The Role of Intrinsic Stacking Fault in Facilitating the Pressure-Induced Phase Transition in CoCrFeMnNi High Entropy Alloys

The pressure-induced phase transitions in CoCrFeNi and CoCrFeMnNi high entropy alloys (HEAs) at ambient temperature at pressure up to 24.0(2) and 19.4(2) GPa, respectively, were investigated using angle-dispersive X-ray diffraction (ADXRD). Structurally at ambient pressure, both CoCrFeNi and CoCrFeMnNi HEAs consist of face-centered cubic (fcc) structure with different lattice constants which are arisen primarily from the cellular growth of alloy during solidification. Insitu ADXRD measurements revealed no evidence of structural transformation in CoCrFeNi HEAs up to 24.0(2) GPa. The intrinsic stacking fault (ISF) begins to appear at 1.7(1) GPa and sustains up to 19.4(2) GPa. Moreover, an fcc to hexagonal close-packed (hcp) structural phase transition emerges at around 7.0(1) GPa in CoCrFeMnNi HEAs. The pressure dependent lattice constants and volume compression yield the zero-pressure isothermal bulk moduli of 187(4) GPa while the normalized c/a ratio 1.636(1) for the resultant hcp phase. The quantitative correlation of the ISF diffraction intensity shows that the appearance of ISF disrupts the crystal lattice to trigger, at around 7.0(1) GPa, fcc-to-hcp phase transition which persists sluggishly to the highest experiment pressure. Neutron powder diffraction (NPD) at pressure up to 8.9(2) GPa was performed in CoCrFeMnNi HEAs at ambient temperature to clarify the significance of pressure induced suppression of local magnetic moment on destabilization of the initial fcc structure. The results, however, suggest that the magnetism may only play a minor role, if not none, in facilitating the pressure-induced fcc-to-hcp phase transition in CoCrFeMnNi HEAs.