Phase-field Study Of The Metallic Solidification In Complex Conditions

Solidification of metal alloys is a complex phase-transition process under variouscomplex conditions, including thermodynamics (phase equilibrium, phase interface,chemical equilibrium, and eta.), kinetics (solute redistribution, nucleation, growth, and eta.)and various kinds of transformation phenomenons (heat, solute, convection, and eta.), andalso, most solidifications take place at high temperature level, and these all lead todifficulties of experimental studies of the solidification process, which make modeling andsimulation methods become the cost-effective means to study the solidification. Phase-fieldmethod, as one of most popular modeling and simulation methods to study phase transitions,has been widely used in studies of solidification in metal alloys. In this thesis, thephase-field modeling and simulations will be carried out to study the solidification of puremetal Ni and Ni-Cu alloys, by introducing lateral constrains, heat flux at boundaries andliquid metal flow, the solute and heat transfer during solidification as well as themicrostructure evolution during solidification will be investigated, which will give aninsight of the solidification control in industries. The main conclusions are listed as follows:1. Dendritic growth in Ni pure metal with the presence of lateral constrains isinvestigatred using phase-field simulations, and results indicate that lateral constrains inpresence of melt has a significantly effect on microstructure evolution of crystal growth ofpure Ni metal., and different properties of lateral constrains have different effects on themicrostructure evolution.2. Lateral constrains of different shapes lead to different changes to the dendrite armspacing developed after constrains during dendritic formation in pure Ni: constrains oftriangle with sharp corner at the bottom have the most significant influence on the dendritespacing changes, and rectangle and triangular constrains with its sharp corner above show acontrolling effect on the dendrite arm spacing; when constrains of trapezoid is introduced,the dendrite arm spacing is determined by changing the size of the hemline of constrains.3. The present of lateral constrains during directional solidification has a significanteffect on the microstructure formation of Ni-Cu alloy. Lateral constrains have an influenceon the solute and heat diffusions during the phase transition, which leads to the changes ofsolute trapping and temperature distribution during phase transitions. Different properties of lateral constrains have different effects on the microstructure evolution, and they cansignificantly affect the tip velocity of the dendrite growing through the lateral constrains,which leads to the microstructure formation changes.4. The dendritic growth with different velocities of liquid metal flow is studied usingphase-field method in a Ni-Cu alloy system. Results indicate that: the dendritic structurebecomes asymmetrical in the flow field, and increasing flow velocity can increase thedegree of this asymmetry. The liquid flow also has an effect on the solute segregation in thesolid/liquid interface, and it rises the temperature during solidification, which makes thetemperature distribution very asymmetrical, too. The maximum, minimum and averagetemperatures of the system are all raised with the increase of the flow velocity, but thetemperature in the upstream is always higher than that in the downstream.5. The effect of heat input/extraction from boundaries on the dendrite structureforming process in Ni-Cu alloy is studied using the phase field method. Results indicate that:heat flux input can raise the temperature near the boundary, and results in the dendrite armgrowing and side-branching prevented, while heat flux extractions form boundaries canenhance the dendrite arm growth and side-branching. The heat flux can also lead to theconcentration distribution changes by changing the temperature distribution. With heatextraction form boundaries at the particular level, a uniform distribution in dendrite patternand solute distribution in the interface can be achieved because the latent heat release fromliquid-solid phase transition can be partly counterbalanced by the heat extractions.6. The columnar structure formation of Ni-Cu alloy during directional solidificationwith heat flux at different boundaries is simulated and studied by the non-isothermalphase-field model for binary alloys. Results indicate that; heat flux input from boundariesparalleling to columnar growing directions can significantly raise the temperature at theinterface, which results in the columnar growing restrained with enhanced solute diffusion,while with heat extractions from these boundaries, columnar near boundaries is speeded upwith low solute segregation and these all leads to irregular columnar structure formations.At the same time with heat input/extraction from other boundaries, typical columnarstructure is still achieved. With temperature changing with time, the solute inside columnararms shows an irregular distribution via time.7. The microstructure formation during solidification of a Ni-Cu alloy is investigatedusing the non-isothermal phase field model coupling with forced liquid metal flow as wellas the boundary heat flux. Results indicate that: large heat flux at boundary can significantlyinfluence the morphology, concentration and temperature distributions, which relatively makes the effect of liquid flow constrained. a dendrite-columnar-transition map is achievedwith all the simulations to give an enlightenment of macrostructure selection duringsolidification with initial liquid metal flow and heat extractional flux at the boundary: withsmall heat extraction flux and large velocity of initial liquid flow, the secondary dendritearms grew along the initial liquid flow direction, and this dendritic microstructure formationwas enhanced in the upstream region and suppressed in the downstream region; while withlarge heat extraction flux and small initial velocity of initial liquid flow, secondary dendritearms changed their direction to the temperature gradient direction and the microstructurebecame columnar, and when the heat extraction was large enough, the steady state shouldbe columnar structure, but with extremely large heat extraction, no secondary dendrite armscould be found, and the microstructure formed into a flat; there was also a transition statewith mix-structure of secondary dendrite arms growing in these two directions, and withincrease of initial velocity of initial liquid flow, this transition state could be move to thelarge heat extraction direction with the scale of this state increased.