Nonbasal Slip Systems Enable a Strong and Ductile Hexagonal-Close-Packed High-Entropy Phase

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{30}{\mathrm{Co}}_{10}{\mathrm{Cr}}_{10}$, at. %) activates three types of dislocations, i.e., $⟨a⟩$, $⟨c⟩$, and $⟨c+a⟩$. Specifically, nonbasal $⟨c+a⟩$ dislocations occupy a high line fraction of $\sim 31\%$ allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a $c/a$ ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal $⟨c+a⟩$ slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the $c/a$ ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the $c/a$ ratio of hcp phases into regimes promoting nonbasal slip activation.

Figure 1

Nonbasal slip in the hcp high-entropy phase. (a) Bright-field TEM image of a region containing hcp phase. (b) Dark-field TEM image showing the morphology of the hcp phase. The inset gives the electron diffraction pattern of the hcp phase taken under $B⫽[1\overline{2}13]$. (c) and (d) Two sequential images from the in situ movie. The yellow dashed lines represent the interfaces between the hcc and hcp phases. The scale bars in (a) and (b) are 500 nm, in (c) and (d) are 200 nm. Full details of the dislocation dynamics are given in movie in the Supplemental Material [23].

Figure 2

Morphology of dislocations in the hcp high-entropy phase under various two-beam conditions. (a) $\mathit{g}=[0002]$, (b) $g=[01\overline{1}0]$, (c) $g=[01\overline{1}1]$, and (d) $g=[2\overline{1}\overline{1}0]$. (e) Schematic illustration of the different types of dislocations. (f) Statistical results of the three types of dislocations in the hcp high-entropy phase after in situ tensile deformation. The inset displays the Burgers vector direction of each dislocation type and some important slip planes. Scale bars in (a)–(d) are 70 nm.

Figure 3

Atomic-resolution images of (a) an $⟨\mathrm{a}⟩$ dislocation and (b) a $1/2⟨c+a⟩$ dislocation core structure in the hcp high-entropy phase. Scale bars in (a) and (b) are 1 nm.

Figure 4

Double cross slip of $⟨c+a⟩$ dislocations. (a) Step-shaped $⟨c+a⟩$ dislocation segment under $\mathit{g}=[0002]$ two-beam conditions around the $[2\overline{1}\overline{1}0]$ zone axis. The scale bar is 40 nm. (b) Schematic illustration of double cross slip.

Figure 5

Overview of the $c/a$ ratios of several Mg and Ti based hcp alloys with varying alloying content compared to the hcp high-entropy phase studied here. Three distinct regimes are indicated according to the activated crystallographic deformation modes [3, 6, 7, 8, 9, 10, 11, 12, 13].