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In the present study, a two-phase Ti 2 AlNb-based alloy with a nominal composition of Ti–22Al–25Nb (at%) comprising the dominant O + B2/β phase was explored. The microstructural evolution and tensile properties are studied by applying different heat treatment methods of water-cooling (WC, 140 °C/s) and furnace-cooling (FC, 0.08 °C/s) after heat treatment at 1015 °C for 40 min, followed by aging treatment at 800 °C for 1, 5, and 100 h. The results indicated that the morphologies of solution-treated alloys after different cooling methods have a significant distinction. The WC alloy consists of a continuous B2/β phase and a few remnant O phase, whereas the FC alloy comprises a mass of lath O phase and discontinuous B2/β phase. The Widmanstätten O + B2 structures were generated in the following aging treatment. The α 2 phase was not detected during the heat treatments in the present work. The B2 phase and O phase in the aged WC and FC alloys follow the classical orientation relationship: (110) B 2 / / (001) O , [ 1 ‾ 11 ] B 2 / / [ 1 1 ‾ 0 ] O. The room-temperature ultimate tensile strength (UTS) and elongation to failure (EL) of the FC and aged FC alloys are higher than the WC and aged WC alloys, except for the 100 h-aging condition. On the contrary, the high-temperature UTS and EL of the aged WC alloys are higher than the aged FC alloys. The existence of the O/O phase boundaries, the coarsening of the B2 grain boundary and discontinuous precipitation harmed the performances of the studied alloys. Beyond that, the fracture mode of the explored alloys with different heat treatment methods was also analyzed. • The morphologies of solution-treated Ti 2 AlNb-based alloys after different cooling methods have a significant distinction. • The B2 phase and O phase in the studied alloys follow the classical orientation relationship: 110B2//001O, [111]B2//[110]O. • Tensile properties and fracture mode are significantly influenced by different cooling rates and aging treatments. • The O/O boundaries, coarsening of the grain boundary and discontinuous precipitation harmed the properties of the alloys. [ABSTRACT FROM AUTHOR]

The effect of primary O phase on thermal compressive deformation behavior of a Ti 2 AlNb-based alloy are investigated at temperatures from 990 °C to 1020 °C and strain rate range from 0.001 s−1 to 1 s−1. The results indicated that the primary O phase has a significant effect on the hot deformation behavior of the studied alloy. The peak stress and the degree of the stress descent of the specimen with primary O lath and Widmanstätten O + B2 structures are higher than those of the specimen with Widmanstätten O + B2 structures under higher strain rate. Besides, both the critical stress and critical strain for the onset of dynamic recrystallization (DRX) of the studied alloy upsurge with increasing the strain rate or decreasing the deformation temperature. The specimen with primary O lath and Widmanstätten O + B2 structures exhibits greater critical stress and critical strain than the specimen with Widmanstätten O + B2 structures. The strain rate sensitivity exponents of the investigated alloy with primary O lath and Widmanstätten O + B2 structures is higher than that of the alloy with Widmanstätten O + B2 structures at low temperature (990 °C), while the reverse is true at high temperature (1020 °C). Beyond that, the DRX mechanisms of the explored alloys with different morphologies of O phase were also investigated by EBSD and TEM analysis. • Thermal deformation behavior of Ti 2 AlNb-based alloy with two morphologies of O phase was studied. • The existence of primary O lath results in more significant effect of flow softening. • Critical conditions for the onset of DRX is sensitive to morphology of O phase. • The studied Ti 2 AlNb-based alloys with different morphologies of O phase undergo DDRX in all deformation conditions. [ABSTRACT FROM AUTHOR]

In order to better understand the thermal stability and mechanical properties of the nominal Ti–22Al–25Nb alloy with different initial microstructures, microstructural evolution after long-term aging, and mechanical properties at room temperature and 650 °C of four differently heat-treated Ti–22Al–25Nb alloy were investigated. The investigation results indicated that α 2 platelets formed by aging at (α 2 +B2) region were not as stable as previous research claimed and spontaneously decomposed into fine (O+β/B2) mixture during the annealing at 650 °C. As a result, the alloy's steady-state creep strain rate dropped dramatically at the secondary creep stage and tensile strength grew. On the contrary, long time aging exposure resulted in the connection of original dispersed lenticular O precipitation and thus degraded the ductility of all specimens. Besides, the discontinuous precipitation B2→β+O caused by supersaturation of solutes in the matrix also harmed the mechanical properties. Under the condition of 650 °C/250 MPa, the steady-state creep strain rates of the Ti–22Al–25Nb specimens show a significant dependence on their grain size. Crept samples showed that most dislocation motion was restrained in the β channels under a small strain and then extended into O phase when creep strain grew over 10%. Finally, by comparing the room temperature tensile property and creep resistance of four samples, supertransus solution and furnace cooling ensured the alloy most balanced mechanical properties among lamellar samples. The supertransus then furnace-cooled sample did not only have an excellent creep resistance but also kept a moderate room temperature elongation to failure of nearly 6%. • Thermal stability and mechanical properties of four different microstructures were investigated and compared. • Metastability of α 2 phase resulted in its decomposition and promoted the creep resistance of the alloy. • Grain size including equiaxed precipitation dominates creep resistance in intermediate stress regime. • Discontinuous precipitation and piling up of dislocations promoted the interfacial cracks. [ABSTRACT FROM AUTHOR]

The Ti 2 AlNb and Mo-modified Ti 2 AlNb alloys were fabricated by hot isostatic pressing, and then underwent annealing treatment at 800 °C for 1, 2 and 3 h respectively. To evaluate the tensile properties of the two kinds of alloys, tensile tests were conducted at ambient temperature and 650 °C in this study. For the Ti 2 AlNb alloy, the room-temperature ultimate tensile strength (UTS) was 700–900 MPa, and the highest elongation to failure (ε f) reached 6.2% after aging for 2 h at 800 °C. Since the Mo addition promoted the precipitation of the coarse B2+O colony structure near the grain boundary, the Mo-modified Ti 2 AlNb alloy exhibited decreased UTS and ε f at room temperature. As for the high-temperature performance of both the Ti 2 AlNb and Mo-modified Ti 2 AlNb alloys, the UTS reached approximately 700 MPa and the ε f exceeded 3.0%. Meanwhile, the two kinds of alloys presented the same tendency that both the high-temperature UTS and ε f increased firstly and then decreased as the aging time increased. Among them the specimens after aging at 800 °C for 2 h for both alloys acquired the highest UTS and ε f. Moreover, the Mo-modified alloy exhibited a better high-temperature ductility than that at room temperature, which is associated with the B2↔O phase transformation during the loading at 650 °C. The Mo addition effectively refined the lath of the precipitates in the grain and the grain size of the matrix, and it induced abundant dislocations. Thus, the Mo-modified Ti 2 AlNb alloy aged for 1 and 3 h exhibited superior high-temperature mechanical properties than those of the Ti 2 AlNb alloy. [ABSTRACT FROM AUTHOR]