Simulation of facet dendrite growth with strong interfacial energy anisotropy by phase field method

Numerical simulations based on a new regularized phase-field model were presented, to simulate the solidification of hexagonal close-packed materials with strong interfacial energy anisotropies. Results show that the crystal grows into facet dendrites,displaying six-fold symmetry. The size of initial crystals has an effect on the branching-off of the principal branch tip along the<100> direction, which is eliminated by setting the b/a(a and b are the semi-major and semi-minor sizes in the initial elliptical crystals, respectively) value to be less than or equal to 1. With an increase in the undercooling value, the equilibrium morphology of the crystal changes from a star-like shape to facet dendrites without side branches. The steady-state tip velocity increases exponentially when the dimensionless undercooling is below the critical value. With a further increase in the undercooling value, the equilibrium morphology of the crystal grows into a developed side-branch structure, and the steady-state tip velocity of the facet dendrites increases linearly. The facet dendrite growth has controlled diffusion and kinetics.

Effect of interface anisotropy on tilted growth of eutectics:A phase field study

Interfacial energy anisotropy plays an important role in tilted growth of eutectics. However, previous studies mainly focused on the solid-solid interface energy anisotropy, and whether the solid-liquid interface energy anisotropy can significantly affect the tilted growth of eutectics still remains unclear. In this study, a multi-phase field model is employed to investigate both the effect of solid-liquid interfacial energy anisotropy and the effect of solid-solid interfacial energy anisotropy on tilted growth of eutectics. The findings reveal that both the solid-liquid interfacial energy anisotropy and the solid-solid interfacial energy anisotropy can induce the tilted growth of eutectics. The results also demonstrate that when the rotation angle is within a range of 30°-60°, the growth of tilted eutectics is governed jointly by the solid-solid interfacial energy anisotropy and the solid-liquid interfacial energy anisotropy;otherwise, it is mainly controlled by the solid-solid interfacial energy anisotropy. Further analysis shows that the unequal pinning angle at triple point caused by the adjustment of the force balance results in different solute-diffusion rates on both sides of triple point. This will further induce an asymmetrical concentration distribution along the pulling direction near the solid-liquid interface and the tilted growth of eutectics. Our findings not only shed light on the formation mechanism of tilted eutectics but also provide theoretical guidance for controlling the microstructure evolution during eutectic solidification.

Formation mechanism of α lamellae during β→α transformation in polycrystalline dual-phase Ti alloys

Phase field simulations incorporating contributions from chemical free energy and anisotropic interfacial energy are presented for theβ→αtransformation in Ti-6 Al-4 V alloy to investigate the growth mechanism ofαlamellae of various morphologies from undercooledβmatrix.Theαcolony close to realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters ofαandβphases with the phase field governing equations.The simulations show thatαlamellar side branches with feathery morphology can form under a certain combination of interfacial energy anisotropy and temperature.αlamellae tend to grow slowly at high heat treatment temperature and become wider and thicker as temperature increase from 800 to 900℃provided that the interfacial energy anisotropy ratio k_(x):k_(y) was set as 0.1:0.6.Besides,higher interfacial energy anisotropy can accelerate the formation ofαlamellae,and the equilibrium shape ofαlamellae changes from rod to plate as the interface energy anisotropy ratio k_(x):k_(y) vary from 0.1:0.4 to 0.1:0.8 under 820℃.Experiments were conducted to study theαlamellar side branches in Ti-6 Al-4 V(Ti-6.01 Al-3.98 V,wt.%)and Ti-4211(Ti-4.02 A1-2.52 V-1.54 Mo-1.03 Fe,wt.%)alloys with lamellar micro structure.Electron backscatter diffraction(EBSD)re sults show thatαlamellar side branches and their related lamellae share the same orientation.The predicted temperature range forαlamellar side branches fo rmation under various interfacial energy anisotropy is consistent with experimental results.