| This thesis focuses on the development of a uniform lamellar structure with controlled minor phase precipitation in an investment cast Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si (at%) near γ-TiAl intermetallic alloy, and on the mechanical behaviour of this alloy with different microstructural conditions.; It is proven difficult to produce a uniform lamellar structure with relatively fine grains through a simple heat treatment, due to the coarse and non-uniform as-cast structure and non-uniformly distributed alloying elements along the cast bar. However, a rather uniform fully lamellar structure can be obtained by a homogenization and solution heat treatment, followed by a furnace cooling + air cooling two-step cooling technique.; Aging the lamellar structure at intermediate temperatures produces precipitate particles predominantly along /γ lamellar interfaces or within lamellae, most of which are W- and Mo-rich β particles. Silicide precipitation is rather limited, compared to the β particles. The morphology of β particles obtained below 850°C is quite different from that at higher aging temperatures, and it is suggested that β particles form at below 850°C via a precipitation process controlled by interfacial diffusion of W and Mo. At higher temperature the formation of the β particles is initially controlled by short range diffusion of W and Mo. The activation energy for β precipitation below 850°C is about 366 kJ/mol.; The fully lamellar structure developed has decreased tensile strength and ductility at both ambient and elevated temperatures but an enhanced creep resistance, compared to the cast duplex structure. Aging the lamellar structure leads to an increased tensile strength, although the strength of aged lamellar structure is still lower than that of the cast duplex structure. Furthermore, the aged lamellar structure has an enhanced resistance to primary creep but a reduced creep life. The activation energy for creep of the lamellar structures is about 390 kJ/mol. |
