Research On Microstructural Refinement Mechanism And Properties Of Mg₂Si_p/Mg-Zn Based Matrix Composites

Magnesium based materials are widely applied in fields which are strongly driven towards weight and vibrate-reduction,such as automotive,aeronautic and astronautic industries,because of their high ratios of strength to density and stiffness to density.The in-situ magnesium matrix composite,in which the high volume fraction of Mg₂Si is reinforced,is one of research directions.The in-situ synthesized Mg₂Si_p/Mg-Zn which has attracted increasing attention has been investigated in this thesis.Aiming at improving its mechanics properties,corrosion and wear resistances,magnesium matrix composite with fine microstructure and excellent performance can be obtained through the refinement of Mg₂Si phase by the single element?Y or Sr?addition firstly and then further refined by the friction stir processing?FSP?.The modification effect of the single element addition?Y or Sr?and FSP on the primary and eutectic Mg₂Si,tensile properties,corrosion and wear resistances of composites was investigated by means of OM,EDS,XRD,SEM,electrochemical workstation,the friction-wear test machine and first principles study.Furthermore,modification mechanisms of addition elements on Mg₂Si crystal growth and properties,the refinement and dispersion mechanisms of FSP and the wear mechanism of magnesium matrix composite has also been analyzed in detail.The appropriate level of Y or Sr addition can remarkably modify the primary and eutectic Mg₂Si in hypereutectic Mg-Zn-Si alloy.The size of the primary Mg₂Si decreases firstly and then increases with the increasing Y or Sr.The morphology of primary Mg₂Si in alloys changes from coarse dendrites to octahedral or hexahedron crystals with the size of 30?m,and that of eutectic Mg₂Si changes gradually from the Chinese script morphology to fine clavate.The optimal content of Y or Sr is 0.5%.When the content of Y or Sr exceeded 0.5%,a new YSi₂ or lath-shaped SrMgSi phase is formed,which reduce their adsorption effect on the crystal plane.The preferential adsorption of Y or Sr atom on the{100}and{111}crystal planes reduces the total energy of systems and the growth rates of{100}and{111}crystal planes,which results in the size reduction of Mg₂Si crystal.The absorption energy of Y atom on{100}crystal plane is similar to that on{111}crystal plane,and thus the obstacle of Y on{100}and{111}is uniforminty,and the morphology of Mg₂Si has no change.The obstruction degree of Sr to the?1 0 0?growth speed is much higher than that to the?1 1 1?growth speed,and thus the growth speed ratio of?1 0 0?to?1 1 1?becomes smaller,resulting in the gradually shift of the lacunarity octahedron crystal,cube-octahedron crystal and hexahedron crystal.With the increasing content of Sr element up to 0.5%,the absorption effect of Sr becomes stronger,and the growth speed ratio of?1 0 0?to?1 1 1?becomes much smaller,and the morphology of primary Mg₂Si in the composites completely changes from coarse dendrites to hexahedron crystal finally.The appropriate level of Y or Sr elements addition can remarkably improve tensile properties at ambient and elevated temperature,and corrosion resistances of the in-situ magnesium matrix composite.The ultimate tensile strength,the elongation at ambient and elevated temperature and corrosion rate increase firstly and then decrease slightly with the increasing content of element addition.When the Y or Sr addition is 0.5%,maximums of the ultimate tensile strength and the elongation as well as the minimums of corrosion rate and corrosion current density can be obtained,mechanical properties and corrosion resistance of magnesium matrix composite is the best.When the Y addition is 0.5%,the maximums of the ultimate tensile and the elongation are 50.5%and 67.8%higher than that of the base alloy respectively,the corrosion rate is reduced by 63.6%,the corrosion current density is reduced by a magnitude,too.With 0.5%Sr addition,the ultimate tensile and the elongation are 53.3%and 55.2%higher than that of the base alloy respectively,and the corrosion rate is reduced by 47.7%,the corrosion current density is reduced by three magnitudes.These properties improvements of materials can be attribute to alloying induced fine and uniformly distributed Mg₂Si phase.Friction stir processing?FSP?can break and refine Mg₂Si phase in magnesium matrix composites with 0.5%Y or Sr addition.When the traverse speed of the tool is 30mm/min,the average size of Mg₂Si particles decreases firstly and then increases slightly with the increasing tool rotation rate.The average size of Mg₂Si particles decreases firstly and then increases slightly with the increasing traverse speed as the specified values of the tool rotation rate.The most tiny and uniformly distributed Mg₂Si particles with micro-size can be obtained employing a tool rotation rate of 900 r/min and a traverse speed of 30mm/min,which is due to intense shear effect between Mg₂Si particles and the microstructure of composites,which is caused by the dual effect of the produced heat and mechanical force during the friction stir processing.FSP can observably improve mechanical properties of composites with Y or Sr addition,whose variation trend is in accordance with that of the size of Mg₂Si phase.The optimal mechanical properties,maximums of the ultimate tensile strength and the elongation,are achieved at a tool rotation rate of 900r/min and a traverse speed of 30 mm/min.Maximums of the ultimate tensile strength and elongation of composites with Y addition after FSP are increased by 43.7%and 35.4%,respectively.Those with Sr addition after FSP are increased by 46.6%and 57.7%,respectively.These are mainly because of the optimal microstructure modification,especially the optimal modification effect of Mg₂Si phase resulting from enough produced heat and mechanical force during the friction stir processing.Reasons for the composite breaking at a nethermore tool rotation rate or an exorbitant traverse speed are as follows:produced heat and mechanical force are not enough,Mg₂Si reinforcements are not broken completely,which leads to the reduction of mechanical properties of composites.Reasons for the composite breaking at an exorbitant tool rotation rate or a nethermore traverse speed are as follows:Mg₂Si reinforcements rotate under the flow field,and cannot been broken,and thus failure of composites occurs due to the formation and extension of cracks during the tensile process.FSP can observably improve corrosion and wear resistances of composites at ambient and elevated temperature,whose variation trend is in accordance with that of the size of Mg₂Si phase.Minimum corrosion rates of composites can be obtained at a tool rotation rate of 900 r/min and a traverse speed of 30 mm/min.Corrosion rates of composites with Y or Sr addition after FSP are reduced by 12.3%or7.9%,respectively.The corrosion resistance of the composite is the best.Meanwhile,the composite has the lowest wear rate and exhibits the best wear resistance.The dominant wear mechanism of composites at ambient temperature is abrasive wear.Adhesive,fatigue and oxidative wear are also observed.The dominant mechanism of composites at elevated temperature is oxidative wear.Abrasive wear,delamination wear,adhesive and fatigue wear are also observed.Coarse Mg₂Si particles peeled off from the wear surface strongly scratches on the surface under high temperature,which cannot form effective and long-term oxide layer.Fine and uniform distributed Mg₂Si particles peels off under high temperature,and are pressed into the oxide layer on the wear surface at the pressure load or adhere to the steel ball surface,and thus a smooth oxide layer is formed,which slows the friction effectively.