Microstructural Evolution And Deformation Behaviour Of Superplastic Mg-9Al-1Zn-0.4Sn Magnesium Alloy Processed By Rolling

In recent years,magnesium(Mg)alloys have gained wide concern due to their low density,high specific strength,good machinability and recyclability.However,it is hard to process by deformation because of the poor plastic formability at room temperature.Meanwhile,the formation of strong crystallographic texture during mechanical processing results in strong anisotropy.These are severe challenges faced up by magnesium alloy.In view of this problem,we investigated the microstructure evolution,effect of texture on mechanical properties and deformation behavior at different temperatures of controlling rolled Mg-9Al-1Zn-0.4Sn(AZT910)magnesium alloy sheets.We developed a new rolling route,i.e.combined hard-plate rolling and controlling rolling referred as HPCR.The main conclusions can be summarized as follows:(1)The study on fine grain formation of rolled AZT910 magnesium alloy sheets was revealed where continuous dynamic recrystallization(CDRX),discontinuous dynamic recrystallization(DDRX)and twin dynamic recrystallization(TDRX)occured.It was found that the spherical Mg17Al12 particle at grain boundaries can significantly retard the growth of fine recrystallized grains.This result is helpful for controlling and refining grain size.(2)The precipitate behavior of spherical Mg17Al12 particles in rolled AZT910 magnesium alloy was investigated.It was found that the regions of grain boundaries,high density dislocation tangles and the ends of lozenge-shaped particles are preferred sites for nucleation and growth of spherical precipitates.The high density of crystal defects were introduced during rolling process,especially dislocations,which can cause dissolution of the precipitates into the matrix and distribute more uniformly.The nucleation sites provided by high density dislocations together with uniform distribution of solid solution aluminum are benefit to activate fine and dispersing distribution of precipitates.(3)Uniaxial tension tests in the rolling direction(RD),transverse direction(TD)and 45° towards RD(45RD)of rolled AZT910 magnesium alloy sheets were investigated,it reveals that the deformation system of rolled AZT910 sheet contains basal slip and non-basal slip at room temperature,which becomes a combination of basal slip,non-basal slip and grain boundary sliding/rotation(GBS)at 200?,and thereby tensile properties exhibit a strong dependence on tensile directions at both room temperature and 200?,i.e.RD,45 RD and TD specimens present apparent tensile anisotropy.At 300?,GBS become the dominate deformation mechanism.It thus leads to near isotropic tensile properties in all three tested directions.Tensile properties exhibit a negligible dependence on tensile directions at 300?.(4)At elevated temperatures,it is believed that the fine recrystallized grains and soft near-spherical Mg17Al12 particles at grain boundaries are beneficial for the occurring of GBS.It suggests that Mg17Al12 particles at grain boundaries exhibiting a pinning effect on grain boundary migration can significantly retard the growth of fine recrystallized grains even during high-temperature deformation,which is beneficial for continual GBS occurring.(5)We proposed a new HPCR route combining the advantages of both hard-plate rolling and controlling rolling.By applying HPCR via merely 3 rolling passes,it is possible to achieve low temperature superplasticity in AZT910 sheets with near-spherical Mg17Al12 phase(~0.5 ?m),finer average grain size(~3 ?m)and weaker texture.More importantly,a tensile strength of ~336 MPa at room temperature and an elongation-to-failure of ~96% at 200 ? were gained in the HPCRed AZT910 alloy.The HPCR process is facile and effective,which can avoid plate cracking that is prone to occur during conventional rolling.It is believed that our strategy effectively reduces rolling passes and hence production cost,which has a promising prospect for industrial application.In summary,we investigated the microstructural evolution of rolled AZT910 magnesium alloy sheets,revealing the formation of fine grains and precipitation behavior of spherical particles.Moreover,the interaction between recrystallization and precipitation has been explored.Results obtained here can be used as a reference to control the microstructure of magnesium alloys.The tensile properties in RD,45 RD and TD of rolled AZT910 magnesium alloy sheets were also investigated to reveal the effect of texture on tensile properties and deformation behaviors at room and elevated temperatures.Additionally,the new HPCR route was proposed,achieving low temperature superplasticity in AZT910 magnesium alloy.It was found to be effective for weakening the rolling basal texture,avoiding plate cracking,reducing rolling passes and hence production cost,which provides a new strategy.