Formation Mechanism of B2 Phase and Micro-Mechanical Property of Rapidly Solidified Ti-Al-Nb Alloy
LIANG Chen, WANG Xiaojuan, WANG Haipeng()
School of Physical Science and Technology, Northwestern Polytechnical University, Xi'an 710072, China
Cite this article:
LIANG Chen, WANG Xiaojuan, WANG Haipeng. Formation Mechanism of B2 Phase and Micro-Mechanical Property of Rapidly Solidified Ti-Al-Nb Alloy. Acta Metall Sin, 2022, 58(9): 1169-1178.
Ti-Al-Nb alloys are widely used in the aerospace industry and are promising candidate materials for turbine engines owing to their relatively low density, high specific strength, and good oxidation resistance. Here, the effects of the cooling rate and undercooling on phase constitution, microstructure evolution, B2 phase formation, and micromechanical properties of the rapidly solidified Ti75 - x Al x Nb25 (x = 22, 45, atomic fraction, %) alloy were investigated. With a decrease in the droplet diameter, the primary B2 phase of Ti53Al22Nb25 alloy transforms from coarse dendrite to equiaxed grain under free fall. For the rapidly solidified Ti30Al45Nb25 alloy droplet, the nucleation and growth of the B2 phase transforms from the center of the γ dendrite to γ-grain boundaries, and the volume fraction of the B2 phase decreases with the droplet diameter. Under the condition of arc melting and vacuum suction casting (VSC), with an increase in the cooling rate, the average diameter of the B2 dendrite of the Ti53Al22Nb25 alloy decreases from 515 to 370 μm. For the Ti30Al45Nb25 alloy, the solidified microstructure changes from irregular (γ + B2) lamellae to regular (γ + B2) lamellar, to acicular (γ + B2) microstructure, and Al segregation is inhibited. The microhardness of Ti75 - x Al x Nb25 alloy increases with a decrease in the droplet diameter, and the maximum microhardness of each alloy is 11.57 GPa and 7.7 GPa, respectively, which are 64% and 22% higher than that of VSC, respectively, thereby indicating that the coupled effect of a large cooling rate and high undercooling can effectively enhance the microhardness of the Ti-Al-Nb alloy.
Fund: National Natural Science Foundation of China(51734008);National Natural Science Foundation of China(51871185);National Key Research and Development Program of China(2018YFB2001800)
About author: WANG Haipeng, professor, Tel: (029)88431669, E-mail: hpwang@nwpu.edu.cn
Fig.1 Calculated cooling rate (Rc) (a) and undercooling (ΔT) (b) of rapidly solidified Ti75 - x Al x Nb25 alloy via drop-tube technique (D—drop diameter)
Fig.2 Temperature distributions of rapidly solidified Ti53Al22Nb25 alloy under different conditions (a) vacuum arc-melting (VAM) (b) vacuum suction casting (VSC)
Fig.3 Cooling rates of rapidly solidified Ti75 - x Al x Nb25 alloy under VAM and VSC (Z—distance) (a) Ti53Al22Nb25 (b) Ti30Al45Nb25
Fig.4 XRD spectra of rapidly solidified Ti75 - x Al x Nb25 alloy via drop-tube technique (a) Ti53Al22Nb25 (b) Ti30Al45Nb25
Fig.5 Microstructure evolutions of rapidly solidified Ti53Al22Nb25 alloy droplets with different diameters (a) 740 μm (b) 417 μm (c) 206 μm (d) 181 μm
Fig.6 Microstructure evolutions of rapidly solidified Ti30Al45Nb25 alloy droplets with different diameters (a) 889 μm (b) 530 μm (c) 340 μm (d) 124 μm
Fig.7 XRD spectra of rapidly solidified Ti75 - x Al x Nb25 alloy via VAM (a) Ti53Al22Nb25 (b) Ti30Al45Nb25
Fig.8 Microstructure evolutions of rapidly solidified Ti75 - x Al x Nb25 alloys processed by VAM (a, b, d, e) and VSC (c, f) (a-c) Ti53Al22Nb25 (d-f) Ti30Al45Nb25
Fig.9 Microstructure and elemental distributions of Ti30Al45Nb25 alloy by VAM
Fig.10 Microhardnesses (H) and typical indentation photos (insets) of the alloys processed by VAM and VSC (a) Ti53Al22Nb25 (b) Ti30Al45Nb25
Fig.11 Microhardnesses and typical indentation photos (insets) of rapidly solidified Ti75 - x Al x Nb25 alloys droplet with different diameters
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