Unveiling the atomic-scale origins of high damage tolerance of single-crystal high entropy alloys

High entropy alloys (HEAs) exhibit an unusual combination of high fracture strength and ductility. However, atomic mechanisms responsible for crack propagation in HEAs are still not clear, which limits further improving the damage tolerance. Here we investigate effect of crystal orientation on the crack-tip behaviors in single-crystal HEA CrMnFeCoNi using atomic simulations to explore fracture micromechanism. The formation of deformation twinning and activation of multislip systems are observed during the propagation crack with the $\left(001\right)\langle 110\rangle$ orientation, consistent with the previous experiments. Under the $\left(\overline{1}10\right)\langle 110\rangle$ orientation, the amorphous region takes place throughout the crack growth, and is difficult to occur in traditional metal materials. Dissimilarly, for the $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ orientation, the blunting and slip bands occur at the front of the crack tip by switching the slip mode from the planar to wavy slip, observed in recent transmission electron microscopy experiments. The chemical disorder leads to the obvious fluctuation of flow stress, but hardly affects the deformation mechanism at the crack tip. Compared to traditional metals and alloys, the high local stress concentration induced by coupling effect of severe lattice distortion and tension strain leads to the structure transformation from crystallization to amorphization at the crack tip in HEA. While the presented atomic simulations and the associated conclusions are based on CrMnFeCoNi HEA, it is believed that the current deformation mechanism at crack tip could also be applied to other face-centered-cubic HEA.

Figure 1

The single-crystal HEA with the edge crack under a mode-I loading condition (a). $•$ Co, $•$ Ni, $•$ Fe, $•$ Cr, and $•$ Mn. Schematic illustration of the type-1 crack with $\left(001\right)\langle 110\rangle$ (b), the type-2 crack with $\left(\overline{1}10\right)\langle 110\rangle$ (c), and the type-3 crack with $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ (d).

Figure 2

Crack-propagation processes at the temperature of 77 K, and crack plane: $\left(001\right)\langle 110\rangle$ (a1)–(a5), $\left(\overline{1}10\right)\langle 110\rangle$ (b1)–(b5), and $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ (c1)–(c5). Various strains are 3% (a1)–(c1), 4% (a2)–(c2), 5% (a3)–(c3), 7% (a4)–(c4), and 10% (a5)–(c5). Here, atoms are colored based on the common-neighbor analysis (CNA): $•$ unknown, $•$ hexagonal-close-packed (hcp), $•$ body-centered-cubic (bcc), and $•$ face-centered-cubic (fcc). The black-line region represents the twinning at the crack tip in (a5), and the black elliptical line region represents the amorphization at the crack tip in (b5).

Figure 3

Distribution of hydrostatic stress at different crack-plane orientations: $\left(001\right)\langle 110\rangle$ (a), $\left(\overline{1}10\right)\langle 110\rangle$ (b), and $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ (c) in HEA. The pink triangle region indicates the crack position.

Figure 4

Distribution of strain at different crack-plane orientations: $\left(001\right)\langle 110\rangle$ (a), $\left(\overline{1}10\right)\langle 110\rangle$ (b), and $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ (c) in HEA. The red atom represents large strain, and the blue atom means low strain.

Figure 5

The stress vs strain at the temperature of 77 K and the different crack-plane orientations: $\left(001\right)\langle 110\rangle$, $\left(\overline{1}10\right)\langle 110\rangle$, and $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ in the HEA (a). Surface energy vs crack-plane orientations (b). Radial distribution function of crystal and amorphous region (c). Microstructure evolution at the strain of 10% and the crack tip (d). The corresponding stress-field distribution along the $z$ direction (e), where the red atom represents the high tension stress, and blue atom represents the low stress.

Figure 6

Element distribution of Co, Ni, Cr, Fe, and Mn along the $x$ direction at the different samples. The different crack-plane orientations are $\left(001\right)\langle 110\rangle$ (a), (d), (g), $\left(\overline{1}10\right)\langle 110\rangle$ (b), (e), (h), and $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ (c), (f), (i).

Figure 7

The stress vs strain at other HEA sample (a), (b). The average stress vs strain from three HEA samples (a), (b).

Figure 8

Microstructure evolution of the crack tip at other HEA sample (a), (b).

Figure 9

The interplanar potentials for the HEA and associated atomic configurations. The figure contains the curve in different color regions: (a) the line in the blue region presents the slip along the leading partial direction; the line in the pink region corresponds to the faulted potential for a crystal containing an intrinsic stacking fault; the line in the light-green region shows three-layer twins. $u_x$ is the total slip, including that of the leading partial. The atomic configurations associated with the labeled extrema on the curves are displayed below them (b), (c). The coordinate systems are the $x$ axis along $\left[1\overline{1}0\right]$, the $y$ axis along $\left[11\overline{2}\right]$, and the $z$ axis along [111].

Figure 10

The evolution of microstructure (a) and strain (b) from MD simulations. The schematic diagram for the occurrence of amorphization under a mode-I loading condition (c). (a)–(c) Amorphization nucleation, and amorphous growth. (c) The resolved shear stress, ${\tau }_3$, induced by the serious lattice distortion and tension strain, drives the occurrence of dislocations and the structure transformation from crystallization to amorphization at the crack tip.

Figure 11

(a) The TEM bright-filed micrographs with the corresponding selected area diffraction patterns inset, and (b) the corresponding high-resolution TEM image of the amorphous band, with the fast Fourier transform pattern inset confirming the presence of amorphous structure [68].

Figure 12

The variation of critical size for a stable amorphous nucleus with dislocation density. The critical nucleation size of the present alloy was estimated on the order of 1 nm. The red dashed line indicates that the corresponding critical dislocation density in which the amorphization would take place is about $9.54\times {10}^{16}{\mathrm{m}}^{-2}$.

Figure 13

Dislocation structure of crack tip at strain of 12% and the different crack-plane orientations: $\left(001\right)\langle 110\rangle$, $\left(\overline{1}10\right)\langle 110\rangle$, and $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$. Dislocations are displayed according to the dislocation extraction algorithm (DXA): Perfect dislocations of deep blue, Shockley dislocations of green, Stair-rod dislocations of pink, Hirth dislocations of yellow, Frank dislocations of light blue, and unidentified dislocations of red.