Dynamic Behaviors Of Metallic Glass-forming Liquids

As a relatively young branch of materials,metallic glass has attracted considerable interest from researchers within the last few decades,owing to its outstanding mechanical and magnetic propeties.The structural evolution and physcial properties of metallic glass-forming liquids during solidification also are hot issues as well as frontier problems in material science.Study on such issues are of great importance not only for better understanding the nature of fragile-to-strong(F-S)transition and the microstructure inheritance during liquid-solid transformation,but also for improving the glass forming abilitity by utilizing liquid properties and promoting the standardization during the process of production and treatment for metallic glasses.Based on the above,in this project,we focus on investigating the dynamic behavior of superheated melts and supercooled liquids of Cu-based and Fe-based metallic glass-forming liquids(MGFLs).Combined with the thermodynamic tests and computer simulation,we also discuss the correlated structural evolution features.The reseach contents and results are as follows:(1)We study the dynamic behavior of Cu-Zr melts above the liquidus temperature.Different from the tranditional Adam-Gibbs theory,the viscosity of Cu-Zr melts shows three-stage tendency with temperature changing,which is also accompanied with abnormal exothermic peaks on heat flow curves.This phenomenon is attributed to the liquid liquid transition(LLT).We find that for Cu-Zr alloys the LLT is beneficial to glass formation,and there is a close relationship between the relative transition strength(RTS)of the LLT and the critical thickness representing the glass forming ability(GFA):the bigger the value of the RTS parameters,the stronger the GFA of Cu-Zr alloys.This work not only uncovers the role of the LLT of melts on the glass formation of solids,but also sheds light on the inheritance of properties of glassy solids from the aspect of the detectable dynamics of high-temperature melts.(2)By thermodynamic and dynamic appoaches,the liquid fragility of Fe-based MGFLs above the liquidus temperature and near the glass transition temperature was investigated.We calculated the liquid fragility parameter m' and m by MYGEA equation and observed the F-S transition in both FeSiB(Nb)and FeSiBNb(Cu)MGFLs,which is possibly a general behavior of Fe-based MGFLs.By comparing F-S transition extent parameter f and LLT extent parameter F,it has been found that there is a negative correaltion between f and F.The smaller extent of LLT,the larger extent of F-S transition.Such correlation implies that the F-S transition might be dynamic transformation accompanied with the polyamorhic transformation.This discovery helps to understand the dynamic evolution during the whole cooling process from melts to supercooled liquids and gives new sight into the structural origin of F-S transition.