Study On The Preparation Technique And Compressive Creep Behaviors Of Micro-alloyed Magnesium Alloys

Under the urgent needs of resource-saving and environmentprotection,it has caught a lot of attention to develop micro-alloyed magnesium alloys with low cost and high performance.In this study,the compression ductility and creep resistance of micro-alloyed Mg-0.4Mn and Mg-0.5Y(at.%)alloys were optimized by low-temperature forging.To discover the contributors of the improved performances,the deformation mechanisms and microstructure evolution during compression and compressive creep were studied through microstructure characterization.The effects of grain size,twinning and dislocation substructure on the compressive creep behaviors were revealed.The compressive performances of a Mg-0.4Mn extruded bar were improved by low temperature forging,especially compressed at high strain rate,the absorption energy of the forged sample increased to 1.35 times of that of the extruded state sample.Lowering the forging temperature and speed could impede grain growth and cracking during the compression process and achieve a large degree of deformation.The microstructure evolution during compression under different strain rates was characterized via various methods.The reasons for the improved performance after forging include:(1)At low strain rates,the increase and expansion of the shear bands took up most of the strain,thus a low strain hardening rate was maintained and no fracture occurred;(2)As the strain rate increased,the deformation was dominated by a large amount of basal slip.The density of<c+a> dislocations was reduced,so that dislocation movement was hardly obstructed by their frequent decomposition,and basal dislocations could move to grain boundaries and be absorbed,promoting continuous deformation.The effect of grain size on the room-temperature compressive creep behavior of Mg-0.4Mn alloy was investigated,and grain size gradients were obtained by different recrystallization annealing processes.The results showed that its compressive yield strength decreases with increasing grain size and their relationship is in accordance with the HallPetch formula.However,the creep resistance was improved with the increasing grain size.When the grain size is smaller than 2.5 μm,grain boundary sliding seriously damaged the creep resistance.When the grain size increases form 2.5 μm to 14.6 μm,the steady creep rate decreased rapidly and the creep process was dominated by dislocation movement.When the grain size is larger than 14.6 μm,dislocation movement and twinning controlled the creep process.And the creep resistance was enhanced to a limited extent,while the room temperature mechanical properties were severely depleted.The creep strain of the first stage increased sharply when the applied stress was higher than the yield strength.In the case of serving at elevated temperature,adding rare earth(RE)elements can improve the creep resistance of magnesium alloy significantly.Taking the trade-off between the performance enhancement and cost-trimming into account,a dilute Mg-0.5Y alloy was applied to optimize its compressive creep resistance through room temperature forging.It was determined that the creep mechanisms of the samples before and after forging were both dominated by basal slip.In addition,the creep resistance of the annealed samples was deteriorated by twinning and crossslip,while pyramidal <c+a> slip was activated in forged samples.Besides,it was found that when twinning was preset in the samples before creep,detwinning happened and cross-slip was retarded,so that the creep resistance was enhanced.While twins formed during creep could not hinder dislocation movement and would gradually broaden,resulting in a large creep deformation.