Pressure-induced structural modifications in high-entropy alloys with varying Al contents, AlxCoCrFeNi (x = 0, 0.1, 0.3, 0.75, 1.5), have been investigated at pressures up to ∼50 GPa by synchrotron X-ray diffraction and, following depressurization, by transmission electron microscopy (TEM). In AlxCoCrFeNi compounds with x ≤ 0.3, all of which exhibit an initial single-phase face-centered cubic (fcc) structure, proportionality between the Al content and the critical pressure for transformation to hexagonal close-packed (hcp) phases, distinguished by a distinct planar stacking sequence, is observed. This is attributed to the structural distortion arising from the large size of Al atoms relative to those of the other constituent elements, which results in an increase in the formation energy of stacking faults and a decrease in compressibility. High-resolution TEM results demonstrate variation of the stacking sequence from ABCABC, typical of fcc materials, to ABABAB, typical of hcp materials, in CoCrFeNi following high pressure. In Al0.75CoCrFeNi, which exhibits an initial dual-phase structure [fcc and body-centered cubic (bcc)], the result again shows the formation of a (hcp) phase despite its higher Al content, suggesting that the bcc phase may be more amenable to pressure-induced phase modification than is the fcc phase, which is absent for lower Al contents. However, the trend of transformation inhibition by increasing the Al content is again observed, with Al1.5CoCrFeNi retaining its initial structure up to the highest pressure achieved. Determination of these compositional trends in the high-pressure phase response of these materials may enable the production of new phase mixtures with precisely tuned phase proportions and potentially desirable properties.

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