Thermal analysis of directional solidification in material processing

Unidirectional solidification techniques are often used to produce uniform and grain-aligned microstructures in crystalline materials. The thermal conditions near the solid-liquid interface have been known to have a strong influence on the microstructure, grain size and mechanical properties of the materials. Misaligned grains and high angle grain boundaries may result from the presence of a radial temperature gradient due to the latent heat of fusion. In this paper, we study the relationship between grain orientations and processing parameters, in an attempt to identify the thermal configurations for producing the required microstructures. We also study the utilization of an auxiliary heat near the interface to improve grain alignments. A boundary value problem is formulated to describe the heat transport in directional solidification using the Bridgman technique. Included in the interface boundary conditions are various factors that affect the interface shape but are neglected in some previous works, such as the coupling effect of temperature and solute concentration, as well as the dependence of interfacial free energy on the interface curvature through the Gibbs-Thomson effect. The governing equations are solved numerically for several material systems. The variations of interface position and shape in processing parameters such as furnace temperature, sample translation velocity,more » and heat transfer coefficients are examined. It is found that the radial temperature gradients are reduced with the interface becoming less curved when the auxiliary heater is introduced. The present results are compared with previous works and experimental measurements.« less