Glass Forming Ability And Molecular Dynamics Of Mg-Zn-Ca Alloys For Biomedical Applications

Mg-Zn-Ca amorphous alloys are regarded one of the most promising biodegradable medical materials exhibit closer densities and elastic modulus to those of human bone,as well as good biocompatibility and degradability.The glass forming ability（GFA）of Mg-Zn-Ca alloys are closely related to their chemical compositions.However,at present the development of Mg-Zn-Ca alloys with high GFA is mainly through trial and error,which is time-consuming and labor-intensive.More importantly,the essential factors especially from the structural issue of liquid-solid transition process affecting the GFA of Mg-Zn-Ca alloys still are not exploited.In this paper,the pair distribution function（PDF）,fragility index m,the fraction of diverse type of icosahedral cluster and coordination number（CN）of Mg_45+xZn_50-xCa₅alloy were calculated based on molecular dynamics（MD）.The glass forming ability（GFA）of Mg_45+xZn_50-xCa₅alloy was predicted using the fragility index m.The accuracy of the fragility index m in predicting the GFA was verified by experiments.The structural factors affecting the GFA were analyzed in terms of icosahedral cluster morphology,surface area and network connectivity.A structural parameterΦ_IMROfor icosahedral clusters was proposed to evaluate the GFA of Mg_45+xZn_50-xCa₅.On this basis,the fragility index and the structural parameterΦ_IMROwere calculated for the Mg_45+xZn_45-xCa₁₀（x=5,10,15,20,25,at%）system alloys to evaluate the GFA of this system,and finally,the validity and correctness of GFA parameters were evaluated experimentally.The main conclusions are as follows.（1）A smaller fragility index m,and a larger value of thermodynamic parameterγ,indicate a greater GFA.Molecular dynamics calculations results show that,the order of the m index from smallest to largest is Mg₆₀Zn₃₅Ca₅65Zn₃₀Ca₅50Zn₄₅Ca₅70Zn₂₅Ca₅55Zn₄₀Ca₅in the Mg_45+xZn_50-xCa₅alloy system.While the sequence for thermodynamic parameterγcalculated from DSC from large to small is:Mg₆₀Zn₃₅Ca₅>Mg₆₅Zn₃₀Ca₅50Zn₄₅Ca₅70Zn₂₅Ca₅55Zn₄₀Ca₅。Mg₅₅Zn₄₀Ca₅.Mg₆₀Zn₃₅Ca₅alloy possesses the largest m and the lowestγvalue,suggesting the highest GFA,which is indicated that m parameter could evaluated the GFA concisely.The icosahedral clusters and the medium range ordered structure formed based on icosahedral clusters are the main structural factors that most influence the GFA.GFA of the studied alloy increases with the content of icosahedral clusters.However,the fraction of icosahedral clusters in Mg₆₀Zn₃₅Ca₅,Mg₆₅Zn₃₀Ca₅and Mg₇₀Zn₂₅Ca₅is very close to each other,it is difficult to distinguish the GFA using the fraction of icosahedral clusters.A structural parameterwere proposed by distinguishing the relationship（2）between the morphology,surface area as well as connectivity of the medium-range ordered structures and the GFA,to evaluate the glass forming ability:Φ_IMRO=（?）·A_s/β（whereβis the scale factor（β=10000）;indicates the average number of filled atoms forming IMRO per cluster center atom of IMRO;A_sis the surface area of the cluster）.The larger the value ofΦ_IMRO,the better the glass forming ability of the alloy.（3）The PDF,type and fraction of cluster,elemental coordination number and fragility index m of the Mg_45+xZn_45-xCa₁₀（x=5,10,15,20,25 at%）system alloys were obtained by molecular dynamics calculations.On this basis,the structural parametersΦ_IMROwere calculated and the glass forming ability of the alloy was predicted.The results show that theΦ_IMROis in the order of smallest to largest is Mg₇₀Zn₂₀Ca₁₀65Zn₂₅Ca₁₀60Zn₃₀Ca₁₀55Zn₃₅Ca₁₀50Zn₄₀Ca₁₀.The order of the m index from smallest to largest is Mg₇₀Zn₂₀Ca₁₀>Mg₆₅Zn₂₅Ca₁₀>Mg₆₀Zn₃₀Ca₁₀>Mg₅₅Zn₃₅Ca₁₀>Mg₅₀Zn₄₀Ca₁₀.The variation ofΦ_IMROand m demonstrates the GFA of Mg_45+xZn_45-xCa₁₀（x=5,10,15,20,25 at%）alloys increases with decreasing Mg content.The trend of thermodynamic parameterγwith alloy composition is the same as that ofΦ_IMRO.In other word,the glass forming ability of alloy increases with increasing Zn content.It is because that the Zn atom content has a greater influence on the average number of filled atoms and the cluster area of the intermediate range ordered structure.Icosahedral clusters centered on Zn atoms are able to form a more connected intermediate range ordered structure,thus improving the GFA.