| With the rapid development of national economy and national defense industry,national defense,civil and other major industries put forward more requirements on the metal structure material.Metal matrix composite materials have attracted widespread attention for its high specific strength,high specific stiffness and good thermal stability.In this paper magnesium borate reinforced aluminum matrix composites（MBOw/Al）were prepared by squeeze casting.In order to exploring the preparation technology and mechanism of deformation,the preparation of MBOw/Al,the mechanical and thermal expansion properties were systematacially investigated.Through the study,the effect factors of the composite material preparation and properties were revealed.In addition,the plastic deformation and the thermal expansion deformation in high temperature were also researched.The non-deformability of the precast magnesium borate whisker was highly improved because the magnesium borate whisker surface was coated by oxides through the chemical precipitation method.Microstructure analysis was showed that the magnesium borate whiskers kept complete morphology in the composite material,thus the magnesium borate whiskers could greatly enhance the matrix.Metal oxides coated on the whisker surface effectively enhanced the strength of the precast whisker and the precast whisker strength increased with the adding of coating content.The composite material preparation and properties can be improved by the suitable metal oxides coating,thus the preparation technology of squeeze casting was optimized.The largest tensile strength and elasticity modulus of MBOw/Al were 330MPa and107GPa at room temperature,respectively,which was larger than that of pure Al matrix.The tensile property of MBOw/Al can be improved through annealing treatment with different time.The fracture mechanism of MBOw/Al by different annealing treatment was same,which was the extracting and debonding of whiskers from the metal matrix.The residual tensile stress can be effectively relaxed in the composites through annealing treatment,which led to the enhancement of the tensile strength.However,the matrix grains would gradually grow up with the heat treatment time increasing,which resulted in a slight tensile strength decline.In the experiment of composite’s high temperature compression deformation,the flow stress of MBOw/Al was affected by the deformation temperature and strain rate,and the flow stress decreased with the deformation temperature increasing and strain rate decreasing.The plastic deformation behaviors of MBOw/Al was mainly the coupling effect of aluminum matrix plastic deformation and whiskers rotation.The plastic deformation of the composite obviously changed when the whiskers were breaked under the continual increasing stress,namely strain softening phenomenon.The strain softening phenomenon was greatly influenced by temperature,the lower the temperature was,the more obvious this phenomenon was.When the deformation temperature of MBOw/Al was higher,the strain softening phenomenon decreased due to the rotation and the reduced break rate of whiskers.The results of thermal expansion experiment of MBOw/Al showed that the residual stress was the key factor of strengthening composite.The soften metal matrix and relaxed residual stress led to the increase of thermal expansion coefficient with the rising temperature.The increase was stabilized at 100℃,the coefficient of thermal expansion was 15×10-6/℃,then it gradually increased and the coefficient of thermal expansion was 18×10-6/℃at350℃.However,the matrix grains would gradually grow up with the increasing heat treatment time,the coefficient of thermal expansion decreased under the coupling effect of residual tensile stress and thermal stress.Therefore,the residual stress was the main influencing factor for the thermal expansion deformation of MBOw/Al composite,because the thermal stress was changed with different external temperature. |
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