| Low-carbon environmental protection is the eternal theme of civilization progress and scientific development.The demand for energy conservation and emission reduction in the fields of transportation and aerospace poses new challenges to the rapidly-developing materials discipline,and it is imperative to generate lightweight materials with low cost and high performance.As the lightest metal structural material,Mg alloy has considerable potential application value in the field of lightweight.Large grain refinement function as essential role for developing high-strength Mg alloys.However,fine-grained/ultra-fine-grained high-strength Mg alloys often exhibit unsatisfactory work hardenability and elongation due to the lacking dislocation storage efficiency.Meanwhile,the asymmetric hexagonal close-packed（HCP）crystal structure of Mg alloys provides insufficient easily-activated deformation modes at room temperature,leading to poor ductility and formability.Therefore,realizing the synergistic strength-ductility enhancement of Mg alloy is the key link to break its limitation of practical application.In recent years,the design concept of hetero-structure has received substantial attention in the field of face-centered and body-centered cubic metals.That is,introducing internal microstrucatrual heterogeneity/soft and hard microdomains with significant strength differences to promote the accumulation and interaction of dislocations during deformation,so that the material can obtain excellent strength-ductility combination.However,for Mg alloy with strong single-crystal anisotropy and orientation dependence,it is still unclear how to design the soft/hard microdomains and how their interactions affect the complex deformation mechanism.In addition,rapid nucleation/growth of dynamic recrystallization are prone to occur inside Mg alloy during thermomechanical processing due to their inherently-low stacking fault energy,which should be a great challenge for optimnizing efficient hetero-structures.According to the remained problems,this work was studied upon low-cost Mg-Al based alloys,which is based on novel processing route combining hard plate assited rolling（HPR）and stress-relieving/recrystallization annealing treatment.（I）The formation mechanisms of hetero-structure were explored.（II）The microstructural evolution and deformation behavior of different micro-domains/hetero-interfaces in hetero-structured Mg alloys were systematically investigated.（III）The influence of microstructural heterogeneities on mechanical properties were explained.The main conclusions were shown here:（1）Processed by hard plate-assisted rolling bonding process（HPRB）and short annealing,the AZ91 Mg features multiscale microstructural heterogeneities,i.e.,consisting of a bimodal-structured center-region sandwiched by two fine-grained section surface-layers.The average size of weak basal-textured fine grains and strong basal-textured coarse grains in bimodal-structured center-region is～2.3 and～46.6μm,respectively.Meanwhile,the surface-layers features a relatively homogeneous fine-grained structure with a strong basal-texture and an average size of～1.9μm.The multiscale hetero-structured AZ91Mg alloy exhibits high strength-ductility synergy,i.e.the yield strength of～303 MPa,tensile strength of～400 MPa and elongation of～14.5%.（2）The influence of strain incompatibility on the deformation mechanisms of multiscale hetero-structured Mg alloy is emphatically discussed.During deformation,the strong basal-textured fine grains in surface-layer（SL）together with basal-textured coarse grains in section center-region（CR）function as the hard domains;while the weak basal-textured fine grains in CR serve as the soft domains.Multiple plastic deformation mechanisms have been activated in multiscale hetero-structured AZ91 Mg alloy,including the non-basal slips with high critical resolved shear stress（CRSS）,non-Schmid dislocation behavior and abnormal twinning activation,for effectively fulfilling local strain compatibility and relaxing local stress concentration.Especially,the fine grains in the bimodal-structured CR coordinate the strain gradient by accumulating dislocations and disclinations near the hetero-interface.The deformation modes of fine grains have been influenced significantly by the neighbouring basal-oriented coarse grain,as reflected by unique“bypassing”of slip traces occurred in the fine grains,for adapting to localized geometry of the bimodal-structured interface,and achiveing uniform plastic deformation.High hetero-deformation induced（HDI）stress and HDI hardening has been developed upon subsequent plastic deformation in the multiscale hetero-structured AZ91 Mg alloy.Actually,HDI stress contributes to more than 50%to the total strength,indicating a significant HDI strengthening effect in sample CR and HPRB.It can be ascribed to the activation of different deformation modes in multiscale hetero-structured HPRB sample,being able to accommodate micron mechanical incompatibility induced by microstructural heterogeneity.Thereby,the multiscale hetero-structured AZ91 Mg alloy exhibits high strain-hardening ability and superior strength-ductility synergy.（3）By using simple rolling and precisely-controlled annealing process,a novel heterogeneous lamella-structured Mg-9Al-1Zn-1Sn alloy（HL Mg alloy）has been designed,introducing a lot of hetero-interfaces.During rolling,the non-uniform deformation results in the non-uniform dynamic recrystallization nucleation and dynamic precipitation of Mg17Al12 particles,i.e.,Mg17Al12particles are concentrated in the fine-grained region but not the coarse-grained region.In the annealing process,the coarse-grained region grows obviously;while the submicron Mg17Al12 particles inhibited the coarsening of fine-grained region by pinning effect,which further enlarges the microstructure heterogenties,and finally developes closely-spaced coarse/fine-grained layers.The HL Mg alloy features alternating micro coarse-grained and fine-grained layers,and exhibits impressive strength-ductility synergy,i.e.a high yield strength of～251 MPa,tensile strength of～393 MPa and superior ductility of～23%.（4）The strengthening and toughening mechanism of the HL Mg alloys was revealed.During deformation,the coarse-grained and fine-grained layers function as soft and hard domains,respectively.Hence,the obvious strain gradient and stress concentration develop at the interlayer hetero-interface,indicated as high-density disclinations,contributing to high HDI stress and the subsequent improvement of yield strength.The high-density disclinations associated with strain gradient and HDI stress are mediated by heterogeneous multi-grain interactions,operating a variety of abnormal deformation modes,e.g.,low Schmid Factor pyramidal II<c+a>slips,inhomogeneous feather-shaped twins and intergranular slip transfer.The activation of various types of crystal defects,such as dislocations,dislocations and twins,renders the improvement of strain coordination and dislocation storage,promoting the strain-hardening behavior under coupling effect,which is of great significance for realizing uniform and continuous plastic deformation under high-stress level.（5）The high strain rate superplastic deformation mechanism of the HLMg alloys was investigated.The HL Mg alloy exhibits superior superplasticity of～705%at 573 K and a relatively high-strain-rate of 10-2 s-1.Under the optimal high-strain-rate condition of 10-2 s-1,grain-boundary-mediated mechanism in fine grains functions as primary and dominant strain vehicle,meanwhile accommodated by efficient dislocation accommodation and self-refinement in coarse-grained layer,which has been accelerated by pronounced grain-boundary-mediated mechanism near vicinities of hetero-interface.The self-refinement procedure,involves sequential dislocation accumulation,recovery and cutting mechanism of low-angle grain-boundaries,which could not only response to strain incompatibility and stress concentration caused by grain-boundary-mediated plasticity around hetero-interface,but also expend stored energy to stabilize the refined microstructure.It declares that the hetero-layers deformed at 10-2 s-1 generates the coordinated effect and dynamic balance of grain-boundary-mediated mechanism of fine-grained layer and induced self-refinement of coarse-grained layer layer,finally achieving superior superplasticity at high-strain-rate of 10-2 s-1. |
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