Grain boundary segregation induced strong UHTCs at elevated temperatures: A universal mechanism from conventional UHTCs to high entropy UHTCs

doi:10.1016/j.jmst.2021.12.074

Abstract

Abstract:

Ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties, including ultra-high melting point, excellent chemical stability, and good oxidation resistance, which make them promising candidates for aerospace and nuclear applications. However, the degradation of high-temperature strength is one of the main limitations for their ultra-high temperature applications. Thus, searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed. To achieve this goal, grain boundary segregation of a series of carbides, including conventional, medium entropy, and high entropy transition metal carbides, i.e., Zr_0.95W_0.05C, TiZrHfC₃, ZrHfNbTaC₄, TiZrHfNbTaC₅, were studied by atomistic simulations with a fitted Deep Potential (DP), and the effects of segregation on grain boundary strength were emphasized. For all the studied carbides, grain boundary segregations are realized, which are dominated by the atomic size effect. In addition, tensile simulations indicate that grain boundaries (GBs) will usually be strengthened due to segregation. Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs, since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.

Key words: UHTCs, High entropy ceramics, Grain boundary segregation, High-temperature strength, Machine learning potential

Fu-Zhi Dai, Bo Wen, Yinjie Sun, Yixiao Ren, Huimin Xiang, Yanchun Zhou. Grain boundary segregation induced strong UHTCs at elevated temperatures: A universal mechanism from conventional UHTCs to high entropy UHTCs[J]. J. Mater. Sci. Technol., 2022, 123: 26-33.

Figures/Tables 7

Fig. 1. Schematic illustration of a Σ5 coincident site lattice (CSL) and the Σ5(210) CSL grain boundary. Red and blue points represent the lattice points of two misoriented crystals. The green line illustrates the trace of the grain boundary, and the green points are coincident sites in the boundary. The box enclosed by the dash lines and the green line is the primitive cell of the CSL, which is 5 times that of the primitive cell of the crystal (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 2. Comparison of energies and forces predicted by the DP model with the DFT-based calculations, and probability density distributions of prediction errors on energy and force.

Fig. 3. Distribution of the normalized concentration cn of Ti, Zr, Hf, Nb, Ta and their corresponding atomic map. In the probability density distribution figures, the dashed line is a Gaussian distribution fitted to the distribution from the data of Ta. PD: probability density.

Fig. 4. (a) Heat map of Pearson correlation coefficients of cn for the elements in the Σ3(112) grain boundary of TiZrHfNbTaC5, (b) Illustration of local potential energies perceived by different elements in grains or GBs.

Table 1. Exceeding normalized concentration δi (i = Ti, Zr, Hf, Nb, Ta) in different GBs of medium entropy TiZrHfC3, ZrHfNbTaC4 and high entropy TiZrHfNbTaC5.

Empty Cell	Empty Cell	Σ3(112)	Σ5(210)	Σ5(310)	Σ9(221)	Σ11(113)
TiZrHfC₃	δ_Ti	0.51	0.35	0.36	0.18	0.33
	δ_Zr	-0.19	-0.15	-0.06	-0.03	0.01
	δ_Hf	-0.31	-0.20	-0.30	-0.16	-0.34
ZrHfNbTaC₄	δ_Zr	-0.02	-0.03	-0.01	0.05	0.05
	δ_Hf	-0.31	-0.29	-0.33	-0.17	-0.34
	δ_Nb	0.27	0.27	0.32	0.10	0.18
	δ_Ta	0.05	0.05	0.03	0.01	0.10
TiZrHfNbTaC₅	δ_Ti	0.32	0.13	0.23	0.01	-0.07
	δ_Zr	0.03	-0.03	0.02	0.18	0.24
	δ_Hf	-0.13	-0.15	-0.23	-0.01	-0.18
	δ_Nb	-0.02	0.08	0.04	-0.07	0.00
	δ_Ta	-0.20	-0.03	-0.07	-0.12	0.01

Fig. 5. Grain boundary strengths of (a) Zr0.95W0.05C, (b) TiZrHfC3, (c) ZrHfNbTaC4, and (d) TiZrHfNbTaC5 with and without segregation.

Fig. 6. (a) Stress-strain curves of the Σ3(112) grain boundary with starting structure being an unsheared one, (b) Unsheared and (c) Sheared structures of the Σ3(112) grain boundary, (d) Stress-strain curves of the Σ3(112) grain boundary with starting structure being a sheared one. With and without MC/MD means whether or not MC/MD simulation is carried out before tensile simulation. In (b) and (c), the carbide is high entropy TiZrHfNbTaC5, where small black spheres are carbon atoms, and the big colorful spheres are metal atoms (Ti: blue, Zr: yellow, Hf: pink, Nb: green, Ta: purple). The dashed line shows the position of the grain boundary. Some of the metal atoms are colored white to illustrate the shear, as shown by the red lines (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

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