Study On The Interfacial And Hybrid Behaviors Of Magnesium Matrix Composites Reinforced With Magnesium Borate Whisker

Magnesium matrix composites have exhibited the wide application prospect inaviation, aerospace, automobile and electrommunication fields ascribed to theiradvantages, such as low density, high specific strength and high specific modulus. Amongthem, whisker-reinforced magnesium matrix composites have been received intensiveinterests by the researchers at home and abroad due to their superior mechanical properties.However, interfacial problems between whisker and matrix have been the key bottleneckto the practical application of whisker-reinforced magnesium matrix composites. In thisstudy, the high cost-effective magnesium borate whisker （Mg2B2O5w） is used as thereinforcement. Magnesium matrix composite reinforced with Mg2B2O5w is successfullyfabricated by optimizing preparation technology and interfacial design. Themicrostructures, formation mechanism of the interfacial reaction layers, mechanicalproperties and strengthening mechanism of composites are studied systematically. Basedon these results, in order to further improve the mechanical properties of composites, thehybrid magnesium matrix composites reinforced with boron carbide particles （B4Cp） andMg2B2O5w are successfully prepared. The microstructures, mechanical properties andstrengthening mechanism of hybrid composites are explored.AZ91D magnesium matrix composites reinforced with20vol.%Mg2B2O5w aresuccessfully prepared by the squeezing casting technology. The microstructure of the composite is dense, and Mg2B2O5w uniformly distributes in the matrix alloy. The additionof Mg2B2O5w can obviously improve the mechanical properties of the composite. Flexuralmodulus, flexural strength and micro-Vickes’ hardness are improved from42GPa,211MPaand56.5to50GPa,407MPa and120.7, respectively.The Mg2B2O5w/Mg interface is clean in the as-cast Mg2B2O5w/AZ91D compositesand is chemically stable at the casting temperature. Although, there is plenty of whiskersdebond in fracture surface of as-cast Mg2B2O5w/AZ91D composite. This observationindicates that the interfacial bonding strength between Mg2B2O5w and matrix is relativelylow. To further enhance the mechanical properties of composites, Al（PO₃）₃coating, ZnOcoating and TiO₂coating are formed on the surface of Mg2B2O5w by various surfacetreatment methods. The TEM analysis of interfacial microstructures in the threecomposites show that all these coatings react with the liquid matrix during the fabricationprocesses of composites. The MgO interfacial layers with different microstructures areformed including:（a） for Al（PO₃）₃coating, MgO particles are relatively large and their size increaseswith the increase in coating thickness;（b） for ZnO coating, there exist a thin bilayer structure which consists of fine MgOparticles and large MgO particles adhering to it;（c） for TiO₂coating, the textured MgO interfacial layer is formed.The mechanical properties results show the thickness and microstructures of the MgOinterfacial layers have significant influence on the flexural properties of composites. Withthe increase in Al（PO₃）₃coating thickness, the flexural strength ofMg2B2O5w/Al（PO₃）₃/AZ91D composite increases firstly and then decreases. When thethickness of the MgO interfacial layer in Mg2B2O5w/Al（PO₃）₃/AZ91D composite is about25nm, the flexural strength reaches the maximum467MPa, which is enhanced by15%compared with that of composite without any coating. The comparative study on interfacemicrostructures and mechanical properties of composites treated by three coatings show that: fine MgO particles introduced by ZnO coating can be much more effective toimprove the interfacial bonding strength and load transfer efficiency than large MgOparticles introduced by Al（PO₃）₃coating, and thus enhance the flexural properties ofcomposites. The textured MgO interfacial layer can most effectively increase the flexuralproperties of composites. The flexural strength and modulus are568MPa and66GPa,which are enhanced by40%and35%, respectively, compared with that of compositeswithout any coating. This is attributed to the increase in interfacial bonding strength andload transfer efficiency. Various strengthening mechanisms including load transferstrengthening, dislocation strengthening and fine-grain strengthening exist in magnesiummatrix composites reinforced with Mg2B2O5w. Load transfer is the main strengtheningmechanism when the interfacial bonding strength is relatively high.To further enhance the overall performance of composites, on the basis of improvingthe interfacial microstructure, the B4Cp is added into Mg2B2O5w and the hybridcomposites are fabricated. The results show that the hybrid of B4Cp remarkably enhancesthe mechanical properties of the composites. The flexural strength reaches the maximum593MPa when the volume fraction of B4Cp is4%, which are improved by35%comparedwith single Mg2B2O5w-reinforced composite.