| Developing high-strength and lightweight materials is one of the important solutions to the energy crisis and environmental pollution.Magnesium alloy,as the lightest metal structural material,has shown great potential for applications in transportation vehicles and aerospace industry due to its ability to improve energy efficiency and minimize carbon emissions.However,the further application of magnesium alloy is limited by its weaknesses such as insufficient strength,poor ductility,and low thermal stability.To address these issues,ultrafine-grained magnesium-based composites have attracted increasing interest.However,due to the fact that the grain size of ultrafine-grained magnesium alloy is close to or even smaller than the scale of plastic deformation mechanism dominated by the internal dislocations in the metal,the initiation and movement of dislocation sources inside grains will be suppressed,resulting in poor plasticity and susceptibility to fracture caused by plastic instability.Based on this,this article proposes to introduce a controllable toughening phase into high-strength ultrafine-grained magnesium alloy through mechanical ball milling and powder extrusion processes,and to coordinate the plastic deformation of the ultrafine-grained structure with the toughening phase,so as to balance the strength and plasticity of the material.The article studies the process of preparing ultrafine Ti-dispersed reinforced nanocrystalline AZ91 magnesium alloy powder by mechanical ball milling,reveals the thermal stability of Ti particle-reinforced nanocrystalline magnesium alloy structure,elucidates the mechanism of the effect of toughening phase structure parameters on the strength and plasticity evolution of the mixed crystal structure material,and realizes the joint improvement of material strength and plasticityIn order to achieve the nanocrystallization of the magnesium matrix,the process of preparing ultrafine Ti dispersed reinforced nanocrystalline AZ91 magnesium alloy powder by mechanical ball milling was studied.The influence of mechanical ball milling parameters on the refinement/nanocrystallization of the composite powder magnesium matrix,as well as the fragmentation and dispersion of Ti particles,was clarified.Submicron Ti particles and nanoscale Mg17Al12 precipitates were dispersed in the Mg matrix.A coherent interface was formed between the Mg matrix and Ti particles and Mg17Al12.68% of the titanium particles were refined to the nanoscale,and the average grain size of the Mg matrix was 72 nm.A magnesium-based solid solution of titanium supersaturation was formed,with a solid solubility of 2.2 at.%.The hardness values of the nanocrystalline AZ91 magnesium alloy and Ti/AZ91 magnesium alloy after ball milling were 1.53 HV and 1.98 HV,respectively.Compared to AZ91 magnesium alloy,the increase in hardness of the Ti/AZ91 magnesium alloy is attributed to the dispersed Ti particles and the acceleration of dislocation pile-up and grain refinement of the magnesium matrix by Ti particles during the ball milling process.The strengthening mechanisms of the ball-milled Ti/AZ91 magnesium alloy include grain refinement strengthening,dispersion strengthening,dislocation strengthening,solid solution strengthening,and load-bearing strengthening,with contributions of 56.3%,18.2%,17.4%,4.7%,and 3.5%,respectively.For Ti-dispersion-strengthened nanocrystalline AZ91 magnesium alloy,Ti particles and precipitates significantly improve its thermal stability.The evolution of microstructure during annealing process was revealed by isothermal annealing experiments of ball-milled Ti/AZ91 and AZ91 powders at 300℃-450℃.During annealing,Al further solid dissolves into the Mg matrix and forms Al3 Ti and Al Ti3 phases.After isothermal annealing at 450℃ for 10 hours,the grain size of Ti/AZ91 was 157 nm,and the average size of dispersed Al3 Ti particles was 17 nm.The kinetic equation for grain growth in Ti/AZ91 was established as follows:.It was found that the activation energy for grain growth of Ti/AZ91 was different when the temperature was raised to 400℃.The grain growth index(n)and activation energy(Q)for nanocrystalline AZ91-Ti were found to be 8 and 126 k J/mol,respectively,while the activation energy was 201 k J/mol.The evolution of material hardness after annealing treatment was explained.After isothermal annealing at 450℃ for 10 hours,the hardness of AZ91-Ti composite powder was 135 HV,which still remained high.Powder consolidation was achieved by using powder extrusion process.The effects of extrusion temperature on microstructure,densification behavior,and mechanical properties of ball-milled AZ91-Ti,Ti/AZ91 magnesium alloys with different volume fractions,and Mg-based alloy with ball-milled Mg powder were studied.The study revealed the influence of extrusion temperature on microstructure,densification behavior,and mechanical properties of magnesium alloys.It was found that the comprehensive mechanical properties of extruded Ti/AZ91 magnesium alloy were optimal at an extrusion temperature of 300℃.The yield strength,compressive strength,and fracture strain of the material were 677 MPa,681 MPa,and 4.9 %,respectively.he average grain sizes of Ti/AZ91 magnesium alloy,ball-milled pure magnesium,and non-ball-milled pure magnesium after extrusion were 201 nm,340 nm,and 2.55 μm,respectively.The influence of microstructure parameters on mechanical properties of mixed-structured alloys was determined.The mixed-structured magnesium alloy with 5 vol.% ball-milled Mg powder exhibited the highest strength,with yield strength and compressive strength of 686 MPa and 748 MPa,respectively.The plasticity was improved with a compressive fracture strain of 5.7%,which was increased by 16%.The mixed-structured magnesium alloy with 15 vol.%non-ball-milled Mg powder exhibited the best plasticity,with a compressive fracture strain of 6.5%,which was increased by 33%.However,the yield strength and compressive strength decreased to 536 MPa and 635 MPa,respectively. |
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