On The Mechanisms Of Grain Boundary Evolution In Magnesium Alloys

Magnesium alloys,featuring light weight,high strength and sufficient deposit,have been widely applied in aerospace,automobile,microelectronics,biomedicine and other industries.However,due to the higher occurence in basal slip than in non-basal slip as magnesium crystal is under loading,magnesium alloys have poor formability which obstacles its further application.Consequently,studies on alloy-mediated defect control address comprehensive interests.On the other hand,the role of twining in performance enhancement of materials has been increasingly valued in the field of nanometallic materials nowadays,which pronounces the significance of study on the mechanisms of grain boundary evolution in magnesium alloys since twins are diverse and the interactions between twins are complicated in magnesium.Inadequate attention has been paid to the evolution mechanisms of {11(?)1},{11(?)2} and other twin boundaries in magnesium as well as low alloying defect control,these be focuses of our study.The work done using molecular dynamics is as following in order to study the evolution mechanisms of several typical twin boundaries in magnesium under multiple working conditions and alloy-mediated control of grain boundary evolution.First,the shear loading tests of {11(?)1},{11(?)2},{11(?)4} and {11(?)6} grain boundary specimens were simulated respectively,the coupling factors of them calculated and the critical ressolved shear stresses of them compared.The coupling factors of {11(?)1},{11(?)2} and {11(?)4}grain boundaries,being attributed to shear dominated migration,are positively correlated with tilt angle and positively correlate critical ressolved shear stress.The coupling factor and the critical ressolved shear stress of {11(?)6} grain boundary,being attributed to atom shuffling dominated migration,are remarkably smaller than those of shear dominated migration.We also analysed the influences of temperature on grain boundary migration and it showed that a higher temperature would decline the critical ressolved shear stresses of grain boundaries and induce the change of migration modes of grain boundaries.Further,we analysed the influences of alloying elements on grain boundary evolution with the effects of temperature and normal stress considered,and we discovered that molybdenum showed the strongest pinning effect,that temperature within a moderate range and normal compression helped grain boundaries to unpin,and that excessively high temperature and normal tension impeded grain boundaries from unpinning.Then,the tension tests of the aforementioned grain boundary specimens were simulated respectively.Results showed that {11(?)1} grain boundary was inactive in plastic deformation process under tension,{11(?)2} and {11(?)4} specimens extended with the help of chevron defects,and {11(?)6} specimen extended by the opposite movements of two incoherent grain boundaries.We also analysed the influences of temperature on grain boundary evolution,and we discovered that irregular surface atomic arrangement impeded the nucleation of chevron defects in the {11(?)2}specimen and that relatively high temperature facilitated the nucleation of chevron defects.Further,we analysed the influences of alloying elements on grain boundaries with multiple spatial distributions and concentrations of alloying elements compared,and we discovered that the spatial distributions of alloying elements were far more crucial than their concentrations.Last,the compression tests of the polycrystal magnesium specimens containing the aforementioned grain boundaries were simulated respectively.The numbers of twins for each specimens were predicted geometrically and were compared with the simulation results.It showed that the number of twins tended to increase as the angle increased between the [0001] directions of initial grains and loading direction,whereas the order of twinning can influence the number of twins as well.We also analysed the effects of alloying elements on the evolution of specimens,and we discovered that alloying elements induced changes of the order of twinning.Further,we analysed the influences of temperature on the evolution of polycrystal magnesium,and we discovered that temperature tuned the order of twining and intensified the competition between twinning and basal slip.The above work summarized the influence factors of grain boundary migration,the nucleation of chevron defects and the nucleation of twins in magnesium systematically and comprehensively,which gave insight on exerting the contribution of twins to the performance of magnesium.In addition,it highlined the significance of the spatial distribution of alloying elemnets as for defect control,which shed light on low alloying defect control of magnesium.