Microstructure Refinement And Strengthening And Toughening Mechanism Of Mg-4Al-1.5Si Magnesium Alloy

Mg-Al-Si series magnesium alloy with low cost is considered a promising high-temperature creep-resistant magnesium alloy within 50MPa/150℃.It has a wide application prospect in the field of aviation and automobile.However,under casting conditions,the eutectic Mg2Si phases in Mg-Al-Si alloy usually exhibit coarse Chinese script shape,and when the Si content is high,coarse polygonal primary Mg2Si phases will appear,which severely cut the magnesium alloy matrix and cause interfacial debonding or crack initiation during service,thus deteriorating the strength and ductility of the alloy.Therefore,it is key to refine and modify Mg2Si for improving the comprehensive mechanical properties of Mg-Al-Si magnesium alloy.In the present study,TiB₂ nanoparticles were added to an Mg-4Al-1.5Si alloy by semi-solid stirring,and then uniformly dispersed in the matrix alloy by ultrasonic vibration.Finally,a hybrid reinforced magnesium matrix composite was obtained.By introducing TiB₂ nanoparticles as heterogeneous nucleation sites of Mg2Si and ultrasonic treatment for refinement,and continuously modifying Mg2Si morphology by solid solution treatment,the strength and toughness of magnesium alloy can be improved synchronously.The effects of TiB₂ nanoparticles content（1vol.%,3vol.%,and 5vol.%）on the size,morphology and mechanical properties of Mg2Si phases in Mg-4Al-1.5Si magnesium alloy were investigated.The influence mechanism of TiB₂ nanoparticles on nucleation and growth of Mg2Si precipitates was revealed by in-depth analysis of morphology and size changes of Mg2Si phases.The modification mechanism of Mg2Si morphology and the influence of solution treatment on the mechanical properties of Mg-4Al-1.5Si magnesium alloy were studied,and the strengthening and toughening mechanism of TiB₂/Mg2Si hybrid reinforced magnesium matrix composites was clarified.The conclusions obtained are as follows:（1）With the increase of the content of TiB₂ nanoparticles,the sizes of primary Mg2Si phases and eutectic Mg2Si phases were significantly refined,the primary Mg2Si phases changed from coarse dendritic to fine blocky shape,and the morphology of the eutectic Mg2Si phases was transformed from coarse Chinese script to fine short strips,meanwhile,the dispersion uniformity of primary Mg2Si was also improved,and the grain size of α-Mg decreased gradually.（2）When TiB₂ content is 3vol.%,the composites have the best comprehensive mechanical properties.The Vickers hardness,tensile strength,yield strength and elongation of the composites were increased by 101.8%,69.8%,10.6%,and 187.5%,respectively,compared with the matrix alloy.The fracture mode of the material changes from brittle fracture（TiB₂ content is 0）to quasi-cleavage fracture（TiB₂ content is lvol.%）and then to plastic fracture（TiB₂ content is 3vol.%）and then returns to quasi-cleavage fracture when TiB₂ content is 5vol.%.The increase in the hardness and strength of the composites is mainly due to the uniform dispersion of TiB₂ nanoparticles and the refinement of Mg2Si phases,which hinder the grain boundaries sliding and dislocation movement,the increase of ductility is mainly attributed to the refinement and homogeneous distribution of the Mg2Si phases.（3）After solution treatment,the primary Mg2Si phases in TiB₂/Mg-4Al-1.5Si composites were passivated by a sharp angle,and some branches of eutectic Mg2Si phases were dissolved into particles.Compared with the as-cast composites,the Vickers hardness,ultimate tensile strength,yield strength,and elongation of the solution-treated composites increased by 11.50%,33.28%,28.57%,and 27.17%,respectively.Compared with the matrix alloy,solid solution treatment has a more significant strengthening effect on the hardness,ultimate tensile strength,and yield strength of the composites.