Fine-grained Microstructure And Thermal Stability Of Rapid Quenched Magnesium Alloy Containing SiC Particles

Along with the development of traditional structured materials in the direction of lightweight and,magnesium alloys were widely used in aerospace,automotive electronics and other fields due to their low density,high specific strength/specific stiffness and good damping performance.However,there are some defects in the traditional method of casting magnesium alloy,such as coarse dendrite,segregation,shrinkage porosity and so on.which limited the further application of magnesium alloys.The primary cause of various defects was coarse grain and grain boundary forβ phase in the high temperature.Grain refinement can improve the high temperature strength and creep resistance of the alloy,reduce the segregation of the elements and inhibit the brittleness of the β phase.Therefore,it is of great value to study the preparation process of the refinement under rapidly cooling magnesium alloy,analysis the refinement mechanism to copper cooling AZ91 magnesium alloy in the presence of micron and nano SiC particles,and reveal in the grain growth behavior under the condition of high temperature..The SiC particles as inoculant and rapid solidification technology were performed on this paper.The effect of SiC particles with different particle size and content to the microstructure and average grain size of the rapidly cooling magnesium alloy was studied.Optimal refinement scheme for magnesium alloy was obtained.Through the solid solution treatment of different temperature and time,combined analysis of alloy hardness after annealing treatment,the impact for grain growth behavior of grain refining magnesium alloy involved in SiC particles were further studied.The high temperature thermal stability behavior of magnesium alloy was discussed.For improving the properties of magnesium alloy at elevated temperature to provide theoretical basisGrain refinement results show that the microstructure of magnesium alloy by copper cooling grains was fine and uniform compared to conventional cooling.The participation of macron SiC particles leads to further refinement of solidification structure of magnesium alloy,the higher the content,the more obvious the refining effect.The best effect was sub rapid solidification AZ91+2wt%SiC micron particles with the diameter of copper 2mm.The macron SiC particles can promote the nucleation of copper cooling AZ91 magnesium alloy.With the solidification process,while excess SiC particles are pushed into the residual liquid phase in grain to hinder the growth of primary phase by pinning boundary,thus the microstructure of alloy was finer and dispersed.Due to micron SiC particles and reduced the inner diameter,the hardness of fast cooling magnesium alloy improved significantly.The copper inner diameter of 2mm and micron SiC content of 2wt%,the hardness was up to 146 Hv,increased by 92% compared to casting.The microstructure of magnesium alloy was obviously refined by nano SiC particles.When the content of nano SiC in 1wt%,the average grain size of fast cooling magnesium alloy was refined to 5μm.The refining effect was obviously better than micron SiC particles.Nano SiC particles was in the fining structure,on the one hand providing heterogeneous nucleation,on the other hand pinning boundary to hinder the growth of grain.Solid solution treatment experiment showed that the fine grain magnesium alloy was not completely dissolved at 320℃ and 370℃ for 2h but in a state of complete solid solution at 400 ℃ for 2h.Under the condition of 400 ℃,the annealing time extended from 2h to 8h,the average grain size of sub rapidly solidified AZ91+2wt%micron SiC alloy just increased from 13 μm to 22 μm.The grain growth of magnesium alloy was effectively suppressed under the condition of high temperature.The presence of micron SiC particles can effectively inhibit the grain growth of magnesium alloy at high temperature,and then improve the thermal stability of the fine grain structure.After solid solution treatment,the divorced eutectic β phase at the grain boundary was disappeared,and alloy hardness decreased.After the annealing of AZ91+2wt% micron SiC alloy at 400℃ for 8h,the β phase was precipitated in the grain improving the hardness,up to 111 Hv,which 63.2% higher than AZ91 alloy.