Microstructure Evolution And Strengthening And Toughing Mechanisms Of Novel High Strength-Ductility Mg Alloys By Controlling Rolling

Owing to low density and high strength-to-weight ratio,Mg alloys are potential in the field of automotive,aerospace and so on.However,it still has to face scientific challenges that disturb the field of Mg alloys nowadays,such as "difficulties of modifying high strength-ductility microstructure".To solve the mentioned problem above,this work devotes into investigating micro structure evolution and strengthening and toughing mechanisms based on the cooperative control of solidification and deformation,as following three aspects:(1)introduction of sub-rapid solidification to achieve supersaturated solution for enhancing the beneficial function of alloying elements;(2)control of micro structure evolution via gradient rolling and hard-plate rolling(HPR)for exploring recrystallization and strengthening and toughing mechanisms;(3)investigating the effect of bimodal-grained deformation behavior and typical texture on strengthening and toughing mechanisms.This work has explored "formation and rolling behavior of supersaturated solution","recrystallization mechanism and microstructure evolution","deformation behavior of bimodal-grained structure and strengthening mechanism" and so on.The cooperative control of solidification and deformation mainly focuses on the modification of casting microstructure and deformation processing.This short-route technique allows for development of high strength-ductility Mg-Zn based and Mg-Al based alloys,which provides essential data for developing high-performance Mg alloys.The main contents are presented as below:(1)The work explores the formation,evolution and modification of as-cast microstructure in Mg-6Zn-0.2Ca(ZX602)alloys.Under sub-rapid solidification (SRS),the as-cast micro structure has been refined significantly.Meanwhile,SRS could remarkably elevate the solubility of Ca element,facilitating the formation of supersaturated solution,which provides favorable conditions for subsequent large-thickness rolling.(2)SRS combined with HPR has achieved high solubility,grain refinement and uniformly dispersed particles simultaneously,which allows for the development of a ZX602 alloy having a superior combination of ductility and tensile strength,i.e.UTS of～290 MPa and elongation of～23%.That contributes to a remarkable grain refinement,which significantly improves strength by grain boundary strengthening.Concurrently,excessive Ca solutes could efficiently suppress the preferred basal orientation,inhibiting the formation of hard oriented grains for preventing early cracking.(3)Novel gradient rolling achieves varying thickness reductions in the same sample,where the evolution of microstructure can be examined precisely and continually.Based on the promotion effect of twinning-induced nucleation vs.the inhibition effect of Mg17Al12 particles pinning for recrystallization,the work explores the mechanisms of dynamic recrystallization and grain refinement in Mg-Al-Zn alloys.During rolling,increasing Al content imposes a remarkable influence on recrystallization.Recrystallization occurs much faster in pure Mg than AZ31 alloy.Conversely,for AZ91 alloy,the size of coarse grains remains unchanged with few indications of recrystallization during the whole rolling process.(4)In pure magnesium,{1011}-{1012} double twinning is regarded as the dominant twin type at the initial deformation stage,which could generate higher local strain areas to trigger the nucleation of DRXed grains.However,with increasing Al content,the fraction of twins is decreased and the main twinning type is transformed into {1012} extension twins,where almost no recrystallization occurs.Moreover,for AZ91 alloy,the Zener pinning pressure(Fz)increases significantly with increasing strain due to the variation of Mg17Al12 particles via dissolution and re-precipitation,leading to a finer average size and a higher volume fraction,which produces stronger pinning effect and hinders dynamic recrystallization extremely.(5)The work reveals the deformation behavior and strengthening mechanism of bimodal-grained Mg-Al-Zn alloys during tension.The yield behavior is controlled by basal dislocation slip of UFGs/FGs and twinning of CGs collectively,followed by the activation of non-basal slip in basal-oriented CGs as deformation proceeds.According to the grain size and typical texture,a modified Hall-Petch relation is proposed to predict the yield strength of Mg-Al-Zn alloys with a bimodal grain size distribution either containing few or numerous second phase particles.A quantitative relation of grain size,texture and yield strength is built with a bimodal grain size distribution,which provides data for modifying microstructure and properties of bimodal-grained Mg alloys.(6)Conventional fabrication of Mg alloys usually consumes long time and significant energy.Severe segregation and the formation of coarse eutectic phase during casting deteriorate mechanical properties.Not only the novel short-route technique(SRS+HPR)contributes to reduce the cost of raw materials(non-RE and simple alloying elements),but also reducing segregation,refining microstructure and improving solid solubility of elements,which provides new insights for fabricating high strength-ductility Mg alloys in a short route.