| The microstructures, convention mechanical property and creep behavior of the Cr-, Mo-, Mn- and minor Sc- microalloyed Ti-Al-Nb alloys processed by directional solidification were investigated. The results showed that microstructures of the Ti-45Al-3Nb and Ti-45Al-3Nb-0.5Sc were fully lamellar. The orientation of the lamellar in both alloys tended to be inclined with an angle of approximately 45°to the growth direction whereas Ti-45Al-3Nb-2Cr-1Mn-0.5Sc and Ti-45Al-3Nb-2Mo-2Cr-1Mn-0.5Sc exhibited duplex microstructure. The lamellar of Ti-45Al-3Nb-2Cr-1Mn-0.5Sc alloy is nearly parallel to the withdrawal direction while theβphase with the band-like shape and vein-like shape were promoted by the addition of Mo and lamellar was miss-aligned totally. Both the two Ti-Ti3Al-Nb alloys displayed basket-weave type microstructures. The laths in Ti-16Al-12Nb-3Cr-1Mo tended to be inclined with a smaller angle to the withdrawal direction than Ti-16-12Nb alloy. The experimental results indicated that the strength, ductility, fracture toughness and creep resistance of Ti-Al-Nb alloy were remarkably enhanced by additions of small amounts of alloying element. However, with the addition ofβ-phase stabilizing elements, a large number of B2 phase emerged, and the structure of the alloy changed, so the high temperature creep resistance of alloys reduced.The stress exponent for creep showed 4.5 for Ti-45Al-3Nb-0.5Sc at 850℃, 5.3 and 6.4 for Ti-45Al-3Nb-2Cr-2Mn-0.5Sc alloy at 850 and 900℃,while, 5.4 and 5.8 for Ti-45Al-3Nb-2Mo-2Cr-2Mn-0.5Sc alloy at 850 and 900℃, respectively. The stress exponent for creep showed a transition and a lower value of 2.5 (LSR-region I) to a higher value of 5.7 (HSR-region II) with increasing stress for Ti-45Al-3Nb-2Mo-2Cr-2Mn-0.5Sc alloy at 800℃. Conversely, the true activation energy for creep of Ti-45Al-3Nb-2Cr-2Mn-0.5Sc alloy was calculated to be 318 kJ/mol,which was close to that for the self-diffusion of multiphase TiAl, while, 486 kJ/mol for Ti-45Al-3Nb-2Mo-2Cr-2Mn-0.5Sc alloy. TEM examinations revealed that ordinary dislocations in the lamina dominated the deformed microstructures. Simultaneously, deformation twins, jogs and large amounts of second phase particles were observed after creep deformation. Minor Sc existing as thin plate Ti3(Al, Sc) compounds laocated both atγ/α2 phase boundaries and withinγphase in the alloys, exerted pinning effect, and therefore inhibited the slipping of dislocations. This feature indicated a sliding/climbing-controlled creep deformation mechanism. The stress exponent for Ti-Ti3Al-Nb 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℃. However, a single stress exponent value of 4.5-6.0 was obtained at 700℃. 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 inα2-Ti3Al (288～312 kJ/mol). 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.
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