Mechanism of Grain Selection in the Channel of a Crystal Selector to Obtain Single Crystal Castings of Nickel-Base Heat-Resistant Alloys

Resumo

Using numerical simulation in the CAFÉ module of the ProCast commercial program, a study was performed of the competitive growth of grains in the seeding unit of the setup to obtain single crystal castings of nickel-base heat-resistant alloy in the process of directional crystallization using the Bridgman method. Using the example of a flat crystal selector, the mechanism of selection of a single grain from several grains with a close axial and arbitrary azimuthal orientation has been considered. It has been found that a significant role in the grain selection is played by the position of the grain in the input section of the crystal selector system. The initial growth advantage comes to those grains that lie in the zone of the crystallization front, from which the way for an unimpeded growth along the crystallization channel is opened. The position of this zone changes as the front moves in the curvilinear channel of the crystal selector. The main tool for selecting a single grain from a variety of closely oriented grains is a change in the position of the zone that is favorable for unimpeded growth. The continuous cyclic change of the zone of favorable growth, which occurs while the grain grows in the helicoid crystal selector, leads to a rapid selection of one grain. In a flat zigzag-shaped crystal selector, the zone of the favorable growth changes only a limited number of times, so this crystal selector is less efficient. The described mechanism of grain selection is applicable in the case of crystal selectors with an angle between the channel axis and the vertical axis Z of no more than 45°, i.e., when the primary axes of dendrites are formed in the crystallographic direction [001].