Phonon instability of a multi-principal element alloy

It is a common wisdom that the emergence of plasticity in crystalline solids is accompanied by the collapse of the softest vibrational mode. However, the recent advance in the multi-principal element alloys (MPEAs) complicates this scenario with essential local chemical inhomogeneity rendered by formation of the chemical short-range order (CSRO) via strong enthalpic interaction between specific atom pairs. Here we develop a set of computational metrics to characterize the features of phonons and the instability pathway in a prototypical ternary CoCrNi MPEA by atomistic simulations. There exists strong anharmonicity in MPEA quantified by a large Gruneisen parameter, which is modulated by the degree of CSRO. Both localized and extended vibrational modes exist at either low-or high-frequency vibrations, whereas extremely high-frequency modes prefer local vibration. Introduction of CSRO increases the phonon density of states at high frequency and reduces the degree of anharmonicity. Therefore, CSRO enhances elastic stability. Upon loading, phonon instability occurs via annihilation of the softest mode, akin to the scenario seen in conventional crystals. However, softening of several low-frequency modes occurs simultaneously in MPEA, and it is the cooperative softening of them that leads to the onset of plasticity. Amid phonon instability, essential variations in the Grueisen parameter and participation ratio are observed, signifying the correlation between vibrational and configurational space in such compositional complex alloys. The results of unusual phonon features could give insight into how the structure, elasticity, and plasticity interact in generic multi-component high-entropy alloys.

Automated summary

It is commonly accepted that plasticity in crystalline solids is accompanied by the collapse of the softest vibrational mode. However, recent developments in multi-principal element alloys have complicated this scenario with the introduction of essential local chemical inhomogeneity. Computational metrics were developed to study the phonon features and instability pathway in a ternary CoCrNi alloy. The presence of chemical short-range order (CSRO) increases anharmonicity in the alloy and enhances elastic stability. Phonon instability occurs through the annihilation of the softest mode, as well as the simultaneous softening of several low-frequency modes, leading to the onset of plasticity.