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To clarify effects of microstructure on tensile properties of high Nb-TiAl alloys after cyclic directional heat treatment (DHT), Ti44Al6Nb1Cr2V alloys are conducted DHT for 1, 2 and 3 cycles (DHT-1, DHT-2 and DHT-3) at 1730 K, respectively. The pulling rate is set as 4.17 µm/s. Results show the phase component mainly consists of γ, α 2 and B2 phases for Ti44Al6Nb1Cr2V alloy after cyclic DHT. Directional microstructure is obtained. Lamellar clusters are columnar and B2 phases present banded morphology. They are distributed alternately. As DHT cycles increase, the radial size of lamellar clusters enlarges as well. Blocky γ phases grow up and coarsen. At room temperature, good tensile properties are obtained in Ti44Al6Nb1Cr2V alloys after cyclic DHT. Tensile strength is about 609 MPa for DHT-1 alloy. Their fractures show that cleavage microstructure is mainly formed for cracks propagation during tensile test. Both DHT-1 and DHT-3 alloys present good tensile properties stability at 900 °C. Their yield strength, ultimate tensile strength and elongation are about 355–487 MPa-3.3% and 355–421 MPa-1.5%, respectively. Combining their microstructures with fractures, it concludes for their tensile properties difference that many blocky γ phases precipitate and coarsen during cyclic DHT, which decreases deformation coordination between lamellar clusters and banded B2 phases. Some microcracks form easily near B2 phases. Consequently, tensile properties of DHT-3 alloy decrease. This study will provide basic experimental data and theoretical support for high Nb-TiAl alloys to further optimize DHT parameters and improve their mechanical properties. • Directional microstructure is obtained after cyclic DHT. • Tensile strength is about 609 MPa for DHT-1 alloy at room temperature. • As DHT cycles increase, the radial size of lamellar clusters enlarges as well. • Many blocky γ phases precipitate and coarsen during cyclic DHT, which decreases mechanical properties of DHT alloys. [ABSTRACT FROM AUTHOR]

To optimize microstructure and improve mechanical properties of directionally solidified (DS) TiAl alloys containing high alloying elements, DS Ti44Al6Nb1Cr2V alloy was conducted directional heat treatment (DHT) at 1730 K. Results indicate that columnar grains grow up straightly after DHT. It decreases from 17.55° to 10.67° for the angle in the axial direction between columnar grains and ingot. Morphology of B2 phases changes obviously, from reticular to banded microstructure. Lamellar clusters grow up greatly and are parallel to the growth direction of columnar grains. These changes are mainly caused by sufficient atoms diffusion during DHT. α grains grow up adequately and residual B2 phases reduce. Compared with that of DS Ti44Al6Nb1Cr2V alloy (505 MPa and 1.40%), ultimate tensile strength and total strain are 609 MPa and 1.86% for DHT alloy, respectively. They are increased by 21% and 33%, respectively. Fracture shows that many tearing edges and some similar dimples morphology are also found in DHT alloy, besides cleavage planes and cleavage steps. TEM image shows that many dislocations are produced in lamellar γ phases of DS alloy. They wind and intercross each other. Dislocation interaction is lower and the stress concentration is less for DHT alloy, due to thin lamellar γ phases. Furthermore, some stacking faults are also produced in lamellar γ phases, which benefits to improving mechanical properties. Therefore, mechanical properties are improved greatly for DS alloy after DHT. • The angle between growth direction of columnar grains and axial direction of ingot decreases from 17.55° to 10.67°. • Lamellar clusters grow up obviously and distribute along the growth direction of columnar grains after DHT. • Morphology of B2 phases changes from reticular to banded structure and its content reduces after DHT. • The ultimate tensile strength and total strain are 609 MPa and 1.86% for DHT alloy, respectively. [ABSTRACT FROM AUTHOR]

• Wide columnar grains are formed along the axial direction after DHT. • Lamellar clusters in columnar grains change from blocky to columnar. • Reticular B2 phases become the banded structure after DHT. • A large number of dislocations and stacking fault bands were formed in lamellar γ phases of DHT alloy after testing. ga1 To improve the directionally solidified (DS) microstructure and mechanical properties of high-Nb containing TiAl alloys, DS Ti44Al6Nb1Cr alloy was directionally heat treated (DHT) with pulling velocity of 4.17 µm/s at 1750 K. Microstructure and grains orientation were characterized by the OM, SEM, TEM and EBSD. Chemical composition and phase component were detected by the EDS and XRD. Results indicate that wide columnar grains are formed along the axial direction after DHT. Microstructure is still fully lamellar and composed of α 2 , γ and B2 phases. Crystallographic orientation relationship among phases (α 2 , γ and B2) is not changed. Lamellar clusters in columnar grains change from blocky to columnar, and the reticular B2 phases become banded structure after DHT. During DHT, the β phase transforms into α phase continuously and α grains grow into columnar clusters. Nb and Cr atoms can diffuse more adequately under the influence of directional heat flow and segregate at boundaries of columnar clusters. Therefore, lamellar clusters and B2 phases are distributed almost in parallel along the axial direction of columnar grains at room temperature. Tensile testing shows that the yield strength, ultimate strength and total strain of the DHT alloy are increased to 574 MPa, 636 MPa and 2.28% from 422 MPa, 523 MPa and 3.03% of as DS alloy, respectively. The tensile strength increases about 36% and this improvement is mainly caused by the directionally aligned B2 phases. In addition, a large number of dislocations and stacking fault bands are formed in lamellar γ phases of DHT alloy after testing, which also plays an important role in improving mechanical properties of DS alloy. [ABSTRACT FROM AUTHOR]

To improve microstructure of high-Nb TiAl alloys after directional solidification (DS) further, DS Ti44Al6Nb1Cr alloy was conducted cyclic directional heat treatment (DHT) for 1 and 4 times under pulling rate of 4.17 μm/s at 1750 K, respectively. Results indicate that columnar grains grow up obviously after DHT. Compared with DS alloy (2.56 mm), the maximum width of columnar grains is up to about 9.07 mm for DHT alloy after 4 times (DHT-4). The SEM images and EDS results indicate that phase component of DS alloy has not changed after cyclic DHT for different times. It is still composed of α 2 , γ and B2 phases. As cyclic DHT times increase, thickness of lamellar phases increases gradually. More massive γ phases precipitate from B2 phases and grow up. The content of B2 phases reduces due to atoms diffusing sufficiently during cyclic DHT. Tensile curves show optimal mechanical properties are obtained in DHT alloy for 1 time (DHT-1). The ultimate tensile strength is 636 MPa and total strain is 2.28%. Compared with that of DS alloy (523 MPa and 3.03%), tensile strength is improved greatly. As cyclic times increase, mechanical properties decrease for DHT alloy, due to massive γ phases coarsening and B2 phases distributing discontinuously. This microstructure could not hinder cracks propagation effectively, which may be the main reasons to result in premature fracture for DHT-4 alloy. At 973 K, the better mechanical properties are still achieved in DHT-1 alloy. The ultimate tensile strength is up to 695 MPa and total strain is 6.21%. The mechanical properties improvement is mainly resulted from directional microstructure, especially the directional lamellar clusters and B2 phases. In addition, dislocations, stacking fault bands and deformation twins are observed in lamellar γ phases of DHT-1 alloy. These microstructure changes play an important role in improving compressive mechanical properties of DHT alloy. [ABSTRACT FROM AUTHOR]

To improve room and elevated temperature mechanical properties of high-Nb TiAl alloys, Ti44Al6Nb1Cr alloy has been prepared by cold crucible directional solidification (CCDS) technique under loaded power of 40 kW and pulling rate of 8.3 μm/s. Macro/micro-structure and phase composition have been characterized. Tensile properties have been tested at room and elevated temperature and the related mechanisms have been revealed. Results show that equiaxed grains change to columnar grains growing up along the axial direction of ingot after CCDS. Phase component is still composed of α 2 , γ and B2 phases. Microstructure consists of lamellar clusters and network structure. The lamellar clusters in a grain possess the same orientation. The content and size of B2 phases decrease and some casting defects with small size decrease as well due to sufficient diffusion of Nb and Cr atoms under action of electromagnetic force during CCDS. Tensile results indicate that comprehensive mechanical properties of Ti44Al6Nb1Cr alloy are improved significantly after CCDS. At room temperature, ultimate tensile strength and total strain are 523 MPa and 3.03%, respectively, which are improved by 18.1% and 64.7% compared with that of as-cast Ti44Al6Nb1Cr alloy (443 MPa and 1.84%). Moreover, fracture morphology also indicates that there are many deep ravines between cleavage steps and some similar dimples microstructure on cleavage steps. They are beneficial to improving comprehensive mechanical properties. At 973 K, ultimate tensile strength of Ti44Al6Nb1Cr alloy does not decrease at all, still reaching 530 MPa after CCDS. When service temperature rises to 1173 K, the strain is less than 4.28% showing excellent mechanical stability under tensile stress less than 436 MPa. The excellent stability for mechanical properties is due to thin lamellar phases, directional microstructure and large plastic deformation region. [ABSTRACT FROM AUTHOR]

For realizing equiaxed to columnar transition, Ti44Al alloy was processed by directional induction heat treatment (DHT) and mechanical properties were tested. The results show that columnar grains can be obtained and elongation is improved at room temperature. During this heat treatment, Ti44Al alloy was heated in single β phase domain. The microstructure is consisted of fully lamellar colonies after DHT. The average lamellar spacing is decreased, and the orientation of lamellar colonies with angle of less than 60° is more than 76%. Moreover, grain boundary presents zigzag. Tensile testing shows that strength and elongation of heat-treated alloy are 217 MPa and 1.78% respectively, they are 253 MPa and 1.63% for as-cast Ti44Al alloy. The elongation is increased about 9.2% although tensile strength is decreased. Cleavage fracture is main way in as-cast Ti44Al alloy but a mixed fracture consisting of some cleavage planes and many tearing edges is in Ti44Al alloys after DHT. The fracture morphology shows that deterioration of strength is mainly resulted from the micropores that reserved from as-cast alloy, it means DHT could not eliminate the micropores. The improvement of ductility is caused by decreasing of lamellar spacing and changing orientation lamellar colonies. [ABSTRACT FROM AUTHOR]

The synergistic effects of B and N on the microstructure evolution, mechanical properties and fracture behavior of Ti46Al4Nb1Mo alloy were investigated by adding BN particles. Ti46Al4Nb1Mo- x B- x N (x = 0, 0.4%, 0.8%, 1.2%, 1.6%, atomic percent, hereafter in at.%) alloys were prepared by vacuum arc melting. Compared with Ti46Al4Nb1Mo alloy, B2 phase disappeared and curvy TiB phase appeared at the interdendritic region in the Ti46Al4Nb1Mo-0.4B-0.4 N alloy. When 0.8% B and 0.8% N were added, Ti 2 AlN particles appeared in α 2 /γ lamellar colonies, tiny TiB particles and a few curvy TiB phases formed both at lamellar boundaries and lamellar colonies. With further increasing B and N content, the volume fraction of Ti 2 AlN and tiny TiB phases increased, and some long TiB phases appeared in the Ti46Al4Nb1Mo-1.6B-1.6 N alloy. Ti 2 AlN and TiB phases formed by the reaction between Ti46Al4Nb1Mo alloy and BN particles. The different formation mechanism between Ti 2 AlN and TiB was related to the different solid solubility of B and N in the Ti46Al4Nb1Mo alloy and solute redistribution during solidification process. When the content of B and N increased from 0 to 1.2%, the average lamellar colony size decreased from 358.4 μm to 48.7 μm, which resulted from synergistic effect of Ti 2 AlN and TiB. Compression results showed that the strength was improved from 1961 MPa to 2493 MPa without decreasing compressive strain. The fracture morphology showed inter-granular and inter-lamellar fracture mode in the Ti46Al4Nb1Mo alloy, while the fracture morphology showed trans-lamellar mode in the reinforced composites. The second phase strengthening of Ti 2 AlN and TiB, grain boundary strengthening by refined α 2 /γ lamellae, and elimination of B2 phase contributed to the improvement of strength. [ABSTRACT FROM AUTHOR]

In order to solve the problem of serious Al segregation and coarse Ti 2 AlN particles in as-cast TiAl composites, ultrasonic treatment was selected to further regulate the macro/microstructure and mechanical properties of Ti46Al4Nb1Mo alloy reinforced with Ti 2 AlN particles, and the effects of ultrasonic treatment time on Ti 2 AlN/Ti46Al4Nb1Mo composites were investigated in this study. Results showed that when the ultrasonic treatment time increased from 0 to 120 s, the region of shrinkage and γ phase between the lamellar colony was reduced. The shape of Ti 2 AlN particles changed from plate to ellipsoid, and the coarse Ti 2 AlN particles with length more than 10 μm gradually became shorter (about 3 μm), especially with the ultrasonic treatment at 120 s. The reduction of shrinkage area, Ti 2 AlN length and γ size resulted from the increase of nucleation supercooling and melt flow capacity by ultrasonic treatment. Compressive results showed that the maximum strength and strain were 2462 MPa and 28.5% with ultrasonic treatment at 90 s and 120 s, respectively, and the improvement of compressive properties was mainly due to the reduced size of Ti 2 AlN and γ phase. [ABSTRACT FROM AUTHOR]

The effect of niobium on microstructure and mechanical properties of Ti46Al2.6C x Nb (at. %) alloys with Nb content ranging from 0 to 8 at.% have been studied. The alloys have been prepared by vacuum arc melting. The content of γ(TiAl) phase increases and content of α 2 (Ti 3 Al) phase decreases with increasing content of Nb. The lattice parameters of γ phase increase and parameters of the α 2 phase remain stable when Nb content is higher than 4 at. %. There is no significant segregation of Nb in the studied Ti46Al2.6C x Nb alloys. The size of lamellar colonies decreases with increasing content of Nb from 0 to 8 at. %. The increase of the lattice parameters and low segregation of Nb without formation of B2 phase are related to a higher solid solubility of Nb in the γ phase. Compressive strength and compressive strain increase with increasing content of Nb. Tensile strength of Ti46Al2.6C8Nb alloy increases from 330 MPa to 442 MPa and the strain increases from 2.9% to 5.3% with increasing test temperature from 750 °C to 950 °C. Solid solution strengthening by Nb, precipitation strengthening by Ti 2 AlC particles, increasing content of γ phase, and grain boundary strengthening are beneficial to improvement in mechanical properties of the studied alloys. • B2 phases did not form in matrix of Ti46Al2.6C x Nb alloys, especially with addition of 8% Nb. • Nb element solid solutes into lamellar colony and Ti 2 AlC phase, which decreases the segregation of Nb. • Mechanical properties have been improved at room temperature and 750 °C of the alloys without B2 phase. • Tensile properties increase when testing temperature rises from 750 to 950 °C of the alloys without B2 phase. [ABSTRACT FROM AUTHOR]

Binary TiAl (Ti29.8Al and Ti33.3Al, weight percent, hereafter in wt.%) alloys with different SiC fibers contents were prepared by arc melting. Microstructure, phase transformation, mechanical properties and related mechanism were elaborately researched. For the microstructure of Ti29.8Al alloy, the matrix consisted of fully α 2 /γ lamellar colonies, the grain morphology changed from columnar to equiaxed when the SiC content reached 0.9%. For Ti33.3Al alloy, the matrix consisted of α 2 /γ lamellar colonies and γ phases, the columnar grains changed to equiaxed grains when the SiC content was more than 0.3%, and the grain size decreased from 150.9 to 69.3 μm with increasing SiC content. Ti 5 Si 3 phases formed at the grain boundaries of both SiC-containing binary alloys. Ti 2 AlC particles would appear in the lamellar colonies when the SiC content was more than 0.9% in Ti29.8Al alloy and 0.3% in Ti33.3Al alloy, respectively. The formation of Ti 5 Si 3 and Ti 2 AlC phases resulted from the reaction of SiC fibers and TiAl alloy. The solid solution of C element in α 2 phase is higher than that in γ phase, Ti 2 AlC will be more easily formed in Ti33.3Al alloy due to less α 2 phases and more γ phases comparing with Ti29.8Al alloy. Compressive testing results showed that the strength and the strain of SiC-containing Ti29.8Al alloy were almost unchanged until the SiC content was 0.9%, which were improved to 1383 MPa and 6.5%, respectively. A good combination of strength and ductility of SiC-containing Ti33.3Al alloy was obtained, the highest values of them were 2196 MPa and 27.5% when the SiC content was 0.5%. Improvement of mechanical properties is caused by the grain refinement, grain boundary strengthening of Ti 5 Si 3 particles, load transfer strengthening of Ti 2 AlC particles and solid solution strengthening of C and Si atoms. [ABSTRACT FROM AUTHOR]

In order to improve high-temperature mechanical properties and oxidation resistance of TiAl alloys, Ho element was added into Ti46Al4Nb1Mo alloy. The microstructure, high-temperature tensile property and oxidation behavior of Ti46Al4Nb1Mo alloy with different Ho content (0, 0.15%, 0.3%, atomic percent, hereafter in at.%) were investigated in this study. The results showed that Ho 2 O 3 particles with square and rod-like shape formed by internal oxidation and the α 2 /γ lamellar spacing was refined with 0.15% Ho addition. When Ho content was 0.3%, irregular and large Al 2 Ho phases appeared at grain boundaries. Compared with Ti46Al4Nb1Mo alloy, the high-temperature tensile strength was improved with 0.15% Ho and decreased with 0.3% Ho. The improvement of the strength resulted from the Ho 2 O 3 second phase strengthening, the decreasing of the strength was caused by the formation of large irregular Al 2 Ho phase. Oxidation testing results indicated that 0.15% Ho improved oxidation resistance of Ti46Al4Nb1Mo alloy, which was due to suppression of inward diffusion of oxygen and better adhesion between oxide film and matrix. • Ti46Al4Nb1Mo alloy doped with Ho is reported the first time. • Ho 2 O 3 particles form in Ti46Al4Nb1Mo alloy with 0.15% Ho content, and Al 2 Ho phases appear with 0.3% Ho content. • High-temperature tensile strength and oxidation resistance of Ti46Al4Nb1Mo alloy are improved with 0.15% Ho addition. [ABSTRACT FROM AUTHOR]