The Research On The Mechanical Behavior Of Magnesium Borate Whisker Reinforced Magesium Matrix Composites Under Thermal Cycling

In this paper,Mg₂B₂O_5w/AZ91D magnesium matrix composites were prepared by the basic vacuum pressure on seepage,based on preparation of magnesium borate whisker under binder-free wet-pressure molding technology.The interface of composite materials,dimension stability,the mechanical properties under the thermal circulation,fatigue crack and so on were studied by means of TEM,XRD, thermal expansion device,thermal simulation testing machine and other modern analytical tools.The results show that composites prepared in this way,which whisker was more evenly distributed,the internal microstructure was relatively dense,and the interface reaction of composite materials was not found.Mg₂B₂O_5w/AZ91D magnesium matrix composites on different heat treatment take place the sawtooth phenomenon at the temperature between 30℃and 400℃in the heat cycle,heat treatment can effectively reduce the sawtooth phenomenon of materials,and the effect of different heat treatment on materials sawtooth is different,sawtooth phenomenon is more serious in as-cast condition,followed by annealing condition,it is small in aging condition while it is not obviously seen in quenched composite materials.In thermal cycle conditions,mechanical properties of the composites will decrease. and with the increase in the number of thermal cycle,mechanical properties decline more and more obviously.Through SEM observations of fracture,the interface between whisker and matrix is damaged,or even debonding after thermal cycle which makes the mechanical properties of the composites decrease,moreover,with the number of thermal cycle increasing,whisker desquamate more and more seriously in fracture section which can reveal the reasons for the decline in mechanical properties.Under 30℃～350℃thermal cycling conditions,average thermal expansion coefficient is great of difference between Mg₂B₂O₅ whisker and AZ91D,which cause their hot large strain differently.Therefore,with their plastic strain accumulating and having big difference,the microcrack occur.During Crack propagation process,the thermal fatigue crack and whisker reinforcements have obvious interaction;crack growth rate is affected by temperature changeâ–³T,the distribution of whiskers and interfacial bonding strength and so on.