Microstructure,Strengthening And Plasticizing Mechanism Of Low-Alloyed Magnesium Alloys With Grain Boundary Segregation

Low-alloyed magnesium（Mg）alloys have attracted much attention as a highly promising Mg alloy system.However,low absolute strength,poor ductility at room temperature have hindered their widespread applications.Unlike common high-alloyed Mg alloys,low-alloyed Mg alloys cannot rely solely on aging strengthening to obtain ideal mechanical properties.The grain structure and dislocation configuration in Mg alloys are crucial for achieving high performance,and grain boundary segregation is an effective method for controlling grain structure.In addition,rare earth（RE）elements are effective alloying elements for improving the comprehensive properties of Mg alloys.In this work,the typical light RE element（Sm element）and heavy RE element（Er element）are mainly used as the main alloying elements.The solid solubility of these two RE elements in Mg alloys varies greatly,making them suitable for Mg alloy designs with different performance requirements.For example,the Er element is more suitable for designing Mg alloys with high ductility,while the Sm element is more conducive to the design of high-strength and high-ductility Mg alloys.Therefore,the combination of micro-RE alloying and grain boundary segregation is expected to achieve ideal microstructures and mechanical properties under specific processes.Finally,the deformation mechanism of high-performance Mg alloys was studied by means of（quasi）in-situ tensile testing to clarify the control mechanism of grain boundary segregation on grain structure and reveal the essential reason for improving the performance of Mg alloys.The following results are drawn:（1）Based on careful experimental design,we clearly demonstrate that the addition of trace Sm（0.15 at.%/1 wt.%Sm）could significantly increase the grain boundary segregation concentration of Zn and Ca in the as-extruded dilute Mg-Zn-Ca-Mn alloy.The increase of segregation level is mainly related to the negative mixing enthalpy between elements and the lower grain boundary free energy.Meanwhile,the Sm/Zn/Ca co-segregation can more effectively drag the grain boundaries,thereby inhibiting the growth of grains.After annealing at 380°C for 30 min,the average grain size of the Mg-2Zn-0.8Mn-0.6Ca-1Sm（wt.%）alloy was only from 2.4μm increased to 3.80μm.The grain growth rate is much lower than that of Mg-2Zn-0.8Mn-0.6Ca（wt.%）alloy.In addition,due to the slow grain growth during annealing in Sm-containing alloy,the solute diffusion may be easier to catch up with the grain boundary motion,resulting in the segregation atmosphere breaks away from the grain boundaries less obviously.（2）In this work,we developed a dilute Mg-RE alloy with ultrahigh ductility（near 50%）at RT by minor Er alloying（0.30 at.%/2 wt.%Er）and moderate grain refinement.On the one hand,minor Er and appropriate fine grains would significantly reduce the critical resolved shear stress（CRSS）of non-basal slips and basal slips,obtaining considerable non-basal dislocations during the tensile process.The CRSS ratios between prismatic<a>slip and basal<a>slip is about 2.88,between pyramidal I<a>slip and basal<a>slip is about 12.60,and between pyramidalΠslip and basal<a>slip is about 15.39,which are lower than those of pure Mg and most Mg alloys.In addition,the inhibition of twinning in the studied alloys is also conducive to ultrahigh ductility.On the other hand,the Mg alloys with the higher Er grain boundary segregation level is mainly responsible for its better ductility.The Er segregation can contribute to activate more non-basal dislocations to accommodate local stress and increase the grain boundary cohesion energy,thus suppressing the grain boundary cracking and finally improving the ductility.（3）Based on the higher grain boundary segregation level of Sm/Zn/Ca co-segregation,a new low-alloyed Mg-2Sm-0.8Mn-0.6Ca-0.5Zn（wt.%）alloy was prepared via low-temperature and low-speed extrusion.The as-extruded alloy shows a higher yield strength（453 MPa）and ultimate tensile strength（465 MPa）,while the elongation of the alloy is only 3.2%.The strength of the alloy is higher than most of the low-alloyed Mg alloys reported so far.The high strength comes from he formation of a fine-grained structure（0.72μm）containing high-density residual dislocations(6.66×10¹⁴m^-2)and a number of nanoparticles.The poor ductility is mainly due to dislocation entanglement and low-mobility straight-segments dislocations.After annealing at 350°C for 15 min,the annealed alloy obtains an excellent combination of high-strength and high-ductility,with the yield strength of403 MPa,ultimate tensile strength of 411 MPa,and elongation of 15.5%.The product of tensile strength and elongation increased from 1.49 GPa%to 6.37 GPa%.The effective inhibition of grain growth by Sm/Zn/Ca co-segregation of is crucial for the annealed alloy to maintain high strength,At this time,the average grain size of annealed alloy is about 1.19μm.Appropriately decreased dislocation density,especially the evolution of immovable straight dislocations towards new grain boundaries,is a key factor for the remarkable increase of ductility for the annealed alloy.（4）Grain boundary segregation would form heterogeneous-grained structure with a weak basal texture under specific conditions（secondary thermal deformation or high temperature annealing）,and this heterogeneous-grained structure can obatin high-formability potential.Firstly,heterogeneous grains are introduced into the Mg-2Sm-0.8Mn-0.6Ca-0.5Zn alloy through annealing,while heterogeneous grains are introduced into the AZ31 alloys through secondary thermal deformation.This is mainly due to the control of grain boundary segregation.Under higher driving forces,part of grains would first break away from drag caused by segregation,while other grains are still loaded with developing solute atmosphere.During curvature driven grain growth,these larger grains with supercritical driving force would be able to overcome solute pinning and exhibit rapid free growth.This special growth mode results in the formation of a heterogeneous-grained structure in which large grains and small grains coexist.Secondly,uneven deformation will occur between heterogeneous grains.In order to adapt to the strain,geometrically necessary dislocations would nucleate at the grain boundaries between large and small grains,resulting in back stress and improving the work hardening of the alloy.High flow stress is also beneficial for activating more non-basal slips,thereby improving the room temperature ductility of Mg alloys.Both Mg-2Sm-0.8Mn-0.6Ca-0.5Zn and AZ31 alloys with heterogeneous grains have achieved good work hardening and high elongation,meaning high formability potential of the two alloys.