The Study On The Mechanisms Of Strengthening And Plasticizing And Orientation Behavior In Mg-Zn Magnesium Alloys

As the metal engineering material with the lowest density,magnesium alloys have attracted widespread attention in lightweight field.However,the application of magnesium alloys was restricted due to poor formability and low absolute strength at room temperature.Moreover,anisotropic mechanical properties and intensive basal texture were exhibited in magnesium alloy sheets,originated from orientation behavior of plastic deformation and recrystallization process.Furthermore,the application of secondary processing was limited.Therefore,modifications of magnesium alloy sheets in chemical composition and fabrication process were research emphasis to simultaneously improve mechanical property and formability.The orientation behaviors in static recrystallization process and plastic deformation were also investigated in the study.To simultaneously promote mechanical property and formability in magnesium alloy sheets,modified Mg-2Zn magnesium alloy sheets,TRC-ZA21,were fabricated through optimization of multi alloying elements with low contents and twin-roll casting.Corresponding mechanical properties and formability at room temperature were also obtained.The comparisons of conventional casting Mg-2Zn(IC-Z2)alloy and conventional casting alloying-Mg-2Zn(IC-ZA21)alloy were conducted.The effects of alloying addition and twin-roll casting on microstructure were analyzed based on investigations in microstructural evolution and precipitation behavior of second phase during the fabrication process.The strengthening mechanism and influence factors of anisotropic mechanical properties were revealed.The results showed that the simultaneous improvements of mechanical property and formability in TRC-ZA21 alloy sheets compared with IC-Z2 alloy sheets.Yield strength along the rolling direction and average IE value at room temperature in TRC-ZA21 alloy sheets increased 41.7%and 60.6%,achieved 201.2 MPa and 6.23 mm,respectively.Refined grain size,weakened basal texture,and altered second phase type were obtained by alloying addition.The formation of nano-scale particles of Al-Mn types with uniform distribution was boosted by twin-roll casting.The balance between strength and formability in magnesium alloy sheets was realized,which was attributed to microstructure characterization of weakened basal texture with diffused distribution,refined grain size,high grain boundary misorientation,and nano-scale second phase particles with uniform distribution in TRC-ZA21 alloy.Finally,the new design concept in magnesium alloy with high strength and high formability was developed.To control the orientation behavior during annealing process and subsequently weaken intensive basal texture in magnesium alloys,static recrystallization behaviors in three experimental magnesium alloys were investigated by salt bath annealing in the study.Combined with variations in softening of hardness,microstructure morphology,and grain orientation during annealing process,the growth behaviors in grains of different orientations were analyzed.The results showed that the significant influence of size and distribution in second phase particles on static recrystallization in magnesium alloys was exhibited,which lead to the formation of two stages in static recrystallization process.Second phase particles with coarse size were beneficial to recrystallization nucleation and increased recrystallization behavior in the initial stage of annealing.Second phase particles with fine size inhibited recrystallization behavior and weakened the softening of hardness.The differences in grain orientation of IC-Z2,IC-ZA21,and TRC-ZA21 alloys were obvious.The intensity in basal texture of IC-Z2 alloy exhibited a trend of decreased firstly and then increased,the texture component maintained basal texture characterization after annealing treatment.The intensities in basal texture of IC-ZA21 and TRC-ZA21 alloys weakened during annealing treatment,and non-basal texture component emerged.The basal texture was weakened by second phase particles at the stage of recrystallization nucleation.The change of basal texture at the stage of grain growth was related to the size of second phase particles.The growth of basal orientation grains was blocked by second phase particles with fine size.The orientation behavior of preferential growth of basal orientation grains was attenuated,and the regulation in basal texture can be envisioned.To analyze the influence factors in anisotropy of mechanical property due to orientation behavior in plastic deformation of magnesium alloys,the tensile deformation tests of different loading directions and strain rates in anisotropic TRC-ZA21 alloy sheets were conducted,and related stress-strain curves and mechanical properties were obtained.Based on analyses in evolution of microstructure,grain orientation and deformation mechanism,the influences of loading direction and strain rate on mechanical behavior were studied.The results showed that variations of anisotropies in mechanical properties presented different with increasing strain rate.Due to the effect of loading stress,the transformation extent in grain orientation exhibited different,and the change of intensity in basal texture along different loading directions also presented diversity.The relationship between orientation change of grains and evolution in plastic deformation mechanisms existed.The main deformation mechanism in TRC-ZA21 during tensile plastic deformation was basal slip.Moreover,tensile twinning and prismatic slip played a content of role and had different activities related to the orientation behavior from loading direction.Due to the change in grain orientation,the effect of twinning decreased with plastic deformation.Moreover,the activity of prismatic slip enhanced to coordinate strain.The anisotropy of mechanical properties and strain rate sensitivity in TRC-ZA21 alloy were dominated by orientation behavior of the deformation mechanism.To analyze the relationship between plastic deformation mechanism and grain orientation in magnesium alloys,the deformation behaviors in grains of different orientations were investigated by nanoindentation deformation in the study.The orientation behavior in deformation mechanism was studied by deformation behaviors in grains of different orientations.Moreover,the effect of grain boundary in plastic deformation was investigated based on orientation difference in adjacent grains,and the behavior in competition and coordination between deformation mechanisms was revealed.The results showed that irregular characterization of nano-mechanical properties was obtained by the discrepancy of grain orientation and grain boundary misorientation angle.Deformation mechanism in grains of direct contact with nanoindenter was controlled by grain orientation,followed by the criterion of Schmid factor.The orientation dependence in plastic deformation can be indicated.The deformation behavior in grains,influenced by plastic deformation and indirectly contacted by nanoindenter,was affected by grain boundary.Based on the orientation difference in adjacent grains,the behaviors in competition and coordination among different deformation mechanisms exhibited.Low c-axes angles were conducive to deformation mechanism pass through grain boundaries,and competitive behavior was presented from activated deformation mechanism in neighbor grains controlled by the criterion of Schmid factor.The propagation of deformation mechanism was blocked with elevated c-axes angles,however,spreading ability in different deformation mechanisms enhanced to coordinate geometric compatibility of neighbor grains.