Preparation Of AZ91/Al₂O₃ Lamellar Composites By Freeze Casting And Reactive Infiltration

Shell with a unique “brick-and-mortar” microstructure usually displays excellent performance.So we can prepare the nacre-like materials to improve their performances.After decades of researches,the lamellar scaffolds can be prepared by directional freezing casting,which can be infiltrated with the melten alloy or the polymer to obtain the lamellar composites.Similarly,we can prepare the lamellar AZ91/Al₂O₃ composites by directional freezing casting and reactive infiltration to improve their compression strength.In this study,we offer a new preparation method of ceramic reinforced Mg matrix composites by adding nano SiO₂ powder and silica sol into the Al₂O₃ slurry based on good wettability between Mg and SiO₂.In this study,the main results are as follows:（1）When the contents of SiO₂ excessed 6 wt.% or the sintering temperature was higher than 1300 °C,there were some cracks at the bottom of Al₂O₃（-SiO₂ sol）scaffolds.The reason was that the stress produced by the dehydration of sol,crystal transition and shrinkage of scaffolds increased with the increase of silica sol contents.The cracks were mainly distributed at the bottom of Al₂O₃（-SiO₂ sol）scaffolds,which may be due to the sedimentation of the silica sol during the process of freezing casting.（2）The compressive strength of Al₂O₃（-SiO₂ sol）scaffold can reach 195 MPa（with 40 vol.% initial solid content,9 wt.% SiO₂,and sintered at 1500 oC）,which is more than 30 times higher than that（6 ± 1 MPa）of pure Al₂O₃ scaffolds（with initial solid content 40 vol.%,and sintered at 1550 oC）.On the one hand,the compressive strength increased with the increase of initial solid contents.On the other hand,the adhesion of the Al₂O₃ particles is enhanced by the addition of the silica sol.In addition,the compressive strength increased with the increase of the sintering temperature,the higher the sintering temperature was,the stronger the adhesion of the Al₂O₃ particles was.（3）When the SiO₂ contents were 1-2 wt.% and the sintering temperature was 1200 oC,there were only some small areas that were not infiltrated by AZ91 in AZ91/Al₂O₃（-SiO₂ sol）composite.When the SiO₂ content was less than1 wt.% or the sintering temperature was higher than 1200 oC,SiO₂ would react with Al₂O₃ to produce mullite.The residual SiO₂ was less and wettability of AZ91 and Al₂O₃（-SiO₂ powder）scaffolds can not be improved effectively,so there were more defects in the composite.（4）When the SiO₂ contents excessed 6 wt.%,there were some areas that were not infiltrated by AZ91 in the center of AZ91/Al₂O₃（-SiO₂ sol）composite.The scaffolds were infiltrated by melt AZ91 from the top and bottom at the same time,which resulted in the higher pressure of Ar in the center of the composite,in which situation AZ91 can not be infiltrated into scaffolds further.When the sintering temperature was higher than 1300 oC,the scaffolds were not infiltrated at all,which was because more SiO₂ reacted with Al₂O₃ to produce mullite with the increase of the sintering temperature and residual SiO₂ in the scaffolds reduced.（5）The highest compressive strength of the AZ91/Al₂O₃（-SiO₂ sol）composites was about 1.7 times higher than that of AZ91,which was becasue the solid contents were higher（41-47 vol.%）and the alloys and ceramics were alternately layered and the lamellas were detailed in the composite.With the increase in silica sol contents and infiltration temperature,the reaction between AZ91 and SiO₂ became more intense,thus leading to the increased structure defects,which weakened the compressive strength of the composites.（6）The smallest wear rate of the AZ91/Al₂O₃（-SiO₂ sol）composites was about 1/4 that of the AZ91,which was mainly because the hardness and abrasion resistance of ceramics were greater,which could inhibit the plastic deformation of the matrix alloy to some extent.SiC particle sizes had significant effects on the wear rates of the composites,and the wear rates of the composites decreased with the decrease of abrasive sizes.However the SiO₂ contents and infiltration temperature had few effects on the wear rate of composites.