The development of columnar grain and single crystal high temperature materials through directional solidification

Abstract

Something exciting has been happening during these last several years. Structural components of a major engineering device were first constructed so that each one consisted of an individual metal crystal. These were alloy single crystal turbine blades for advanced aircraft jet engines. A new precision casting technique, based on directional solidification, which imparts significantly improved ductility and thermal shock resistance to high temperature creep resistant, nickel-base superalloys, has been carried through from research to production. This controlled solidification technique has been used to produce both columnar grain and alloy single crystal gas turbine components. The improvement in physical properties is achieved by controlling the solidification process to produce either columnar grains throughout a cast-to-size part, or a complete single crystal throughout a cast-to-size part, with a preferred [001] crystallographic orientation. This orientation is established parallel to the major stress axis of the part without the use of separate “seeding”. These parts have exhibited superior structural strength and stability in the severe operating environments associated with gas turbine engine operation. A comparison is made between the properties of superalloys having conventional equiaxed grains, directionally solidified columnar grains and [001] oriented single crystals. The evolution of this new process is traced from its beginning in columnar grain directional solidification experiments through the pilot-plant operation. The feasibility of producing parts using the “directional solidification process” has been demonstrated in production foundry facilities where several thousand gas turbine blades and vanes have been cast-to-size in various complex shapes.