Microstructure Evolution and Deformation Mechanisms by Direct Hot-Pack Rolling for As-Cast Ti-46Al-8Nb Alloys
LI Tianrui1, LIU Guohuai1(), YU Shaoxia2, WANG Wenjuan2, ZHANG Fengyi2, PENG Quanyi2, WANG Zhaodong1
1 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China 2 School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
Cite this article:
LI Tianrui, LIU Guohuai, YU Shaoxia, WANG Wenjuan, ZHANG Fengyi, PENG Quanyi, WANG Zhaodong. Microstructure Evolution and Deformation Mechanisms by Direct Hot-Pack Rolling for As-Cast Ti-46Al-8Nb Alloys. Acta Metall Sin, 2020, 56(8): 1091-1102.
TiAl alloys are considered attractive structural materials because of their low density, excellent high-temperature strength, and oxidation resistance. However, their intrinsic characteristics, including low-temperature brittleness, poor workability, and narrow processing window, restrict their wide use in industrial applications. Various hot-work processes are conducted to enhance the inherent ductility of TiAl alloys, especially hot-pack rolling. In this work, the hot processing maps at different strains were developed based on isothermal compression tests and dynamic material model (DMM). The optimum hot-working parameters were selected and a crack free Ti-46Al-8Nb (atomic fraction, %) sheet was directly fabricated by hot pack rolling from ingot. Moreover, microstructure evolution and hot deformation behavior of the as-rolled alloys were investigated. The processing maps showed two typical dynamic recrystallization (DRX) domains which would facilitate the hot-work process, of which the temperature was at 1200 ℃, strain rates was 1 s-1 with a peak efficiency of power dissipation of 0.38 and temperature of 1150~1200 ℃, strain rate of 0.01 s-1 with a peak efficiency of power dissipation of 0.45. The instable-area temperature was 1100~1200 ℃ and strain rate was 0.06~1 s-1 at low strain, which was expanded to low strain rate with the increasing strain. As the strain increased to 0.4, the region with the temperature of 1250 ℃ and strain rate of 0.006 s-1 always became instable. The Ti-46Al-8Nb alloy sheet with thickness of 0.85 mm was produced within processing windows of 1150~1200 ℃, 0.01~0.03 s-1 with engineering strain 18% per pass. The produced sheet showed uniform microstructure as a whole, though the local flow softening and deformation bands were inevitable. Furthermore, the main softening mechanism of Ti-46Al-8Nb alloy was DRX which began with the pile-up of dislocations, the formation of sub-boundaries and mechanical twins. Then the substructures would rearrange to inducing the formation of DRXed grains with the cumulative reduction increasing. The phase transitions of Lamellae (α/γ)→γ+α+B2/β and α→γ during hot-pack rolling combining with the growth of DRXed grains were simultaneously a main softening mechanism. The formations of plentiful mechanical twinning and twin lamellae also contributed to the uniformity of as-rolled microstructure.
Fund: National Key Research and Development Program of China(2016YFB0301200);National Natural Science Foundation of China(51504060);National Natural Science Foundation of China(51301140);Fundamental Research Funds for the Central Universities(N160713001)
Fig.1 Hot processing maps of Ti-46Al-8Nb alloys at ε=0.1 (a), ε=0.2 (b), ε=0.3 (c), ε=0.4 (d) and ε=0.5 (e) (ε—true strain, —strain rate, T—deformation temperature; The contour line shows power dissipation efficiency (η) and gray regions correspond to the instability regions; Regions 1~3 show different deformation mechanisms in the safe regions)
Fig.2 SEM-BSE image of as-cast Ti-46Al-8Nb alloys (a), TEM images of the lamellar colonies (b), and γ grains at colony boundaries and the corresponding selected area electron diffraction (SAED) pattern (inset) (c)
Fig.3 SEM images showing deformed microstructures in hot compression tests of Ti-46Al-8Nb alloys with different deformation conditions (a) 1100 ℃, 1 s-1 (Inset shows the crack defects) (b) 1150 ℃, 0.001 s-1 (c) 1200 ℃, 0.1 s-1 (Region 1, inset shows the high magnified image of kinked lamellae) (d) 1150 ℃, 0.01 s-1 (Region 2) (e) 1200 ℃, 0.01 s-1 (Region 2) (f) 1250 ℃, 0.001 s-1 (Region 3)
Fig.4 EBSD maps showing the deformed microstructure of Ti-46Al-8Nb alloys under deformation conditions of 1150 ℃,0.001 s-1 (Dotted boxes show grains in Al-segregation regions at colony boundaries) (a~c) and 1200 ℃, 0.1 s-1 (d~f) Color online (a, d) image quality (IQ) maps (b, e) phase maps with grain boundary misorientation angles (GB—grain boundary) (c, f) grain distribution maps
Fig.5 Variations of strain rate sensitivity exponent (m) with T and at ε=0.2 (a) and ε=0.5 (b)
Fig.6 SEM-BSE images of the as-rolled microstructure evolutions of Ti-46Al-8Nb alloy (a) macro-structure (ND—normal direction, RD—rolling direction) (b1, b2) decomposition of lamellar colonies at low rolling reduction (c1, c2) decomposition of lamellar colonies at higher rolling reductions (d1, d2) fully DRXed microstructure (DRX—dynamic recrystallization)
Fig.7 TEM analyses of the as-rolled microstructure of Ti-46Al-8Nb alloy (a) dislocations in γ grains (b) sub-structures in γ grains (c) twinning in γ grains at colony boundaries (d) twinning lamellae and the corresponding SAED patterns (inset) (e) interactions of dislocations and deformation twins (f, g) decomposition of lamellar colonies (Inset in Fig.7f shows the SAED patterns of γ grains) (h) fully DRXed microstructure (Inset shows the SAED patterns of α2 grains)
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