Microstructure Control And Mechanical Properties Of Hard-plate Rolled Multimodal-grained Mg-Al(-Sn)-Zn Alloys

Magnesium?Mg?alloys have outstanding properties such as low density and high specific strength,but the applications are still limited due to the poor room-temperature mechanical properties.Therefore,improving mechanical properties becomes the focus of researches on Mg alloys.Rolling is one of the main methods for improving the mechanical properties of Mg alloys.However,Mg alloys tend to be cracked upon conventional rolling due to the strong shear stress along the rolling direction?RD?,limiting the achieving of large strain by one rolling pass.One effective method to solve this problem is reducing the shear stress along RD and increasing the compressive stress along the normal direction?ND?by Hard-plate rolling?HPR?.Our previous works proved that HPRed multimodal-grained Mg-9Al-1Zn?wt%,AZ91?alloy has high strength and ductility.Whereas the works on the deformation mechanism of multimodal-grained Mg alloys are still insufficient,comparing to the works on ultrafine-grained and coarse-grained Mg alloys.Besides,the formation mechanism of multimodal structures is also inadequate.Therefore,the present work investigates the effects of precipitation particles,initial grain size and initial texture on the microstructures and tensile properties of Mg-Al-Zn alloys and reveals the microstructure control rules of HPRed Mg-Al-Zn alloys.Based on these rules,the deformation mechanism of multimodal-grained Mg-8Al-2Sn-1Zn?wt%,ATZ821?alloy at room temperature is investigated.The main conclusions can be summarized as follows:?1?By comparing microstructures and tensile properties of HPRed Mg-3Al-1Zn?wt%,AZ31?,Mg-6Al-1Zn?wt%,AZ61?and AZ91 alloys,the effect of Mg₁₇Al₁₂ particles on microstructure evolution of Mg-Al-Zn alloys and the formation mechanism of a multimodal structure during HPR processing was investigated.The high-volume fraction Mg₁₇Al₁₂ particles strongly retard the dynamic recrystallization?DRX?and preferential growth of DRXed grains by Zener drag mechanism upon HPR.The retarded DRX leads to the remaining of deformed coarse grains with a strong basal texture.The retarded preferential growth of DRXed grains results in the formation of ultrafine/fine grains with weak texture.It is feasible to achieve a multimodal structure by controlling the size and volume fraction of precipitation particles.?2?The influence of initial grain size on the microstructure of HPRed AZ91 alloy was investigated.The results indicate that the initial grain size distribution directly decides the grain size distribution of coarse grains but has a limited effect on the recrystallized degree and grain size distribution of ultrafine/fine grains,second-phase particles and texture.The grain size distribution of coarse grains is similar to that of initial grains.?3?The effect of initial texture on the microstructure of HPRed AZ91 alloy was investigated.The results indicate that the initial texture affects the recrystallized degree and texture of coarse grains in the HPRed sample.The driving force of DRX increases with the increasing angle between the c-axis of grain and ND.When the basal plane of grain is parallel to the rolling plane and the {101?0} plane is parallel to the transverse direction?TD?or at a 30° angle,the driving force of DRX is minimum.The weaker the initial texture is,the higher recrystallized degree the HPRed sample has.The initial texture has a limited effect on the precipitation particles,texture and grain size distribution of ultrafine/fine grains.?4?The deformation mechanism of ATZ821 alloy at room temperature was investigated by an interrupted tensile test.At the initial stage of the tensile test,deformation is mainly accommodated by fine grains with a weak basal texture,favouring of activating basal slip systems.Nevertheless,when the applied resolved shear stress exceeds the CRSS value of pyramidal slip in Mg alloys,the coarse grains begin to play a prominent role in storing dislocations and hence promote work hardening.The synergy effect of ultrafine/fine and coarse grains leads to a simultaneous higher strength and ductility compared to fine-grained ATZ821 alloy.?5?By comparing tensile properties of multimodal-grained AZ91 and fine-grained AZ31 alloys as well as comparing tensile properties of multimodal-grained and fine-grained ATZ821 alloys,we confirm the high work-hardening ability of multimodal-grained alloys is the main source of their high strength and ductility.The yield strength?YS,246 MPa?and ultimate tensile strength?UTS,370 MPa?of multimodal-grained AZ91 alloy are higher than those of the fine-grained AZ31 alloy?a YS of 182 MPa and an UTS of 260 MPa?,and the uniform elongation of multimodal-grained AZ91 alloy is same as that of fine-grained AZ31 alloy?14%?.The YS?243 MPa?of multimodal-grained ATZ821 alloy is slightly lower than that of fine-grained sample?257 MPa?,but the UTS?377 MPa?and uniform elongation?17%?of multimodal-grained ATZ821 alloy are significantly higher than those of fine-grained sample?an UTS of 330 MPa and an uniform elongation of 11%?.?6?Microstructure control principles for multimodal-grained Mg alloys are proposed based on the deformation mechanism of multimodal-grained ATZ821 alloy.The recrystallized grains should be submicron-sized and have a weak texture to facilitate the activation of basal slips and achieve a high work-hardening rate at the initial deformation stage.The texture of deformed grains should facilitate non-basal slips and high dislocaition-storage ability at the late deformation stage.Let the ultrafine/fine and coarse grains have a synergy effect during deformation to enhance the work-hardening of Mg alloys.