Tensile properties and high temperature creep behavior of microalloyed Ti–Ti 3Al–Nb alloys by directional solidification

Abstract

The microstructures, tensile properties and creep behavior of the Cr- and Mo-microalloyed Ti–Ti 3Al–Nb alloys processed by directional solidification were investigated. The experimental results indicated that the strength, ductility and creep resistance of ternary Ti–Al–Nb alloy were remarkably enhanced by additions of small amounts of Cr and Mo. The stress exponent for creep showed a transition and varied from a lower value of 3.5–5.3 (LSR-region I) to a higher value of 8.3–12.6 (HSR-region II) with increasing stress at 600 and 650 °C whereas a single stress exponent value of 5.3–6.0 was obtained at 700 °C. Conversely, the true activation energy for creep was calculated to be 303–324 kJ/mol and fell into or was quite close to that for the self-diffusion of Ti in α 2-Ti 3Al (288–312 kJ/mol). TEM examinations revealed that ordinary dislocations in the lath dominated the deformed microstructures. However, the initial microstructure of the alloys was unstable during long-term creep exposure and dynamic recrystallization occurred at high stress and moderately high temperature. The creep deformation in region I was dislocation-controlled whereas the abnormally high stress exponent in region II was associated with the effects of recrystallization resulting in a reduction in the overall grain size of the initial structure. The fracture modes and total strain were dependent on the composition, test temperature, and stress level. The accelerating strain rate in the extended tertiary stage was attributed to the microstructural instabilities or the nucleation, coalescence and merge of voids or microcracks.