Molecular Dynamics Simulation Study On Medical Magnesium Alloys During The Solidification Process

In this dissertation,molecular dynamics simulations have been performed concerning microstructure evolution of magnesium alloys,which are applied in medical use,and the connection between microstructure and macroscopic mechanical properties during their rapid quenching.Firstly,the dissertation overviews the application and progress of biomedical magnesium and its alloys and the relevant computational work as well as the Molecular Dynamics methods.On top of that,several analytical methods,E-T?Energy versus Temperature?curve,PDF?Pair Distribution Function?,H-A index method,CTIM?the types of clusters index method?,and 3-D visualization are applied to figure out the law of microstructural formation and evolution during their solidification.Finally,computational efforts are also paid on calculating the modulus and poisson's ratio of magnesium at room temperature,which the mechanical properties are found to be related to those microstructureMg element,which deserves attention,is the fundamental matrix of Magnesium alloys.Thus the evolution process of single Mg was systematically during quenching at different cooling rates.At first,the validity of the simulation was verified by the experimental data.Through analyzing,it is found that the meta-sable bcc phase plays an important role in the forming process.Under the lower cooling rate,bcc phase at first emerged in the system as a transition state,then transformed into a more stable hcp and fcc phase.As the cooling rate increasing,bcc phase remained as stable phase at room temperature,in the system co-existed hcp and fee phases.Once the cooling rate reached the critical value,only a small amount of long-range order structure formed in the system.In addition,it is also found that the birth of the short-range ordered structure is directly related to the formation of bcc structure.It is reported that trace element yttrium?Y?is effective in enhancing the strength and creep resistance of Magnesium alloys,whose enhanced properties are required in application of biomedical-use magnesium.So further studies were also performed on the connection between microstructure and mechanical properties when adding Y on magnesium alloy.Through solidification process of Mg-Y alloys?Y content is 1%-5%?,it is found that the composition of Y has a great influence on the microstructure of this magnesium alloy.The crystallization degree is relatively higher when the composition of Y was 1%and 2%than that of other compositions.Actually,fee was predominant in the two systems with a few hcp structure.Furthermore,that was also a trend that with the increase of Y,the amount of fcc decreased and,by contrast,the percentages of hcp and intermetallic compound increased.When the composition of Y reached 5%,only a small amount of long-range order structure exited in the system.In addition,adding Y atoms is found to facilitate the formation of amorphous structure and metal compounds.From the perspective of thermodynamics,with the increasing content of Y,the onset temperature of crystallization decreased and the range of crystallization temperature increased gradually.As for the mechanical performance,the alloy with I%is calculated to be of great strength and brittleness;then with the increase of Y atom,the strength of the material decreases gradually,by contrast,plasticity and toughness increase gradually.By comparing and analyzing the relationship between microstructures and mechanical properties of the five kinds of alloy,it was found that the fcc phase play an important role in terms of material mechanical properties.In conclusion,the composition and distribution of the microstructure have vital influence on the mechanical properties of material.Layer plate structure and twinning explained why Y atom content of 1%has the highest strength and maximum brittleness.Finally,simulation studies were also performed on the microstructural evolution of other four alloys,which are Mg-Y,Mg-Zn,Mg-Al and Mg-Cu consecutively.It is found that different alloys formed different structures in room temperature.The predominant structure of three kinds of alloy,Mg99Y₁,Mg99Zni and Mg99Cu₁,is fcc while hcp is complementary.By contrast,in Mg99Al₁ alloy,metastable bcc emerged at first and ultimately transformed into a more stable hcp structure.Furthermore,considering atom radius and atom mass of solvend elements,it was clearly seen that relative radius and mass of Mg atom is close to that of Al,which cause less distortion.As a result,structure of Mg99Al₁ alloy at room temperature is similar to that of single Mg.As for the other three alloys,the atomic parameters of the solvend atoms differ from that of Mg atoms so that fcc instead of hcp formed at room temperature.At last,it is about the mechanical performance,whose results showed that Mg99Y₁ alloy had high strength and brittleness while Mg99Cu₁ alloy has great plasticity and toughness.Still,the properties of the other two alloys,Mg99Zn₁,Mg99A₁ were mediocre,suggesting that Zn and Al contributed little to strengthening the Mg alloys.Comparing the microscopic structures and macroscopic properties,it can be clearly found that Mg99Y₁ alloy microstructure is very neat with obvious twinning structure as enforced phase,thus the Mg99Zn₁ alloy have the greatest strength and brittleness.By contrast,in Mg99Cu₁ co-exist multiple structures,such as fcc,hcp and short range order structure,which impede the forming of coarse crystal so that the Mg99Cu₁ alloy have good plasticity and toughness.