Dominant factors for long-term radiation damage evolution revealed in multi-principal elemental alloys

One major distinguishing characteristic of multi-principal elemental alloys (MPEAs) is that they contain two or more elements with equal/nearly equal molar ratios. The coexistence of multiple elements endows MPEAs with unique irradiation resistance and mechanical properties, making them attractive materials for application in fission and fusion reactors. Numerous studies have been conducted to understand the underlying mechanism behind their extraordinary properties. However, the inconsistencies in the obtained results remain a big hindrance to the practical applications of MPEAs.

Chemical complexity is one of the main factors influencing the irradiation performance of MPEAs. Generally, the irradiation damage of materials occurs in two stages: ballistic collision and kinetic diffusion. Several studies on MPEAs attributed their irradiance tolerance to the long-term defect evolution emanating from the kinetic diffusion phase instead of the ballistic collision phase. The defect evolution characteristics, such as defect migration and clustering, are significantly influenced by several factors including the nature of chemical orders. Moreover, extreme elemental inhomogeneity has also been shown to influence the microstructural evolution of MPEAs under radiation.

Although defect evolution in MPEAs has been extensively studied at the atomic scale, few studies are based on the larger spatial and longer time scales. As a result, it is often difficult to compare experimental and simulation results.Determining the dominating factors governing defect evolution and understanding the long-term evolution of MPEAs is, therefore, extremely challenging. This is mainly attributed to intrinsic chemical disorders. While cluster dynamics is a suitable method for studying the long-term dynamic evolution of materials, its application to MPEAs is still underexplored. In particular, it is challenging to incorporate the effects of chemical disorder in cluster dynamics.

On this account, Dr. Yaoxu Xiong, Dr. Jun Zhang, Dr. Shihua Ma, Dr. Biao Xu, and Professor Shijun Zhao from City University of Hong Kong introduced the use of atomistically-informed cluster dynamics to study the long-term radiation damage and defect evolution in MPEAs. They use equiatomic NiFe alloys as a model system, and the defect evolution in pure Ni was compared to NiFe. The influence of different factors on defect evolution, such as cluster geometry and defect migration energy heterogeneity, was evaluated. Their work is currently published in the peer-reviewed journal, Materials and Design.

The authors showed that by using appropriate parameters and proper accounting of irradiation conditions, cluster dynamics could reproduce observable cluster size distributions that are in good agreement with experimental results. The chemical disorder was identified as the main factor promoting the formation of small-sized clusters during the nucleation stage and suppressing the formation of large-sized clusters during the growth stage. In other words, it resulted in the production of smaller clusters during the collision stage in MPEAs, which suppressed the long-term formation of large-sized defect clusters.

As depicted by the broad distribution of the migration barriers attributed to chemical disorder, the atomic-level heterogeneity has negligible influence on the growth of large-sized defect clusters. Instead, it smoothened the cluster distribution under irradiation by increasing the number density of small-to-medium clusters. Furthermore, results showed that the competition between kinetics and thermodynamics was responsible for the limited cluster growth.

In summary, this is the first study to employ cluster dynamics to investigate the factors governing long-term radiation damage evolution in MPEAs, allowing the evaluation of critical features influencing the irradiation performance of MPEAs. From the results, it was deduced that the main role of chemical complexity in MPEAs was modifying the cluster growth mechanism by promoting the formation of small clusters and suppressing defect diffusivities. In a statement to Advances in Engineering, Professor Shijun Zhao noted that their study provided a comprehensive understanding of defect evolution and proposed two essential guidelines for irradiation-resistant MPEA development, which would contribute to designing high-performance MPEAs.