Precipitation of Ti2Al phases at lamellar interfaces in a high-Nb-containing TiAl alloy during thermal exposure

doi:10.1016/j.jmst.2022.02.048

Abstract

Abstract:

The thermal stability of microstructures is crucial for determining the performance of alloys in extreme environments. In the present work, the microstructural evolution and precipitation behavior in a high Nb-containing Ti45Al8Nb alloy during thermal exposure at 950 °C were investigated. It was found that excess α₂ phases in the as-cast microstructure were unstable and tended to decompose during thermal exposure. Hexagonal Ti₂Al phases precipitated at lamellar interfaces and had a [1$\bar{1}$0]_γ//[11$\bar{2}$0]_α2//[11$\bar{2}$0]_Ti2Al, (002)_γ//(1$\bar{1}$00)_α2//(1$\bar{1}$00)_Ti2Al crystallographic orientation relationship (OR) with the matrix. Stacking faults (SFs) generated in α₂ phases during the α₂ → γ phase transformation provided favorable nucleation sites for Ti₂Al phases.

Key words: TiAl alloy, Thermal exposure, Phase transformation, Precipitation

Dongxu Li, Guoying Zhang, Gang Lu, Yujie Liu, Jianjun Wang, Chunming Liu. Precipitation of Ti₂Al phases at lamellar interfaces in a high-Nb-containing TiAl alloy during thermal exposure[J]. J. Mater. Sci. Technol., 2022, 126: 132-140.

Figures/Tables 11

Fig. 1. Basic microstructure of the as-cast Ti45AlNb alloy: (a) SEM image taken in the BSE mode, (b) OM image, (c) BF TEM image, (d) SAED pattern.

Fig. 2. (a-d) Microstructures of Ti45Al8Nb alloy after 1, 12, 24, and 48 h of thermal exposure at 950 °C, (e) magnified EPMA image of precipitates and corresponding WDS mapping results.

Fig. 3. XRD patterns of Ti45Al8Nb alloy before and after thermal exposure at 950 °C.

Fig. 4. TEM morphologies of the alloy after thermal exposure for different times at 950 °C: (a, b) BF images of α2 lamellae after 12 h of thermal exposure, (c) BF image of precipitates after 24 h of thermal exposure, (d) dark-field (DF) image of precipitates after 48 h of thermal exposure, (e) SAED patterns of position E in Fig. (c), (f) SAED patterns of position F in Fig. (c), (g) Schematic diagram of SAED patterns in Fig. (e) and (f).

Fig. 5. (a) STEM image of precipitates in Fig. 4(c) and (b) corresponding EDS mapping results.

Table 1. Local chemical compositions (at.%) at different points in Fig. 5(b).

Number	Ti	Al	Nb
1	47.60	40.75	11.66
2	69.07	29.65	1.28
3	46.70	41.18	12.12
4	61.48	28.75	9.78

Fig. 6. (a) HRTEM image of Ti2Al shown in Fig. 4(c), (b) one-dimensional IFFT image of the selected area, (c) magnified HRTEM image of Fig. (a), (d) atom arrangement of (11$\bar{2}$0)Ti2Al.

Table 2. Lattice parameters and crystal structures of α2-Ti3Al and Ti2Al.

Phase	Lattice parameters (nm)		Structure type	Space group
Phase	a	c	Structure type	Space group
α₂-Ti₃Al	0.57	0.47	D0₁₉	P63/mmc (194)
Ti₂Al	0.29	1.39	D0₁₉	P63/mmc (194)

Fig. 7. (a) HRTEM image of areas near decomposed α2 lamellae (FFT patterns of α2 and Ti2Al are displayed in the inset), (b) one-dimensional IFFT image showing the distribution of dislocations (marked by red ┴ symbols) and SFs (marked by yellow arrows), (c) magnified HRTEM image of the rectangular area in (a).

Fig. 8. Volume fractions of α2 and Ti2Al phases in the alloy at different thermal exposure times.

Fig. 9. Schematic illustration of the precipitation of Ti2Al phases during thermal exposure at 950 °C.

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Precipitation of Ti₂Al phases at lamellar interfaces in a high-Nb-containing TiAl alloy during thermal exposure

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