The Effect Of HPT On Solid Solution And Aging Behavior Of AZ91D Magnesium Alloy

As one of the most promising materials,magnesium alloy has the advantages of small density,good toughness,strong damping capacity,fast solidification speed and excellent machinability,etc.,but the disadvantages of magnesium alloys are also very prominent,such as low room temperature plasticity,low strength,corrosion resistance,etc.,if we want to make a wide range of magnesium alloy applications,we must solve these sharp problems.In this paper,the solid solution treatment,high pressure torsion+solid solution treatment,solid solution+aging treatment,solid solution+high pressure torsion+aging treatment are respectively carried out in the as-cast AZ91D magnesium alloy.The microstructure was studied by X ray diffraction?XRD?,optical microscope?OM?,scanning electron microscope?SEM?and transmission electron microscope?TEM?,and the hardness of magnesium alloy was tested by microhardness test?HV?.The effects of high pressure torsion on the dissolution and precipitation of the second phase of AZ91D magnesium alloy were investigated.The experimental results show that for the?high pressure torsion+?solid solution treatment contrast experiment,high pressure torsion on the microstructure refinement is obvious,the average grain size from 171³50?m of solid solution state refines to 30⁴9?m of HPT+solid solution state,grain size gap more than 10 times;After high pressure torsion,the solid solution temperature was reduced,and the solid solution time was shortened,and the second phase,the Mg₁₇Al₁₂2 phase,was able to dissolve into the matrix faster,but the microhardness improvement was not obvious,only 4 HV was improved.For the solid solution?+high pressure torsion?+aging treatment contrast experiment,high pressure torsion is very obvious in the microstructure refinement by the solution treatment.The grain size is refined from hundreds of micrometers to nanocrystalline grades.After high-pressure torsion,the aging temperature is reduced,the aging time is shortened,and the second phase?-Mg₁₇Al₁₂2 phase of long rods and lumps in the solution state become elliptic and granular,becoming finer,more evenly distributed,and the microhardness increased significantly,which is 42%higher than the solid solution aging treatment,to 128 HV.High pressure torsion can promote the precipitation and dissolution of the second phase,and change the morphology of the?-Mg₁₇Al₁₂2 phase precipitated by subsequent aging treatment,and significantly increase the microhardness of the solution-aged and aged treatment state,but it has a smaller effect on the hardness of the solution-only treatment.