Studies On Mechanical Properties And Thermal Stability Mechanisms Of New Bimodal-grained Al-Mg Based Alloys

Energy saving and emission reduction are inevitable requirements under the development of the times and the overall trend of material science development.The demand for light metallic materials with high performance in transportation and aerospace fields has promoted Al alloys received considerable application and development.Thereinto,low-mass density Al-Mg alloys,due to low cost,high-specific strength and excellent corrosion resistance,play important roles in industrial applications.However,the strength of Al-Mg alloys cannot be comparable to steels.The high Mg concent can effectively decrease the stacking fault energy of Al alloys,which is beneficial to refining grain size into submicron or even into nano scale,with strenghs greatly increased.Although high strength is obtained in uniform fine-grained Al-Mg alloys,ductility is decreased remarkably.Moreover,the poor formability at room temperature cannot meet the production requirement for complex stamping parts.In contrast,the superplastic forming(SPF)technique at elevated temperatures enables near-net-shape manufacturing,improving production efficiency notably.Nevertheless,the poor microstructural thermal stability of deformed binary Al-Mg alloys results in grain coarsening easily during annealing or deformation at high temperatures,which makes controlling the superplastic microstructure difficult.Our previous work demonstrated that preparing bimodal grain structure was a feasible strategy to solve the“trade-off”between strength and ductility in Al-Mg alloys.Solute segregation along grain boundaries(GBs)and dispersed nano-sized precipitates,such as Al₃Sc and Al₃(Sc,Zr),in the matrix can not only improve strength but also effectively enhance the microstructural thermal stability of Al-Mg alloys.However,studies about the effect of solute segregation along GBs on thermal stability mainly focus on statically annealed Al-Mg alloys.It is unclear about effect mechanisms of solute segregation along GBs on the superplastic deformation of bimodal-grained Al-Mg alloys.Also,roles of dispersed nano-sized particles on microstructural evolution,strength and ductility,as well as the microstructural thermal stabiltiy of bimodal-grained Al-Mg alloys remain to be revealed.According to above associated researches and remained problems,this work was studied on high solid solution Al-7Mg based alloys.Equal-channel angular pressing(ECAP)and short-route hard-plate rolling(HPR)were employed to process binary Al-7Mg and Al-7Mg-Sc-Zr,respectively.The main conclusions were shown here:(1)The tensile properties at high temperatures and corrreponding superplastic deformation mechanisms of the bimodal-grained Al-7Mg alloy were discussed.A bimodal grain structure,containing nano/ultrafine grains and micron-sized coarse grains,was obtained in an Al-7Mg alloy prepared by ECAP.Superplasticity of?523%elongation was achieved in the Al-7Mg alloy when tension deformed at 573 K and 1×10^-3 s^-1.The early stage of superplastic deformation is accommodated by continuous dynamic recrystallization(CDRX)via relaxing local strain concentration in coarse deformed grains.Grain boundary sliding(GBS)is dominated once a uniform fine grain structure forms in the late stage.(2)The mechanisms of improved thermal stability of fine grains in the bimodal-grained Al-7Mg alloy were clarified.A uniform fine grain structure(?3.6?m)was well kept after?1.5 h tensile deformation at 573 K.The enhanced thermal stability of fine grains is due to that:(i)the strong solute Mg segregation along GBs of initial nano-sized/ultrafine grains decreases GB energy and drags GBs effectively;(ii)Al₃Mg₂ precipitates via pinning GBs also retard grain growth to some degree and(iii)the coarse deformed grains feasuring high stored energy occur CDRX preferentially upon tensile deformation,which decreases the driving force for growth of fine grains in regions adjacent to GBs of coarse grains.(3)The formation mechanisms of bimodal grain structures in Al-7Mg-Sc alloys prepared by short-route HPR were elucidated.The bimodal grain structure formation is associated with distinct grain stability determined by their orientations.Compared with stable<101>//ND-oriented ones,new ultrafine/fine(sub)grains tend to form preferentially along GBs of<001>//ND-and<111>//ND-oriented coarse grains during HPR.Moreover,the increased volume fraction of Al₃Sc precipitates favors dislocation accumulation and enhances the driving force for DRX.Meanwhile,grain growth is restricted by Al₃Sc via pinning GBs,contributing to the formation of more and refined ultrafine/fine grains.(4)The influence mechanisms of dispersed nano-sized Al₃Sc precipitates on tensile properties at room and high temperatures in HPRed Al-7Mg-Sc alloys were studied.The Al-7Mg-0.4Sc alloy exhibited ultimate tensile strength(UTS)of?520 MPa,yield strength(YS)of?482 MPa and?8%elongation,much higher than Al-7Mg.The cooperative effects of bimodal grain structure,dispersed nano-sized Al₃Sc precipitates plus high-density dislocations induced contribute to the improved tensile property.However,upon tension at?250 ?,the great dislocation loss related to DRX in Al-7Mg-0.4Sc results in much lower YS,compared with Al-7Mg.(5)The mechanisms for good strength-ductility synergy in the HPRed Al-7Mg-0.3Sc-0.1Zr alloy with a high Sc/Zr ratio were revealed.Remarkably improved YS and UTS,i.e.?548 and?605 MPa were achieved,much higher than that of Al-7Mg-0.1Sc-0.3Zr.Moreover,both alloys had a similar elongation of?10%.The good strength-ductility synergy is ascribed to cooperative effects of the hierarchical inhomogeneous microstructure tailored by the high Sc/Zr ratio.The high strength originates from bimodal grain structure,dispersed nano-sized Al₃(Sc,Zr),high-density dislocations and intragranular Mg-Zr co-clusters segregated on dislocations.The synergetic effects of coarse grains,nano-sized Al₃(Sc,Zr)and solute-dislocation clusters lead to the improved work-hardening ability,promoting good elongation.(6)The enhanced microstructural thermal stability mechanisms of the HPRed Al-7Mg-0.1Sc-0.3Zr alloy with a low Sc/Zr ratio were studied.The Al-7Mg-0.1Sc-0.3Zr presented higher microstructural thermal stability when annealed at?400 ?,than alloys with higher Sc/Zr ratios.It owes primarily to that the concurrent Al₃(Sc,Zr)precipitation in Al-7Mg-0.1Sc-0.3Zr is promoted greatly upon high-temperature annealing.Moreover,the Al₃(Sc,Zr)coarsening kinetics is decreased via enhancing Zr concentration in the precipitate shell.It concomitantly leads to stronger pinning force on dislocations and GBs,delaying recrystallization and restricting grain growth.