| As the lightest engineering structural metal material,magnesium(Mg)alloys possess many advantages such as low density,high specific strength,etc,which have great potential for applications in fields such as aerospace,automobile,3C,biomedical,and energy.Among them,low-alloyed Mg alloys have received extensive attention from researchers due to their advantages of low cost and excellent hot workability.However,during the hot deformation process,strong textures are usually developed in Mg alloys due to its hexagonal close-packed structure,which greatly limits the industrial application of dilute Mg alloys.Weakening texture can improve the ductility and formability,but also accompanied by a considerable sacrifice of strength.Recently,heterogeneous structure materials have been widely studied due to their excellent strength-ductility synergy.Researchers believe that the main reason for the strengthductility synergy is the hetero-deformation strengthening induced by the mechanical incompatibility among heterogeneous domains.Due to the strong anisotropy of single Mg crystal,present work proposed to fabricate orientation heterostructured microstructure comprising of soft/hard orientated domains,aiming to attain excellent strength-ductility synergy.By studying the recrystallization mechanism of Mg-Zn-Sn-Ca alloy,a series of preparation parameters were proposed to achieve controllable preparation of the orientation heterostructured microstructure.The interactions between soft/hard-oriented grains were revealed and the effects of the volume fraction ratio and spatial distribution of soft/hardoriented grains on deformation behavior and mechanical properties were explored.Based on these factors,the optimum orientation heterostructured sample was obtained in the present work.The main conclusions obtained in this work are as follows:(1)The dynamic/static recrystallization behavior of Mg-1Zn-1Sn-0.2Ca(wt.% ZTX110)alloy and their effects on the final texture were investigated.Large reduction rolling at high temperature will facilitate the continuous dynamic recrystallization mechanism,which leads to the formation of recrystallized grains with intense basal texture.It is hard to be weaken through subsequent annealing process.In contrast,numerous shear bands,deformation twins,and dislocations are developed in the ZTX110 alloy after the small reduction rolling at low temperature.During annealing,fresh recrystallized grains with off-basal orientations nucleate in these highly strained areas and quickly consume adjacent deformed grains.Based on the above fabrication route,i.e.,large reduction rolling at high temperature to produce hard-oriented grains and small reduction rolling at low temperature to produce soft-oriented grains,the controllable preparation of orientation heterostructured microstructure was achieved.Hereinto,the fraction of off-basal grains can be increased from 0% to 67%.(2)The influence of twinning induced recrystallization on the formation of orientation heterostructured microstructure was investigated.Different with the view reported in previous research,recrystallized grains originating from double twins can be expended into surrounding deformed parent grains,contributing to the formation and growth of softoriented grains.This phenomenon is mainly attributed to the activation of numerous <c+a>dislocations in the double twins during low temperature rolling.These <c+a> dislocations have a larger Burgers vector and provide more recrystallization driving force during the annealing process,facilitating the soft-oriented grains to break through the restriction effect of twin boundaries.(3)The disparity of deformation mechanisms between orientation heterostructured and basal textured samples were compared.It was found that the deformation of hard-oriented grains in the basal textured sample is dominated by basal slip,while most of the hard-oriented grains in orientation heterostructured samples are deformed by non-basal slips.The interactions between soft and hard-oriented grains were revealed.When a soft-oriented grain is adjacent to a hard neighbor,soft-oriented grain deforms first by basal <a> slip and generates strain gradients between the heterogeneous interface.The strain gradient will facilitate the activation of non-basal slip in neighboring hard-oriented grain to release the stress concentrations at the soft/hard interface.In contrast,the local stress generated between the grains with similar orientations(soft-soft or hard-hard grains)is relatively low,and insufficient to change the deformation mode of neighboring grains or trigger the second deformation mechanism.(4)The effect of spatial distribution and volume fraction of the soft/hard-oriented grains on the intergranular interaction was revealed.A higher percentage of soft-oriented grains will weaken the intergranular interaction and reduce the contribution of hetero-deformation strengthening to the mechanical properties.If the volume fraction of soft grains is too small,limited soft-oriented grains are not sufficient to change the local strain states of all the hard-oriented grains,and the deformation behavior is still similar to the basal textured samples.Besides,it is found that uniformly distributed soft/hard-oriented grains contribute to homogenous plastic deformation.Therefore,when the soft/hard-oriented grains are in appropriate proportion and uniform distribution,the orientation heterostructured sample can achieve more uniform plastic deformation,and benefical for strength-ductility synergy.(5)The optimum volume fraction between soft /hard grains was obtained in present work.It is found that the orientation heterostructured sample with the proportion of soft and hardoriented grains approaching ～3:7 exhibits the best combination of strength and ductility.Under this proportion,the yield strength of the orientation heterostructured sample is almost comparable to that of the basal texture counterpart,and its ductility is even slightly higher than that of the sample with weaker texture intensity.The unusual high YS originates from the constraint imposed by the hard domains,which makes the soft-oriented grains almost as strong as hard-oriented grains.The high ductility originates from the intergranular intreactions,which facilitates the activation of non-basal slip and inhibits the nucleation of tension twinning.(6)The orientation heterostructured Mg alloys with fine grain size were successfully prepared by a short process.The orientation heterostructured microstructure with the optimal proportion of soft and hard-oriented grains was produced through a short process: subrapid solidification + one pass reduction rolling + recrystallization annealing.The orientation heterostructured sample exhibits outstanding strength-ductility synergy as compared to the basal textured countpart,i.e.,YS of ～170 MPa,UTS of ～255 MPa,uniform elongation of ～19.6%,and failure selongation of ～ 26.6%,respectively.This study provides a reference for the development of high performance orientation heterostructured alloy sheets. |
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