn promotes pyramidal slips, inhibits twinning, and enhances GB cohesion. • Zn segregation enables crack buffering with a stepped interfacial transition zone. • In-situ TEM proves the emission of 〈 c + a 〉 dislocations despite microcracking. • Higher strength is mainly due to enhanced solid-solution strengthening. Effects of solute segregation at grain boundaries (GBs) on the deformation mechanism and fracture behavior remain obscure for magnesium (Mg) alloys. Here, by introducing Zn segregation at GBs, we obtained an Mg-0.5Al-0.4Mn-0.2Ce-0.4Zn (wt.%) alloy achieving fracture elongation (FEL) of ∼33.6 %, with a remarkable FEL improvement by 100 % in comparison to Zn-free counterpart. Meanwhile, the tensile yield strength (TYS, 195.5 MPa) is increased by ∼27.5 MPa after trace Zn addition. Although trace Zn addition improves the fraction of GBs with high misorientation, it reconciles crack tolerance with enhanced strength. The introduction of Zn not only promotes pyramidal 〈 c + a 〉 slips and inhibits twinning nucleation, but also enhances the GB cohesion by Zn segregation. Based on in-situ microstructure observation, we found that the enhanced GB cohesion enables the segregation-inspired hierarchical crack buffering, as well as deflecting and branching. Enhanced GBs can also facilitate the continuous emission of 〈 c + a 〉 dislocations in neighboring grains irrespective of the onset of microcracks, forming a plastic zone to retard local strain concentration, thus avoiding microcrack percolation and attaining a crack-mediated elongation reserve of above 15 %. Besides, the higher TYS in the Zn-containing alloy mainly stems from the enhanced solid-solution strengthening of Zn solutes, thus achieving strength and crack tolerance synergy. [Display omitted] [ABSTRACT FROM AUTHOR]

• Through using a simple accumulative roll bonding process assisted by hard plates, we tailored a novel multiscale hetero-structure in Mg-9Al-1Zn (AZ91) alloy, consisting of bimodal-structured center-region and fine-grained surface-layer. The multiscale hetero-structured AZ91 alloy exhibits a high yield strength of ∼303 MPa, tensile strength of ∼400 MPa and elongation of ∼14.5 %. • The strain incompatibilities stimulated by the multiscale microstructural heterogeneities could influence the dominant deformation modes, and further activate diversified strain accommodation mechanisms, including non-Schmidt 〈 c + a 〉 dislocations. • A unique slip trace distribution has been developed in the fine grains around the basal-oriented coarse grain in bimodal-structured center-region, which effectively releases the micro-stress concentration. • High hetero-deformation induced (HDI) stress and HDI hardening could be attributed to the activation of multiple deformation modes, resulting in a mediated plastic deformation at high-stress-state and dislocation accumulation, which is beneficial to enhanced strength-ductility synergy. Most of the high-strength Mg alloys exhibit poor ductility at room temperature. Architecturing appropriate heterostructure in metallic materials gives rise to the breakthrough of the inherent strength-ductility trade-off dilemma. Here, we prepared a novel multiscale hetero-structure in Mg-9Al-1 Zn (AZ91) alloy, consisting of bimodal-structured center-region and fine-grained surface-layer, providing high strength and high ductility, i.e. yield strength of ∼303 MPa, tensile strength of ∼400 MPa, and elongation of ∼14.5 %. The multiscale microstructural heterogeneities were induced via a simple rolling process that combines the advantages of both accumulative rolling bonding (ARB) and hard plate rolling (HPR). By utilizing quasi-in-situ electron backscattered diffraction (EBSD)-digital image correlation (DIC) characterizations combined with the determination of dislocation/disclination distributions, it has been found that diversified strain accommodation mechanisms are stimulated by the strain incompatibilities near hetero-interfaces in the multiscale hetero-structure. In particular, non-basal slips have been activated largely in both the bimodal-structured center-region and fine-grained surface-layer. Moreover, a unique slip trace distribution has been observed in the fine grains adjacent to the basal-oriented coarse grain. Such a strain accommodation can better coordinate local strain incompatibility among the hetero-interface. The uniaxial loading–unloading–reloading tensile loop tests demonstrate that the multiscale heterogeneities impart a pronounced hetero-deformation induced hardening during tensile deformation. Multiple deformation modes, including the non-Schmid and non-basal slips, have been activated substantially in the multiscale hetero-structured Mg alloy, effectively relaxing local stress concentration and achieving stable plastic flow at a high-stress state, which is beneficial to improving strain-hardening ability and strength-ductility synergy. Our work could shed light on designing and processing strong and ductile Mg alloys, by tailoring proper microstructural heterogeneity to activate cooperated accommodating deformation modes. [Display omitted] [ABSTRACT FROM AUTHOR]

• A bimodal-grained WE43 alloy consisting of fine grains (FGs) and coarse grains (CGs) with superior strength-ductility synergy has been achieved by one pass rolling. • Statistical slip trace analysis reveals that the deformation behaviors of FGs and CGs are quite different, i.e., basal slips are dominate in FGs while non-basal slips are prevalent in CGs. • Multi-faceted equilibrium β phase has precipitated along grain boundaries of FGs, while short rod-shaped metastable β 1 phase has formed within CGs. • Interactions between pyramidal dislocations and β 1 precipitates dominant the Orowan strengthening. • The stress concentration near grain boundaries can be released by activating plenty of non-basal slips in CGs and slip transfers between FGs, preventing effectively the premature failure and leading to enhanced ductility. Achieving a high strength-ductility synergy in rolled Mg alloys remains a major challenge, especially for Mg alloys with high contents of rare earth (RE) elements, which are hard-to-deform and exhibit poor formability. In the present work, a bimodal grain structure consisting of fine grains (FGs) and coarse grains (CGs) has been achieved in a novel WE43 alloy sheet prepared by the hard-plate rolling (HPR) technique, which breaks the strength-ductility trade-off dilemma of the conventional WE43 alloy. Notably, the yield strength (YS) reaches as high as ∼312 MPa, presenting a remarkable improvement of YS (∼60 MPa) in comparison to conventionally rolled counterparts. Meanwhile, the ultimate tensile strength (UTS) and elongation-to-failure (EF) are ∼332 MPa and ∼11.8%, respectively, much higher than those of the wrought WE43 alloy reported in literature. Specifically, the disparate dislocation slips and dynamic precipitation behaviors in FGs and CGs have been studied in detail by combining EBSD and TEM analysis. The discrepancy of dynamic precipitation behavior in FGs and CGs is originated from the heterogeneous dislocation slip-dominated deformation behavior during HPR, related to initial grain orientations. It reveals that basal slips are dominant in the FG area, where equilibrium β-Mg 14 Nd 2 Y phase has precipitated along grain boundaries. In contrast, non-basal slips are prevalent and the metastable short rod-shaped β 1 phase has formed within CGs. The enhanced YS of the present HPRed WE43 alloy sheet is mainly attributed to the formation of large amounts of extremely fine grains with an average size of ∼0.94 μm. The enhanced Orowan strengthening comes from the interaction between short rod-shaped β 1 particles and non-basal dislocations (prismatic , pyramidal and pyramidal dislocations) in CGs. In particular, the interactions between β 1 particles and pyramidal dislocations dominate the Orowan strengthening, leading to an increase of ∼34 MPa in the YS. Basal to basal slip transfers have been observed in FGs while non-basal slips are prevalent in CGs, which could be helpful for release the stress concentration near GBs. It is beneficial for preventing earlier fracture and thus enhancing the tensile strength combined with decent ductility. [Display omitted] [ABSTRACT FROM AUTHOR]

• Plasticity can be achieved through multiple twinning modes and disclinations. • { 11 2 ¯ 6 } type extension twinning mode has been observed in mg alloys. • { 11 2 ¯ 6 } twins can be generated through reactions between twins and disclinations. • Three formation mechanisms of { 11 2 ¯ 6 } twins have been identified. Twinning-induced plasticity plays a critical role in determining the deformation behaviors of hexagonal close-packed metals (e.g., Mg and Ti), due to the lack of five independent slip systems required for a general deformation. In particular, twinning modes that can accommodate c-axis strain (tensile or compressive) are especially important, which are necessary complementary deformation modes to < a >-type dislocations. However, only limited types of extension twins, e.g., { 10 1 ¯ 2 } type twin, have been unambiguously identified in the literature, which is theoretically inadequate to accommodate a complex deformation with c-axis extension. Using topological defect theory, here we show that another extension twinning mode, { 11 2 ¯ 6 } twin, can be formed through reactions between disclinations and { 10 1 ¯ 2 } twins in Mg-alloys. Based on symmetry of the deformation space, we demonstrate that the { 11 2 ¯ 6 } twin originates from the inter-connection of correlated deformation paths, which can provide a tensile strain of 5.2% along c-axis and a compressive strain of 4.5% along 〈 11 2 ¯ 0 〉. { 11 2 ¯ 6 } twins can be formed through three formation mechanisms, i.e., direct formation, twin-twin intersection, and double twinning, which are validated by a combination of experimental characterizations and phase field simulations. Our work not only suggests a symmetry-based method to analyze twinning-induced plasticity, but also provides a new insight into the deformation mechanisms and mechanical properties of Mg-alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

• The superplastic elongation of ∼450% is achieved in low-alloyed Mg system by utilizing co-segregation of Zn, Ca, Zr and Ag atoms along the grain boundaries. • Superplastic deformation of low-alloyed Mg system can be enhanced by stress-driven grain boundary migration mechanism. • Based on disclination-dislocation reactions, a new grain boundary migration model has been proposed. • The grain boundary migration model can be used to understand the cooperative deformation mechanisms of dislocation slip and grain boundary migration. Superplastic forming, as an advanced manufacturing method, is especially important for producing complex-shaped components for Mg-alloys. Superplasticity can be achieved through grain boundary sliding at elevated temperatures in high-alloyed Mg systems, with a typical grain size < 10 μm stabilized by high densities of intermetallic precipitates. However, it is difficult to stabilize small grains and facilitate grain boundary sliding in low-alloyed systems due to insufficient precipitates. Here, by utilizing a distinctive design strategy, i.e. introducing solute segregation to enhance stability of the fine-grained structure, we obtained an Mg-1Zn-0.2Ca-0.2Zr-0.1Ag (wt.%) alloy achieving ∼450% superplastic strain. Through quasi-in-situ Electron Backscatter Diffraction analyses, we investigated systematically microstructure evolutions during superplastic deformation of the alloy at different strains. It has been found that superplasticity is achieved through the absorption of intragranular dislocations by grain boundaries, which are associated with stress-driven grain boundary migration mediated by disclination-dislocation reactions. By using a theoretical description based on stress-driven grain boundary migration and disclination-dislocation reactions, our model can capture multiple superplasticity mechanisms (e.g., grain boundary sliding and grain rotation), which establishes a fundamental relationship between the evolution of microstructural defects and the strain accommodation during superplastic deformation. Our results not only suggest an effective way to achieve superplasticity in low-alloyed Mg systems, but also provide a new insight to elucidate the nature of superplasticity mechanism by revealing the intrinsic correlation between stress-induced grain boundary migration and disclination/dislocation motion. [Display omitted] [ABSTRACT FROM AUTHOR]