The creep behavior of gamma alloy Ti-46.5Al-3Nb-2Cr-0.2W (alloy K5) in two microstructural conditions

The creep behavior of a wrought gamma alloy Ti-46.5Al-3Nb-2Cr-0.2W (Alloy K5) has been investigated in both a fine-grained duplex (∼10 microm grain size) and a refined fully-lamellar (RFL) microstructural condition (average lamellar grain size of ∼ 300 microm). Creep tests were conducted in laboratory air under constant tensile load at temperatures between 676 and 870 C and stresses between 69 and 242 MPa. The results show that the RFL condition exhibits creep resistance significantly improved over the duplex microstructure. At 800 C and 138 MPa the RFL material exhibited a secondary or minimum creep rate of 3.8 x 10-5 h-1 which is nearly two orders of magnitude slower than for the fine duplex. Activation energies and stress exponents for power law creep were measured and found to be similar for the two microstructures over the range of conditions investigated. Microstructural studies conducted on crept RFL specimens showed evidence of bulk deformation within the lamellar grains which lead to break up or spheroidization of the α2 laths at high creep strains. Creep failure occurred by intergranular cracking and cavity formation along grain boundaries in the material