Microstructure Evolution Behavior Of Directionally Solidified Al-1.2 Wt.%Fe Hypoeutectic Alloy Under High Magnetic Fields

Directional solidification technique achieves control of solidification structure by controlling solidification rates(pulling rates)and temperature gradient,thereby obtaining materials with special orientation and excellent properties,which have attracted great attention on industrial production and theoretical research.In the solidification process,the application of high magnetic field can control the melt flow,change the solute distribution and induce crystal orientation,and ultimately achieve the regulation of solidification structure.The combination of high magnetic field and directional solidification provides a new way to study solidification theory,improve material properties and develop new materials.However,the mechanism of action of directional solidification on structure of high magnetic field is relatively complicated.and the related theoretical research still needs to be deepened and improved.In this thesis,Al-1.2 wt.%Fe hypoeutectic alloy was chosen to be studied under magnetic field by directional solidification.The effects of pulling rates and magnetic flux density intensity on the evolution of solidification structure,the stability of solid/liquid interface and crystal orientation were investigated by means of optical microscopy,Xray diffraction,scanning electron microscope and electron backscattered diffraction.Firstly,the effects of magnetic flux density and pulling rates for the microstructure evolution of directional solidification stability zone were studied.Under the condition of no magnetic field,the primary ?-Al phases are aligned directionally along the solidification direction.With the increasing of pulling rates,the solidification structure undergoes celluar/dendritic transformation.The primary dendritic arm width and spacing of primary ?-Al gradually decrease to the increasing of the pulling rates,and the uniformity of microstructures distribution enhances.The application of magnetic field induces the instability of solidification structure,changes into morphology of primary dendrite and Fe-rich phase,and decreases primary dendritic spacing.When the pulling rates are constant,the increase in magnetic flux density decreased the instability of solidified structure,and the decreasing trend of dendritic spacing of ?-Al is gradually weakened.This is the result of the lorentz force and thermal electromagnetic force induced by strong magnetic field on the convection competition during directional solidification;The effect of magnetic field on solidification structure decreases with the increasing of pulling rates.When the pulling rates are more than 100 ?m/s,the morphology of solidification structure does not changed significantly.In the study of the evolution of the solid/liquid interface and mushy zone of Al1.2 wt.%Fe hypoeutectic alloy under high magnetic field.It was found that the increasing of pulling rates decrease the stability of the solid/liquid interface and induce the transformation of the solidified structure of the interface from celluar/dendrite mixed structure to regular dendrite growth without magnetic field.The length of the mushy zone increases to the increasing of the pulling rates,and the increasing trend is gradually weakened.The application of 1 T magnetic field induces the instability of solid-liquid/interface,and the effect decreases to the increasing of the pulling rates.When the pulling rates are less than 10 ?m/s,the solid/liquid interfaces to become uneven and the regular growth cellular/dendrite structures destroyed.When the pulling rates are greater than 10 ?m/s,the solid/liquid interfaces does not change,and the secondary dendrite promoted by the application of magnetic field.The application of magnetic field promotes the growth of secondary dendrite and aggravates the cracking of dendrite tip.The effects of pulling rates and magnetic flux density on crystal orientation during solidification were studied.It was found that the orientation of {110}?-Al//(010)Al₃Fe existed in the solidified structure.The orientation of ?-Al phase grew in the direction of<100>and the orientation of ?-Al phase couldn't be changed by the application of magnetic field.At 5?m/s,Al₃Fe phase forms texture along[010]direction driven by heat flow,which disappears when the pulling rates or magnetic field conditions change.