Microstructure development in gamma alloy mill products by thermomechanical processing

Gamma titanium aluminides are emerging as a revolutionary high-temperature material. The last decade led to significant engineering advances and component demonstrations for cast gamma-alloy products. However, cast processing has yet to permit the full potential of these materials to be realized, principally as the result of the limited microstructural control available. Conversely, thermomechanically processed gamma alloys are leading to a wide spectrum of microstructures, an outstanding balance of properties within the alloy class, and prospects for improved alloy durability. The manuscript succinctly describes the general field of thermomechanical processing for gamma alloys, giving particular attention to homogenization of large ingots in wrought processing. Aspects of producing fine-grained fully-lamellar microstructures, having controlled lamellar characteristics in wrought mill products are discussed. Some influences of alloy chemistry are discussed to show the feasibility of producing high-strength alloys across the gamma alloy class. Both a β-phase forming element (1 at.% Mo) and boron in the alloys are examined as grain-size controlling agents. These alloys are compared along with traditional alloys containing neither of these elements. Thermal-treatment windows are identified and discussed for producing fully-lamellar materials. When grain-size controlling agents such as boron or beta-phase are used, the lamellar transformation kinetics may be significantly altered relative to conventional gamma alloys, thus changing the thermal process path and affecting the perfection of the lamellar microstructures. These lead to concomitant changes in alloy properties. The prospects for attaining such structures and properties in large product scales are discussed.