Microstructure - Property Relationships in Newly Developed Multiphase Ti_2AlNb-Based Titanium Aluminides

摘要

Ti_2AlNb-based titanium aluminides have recently received considerable attention as potential materials for high-temperature applications in aeroengines. Several compositions based on Ti-22Al-25Nb with molybdenum, silicon, zirconium additions were developed. The micro-structures consist of α_2-Ti_3Al nodules, together with primary O-Ti_2AlNb laths surrounded by a B2 matrix that is hardened by submicron secondary O laths. Multiscale hardening ensues from a careful control of the volume fractions of α_2 and primary O phases. The secondary O lath precipitation occurs during prolonged ageing at low temperature ( <650℃) and leads to a stabilization of the O+B2+α_2 microstructure. A careful optimization of the multiphase microstructures was carried out bringing the RT ductility of aged alloys up to 5%. The effects of microstructural parameters, such as α_2 and O volume fractions and O lath size, on mechanical properties are discussed. It is especially shown that a decrease in secondary O lath size improves the tensile strength without affecting creep rate. The best combinations of composition and microstructures exhibit attractive mechanical properties. Zirconium additions improve the creep resistance significantly without impairing ductility or yield stress. The density normalized creep performance at 650℃ is comparable to those of nickel base superalloys such as IN718. The specific tensile strength is superior to that of disc superalloys and γ-TiAl alloys. Recent results indicate that adequate design of alloy chemistry can lead to an oxidation resistance comparable to that of γ-TiAl alloys.