Preparation And Mechanical Properties Of Nanocrystalline LPSO Particles Reinforced Magnesium Composites

Long period stacking order(LPSO)has attracted much attention of the scientific community due to its high hardness,elastic modulus,thermal stability and good coherence with the magnesium matrix interface.However,most of the studies about LPSO phase are focused on in-situ method,and LPSO phase can only exist in Mg-TM-RE system,which greatly limits the application of LPSO phase alloy.In this paper,nanocrystalline LPSO powder(LPSOp)was introduced to Mg and AZ91 as extra reinforcing phase,respectively,to enhance the strength of the composite materials and also expanded the application of LPSO phase.The method comprises the following steps:preparing a Mg85Zn6Y9(at.%)master alloy containing almost 100%LPSO phase by a conventional casting method;fabricating nanocrystalline LPSOp by high energy ball milling after the initial crushing of the milling machine;sintered LPSOp/Mg and LPSOp/AZ91 composites by ultrahigh pressure(SHP)and spark plasma sintering(SPS),respectively.Both milling time and annealing process was optimized.The content and distribution of LPSOp and heat treatment on the microstructure and room temperature mechanical properties of composites were investigated systemetically.Nanocrystalline LPSOp can be successfully prepared by as-cast Mg85Zn6Y9 alloy using high energy ball milling method,which can significantly reduce the particle size,grain size of LPSOp and improved the microhardness of LPSOp with the increase in milling time.The improvement hardness of LPSOp is mainly attributed to synergy effects of grain refinement and cooperative reinforcement of LPSO phase and W phase nano clusters.Moreover,the microhardness of the LPSOp block specimen seems not sensitive to the annealing temperature,indicating that the LPSOp has good thermal stability.The LPSOp/Mg composite was successfully prepared by super high pressure(SHP)method with high density.The addition of LPSOp is beneficial to improve the mechanical properties of composites,i.e.specimen annealing at 400? makes the LPSO phase precipitate,leading the enhancement of yield compressive strength and ultimate compression strength at room temperature.The 30wt%LPSOp/Mg composite exhibits the yield strength and compressive strength of 225MPa and 360MPa,respectively.The main strengthening mechanism of the LPSOp/Mg composites is attributed to the good interfacial bonding that leads to the load transfer and the synergistic enhancement of the three-dimensional skeleton structure.LPSOp/AZ91 composites were also successfully prepared by SPS and the density is more than 97%,but the porosity increased with the increase of LPSOp content.The yield compressive strength of the composite is significantly higher than that of AZ91 counterpart.The yield compressive strength of the LPSOp/AZ91 composite after 400?,24h treatment is improved without degradation of plasticitiy.The yield compressive strength and ultimate compressive strength after solution treatment were improved to 230MPa and 400MPa from 200MPa and 430MPa in case of 20wt%LPSOp/AZ91,respectively.In addition,LPSOp maintained at a high level of hardness after solid solution treatment,indicating LPSOp has excellent high temperature performance.The strengthening mechanism of the LPSOp/Mg composites is mainly the load transfer mechanism and dislocation multiplication.