Study On The Microstructure Evolution Of Mg-RE Alloys During Solidification:the Phase-field Simulation And In-situ X-Ray Radioactive Characterization

With the increasing demand for lightweight,energy-saving and emission-reduction from industry,magnesium-rare earth?Mg-RE?alloys as the lightest structural metal,owing to its high specific strength,good resistance characteristics and excellent machining property,has received great attention in the field of aviation,aerospace,marine,military and automotive.Studying the microstructure evolution rules and patterns during solidification is the corner stone for precise control and further imoproment of material's mechanical properties,because the microstructure formed in casting process always plays a key role in determining the mechanical properties of alloys.In this work,the microstructure evolution during solidification in Mg-6Gd?wt.%?alloys is studied using the phase-field method,taking into account the effect of thermal diffusivity,undercooling,cooling rate and temperature gradient.A polycrystalline quantitative phase-field model for dilute solution of binary alloys has been established to systematically study the evolution path of the equiaxed and columnar dendritic growth during solidification with a discussion of the influences of phase-field parameters such as the noise coefficient,spatial step and anisotropy intensity.In addition,the evolution of dendritic morphology and growth are further studied from experimental point of views.The real-time dynamic process of solidification has been observed and recorded using in situ synchrotron radioactive technique.Through the comparison of phase-field calculations and experimental results,the effect of some factors which could not be taken into account in convential experiment and simuliation has been revealed,and more in-depth understanding of the microstructure evolution in Mg-Gd alloys are obtained.The major work and conclusions are listed as follows.a)The single dendritic growth in isothermal solidification of Mg-Gd alloys has been studied using the quantitative phase-field model developed by Karma.The anisotropy intensity,noise coefficient and spatial step have effect on the morphology of dendrite growth.Simulation results indicated that with the increase of anisotropy intensity at interface the non-steady behavior of dendrite tip is more intense,and the secondary dendrite arm appears more easily.The dendrite tip velocity(1₄)increases linearly with the increase of the anisotropic strength,while the tip radius₄)decreases in the exponential parabola form.The larger the noise coefficient is,the more unstable the dendrite tip becomes,and the branching and secondary dendrite emerge more easily.The selection of the spatial step should be controlled within the range of 0.2?2₀⁰.8?2₀.Steps too large will cause changes in dendritic morphology,and too small will lead to unacceptable computation costs.b?In order to study the evolution of multi dendritic growth,a quantitative phase-field model for polycrystalline solidification in dilute solute binary alloys is developed.It is found that the dendrite arm deforms and bends due to the competition of dendrites growth.The solute enriches among dendrites,which also inhibits the dendrite growth.The phase-field simulation results are compared with classical JMAK theory and Starink phenomenological theory from dendrite growth dynamic point of view.It shows that the simulated results are in good agreement with prediction of the Starink phenomenological theory,while the JMAK theory fails to describe dendritic growth behavior because of the negligence of the blocking effect and the soft collision effect among multiple dendrites.c?The single and multiple dendrites growth in non-isothermal solidification have been studied by establishing a polycrystalline phase-field model that coupled with temperature field.The larger the thermal diffusion coefficient presents,the faster the solidification latent heat is released,as well as higher dendrite growth rate and larger dendrite size.The driving force of solidification increases by enlarging the undercooling,which also leads to the increase of dendrite growth rate and the solid volume fraction,and more rapid release of solidification latent,as well as higher degree of solute enrichment at solid-liquid interface and dendritic intervals.d)The columnar dendrite growth has been investigated using the modified Karma phase-field model for directional solidification.Larger anisotropy results in smaller columnar dendrite radius,faster columnar dendrite growth.Under the condition of fixed temperature gradient,both the radius of columnar dendrite tip and the solid volume fraction increase by increasing the cooling rate,which also leads to higher interface velocity and faster dendrite growth.And the mushy zone increases with the increase of cooling rate,while both the columnar dendrite tip zone and the primary dendrite spacing decrease.Under the condition of fixed cooling rate,both the radius of columnar dendrite tip and the solid volume fraction increase by increasing the temperature gradient,while the interface velocity and dendrite growth rate have no obvious change.And the mushy zone increases with the increase of temperature gradient,while both the columnar dendrite tip zone and the primary dendrite spacing decrease.In addition,different angles between dendrite preferred orientation and heat flow direction have been studied.The results shows that the columnar dendritic morphology changes as the angle varies.The primary dendrite spacing and the number of secondary dendrite arm increase with the increase of the angle,while the interface velocity decreases.e)The real-time process of solidification of Mg-6Gd?wt.%?alloys has been obtained under a fixed temperature gradient?=5K/mm?using the synchrotron X-ray radiographic technique.The effect of different cooling rates on the transition of dendrite morphology were semi-quantitatively analyzed.Results show that equiaxed dendrites,including exotic‘butterfly-shaped'dendrite morphology,dominate at high cooling rate?>1 K/s?.Different from the equiaxed dendrite,solute segregates at the center of two long dendrite arms?LDA?for the‘butterfly-shaped'dendrite case.When the cooling rate falls in the range of 0.5 K/s¹ K/s,the equiaxed-to-columnar transition takes place.When the cooling rate is lower than0.5 K/s,directional solidification occurs and the columnar dendritic growth direction gradually rotates from the crystalline axis to the thermal gradient direction with the increase of cooling rates.The floating,collision and rotation of dendrites under convection have been characterized and studied.f)The effect of low cooling rate?0.033⁰.25K/s?has been investigated by coupling the phase-field method and in situ synchrotron X-ray radiographic technique.It is found that the primary dendrite spacing decreases with the increase of cooling rate,and the columnar dendritic arrangement becomes more compact.In addition,the simulation shows that both the interface moving velocity and the transferred solid volume fraction increase with the increase of cooling rate.The simulation shows good agreement with the experimental results.