Simulation Study On Microstructure And Properties Of Mg Alloys With Different Alloying Elements During Quenching

Magnesium alloys are widely applied for their characteristic properties.The unique alloys are applied as aeronautic and astronautic materials as well as structura l parts in lightweight design for their specific strength and specific stiffness,as electronic and electrical components for the electromagnetic shielding and electro conductibility,as bio-implants such as bio-stents and biodegradable screws for the bio-degradability and nontoxicity.Solidification is the first step of metal formation,and metastable structures formed in the non-equilibrate process sometimes show unique properties.However,there are not enough and systematic research about the melt structure and solidification process of Mg alloys.Therefore,Molecular Dynamics?MD?simulations are performed to shed light upon the effects of different adding elements on the microstructures and mechanical property of Mg alloys.Firstly,MD simulations are performed to simulate the effects of adding trace elements on the evolution and final structure of Mg alloys.Mg_98.5M_1.5?M=Cu,Y,Zn?are rapidly solidified form 1100K to 300K under two different cooling rates,10¹¹K/s and 10¹²K/s.Results show that the final structures of the three alloys are close to each other when the cooling rate is controlled.At 10¹²K/s,the final structures are all metal glass.In the three systems,there is not one type prominent cluster and all the clusters are less than 5at.%,but the types and percentages of the main clusters are similar.And the amorphous clusters have numerous 1551 bond type,which is five-fold in short range.At 10¹¹K/s,the three structures are FCC matrix with HCP as stacking fault.However,g?r?curves and structural evolution results show Mg _98.5Y_1.5 and Mg_98.5Zn_1.5 enjoys more similarity in structure configuration.When r>9?in g?r?curves,the trends of Mg_98.5Cu_1.5 are no longer close those of Mg_98.5Y_1.5 and Mg_98.5Zn_1.5,although the valleys and peaks of g?r?curves are almost same as each other's when r<9?.An interesting phenomenon is that the HCP stacking faults in Mg_98.5Cu_1.5 are odd levels,but in Mg_98.5Y_1.5 and Mg_98.5Zn_1.5 are even levels.Adding elements influence the structures through influencing solidification orders.In the three systems,Mg elements separate out from the melt and form the crystal nuclei,resulting in the accumulation of solute elements in melts.In Mg _98.5Cu_1.5,Mg atoms continue to separate out from the melt and contribute to crystal growth,meanwhile most of the Cu atoms maintain in the non-crystal structures.In Mg_98.5Y_1.5 and Mg_98.5Zn_1.5,solute elements and Mg elements together separate out from the melt and non-crystal structures,thus contributing to the crystal growth,where the solute elements transform into FCC structure.In each solidification model,most solute elements are in non-crystal structure,a small part in FCC and an ever small part or none in HCP.Secondly,the solidification processes of Mg-Ti alloys are simulated by MD method to explore the effects of Ti content on the non-equilibrium structures and mechanical properties of immiscible Mg-Ti alloys.Mg_100-XTi _X?X=1,10,20,30,40,50?are rapid-solidified under the cooling rate of 10¹¹K/s and then stress-strain fluctuation method are applied to calculated the elastic constants.Results show that the contents of Ti largely influence the solid-state structures and elastic constants.As Ti content is less than 10at.%,the structures are mainly FCC with minor HCP.By contrast,as Ti content is in the range from 20at.%to 50at.%,the structures are BCC.The mechanical properties are highly related to structure,i.e.similar structures with close values of elastic constants.The Young's Modulus E and Shear Modulus G of the alloys with FCC and HCP structures are close 40 GPa and 15 GPa,respectively,compared to those of BCC structures,which are close 70 GPa and 25 GPa,although all the six alloys show ductility.The degree of crystallization is also related to the Ti content,but the relation between them shows non-linearly.Through the tracking of structural evolution,the average size of BCC cluster is 26 atoms at nucleation,far less than the size of thermal stabilization.That is due to the existence of numerous free basic BCC clusters in the system,and the possible critical nuclei size should be 203 atoms formed by 47 basic clusters.