Unidirectionally-solidification process and castings formed thereby

A process of producing a metallic casting having a length greater than one hundred centimeters and a unidirectional crystal structure that is substantially free of freckle defects, the process comprising:

• pouring a molten metal alloy into a cavity in a preheated mold located within a heating zone, the cavity having the shape of the casting;
• withdrawing the mold from the heating zone, through a heat shield, and into a cooling zone to directionally solidify the molten metal alloy, the heat shield operating as a barrier to thermal radiation between the heating zone and the cooling zone, the mold being withdrawn at a rate that, in combination with the heat shield, maintains a thermal gradient of greater than 50° C./cm in the molten metal alloy to solidify the molten metal alloy and form primary dendrite arms having an average spacing therebetween of about 150 micrometers to about 500 micrometers;
• and then
• cooling the mold to produce the casting and the unidirectional crystal structure thereof that is substantially free of freckle defects having a size greater than one hundred centimeters.