Research On Competitive Grain Growth During Directional Solidification

Grains are the basic elements of metal materials. The orientation of grains and the morphology of grain boundaries have great influence on the material properties. Directional solidification is an effective process to obtain material with high performance along specific directions. Especially, single crystal superalloys manufactured by directional solidification are important materials widely used in aviation engine and turbine blades. In order to get single crystal material, both crystal selection and crystal growth should be considered, while competition growth of grains with various orientations is involved in both these processes. The existing theoretical description of competition grain growth only stays on that the supercooling affects the dendrite tip position and in turn affects the trend of dendrite overgrowth. Unfortunately, it does not take into account the effect of changes of dendrite morphology, dendrite tip shape and primary dendrite spacing when the orientation of dendrites are different. Thus it cannot provide valid explanations to some new phenomena.This paper uses a succinonitrile-acetone transparent model alloy to investigate morphologies of dendrite with different orientations under directional solidification. The effect of solidification parameters including pulling velocity and forced convection on single crystal growth and bi-crystal grain competition is studied. The purpose of this paper is to provide some beneficial supplement to the competitive grain growth theory. Some main findings and conclusions are listed below. 1. Directional solidification of single crystal with different orientations is investigatedThe difference of dendrite morphology, primary dendrite spacing, secondary dendrite arm spacing between favorably oriented(FO) dendrites and unfavorably oriented(UO) dendrites are studied. It is found that when the dendrite orientation changes, the tip shape is still symmetric and can be fitted well with a parabola. The axis of symmetry is parallel to the crystallography preferential growth direction. Compared to the dendrites whose preferred orientation is along the heat flow direction, the dendrites who grow deviating from the heat flow direction have lager primary spacing. 2. Competitive grain growth is investigated under diffusion conditionsBoth converging and diverging bi-crystal grain growth are studied. In converging grain growth, the anomalous over growth in which the FO dendrite is overgrown by the UO dendrite is observed and analyzed. It is found that the growth of FO dendrite at the grain boudary is retarded and primary dendrite spacing between this dendrite and its neighbor decreases continuously. This phenomenon can be explained by the solute interaction between the FO and UO dendrites at the gran boundary. As for diverging dendrite grain growth, it is found that when the FO dendrites grow slightly deviating from the heat flow direction, the grain boundary can also develop towards FO dendrite direction. The selection process is directly related with the orientation of FO dendrite and its frequency of new primary dendrite arms. 3. The effect of convection on competitive grain growth is studiedForced convection is introduced in front of the solid-liquid interface, and the change of single dendrite morphology and orientation as well as the competitive grain growth is investigated. It is found that convection increases the interface fluctuation, making the anomalous overgrowth in which FO dendrites are eliminated easier to take place. Meanwhile, at low pulling velocity, convection also changes the interface morphology from a straight line into a curve one, which causes tip position variety, making the overgrowth results unexpectable. New phenomenon of converging UO dendrites overgrown by FO dendrite is observed under this condition.