Study On The Dynamic Behaviors Of Iron-based Metallic Glass Forming Liquids With Rare-earth Element Addition

Metallic glasses are new advanced material developed in recent decades due to its excellent mechanical,magnetic properties and good corrosion resistance.At present,it has been widely used in many fields such as amorphous transformer,information and electronic devices,sports equipment,military technology and so on.Compare with traditional alloys.metallic glasses are more closely related to the structure and properties of melts.Therefore,studies on properties of glass forming liquids not only help to further understand the nature of glass transition and uncover the liquid-solid heredity,but play important roles on the development of new bulk metallic glasses.Considering that the importance of rare-earth element on enchancing the glass forming ability of iron-based alloys,this work investigated systematically the dynamic features of iron-based melts with rare-earth element addition,and further uncovered the correlation between dynamic features and glass forming ability in iron-based alloys.The main contents and conclusions of this paper are as follows:(1)The dynamic behavior of the iron-based melts containing rare earth elements has been investigated well above TL.In the iron-based melts without rare earth element.a distinct transition in viscosity changes during the cooling process has been found.There is no obvious dynamic transition for iron-based liquids containing rare earth elements and the good glass forming ability.By using fragility parameter M of super-heated liquid approaching TL,the degree of these dynamic transitions has been quantified by F values.The negative correlation between the degree of dynamic transition in melts and the GFA of iron-based alloys has been established.When the degree parameter of the dynamic transition F in alloy melts exceeds(or is equal to)1,the GFA of the alloy is low.In this scase,the clusters in melts tend to transform into crystal-like clusters with larger size during the dynamic transition,and the melt changes from fragile at HTZ to strong at LTZ.When F is less than 1,the clusters in HTZ is inclined to decompose into the SROS clusters with smaller size,and the melt changes from strong to fragile.A smaller Fin iron-based melts suggests a more distinct dynamic transition towards the glass formation and less exothermic effect in the rare earth-contained iron-based glass-forming liquids.This is important for us to find and predict the origin of the bulk metallic glasses(2)Based on the above findings,we further systematically investigated the viscosity changes of different iron-based melts after the additions of rare-earth elements It is found that the change of dynamic perperties is obvious during the cooling process in the iron-based glass forming liquid without rare earth element.This obvious dynamic transition becomes less apparent when some rare earth elements were added to improve the glass forming ability.By using fragility parameter M of super-heated liquid approaching TL and further the values of F,it is found that there is a negative correlation between the F value of iron-based glass forming liquid and the glass forming ability of iron-based alloy.With the dynamic transition F=1 as the boundary,F greater than 1 indicates poor glass forming ability,while F less than 1 indicates strong glass forming ability.When the added rare earth elements replace the main elements(Fe or FeCo),they must show negative mixing enthalpy with the main elements to enhance the glass forming ability.It can be seen from SEM and EDS analysis that when the added rare earth elements and the main elements exhibit a positive enthalpy of mixing,the rare earth elements tend to collect in a phase.This work uncovers the dynamic features of bulk iron-based metallic glass forming melts with rare earth elements addition,and provides new idea for predicting the glass forming ability of iron-based alloys from high temperature melts.