Growth Behavior of Grain Boundary α Phase and Its Effect on the Microtexture During β → α Phase Transformation in Ti6246 Titanium Alloys
QI Min1,2, WANG Qian2, MA Yingjie1,2(), CAO Hemeng1,2, HUANG Sensen2, LEI Jiafeng1,2, YANG Riu1,2
1 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 2 Shichangxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
QI Min, WANG Qian, MA Yingjie, CAO Hemeng, HUANG Sensen, LEI Jiafeng, YANG Riu. Growth Behavior of Grain Boundary α Phase and Its Effect on the Microtexture During β → α Phase Transformation in Ti6246 Titanium Alloys. Acta Metall Sin, 2025, 61(2): 265-277.
The initial lamellar microstructure of titanium alloys significantly affects their microstructural evolution and mechanical property during thermo-mechanical treatments. Thus, the evolution of the initial lamellar microstructure must be explored to control the microstructure and enhance the mechanical property. The present work focuses on the evolution of the lamellar microstructure during β→α phase transformation via interrupted furnace cooling experiments to analyze the growth behavior of the grain boundary α phase (αGB) and its effect on the subsequent growth of intragranular α lamellae and microtexture. Results show that when titanium alloy furnace cools from the β phase field to the α + β phase field, the αGB holding Burgers orientation relationship (BOR) with both sides of β grains (2-BOR αGB) has an advantage for early transformation at the β grain boundary. In particular, type II (49.5°/<110>) and type III (60°/<110>) β grain boundaries are preferential sites for the early nucleation of αGB particles. As the temperature decreases, α lamellae holding similar orientation to 2-BOR αGB grow to both sides of β grains. Thus, 2-BOR αGB and both sides of α lamellae form a strong microtexture at the grain boundary. At the early growth period of the α phase, the smaller the θ2-BOR (misorientation of the close-oriented αGB variant pair of parents β1 and β2) of 2-BOR αGB, the earlier the formation of a strong microtexture at the grain boundary. 2-BOR αGB preferentially precipitates, whereas the αGB holding BOR with only one side of the β grain (1-BOR αGB) nucleates. α lamellae holding a similar orientation to 1-BOR αGB grow to one side of the BOR-β grain (holding BOR with αGB), whereas α lamellae with a different orientation grow in the non-BOR-β grain. Thus, 1-BOR αGB and one side of α lamellae form a weak microtexture at the grain boundary.
Fig.2 Low (a) and high (b) magnified SEM images of the as-received Ti6246 alloy (αp—primary α, αs—secondary α)
Fig.3 OM image of the lamellae microstructure of Ti6246 alloy after β annealing treatment
Fig.4 EBSD analyses of Ti6246 alloy after β annealing treatment, showing typical Widmanstätten microstructures (a, b) band contrast map (a) and the corresponding α orientation map (b) (αGB—grain boundary α phase) (c, d) SEM image (c) and the corresponding α orientation map (d)
Fig.5 OM images of α phase in the Ti6246 alloy after furnace cooling from β phase field to 920 oC (a), 880 oC (b), 840 oC (c), and 810 oC (d)
Fig.6 SEM images of α phase in the Ti6246 alloy after furnace cooling from β phase field to 920 oC (a), 880 oC (b), 840 oC (c), and 810 oC (d)
Fig.7 Volume fractions of the αGB and α colony as a function of temperature below Tβ in Ti6246 alloy
Fig.8 EBSD analyses of the α phase in the Ti6246 alloy after furnace cooling from β phase field to 920 oC (a), 880 oC (b), and 840 oC (c) (The area enclosed by dashed line in Fig.8c indicates that the β phase has not yet transformed to α phase)
Temperature / oC
Ⅰ
Ⅱ
Ⅲ
Ⅳ
HAGB
LAGB (< 10o)
920
3.3
20.0
16.7
11.7
48.3
0
880
5.0
13.3
01.7
08.3
71.7
0
840
3.3
08.3
01.7
08.3
76.7
1.7
Table 1 Fractions of the β phase grain boundary transformed to αGB in Ti6246 alloy after furnace cooling from β phase field to different temperatures
Fig.9 EBSD analyses of the α phase grain at multiple positions (a-d) of Ti6246 alloy after furnace cooling from β phase field to 880 oC, pole overlaps as shown in black boxes
Fig.10 EBSD analyses of αGB in the Ti6246 alloy after furnace cooling from β phase field to 880 oC (a) SEM image (b, d) orientation map (b) and pole figure (d) of α phase corresponding to the square area in Fig.10a (c, e) orientation map (c) and pole figure (e) of β phase corresponding to the square area in Fig.10a
Fig.11 EBSD analyses of 2-BOR αGB in the Ti6246 alloy after furnace cooling from β phase field to 880 oC, pole overlaps as shown in black boxes (a, c) orientation map (a) and corresponding pole figure (c) of α phase (b, d) orientation map (b) and corresponding pole figure (d) of β phase
Fig.12 Probabilities of two colonies emitted by αGB grains as a function of θ2-BOR in Ti6246 alloy after furnace cooling from β phase field to 880 oC (Numbers of observed boundaries are indicated in the figure)
Fig.13 Schematics of the transformation of the microtexture when the Ti alloy was cooled from a β phase field (a) 2-BOR αGB and both sides of α lamellae forming strong microtexture (b) 1-BOR αGB and one side of α lamellae forming weak microtexture
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