Post-irradiation defect evolution in alloys with varying degrees of lattice distortion.

Professor Lu Chenyang's team at Xi'an Jiaotong University (XJTU), in collaboration with the City University of Hong Kong and Hunan University, has proposed a brand-new design strategy for radiation-resistant materials.

By combining experimental work, molecular dynamics (MD) simulations, and theoretical diffusion models, the researchers demonstrated that enhancing local lattice distortion can simultaneously "freeze" the migration of both interstitials and vacancies.

The findings have been published in the internationally renowned journal Nature Communications under the title Realizing irradiation-resistant metallic alloys by immobilizing induced defects.

The study designed four face-centered cubic (FCC) alloys with varying degrees of lattice distortion: Ni, NiFe, NiCoV, and Ni₈₀Mo₂₀. These alloys were subjected to 3-MeV Ni-ion irradiation experiments at room temperature. The team then analyzed the formation and diffusion behaviors of radiation-induced defects using cumulative cascade collision MD simulations.

These experimental results revealed that as lattice distortion increases, the size of dislocation loops continuously decreases. In the Ni₈₀Mo₂₀ alloy, not only was the defect size the smallest, but the defect density was also significantly reduced. This demonstrates that high lattice distortion can effectively suppress the growth of radiation-induced defects.

In Ni, defects readily migrate and aggregate into large clusters. In NiCoV, interstitial diffusion slows down, but vacancy diffusion is unexpectedly enhanced. In Ni₈₀Mo₂₀, both interstitials and vacancies exhibit highly localized migration characteristics.

Most notably, vacancies in Ni₈₀Mo₂₀ stop undergoing long-range diffusion. Instead, they repeatedly "hop back and forth" between localized lattice sites, resulting in a diffusion behavior that resembles being "frozen." This discovery challenges conventional understanding, proving that high lattice distortion can not only suppress interstitial diffusion but also significantly localize vacancy motion under extreme conditions.

To uncover the root cause of these frozen vacancies, the team further analyzed the distributions of vacancy formation and migration energies, where they discovered that enhancing lattice distortion significantly broadens the local energy distribution.

Although the average migration energy barrier decreases, much stronger local energy traps form between different lattice sites. Consequently, vacancies tend to engage in forward-and-backward hopping between adjacent lattice sites, making effective long-range diffusion difficult.

The paper proposes that two competing mechanisms ultimately dictate vacancy diffusion:

1. Lower migration energy barriers that promote vacancy hopping.

2. Stronger local energy fluctuations that enhance vacancy trapping.

When the lattice distortion exceeds approximately 4 percent, the latter mechanism begins to dominate, ultimately freezing the vacancy diffusion.

Based on systematic experiments and theoretical analysis, the paper establishes a unified framework for "tuning defect evolution via lattice distortion":

1. Low-to-Medium Lattice Distortion: Suppresses interstitial diffusion, enhances vacancy migration, and improves defect recombination efficiency.

2. Extremely High Lattice Distortion: Simultaneously freezes both interstitials and vacancies, suppresses defect aggregation, and forms a more stable, radiation-resistant structure.

This mechanism resolves conflicting experimental results in past literature on the evolution of vacancy clusters, providing a fresh theoretical foundation and a material design pathway for developing radiation-resistant structural materials using solid-solution and high-entropy alloys (HEAs).