Effects of non-hydrostaticity and grain size on the pressure-induced phase transition of the CoCrFeMnNi high-entropy alloy

Abstract

Recently, an irreversible polymorphic transition from face-centered cubic to hexagonal close-packing was surprisingly observed under high pressure in the prototype CoCrFeMnNi high-entropy alloys (HEAs) by various research groups. This unexpected phase transition brings new insights into the stability of HEAs, and its irreversibility stimulates exploration for new HEAs via high-pressure compression synthesis. However, the onset pressure for the phase transition was reported to fluctuate over a vast range from ∼7 to above 49 GPa in the reported experiments. The reason for this inconsistency remains unclear and puzzles the HEA community. To address this problem, this work systematically investigates the effects of non-hydrostaticity and grain size. Our results demonstrate that larger deviatoric stress induced by the non-hydrostaticity of the pressure medium and larger grain size of the initial sample can both promote a phase transition and, therefore, considerably depress the onset pressure.

Microstructure, Texture, and Strength Development during High-Pressure Torsion of CrMnFeCoNi High-Entropy Alloy

“…To transform the FCC phase to HCP a certain onset pressure is necessary depending on hydrostaticity and grain size. On the one hand, by using different media to apply pressure, the onset pressure at RT decreases from 22.1 GPa (helium) to 6.9 (silicone oil) and 2.2-6.6 GPa (amorphous boron), i.e., with increasing non-hydrostaticity [34,35] (see also effect on iron [36]). On the other hand, for a given medium, silicone oil, decreasing the grain size from 5 µm to about 0.01 µm increases the onset pressure from 6.9 to 12.3 GPa [35].…”

“…On the one hand, by using different media to apply pressure, the onset pressure at RT decreases from 22.1 GPa (helium) to 6.9 (silicone oil) and 2.2-6.6 GPa (amorphous boron), i.e., with increasing non-hydrostaticity [34,35] (see also effect on iron [36]). On the other hand, for a given medium, silicone oil, decreasing the grain size from 5 µm to about 0.01 µm increases the onset pressure from 6.9 to 12.3 GPa [35]. As the FCC to HCP martensitic transformation takes place by slip of 1/6 <112> partial dislocations on every second {111} plane [37], it is surprising, that in the present case under high shear stress, HPT at RT under a pressure of 7.8 GPa did not lead to the transformation.…”

Mechanical and Magnetic Properties of the High-Entropy Alloys for Combinatorial Approaches

“…In summary, the element effect can contribute to local heterogeneous atomic sizes, electronegativity [9], and magnetic properties [20,22] of the high-entropy alloys among the elements. These differences result in low stacking fault energies [20,21], twinning [5,42], and phase transformation [3,15,30,43,[66][67][68][69][70], which accommodate the deformation and enhance the overall performance in terms of strength and ductility.…”

Unveiling the Electronic Origin for Pressure-Induced Phase Transitions in High-Entropy Alloys

“…Such an abnormal phenomenon could be attributed to the presence of shear stress in the diamond-anvil cell (DAC) ( Figure S1), which has been proved to promote the onset of the fcc to hcp transformation in the CrMnFeCoNi alloy. 33 The pressure medium used in our experiment was alcohol, which solidifies above 10 GPa and leads to a non-hydrostatic pressure environment. Thus, shear stresses possibly generated inside the DAC.…”

“…[1][2][3][26][27][28][29][30][31] On the one hand, it was generally recognized that the fcc phase in multicomponent HEAs observed by high-temperature quenching was metastable at room and low temperatures. 1,[27][28][29]32,33 On the other hand, possibly due to the kinetic limitations, until now no fcc to hcp transformation has been experimentally observed in equiatomic fcc HEAs at ambient temperature and pressure. Excitingly, the hydrostatic pressure-induced fcc to hcp transformation [1][2][3]34 has made it possible to distinguish the intrinsic stability of the fcc and hcp structures at room temperature; however, the pressure-related fcc to hcp transformation in multicomponent HEAs is quite complicated.…”