High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi

Abstract: High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.

• The paper demonstrates a martensitic fcc-to-hcp transformation in CrMnFeCoNi starting at ~14 GPa with the hcp phase persisting at ambient pressure.
• The paper reports a minor 0.6% volume contraction and similar bulk moduli (141 GPa for hcp, 150 GPa for fcc), indicating robust compressibility.
• The paper identifies pressure-induced magnetic moment suppression, particularly from Fe, as key to stabilizing the hcp phase, suggesting new routes for HEA processing.

Hexagonal Close-Packed Phase Synthesis in High-Entropy Alloy CrMnFeCoNi

This study investigates the synthesis of a hexagonal close-packed (hcp) phase in the high-entropy alloy CrMnFeCoNi via high-pressure techniques. High-entropy alloys (HEAs) are a class of materials defined by their multi-principal-element compositions and potential for unique mechanical properties not attainable in conventional alloys. Traditionally, these alloys, especially those based on transition metals, have been constrained to face-centered cubic (fcc) and body-centered cubic (bcc) structures. The current study expands the phase space of HEAs by synthesizing an hcp phase in CrMnFeCoNi, which displays properties that could lead to new applications in materials science.

Key Findings

• High-Pressure Transformation: The authors report a phase transformation from fcc to hcp beginning at approximately 14 GPa. This transformation is characterized as martensitic, occurring with suppression of local magnetic moments, thereby destabilizing the initial fcc structure. Remarkably, this phase change takes place over a pressure range extending to 54.1 GPa. Even after decompression to ambient pressure, the hcp phase endures, yielding metastable mixtures of fcc and hcp phases with variable volume fractions.
• Structural and Mechanical Insights: The hcp phase demonstrates a small volume contraction of about 0.6(4)%, similar to differences noted in ab initio predictions. The bulk modulus remains relatively consistent between phases, with a value of B0 = 141(8) GPa for the hcp phase compared to 150(3) GPa for the initial fcc phase, reflecting the alloy’s robustness in terms of compressibility characteristics.
• Mechanism and Implications: The transformation mechanism involves the stacking sequence reordering typical of martensitic transformations but is precipitated by pressure-induced magnetic moment suppression. This suppression of magnetism, primarily from Fe, plays a crucial role in stabilizing the high-pressure hcp phase over the fcc phase. This discovery prompts further investigation into how magnetic interactions might be leveraged for tailoring HEA structures.

Implications and Future Development

The retention of the hcp phase at ambient conditions introduces a new lever for property modification in HEAs, potentially enhancing the design of alloys with high hardness and ductility. The quasi-static nature of the hcp phase upon decompression highlights new venues for permanent phase modulations under usable conditions. Practically, this allows for innovative high-pressure processing routes to achieve and maintain desirable phase mixtures, providing a spectrum of mechanical properties for advanced applications, including those requiring high strength and corrosion resistance.

Future work may focus on the exploration of pressure-induced phase transitions across a broader range of HEAs, especially considering the promising results regarding the utility of suppressed magnetic moments to control phase stability. Furthermore, reducing critical pressure for phase transition through composition modification, such as Fe removal, could make these processing techniques more economically viable and easily scalable for industrial applications. Understanding such transformations could also contribute to the development of resilient materials for extreme environmental conditions, such as deep-sea or extraterrestrial applications where high-pressure environments naturally prevail.