Heat-Resistance Behavior Of Magnesium Based Composite Foams With Hollow Ceramic Microspheres

As one kind of structural and functional material,the composite foams with magnesium matrix have promising prospects in automobile and aerospace industries due to its low density,high specific strength and stiffness,excellent energy absorption capacity and electromagnetic shielding abilities,etc.In order to access the possibility that whether the heated Mg composite foams can be reused in the practical application,AZ31B composite foams with 10 vol.%ceramic microspheres are fabricated by using the modified melt foaming method.The heat resistance properties of the composite foams are investigated by using the samples subjected to various heating temperature（150℃,250℃,320℃,400℃and 500℃respectively）and holding times（1h,2h,4h,6h and 24h respectively）.The microstructures and mechanical properties before and after the heating processes are examined by applying the X-ray diffraction technique,the optical microscopy,the scanning electron microscopy and the quasi-static uniaxial compression test.The results show that the micro-hardness and yield strength are gradually decreasing when the foams are heated at 150℃for 1h to 4h while turned to increasing when heated for 6h and 24h;when the heating temperature increased to 250℃,the micro-hardness and yield strength are decreasing when the foams heated from 1h to 2h,while increasing from4h and achieved the maximum value at 24h.Further increasing the temperature to 320℃,400℃and 500℃,the micro-hardness and yield strength decreased to the lowest level at1h and then gradually increasing to the maximum value at 24h.It’s examined that when the foams are heated for 1h,the micro-hardness and yield strength decrease rates are increasing as the temperature gradually increasing to 320℃,while the decrease rates are reduced when the temperature further increased to 400℃and 500℃.In addition,it is found that the differences of the micro-hardness and yield strength for the foams heated at different times are gradually decreasing with the heating time increasing.It’s also found that with the heating time increasing,the strain hardening exponents are increasing first and then decreasing,while the plateau stresses and energy absorption capacities are show a trend of decreasing first,then increasing and finally decreasing again.Comparing to the unheated foams,almost all the heated composite foams possess lower micro-hardness,yield strength and energy absorption capacity.It can be explained by that the phases in the foams are not changed except theβ-Mg₁₇Al₁₂ phase.Meanwhile,with the heating temperature and time increasing,β-Mg17Al12 phases are gradually dissolved from the grain boundaries into the grain interior,resulting in the strengthening effect,meanwhile,the internal stresses in the matrix are released,leading to the strength and hardness reductions.The mutual influence between theβ-Mg17Al12 phases and the internal stresses are responsible for the variation regularity of mechanical properties.Accordingly,it’s proposed that more considerations are deserved when the heated composite foams are to be reused in the practical application.