Deformation-induced crystalline-to-amorphous phase transformation in a CrMnFeCoNi high-entropy alloy

Abstract

The Cantor high-entropy alloy (HEA) of CrMnFeCoNi is a solid solution with a face-centered cubic structure. While plastic deformation in this alloy is usually dominated by dislocation slip and deformation twinning, our in situ straining transmission electron microscopy (TEM) experiments reveal a crystalline-to-amorphous phase transformation in an ultrafine-grained Cantor alloy. We find that the crack-tip structural evolution involves a sequence of formation of the crystalline, lamellar, spotted, and amorphous patterns, which represent different proportions and organizations of the crystalline and amorphous phases. Such solid-state amorphization stems from both the high lattice friction and high grain boundary resistance to dislocation glide in ultrafine-grained microstructures. The resulting increase of crack-tip dislocation densities promotes the buildup of high stresses for triggering the crystalline-to-amorphous transformation. We also observe the formation of amorphous nanobridges in the crack wake. These amorphization processes dissipate strain energies, thereby providing effective toughening mechanisms for HEAs.

Fig. 1. Microstructures near a crack tip of a UFG Cantor alloy sample, from in situ TEM tensile straining experiment.

(A) TEM image showing all three types of amorphous, spotted, and lamellar areas near a crack tip. The inset of diffraction pattern is taken from the amorphous area. (B) HRTEM image of a spotted area taken from the blue square area in Fig. 1A, showing the coexistence of crystalline and amorphous regions, as marked by C and A, respectively. (C) HRTEM image of a lamellar area taken from the red square area in Fig. 1A. (D) Low-magnification TEM image of a crack tip captured during in situ straining experiment. The black arrow points to the direction of crack propagation. A high density of dislocations is indicated by the white arrow. An amorphous area in the crack wake is indicated by the symbol A.

Fig. 2. Analysis of a lamellar structure.

(A) HRTEM image of a typical lamellar area. Four crystalline bands are marked as 1, 2, 3, and 4. Inset is an FFT pattern from the squared area. The four spots (marked by red arrows) in the FFT pattern correspond to two different sets of planes (33). (B and C) One-dimensional Fourier-filtered image of the squared area in (A), showing each of the two sets of {111} planes. Dislocations are marked with the symbol ⊥.

Fig. 3. Crack-tip dynamic process of crystalline-to-amorphous transformation from in situ straining experiment.

(A) HRTEM image indicates the coexistence of amorphous and crystalline structures at time t = 7 s. The crystalline region is 3 nm in front of the crack tip and has a length of 8 nm along the crack propagation direction. (B to D) Time-series HRTEM images showing the decreasing crystalline region and the increasing amorphous region.

Fig. 4. Amorphous nanobridges formed behind the crack tip.

(A) Low-magnification bright-field TEM image of a crack tip with nanoscale amorphous bridges. (B) Magnified TEM image of the boxed area in (A) showing a nanobridge with an amorphous structure behind the crack tip. The red arrow indicates an area with a high density of dislocations. (C) HRTEM image of an amorphous nanobridge that was ~600 nm behind the crack tip. (D) FFT pattern from an HRTEM image of the fractured amorphous bridge. The nonuniform intensity distribution is shown by two strong diffraction arcs (marked by red arrows) and thus demonstrates the existence of residual crystalline phases in the amorphous structure.

Fig. 5. MD results of crack-tip amorphization in a model binary alloy.

(A to F) MD images showing the deformation-induced crystalline-to-amorphous transformation in a near-tip region (as extracted from the large simulation cell shown in fig. S5). The atomic structure visualization tool OVITO (43) is used for a common neighbor analysis, so as to identify the amorphous structure. Three types of atoms in local FCC (green), HCP (red), and amorphous (gray) structures are identified. (G and H) Magnified atomic images in the rectangle area in (B) with associated dislocation analysis, showing different types of dislocations near the crack tip. (I) Magnified image in the rectangle area in (C), showing the nucleation of a nanovoid ahead of crack tip. (J) Magnified image of the rectangle area in (F), showing the formation of an amorphous nanobridge during crack growth.