The Investigation Of The Micro Alloying Effect On The Microstructure Evolution And Mechanical Propertie's In AZ80

Microalloying is one of the attractive development paths for low-cost and high-performance magnesium(Mg)alloys.The novel forming-property adjustment mechanism of magnesium alloys needs to be researched based on microstructure optimization and design,which will contribute to the eff-icient preparation of lightweight components and service quality improvement.In this dissertation,three different AZ80M microalloyed Mg alloys:AZ80+0.2-0.8Y,AZ80+0.1Y+0.1Nd,AZ80+0.2Y+0.15Ca(wt.%)were prepared using Y,Nd,and Ca alloying elements with high solid solubility difference.First,we studied the modulation effect of the Y,(Y+Nd),and(Y+Ca)microalloying groups on the microstructure evolution behaviors:static recrystallization and aging precipitation.Furthermore,the 1)plastic deformation mechanisms and dynamic microstructure evolution at wide-temperature,and 2)the power-law-breakdown type creep behavior and damage mode at medium-temperature were investigated under the microalloying effect.The research results illustrate that:The minor addition of Y,Nd and Ca resulted in the grain coarsening in the as-cast and as-extruded AZ80,formed Al-x(x=Y,Nd,Ca)intermetallic compounds in the grain interior(GI)and grain boundaries.The grains size heterogeneity of the extruded sheets also increased with the increasing Y content.Y microalloying could refine and spherify the?-Mg17Al12 phase.In addition,(Y+Ca)microalloying significantly enhanced the thermal stability of the ? phase and increased its melting point of about 32 0C.Microalloying reduced the AZ80 GB migration velocity during the static recrystallization(SRX)which was attributed to the GB drag stress from the microalloying elements,and the pinning effect from the Al-x particles.AZ80+0.2Y+0.15Ca exhibited the lowest grain growth rate among all alloys.The GB migration was accompanied by GB polygonization and orientation changes.The microalloying elements regulated the recrystallization texture evolution,Y or(Y+Nd)promoted the basal orientation of the preferentially grown grains to tilt along the extrusion direction,and weakened the basal texture.(Y+Ca)microalloying continuously strengthened the basal texture while maintaining uniform grain size.Microalloying could suppress the discontinuous precipitation(DP)during the AZ80 aging,reduce the DP volume,facilitate the precipitation-free-zone formation near GBs,and decrease the peak hardness.The recovery and precipitation coexisted in the primary aging stage,and the recovery softening reduced the hardness by 16%.Avrami index was close to 1 during AZ80+0.2-0.8Y aging process,which suggested that GB nucleation dominated the precipitation.Little aging hardening was exhibited when(Y+Ca)further suppressed the DP.The DP process was accompanied by diffusion induced grain boundary migration(DIGM),and the high-density precipitation in twins hindered DIGM,and at the same time,DP stimulated the twins multiplication.The a crystal in the DP exhibited a similar orientation with the original orientation of the aside grain.DIGM preferentially occurred in the 32°misorientation orientation GBs.The DP fronts preferentially migrated from the grain,inclined to extrusion texture,to the aside one,resulting in the texture strengthening.Y microalloying activated more<c+a>non-basal slips at room temperature,which resulted in the improved plastic deformation homogeneity.0.2-0.8 wt.%Y increased fracture strain,slightly reduced yield strength.In addition,the Y microalloying weakened the tension-compression asymmetry,and reduced the anisotropy of mechanical properties.(Y+Ca)microalloying weakened the cross-slip stress which resulted in the hardness reduction of AZ80.The AZ80+0.2Y extruded sheets exhibited grain rotation and coordinated deformation between grain groups during room-temperature plastic deformation.The basal<a>slip in the matrix and prismatic<a>slip in the twin exhibited a coordinate motion mode in the twin boundary.The tensile deformation damage mode in annealed AZ80+0.2Y was composed of the Al2Y particle break,cavities induced by the interaction between twin and GB,and grain boundary sliding(GBS)assisted intracrystal shear cracks.Microalloying improved the high temperature(300-400?,10-4-10-2 s-1)deformation performance for AZ80.Y microalloying activated more micro shear band(MSB)which increased the microstructure uniformity in the as-cast AZ80.Y microalloying also increased the tensile elongation-to-failure for the AZ80.Moreover,the high-temperature plastic deformation mechanism changed from GBS to dislocation creep after microalloying.The dynamic recrystallization(DRX)was accompanied by the dynamic strain precipitation(DSP)during high-temperature deformation.DRX grain size decreased with the increasing Zener-Hollomon(Z)parameter.As for the ? phase in DSP,its average growth rate,volume and density increased with the increasing Z parameter.DRX,DSP and micro cracks coexisted in the MSB.The width of the MSB decreased with the increasing Z parameterAll three kinds of microalloying groups,Y,(Y+Nd)and(Y+Ca),could reduce the steady-state creep rate,?min,for the as-cast AZ80 at 100-150?/50-100 MPa,and improve the medium temperature creep resistance.The creep activation energy of the annealed AZ80,AZ80+0.2Y,and AZ80+0.1Y+0.1Nd was 63-67 kJ/mol,which followed the GBS and grain boundary diffusion mechanisms coordinated by the dislocation climbing.Micro-alloying could also significantly increase the creep resistance of the extruded sheet,especially,the(Y+Ca)microalloying increased the creep resistance by an order of magnitude.The hardness of the microalloyed sheet was lower than AZ80 after ultra-long time duration(t>600 h)creep,in which the hardness of AZ80+0.2Y+0.15Ca was about 30%lower than that of AZ80Under the regulation of the microalloying effect,DP content,grain size,and texture intensity have a synergistic effect on the creep performance of AZ80 at medium temperature The microalloyed alloys presented less DP volume during the creep process,which resulted in the creep resistance improvement.The ?min first decreased and then increased around the grain size of 50 ?m;it also decreased with the basal texture strengthening.In addition,the creep damage mode was regulated by microalloying.The cavities initiation and propagation in the GBs and GI served as the dominant creep damage mode in AZ80.And the cavity damages also resulted from the interaction between twins and GB,second phases and other twins.For the 125? creep in AZ80+0.2Y,the creep damage mode changed from the GB/GI cavities to GB or twin boundary crackings;the fracture mode also changed from ductile fracture to mixed ductile-brittle fracture.