Microstructure-Property Relationships of Two Ti_2AlNb-based Intermetallic Alloys: Ti-15Al-33Nb(at.) and Ti-21Al-29Nb(at.)

摘要

Two Ti_2AlNb intermetallic orthorhombic (O) alloys, Ti-15Al-33Nb and Ti-21Al-29Nb(at.%), were subtransus processed into sheets, using pancake forging and hot-pack rolling, and evaluated in tension (25 and 650°C) and creep (650-710°C) and the properties and deformation behavior were related to microstructure. Some of the microstructural features evaluated were grain boundary character, grain size, phase volume fraction, and morphology. The alloy Al content was important to strength and elongation-to-failure (ε_f), where higher Al contents lead to greater tensile strengths and lower ε_f values and a corresponding brittle fracture response. However, the room temperature (RT) strengths of Ti-15Al-33Nb, which exhibited greater BCC phase volume fractions than Ti-21Al-29Nb and ductile failure (ε_f > 2%), were always greater than 775 MPa. The creep stress exponents (n) and activation energies (Q_(app)) suggested that a transition in the dominant creep deformation mechanism exists and is dependent on stress and microstructure. Supertransus heat treatment, which increased the prior-BCC grain size and resulted in a lath-type O+BCC microstructure, resulted in reduced creep strains and strain rates. In fact, the supertransus heat-treated Ti-15Al-33Nb microstructures exhibited greater creep resistance than subtransus heat-treated Ti-21Al-29Nb microstructures. Combining the creep observations with the tensile response, the supertransus heat treated Ti-15Al-33Nb lath O+BCC microstructures exhibited the most attractive combination of tensile strength, ε_f values, and creep resistance.