Research On The Mechanical Behavior Of Magnesium Borate Whisker Reinforced Magnesium Matrix Composites Under Thermal Cycling With Different Applied Stresses

In this paper, Mg₂B₂O₅w/AZ31B magnesium matrix composites were preparedvia vacuum pressure infiltration processing. The dimensional stability of thecomposite under applied load and thermal cycling was investigated by means ofSEM, thermal analysis system and thermal/mechanical simulator testingmachine.The effect of applied stress and thermal cycling on strength and hardness ofthe composites was also studied.The results showed that the variation of the volume fraction of the compositesprepared via vacuum pressure infiltration processing is very small and whiskerswere distributed uniformly in the composite. The thermal ratcheting phenomenontook place as Mg₂B₂O₅w/AZ31B magnesium matrix composites under thermalcycling with applied stresses. The heat treatments could reduce the thermalratcheting of the composites effectively; it was in favor of the dimensional stabilityof the composite under applied load and thermal cycling. The axial tensile thermalcycling was divided into three stages: the primary creep stage, the steady creep stageand the collapsing deformation stage. The strain rate at steady creep stage increasedwith the applied stress. The lower stress was, the longer creep time was, and theelongation value at break would be bigger. Hysteresis loops are produced duringdead-load thermal cyclings （TCs） due to the symmetric stress relaxation, while theresidual strain is produced due to the asymmetric stress relaxation. The dead-loadthermal cycling showed superplasticity. The strain exponent, m, would becomehigher if the applied stress fell, and the superplasticity would be greater then. Underisothermal creep conditions, superplasticity appeared too, but the value of, m, wassmall. Furthermore, the strain rate was much lower than that under thermal cyclingcreep conditions with the same tensile stress.Thermal cycling with applied stresses has many effects on the mechanicalproperties of Mg₂B₂O₅w/AZ31B composites.The tensile strength of Mg₂B₂O₅w/AZ31B composite increased first, and thendecreased after certain dead-load TCs, There was a maximum tensile of Mg₂B₂O₅w/AZ31B composite after certain number of cycles. During dead-load TCs,the tensile strength increased first, and then decreased with the maximum cycletemperature Tmax due to the interaction between the weakening effect caused bydefects and the strain harding effect in the materials. After certain cycles the tensilestrength increased rapidly due to the interaction between stress relaxation effect andstrain hardening effect which is more obvious.The hardness of Mg₂B₂O₅w/AZ31B composites decreased with increasingapplied load and the reducing margin was getting smaller and smaller after250TCs,After dead-load TCs, the hardness of quenched composites was maximum, followedby the annealed composites, and the as-casted composites was minimum.