Variant selection, coarsening behavior of α phase and associated tensile properties in an α+β titanium alloy

doi:10.1016/j.jmst.2021.04.069

Abstract

Abstract:

Effect of cooling rates, i.e., air cooling and furnace cooling, after solution in α+β phase-field on variant selection, coarsening behavior of α phase and microstructure evolution were investigated in α+β TC21 alloy. The textures of primary α (α_p) and lamellar α (α_L) in β phase transformation microstructure (β_t) were analysed separately, and the orientation relationship among α_p, α_L and the parent β phase were studied. In addition, the influence of the microstructure characteristics on the tensile properties was investigated. The results showed that all parent β grains, despite their different orientations, produced 12 ideal α_L variants with the same texture components and interweave to form a basketweave β_t structure under the air-cooling condition. The α_p without Burgers orientation relationship (BOR) with β phase exhibited obviously texture component without overlapping the α_L texture component. The volume fraction of α_p in the furnace-cooled sample (about 50%) was higher than that of the air-cooled sample (about 12%), while the size of it slightly increased with decreasing the cooling rate. In each β grain, the thick α_L in the same orientation formed an α colony. A typical 3 variant colonies which were related to each other were observed. Consequently, the α_L spatial orientation distribution showed more heterogeneity. Moreover, the BOR between α_p and β and the same orientation of some α_L and the surrounding α_p grains resulting in the overlapping of α_p texture component and α_L texture component. At last, the relationship between microstructure and tensile properties was analysed.

Key words: TC21 alloy, Crystallographic orientation, Texture, Variant selection, Tensile properties

Lei Lei, Qinyang Zhao, Cong Wu, Yongqing Zhao, Shixing Huang, Weiju Jia, Weidong Zeng. Variant selection, coarsening behavior of α phase and associated tensile properties in an α+β titanium alloy[J]. J. Mater. Sci. Technol., 2022, 99: 101-113.

Figures/Tables 15

Fig. 1. SEM images of the as-received TC21 alloy: (a) low magnification image and (b) high magnification image.

Fig. 2. SEM images of the TC21 alloy after heat treatment: (a) BM1 and (b) BM2.

Fig. 3. The phase map, IPF map of α phase and IPF map of β phase of (a-c) BM1 and (d-f) BM2.

Fig. 4. The IPF map of αp phase, pole figure of αp, β and αL of (a-d) BM1 and (e-h) BM2.

Table 1 Twelve possible α variants generated during β to α phase transformation through the BOR.

Variant	Plane	Direction	Rotation angle/axis from V1
V1	(1-10)(0001)	[111][11-20]	-
V2	(10-1)(0001)	[111][11-20]	60°/[1 1 -2 0]
V3	(01-1)(0001)	[111][11-20]	60°/[1 1 -2 0]
V4	(110)(0001)	[-111][11-20]	90°/[1 -2.38 1.38 0]
V5	(101)(0001)	[-111][11-20]	63.26°/[-10 5 5 -3]
V6	(01-1)(0001)	[-111][11-20]	60.83°/[-1.377 -1 2.377 0.359]
V7	(110)(0001)	[1-11][11-20]	90°/[1 -2.38 1.38 0]
V8	(10-1)(0001)	[1-11][11-20]	60.83°/[-1.377 -1 2.377 0.359]
V9	(011)(0001)	[1-11][11-20]	63.26°/[-10 5 5 -3]
V10	(1-10)(0001)	[11-1][11-20]	10.53°/[0 0 0 1]
V11	(101)(0001)	[11-1][11-20]	60.83°/[-1.377 -1 2.377 0.359]
V12	(011)(0001)	[11-1][11-20]	60.83°/[-1.377 -1 2.377 0.359]

Fig. 5. Misorientation angle distribution of αL along with misorientation angles/axes in (a) BM1, (b) selected βt region and (c-f) pole figures of αL and β phase in region I-IV selected from Fig. 5(b).

Fig. 6. Misorientation angle distribution of αL along with misorientation angles/axes in (a) BM2, (b) selected region and (c, d) pole figures of αL and β phase in region 1 and 2 selected from Fig. 6(b), (e) pole figures of α phase and (f) IPF image of α phase selected from Fig. 6(e).

Fig. 7. Tensile properties of BM1 and BM2: (a) engineering stress-strain curves, (b) true stress-strain and work hardening rate curves.

Fig. 8. Fracture surfaces and cross-section of (a-c) BM1 and (d-f) BM2.

Fig. 9. The IPF map and misorientation angle distribution of α phase after tensile deformation: (a, b) BM1 and (c, d) BM2.

Fig. 10. KAM maps of (a-c) BM1 and (d-i) BM2.

Fig. 11. Schmid factor maps and distributions of α and β phase: (a-f) <11-20> direction of α phase and (g-l) <111> direction of β phase for BM1 after deformation.

Fig. 12. Schmid factor maps and distributions of α and β phase: (a-f) <11-20> direction of α phase and (g-l) <111> direction of β phase for BM2 after deformation.

Fig. 13. TEM morphologies of BM1 after tensile deformation: (a) kink deformation of αL and dislocation is blocked at the intersection of αL, (b) sub-grains formed in αp, (c) dislocation accumulate at αp/βt interface and (d) dislocation tangles formed in αp and αL that close to αp/βt interface.

Fig. 14. TEM morphologies of BM2 after tensile deformation: (a) kink deformation of αL and dislocation cell formed in αp, (b) slip line, dislocation arrays and dislocation cell formed in αp, (c, d) deformation twin formed in αp, (e) dislocation cell formed in αL and (f) slip line formed in αs.

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