Composite geometrical design for a grain starter in a bridgman investment casting process

A structure comprising:

• a metallic grain starter, wherein the grain starter comprises a grain-starting material;
• an article attached to the grain starter, wherein the article is characterized by directionally solidified grains, the grains aligned in a preselected crystallographic direction, the article further characterized by a substantial freedom of stray grains oriented at angles to the preselected crystallographic direction;
• and
• means for transferring heat away from the grain starter, wherein the means for transferring heat away from the grain starter modifies heat flow from the grain starter, the means for transferring heat away from the grain starter being solely a feature of the grain starter.

A method for forming an article having directionally solidified grains comprising the steps of:

• providing a mold;
• providing a metallic grain starter, the metallic grain starter including means for transferring heat away from the metallic grain starter;
• attaching the metallic grain starter to the mold;
• supplying molten metal to the mold;
• providing a heat source to maintain the molten metal in the mold in a preselected temperature range;
• slowly withdrawing the molten metal-containing mold from the heat source into a cold zone adjacent to the heat source while forming an advancing solidification front within the molten metal as heat is removed from the molten metal within the mold, while the grain starter initiates the formation of grains in a preselected crystallographic direction, wherein the means for transferring heat away from the grain starter alters a profile of the advancing solidification front within the mold;
• continuing to slowly withdraw the molten metal-containing mold from the heat source as heat is withdrawn through the grain starter and the means for transferring heat away from the grain starter so that the formed grains advance into the molten metal in the preselected metallurgical direction with the substantial absence of a nucleation and growth of stray grains in the molten metal in front of the advancing solidification front;
• and
• completing the solidification of molten metal in the mold.