Study On The Fabrication Of Ultra-fine Grained N-SiCp/AZ91D And CNTs/AZ91D Magnesium Nanocomposites By Cyclic Extrusion And Compression

Cyclic extrusion and compression?CEC?was taken to process the n-SiCp/AZ91D and CNTs/AZ91D nanocomposites prepared by ultrasonic-assisted casting.Effects of the CEC parameters?pass,temperature?on the microstructure evolution and distribution of nano-reinforcements were investigated by using optical microscopy?OM?,scanning electron microscopy?SEM?and electron backscattered diffraction?EBSD?.Effects of the nano-reinforcements?category,amount?on the microstructure of the magnesium nanocomposites were then revealed by taking the monolithic magnesium alloy as a reference.The grain refinement mechanism for magnesium nanocomposites during CEC was discussed.Tensile tests were carried out to evaluate the mechanical properties of the magnesium nanocomposites.By correlating the measured mechanical property to the microstructure of the post-processed magnesium nanocomposites,the strengthening and toughening mechanism was established.Damping capacity of the magnesium nanocomposites was studied by using the dynamic thermo-mechanical analyzer?DMA?.Changes of the damping mechanism with the increasing of temperature and the critical temperature were discussed.At last,the tribological property of the magnesium nanocomposites was investigated and the wear mechanism of the ultra-fine grained magensium nanocomposites were revealed by analyzing the worn surfaces and the wear debris.The obtained conclusions can be summarized as follows:The matrix grains of both n-SiCp/AZ91D and CNTs/AZ91D composites were continuously refined with the progress of CEC.Mg₁₇Al₁₂ was mainly precipitated along grain boundaries and the twinning boundaries.After 8 passes of CEC at 300°C,the matrix grains were greatly refined down to 100¹30 nm and Mg₁₇Al₁₂ was uniformly precipitated within the magnesium matrix with an average particle size of⁰.21?m.Increasing in the CEC temperature led to an obvious growth of the matrix grain and coarsening of the Mg₁₇Al₁₂.Besides the normal extrusion and compression components,multi-pass of CEC also induced other complex texture components,which was attributed to the activation of various twinning,especially{10???2}twinning,{10???1}twinning and{10???1}-{10???2}double twinning.Clusters of SiC nanoparticles could be effectively dispersed after 8 passes of CEC,while this was not the case for CNTs.Though the initial highly-agglomerated CNTs could be separated by CEC to some extend,there were still CNTs-dense zones and CNTs-sparse zones.Degrading of CNTs occurred after the CEC was introduced.The addition of CNTs led to a reduced matrix grain for the post-processed mangesium nanocomposites,while the SiC nanoparticles did not.The incorporated nano-reinforcements exerted a trivial effect on the precipitation of Mg₁₇Al₁₂.The grain refinement mechanism of magnesium nanocomposites during CEC was verified as twinning-assisted rotational DRX accompanied by continuous DRX at relatively low temperatures??350°C?.At high temperature??400°C?,the grain refinement mechanism was rotational DRX accompanied by continuous DRX.The dynamically precipitated Mg₁₇Al₁₂2 and the incorporated SiC nanoparticles were mainly acted as obstacles for the growth of the fine grains.The hardness,yield strength?YS?,ultimate tensile strength?UTS?and elongation to facture?Elongation?of the n-SiCp/AZ91D and CNTs/AZ91D composites were continuously enhanced with the progress of CEC,but increasing in the CEC temperature would reduce in hardness,YS,UTS and Elongation of the magnesium nanocomposites.The incorporated nano-reinforcements increased the YS and UTS of the magnesium nanocomposites both prior to and after CEC,but decreased the Elongation.Besides,the strengthening efficiency of SiC nanoparticles outweighed that of CNTs for the initial solution treated magnesium nanocomposites,but a reversed result was obtained for the post-processed magnesium nanocomposites.The dominant strengthening mechanisms of the nano-reinforced ultra-fined grained magnesium composites were Hall-Petch strengthening,precipitation strengthening and compound strengthening,with the Hall-Petch strengthening contributing most.The increased toughness of the magnesium nanocomposites was mainly attributed to the greatly refined matrix grain.Both the room temperature?25°C?and high temperature?250°C?damping capacity of the magnesium nanocomposites were enhanced after the implement of CEC.Specifically,for the n-SiCp/AZ91D nanocomposites,with the progress of CEC,the room temperature and high temperature damping capacity increased at the initial stage and then decreased.For the CNTs/AZ91D nanocomposites,both the room temperature and high temperature damping capacity increased continuously with the progress of CEC.What's more,with the increasing of CEC temperature,the room temperature damping capacity of n-SiCp/AZ91D nanocomposites showed an upward trend,especially that obtained at 400°C,which was nearly doubled.As for the high temperature damping capacity,the variation was negligible.The incorporation of SiC nanoparticle generally reduced the room temperature and high temperature damping capacity of the matrix alloy,while effects of CNTs varied depending on its amount.0.5 wt.%of CNTs could reduce the room temperature damping capacity of the matrix alloy,but 2.0 wt.%could increase that of the matrix alloy.Besides,the addition of CNTs was beneficial to the high temperature damping capacity of the matrix alloy.For the solutioned treated magnesium nanocomposites,the damping was generated by the vibration of dislocations between weak/strong pinning points below a critical temperature?Tcr?.Afterwards,the damping mechanism was grain boundary sliding.But for the CEC processed magnesium nanocomposites,below a critical temperature?Tcr?,the damping was still generated by the vibration of dislocations between weak pinning points.Once the tested temperature was higher than Tcr,dislocations started to break away from weak pinning points but were still pinned by strong pinning points.The snowslide-like unpinning occurred when the peak temperature?Tp?was reached.Afterwards,although the unpinning of dislocations still occurred,the extent was greatly reduced.Therefore,the damping capacity within the temperature range of Tcr^Tv was mainly controlled by the vibration of dislocation between the strong pinning points.Above the valley temperature?Tv?,grain boundary/interface sliding occurred,which contributed to the greatly enhanced damping capacity.The tribological property of the magnesium nanocomposites was enhanced after the implement of CEC.The incorporation of SiC nanoparticles exerted a trivial effect on the wear loss of the matrix alloy,while the addition of CNTs could reduce the wear loss of the matrix alloy.This was mainly attributed to the extraordinary thermal conductively of CNTs,which helps to liberate the generated friction heat,retain a reasonable strength for the CNTs/AZ91D composites.Increasing in the applied load led to an increasing in the wear loss for the magnesium nanocomposites.The wear loss decreases with the increasing of the sliding speed from 0.10 to 0.15 m/s,while an opposite trend was observed with the sliding speed further increasing to 0.20 m/s.Changes in the applied load and sliding speed could alter the operating wear mechanism.