Study On Fabrication And Aging Behavior Of Mg₂B₂O_5W/ZK60 Magnesium Matrix Composites

Magnesium alloy is the lightest structure material, which has many traits such as high specific strength and stiffness, high thermal conductivity, good electromagnetic shielding characteristics, good vibration and noise reduction capability and well recycling property etc. However, the use of magnesium alloy is limited because the strength, corrosion resistance and creep resistance of the magnesium alloy are relatively low. Until now, much attention has been paid to the development of magnesium matrix composites because of their high specific strength and stiffness, good size stability and superior damping capacity.This article reports the preparation and aging behavior of Mg2B2O5W/ZK60 magnesium matrix composites with 15% volume fraction of boric acid magnesium whisker (Mg2B2O5W) by powder metallurgy. The effect of solution and aging treatment for different time of the Mg2B2O5W/ZK60 composites on the microstructure and Vickers hardness of the composite was investigated by metallography, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and micro hardness measurements.The results showed that the microstructure of Mg2B2O5W/ZK60 composites is consisted ofα- Mg (matrix), Mg2B2O5W, eutectic phase MgZn2 and zirconium. The strip or irregularα- Mg is white. The eutectic phase MgZn2 is located at the boundary of the grains. Boric acid magnesium whiskers are distributed uniformly in the composite. The zirconium hydride (ZrH) was also observed in magnesium matrix composites. With the addition of Mg2B2O5W, grain size of composite was uniform and smaller than that of ZK60 alloy. Therefore, the whiskers play a certain role in grain refinement. The interface reaction products and attachments at boundary of whisker were not observed. After solid solution treatment, zinc at the zinc-rich grain boundaries of the composite is partly oxidized into ZnO. Moreover, the MgZn2 phase formed in the process of sintering is stable without further phase transformation in the following heat treatment. Essentially, the addition of Mg2B2O5W didn't change mechanism of age hardening because the curve of aging hardening is the same as that of ZK60 alloy with two peaks although MgZn was a strengthening phase during the aging process.After analysis of microstructure change of Mg2B2O5W/ZK60 composites, it is determined that the composites mainly recrystallized as subgrains at the aging process of 170℃. The nucleation of recrystallization occurred both in the grains and at the grain boundary of magnesium matrix. Hardness of the Mg2B2O5W/ZK60 composite was increased by 28% than that of ZK60 alloy.