Microstructure Evolution And Strengthening Mechanisms Of SCFs/AZ31 Magnesium Matrix Composites Fabricated By Solid-State Synthesis

Magnesium matrix composites have good development prospect in the fields of transportation and architectural engineering by the advantages of high specific strength,high specific stiffness and light weight.At present,the preparation method of Mg-matrix composite is based on the liquid-state synthsis,however,the liquid-state synthsis has some shortcoming,such as low utilization ratio of raw materials,high production cost and polluting environment.In this paper,the short carbon fiber reinforced magnesium matrix composite was prepared by solid-state synthesis technique and solid-state synthesis assisted by semi-solid technique.The microstructure evolution during hot-press process,semi-solid isothermal treatment and hot-extrusion deformation process were investigated,the influence of the changing extrusion deformation technology parameters,SCFs contents and semi-solid isothermal treatment parameters on the mechanical properties were also analyzed.The effect of SCFs on the?-Mg matrix dynamic recrystallization(DRX)behavior,the strengthening and toughening mechanisms of composite and the synergetic effect of dynamic recrystallized grain size and second phase on composite mechanical properties were discussed.Effect of SCFs content on microstructure and mechanical properties of composite were studied.The result indicated that the addition of SCFs could promote the dynamic recrystallization process,reduce the DRXed grain size and as the carrier of interface shear stress,thus improving the mechanical properties of the composite.When the SCFs content was 3 wt.%,the optimal comprehensive mechanical properties were obtained,the yield strength,utilmate tensile strength and elongation is 171 MPa,258 MPa and 6.0%,respectively.The significant improvement in the mechanical properties of the composite was mainly attributed to load transfer strengthening,fine grain strengthening,thermal residual stress strengthening and Orowan strengthening.Effect of extrusion deformation technology parameters on microstructure and mechanical properties of composite were studied.When the extrusion ratio was25:1,the lower extrusion temperature is beneficial to obtain the finer DRXed grains.When the extrusion temperature was 400°C,with the extrusion ratio increasing,the nucleation and growth of recrystallizing grains were promoted by the higher deformation degree and more deformation heat during plastic deformation process.Meanwhile,with the extrusion ratio increasing from 11:1 to25:1,the brokened and refined eutectic compounds were dispersed with movement of matrix,which contributed to both the higher strength and ductility of the composite.By observing the microstructure of AZ31 Mg alloy and composite extrusion transition section,under increasing stress,the volume fraction of DRX increased,for the composite has higher volume fraction of DRX and finer DRXed grains.The microstructure evolution of pre-forming billets during semi-solid isothermal treatment was investigated,it was found that near-spheroidal solid phases and network-like distributed secondarily solidified structures were obtained after semi-solid isothermal treatment.After hot-extrusion deformation,?-Mg₁₇Al₁₂ phases distributing in secondarily solidified structures were cracked to fine particles,and combined with SCFs as second phases that have impact on microstructure and mechanical properties of composite.Meanwhile,as-extruded composite occurs obvious DRX behavior.Under the effect of?-Mg initial grain size and second phases content,the DRXed grain size presented a trend of first decrease and then increase with rising the reheating temperature and extending the reheating time.Under the synergistic effect of grain size,SCFs,and?-Mg₁₇Al₁₂phases,the optimal mechanical properties were achieved after reheating at 610°C for 30 min.Compared with the composite without semi-solid isothermal treatment,the yield strength and utilimate tensile strength were increased 21.1%and 12.0%,respectively.Relationship between DRXed grain size,?-Mg₁₇Al₁₂ phases contents and mechanical properties were studied for the composite.The reciprocal of the square root of the DRXed grain size is directly proportional to the yield strength,the specific fitting formula is R_{e L}=121.5+243.1d^-1/2.The fitting relation curves between?-Mg₁₇Al₁₂ phases contents and yield strength of composite showed a significant turning point,when the?-Mg₁₇Al₁₂ phases content was 1.6%.When the?-Mg₁₇Al₁₂ phases content is 1.3%-1.9%,the fitting equation of the?-Mg₁₇Al₁₂phases contents and yield strength of composite is R_{e L}=177.4+467.3A-145.7A².