Microstructural stability and aging behavior of refractory high entropy alloys at intermediate temperatures

doi:10.1016/j.jmst.2021.12.057

Abstract

Abstract:

Several body-centered-cubic (BCC) refractory high entropy alloys (HEAs), i.e., HfNbTaTiZr, NbTaTiZr, HfNbTiZr and NbTiZr, were annealed at intermediate temperatures for 100 h, and their microstructures and aging behaviors were studied in detail. All these HEAs start to decompose into multiple phases at around 500 °C, but reenter the single-phase region at significantly different temperatures which were determined to be 900, 1000, 1100 and above 1300 °C for HfNbTiZr, NbTiZr, HfNbTaTiZr and NbTaTiZr, respectively. Our analysis indicates that the onset decomposition temperature in these four HEAs is closely related to the elemental diffusion rates while the ending decomposition temperature is strongly dependent on the elemental melting points. Our findings are important not only for understanding phase stability of HEAs in general, but also for adjusting processing parameters to optimize mechanical properties of these HEAs.

Key words: Refractory high entropy alloys, Phase decomposition, Diffusion rate, Melting point

Meng Dai, P.P. Cao, H.L. Huang, S.H. Jiang, X.J. Liu, H. Wang, Y. Wu, Z.P. Lu. Microstructural stability and aging behavior of refractory high entropy alloys at intermediate temperatures[J]. J. Mater. Sci. Technol., 2022, 122: 243-254.

Figures/Tables 15

Fig. 1. Inverse pole figures from EBSD of the HfNbTaTiZr(a), NbTaTiZr (b), HfNbTiZr (c) and NbTiZr (d) after homogenized treatment and the corresponding XRD results (e).

Fig. 2. XRD patterns (a) and lattice parameters (b) of the HfNbTaTiZr HEA annealed at different temperatures for 100 h. The red arrows in (a) indicate the diffraction peaks of the third phase in HfNbTaTiZr annealed at 650 °C for 100 h.

Fig. 3. (a-i) Backscattered electron images of the HfNbTaTiZr HEA after annealing at different temperatures for 100 h. The yellow squares in (f) show three contrast phases in HfNbTaTiZr annealed at 750 °C for 100 h.

Fig. 4. Bright-field (BF) TEM and the corresponding elemental-distribution maps obtained by EDS in HfNbTaTiZr annealed at 750 °C for 100 h.

Fig. 5. HRTEM images and SAED patterns of the phase interfaces in HfNbTaTiZr annealed at 750 °C for 100 h.

Fig. 6. XRD patterns (a) and lattice parameters (b) of the NbTaTiZr HEA annealed at different temperatures for 100 h.

Fig. 7. Backscattered electron images of the NbTaTiZr HEA after annealing at different temperatures for 100 h.

Fig. 8. STEM image (a) and the elemental-distribution corresponding to the yellow wireframe region of NbTaTiZr annealed at 750 °C for 100 h. HRTEM image (b) and SAED pattern (c) of the phase interfaces.

Fig. 9. XRD patterns (a) and backscattered electron images (b-g) of the HfNbTiZr HEA annealed at different temperatures for 100 h.

Fig. 10. . STEM image (a) and the elemental-distribution maps corresponding to the yellow square region of HfNbTiZr following 100 h exposure at 750 °C. SAED patterns of the precipitates (b, c) and the phase interfaces (d).

Fig. 11. XRD patterns (a) and backscattered electron images (b-g) of NbTiZr annealed at different temperatures for 100 h.

Fig. 12. STEM image (a) and the elemental-distribution corresponding to the yellow square region of NbTiZr following 100 h exposure at 750 °C. SAED pattern (b) of the phase interfaces.

Table 1. The onset temperature (Tc), the ending temperature (Ts) and the temperature range (ΔT) of phase decomposition after annealing for 100 h obtained by SEM and XRD data.

Empty Cell	Temperature range of phase decomposition ( °C)
	XRD			SEM
	T_c	T_s	ΔT	T_c	T_s	ΔT
HfNbTiZr	500			500	900	400
NbTiZr				500	1000	500
HfNbTaTiZr	500	1000	500	500	1100	600
NbTaTiZr	500	1300+	800+	500	1300+	800+

Fig. 13. . Self-diffusion coefficients of Hf, Nb, Ta, Ti, Zr at different temperatures. The intrinsic diffusion constant D0 and activation energy for diffusion QD for each element were taken from Ref [26]., and their units are 10-4 m2/s and kJ/mol, respectively.

Fig. 14. Dependence of Ts on the configuration entropy (a), atomic size difference (b), the enthalpy of mixing (c), parameter Ω (d), electronegativity difference (e), and the melting point difference (f) of the four RHEAs after annealing for 100 h.

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