Microstructural Evolution of Compositionally Complex Alloys Under Heavy Ion Irradiation

Abstract

To improve the alloy discovery and development process for radiation resistant nuclear structural materials, the new design parameter of an atomically disordered base matrix was investigated experimentally using heavy-ion irradiation. Candidate compositionally complex alloys (CCAs) were compared to less compositionally complex reference materials and conventional structural materials. Cr₁₈Fe₂₇Mn₂₇Ni₂₈, Cr₁₅Fe₃₅Mn₁₅Ni₃₅, and NbTaTiV were irradiated using in-situ transmission electron microscopy (TEM) at 50 K, 500 and 600 °C and compared to structurally similar but less compositionally complex reference materials. The cryogenic temperature irradiation allowed confirmation of a lower primary defect accumulation in CCAs with diffusion limited while higher temperature irradiations with and without a bubble-stabilizing He-environment characterized the extended defect evolution at the early nucleation and incubation stages of void formation. Void swelling was also lower in He-implanted CCAs compared to less complex reference materials, which was linked to faulted interstitial dislocation loop sink strength and stacking-fault energy. Bulk irradiations were performed at 50, 75, 100, 150, and 200 dpa using various temperatures to investigate the influence of compositional complexity and composition on the temperature and dose effect on void swelling. Cr₁₈Fe₂₇Mn₂₇Ni₂₈ and Cr₁₅Fe₃₅Mn₁₅Ni₃₅ both experienced redistribution of Mn with depth, indicating that vacancies exchange preferentially with Mn atoms as they diffuse along the vacancy gradient, while Ni became enriched in the areas near voids in all materials. Voids nucleate before 50 dpa in the plateau region in Cr₁₈Fe₂₇Mn₂₇Ni₂₈ while they nucleate deep beyond the damage peak in Cr₁₅Fe₃₅Mn₁₅Ni₃₅ by 75 dpa, and not in the plateau region until between 75-100 dpa. The swelling increase from 500 to 600 °C was far less in Cr₁₅Fe₃₅Mn₁₅Ni₃₅ than 316H stainless steel at 150 dpa. Cr₁₅Fe₃₅Mn₁₅Ni₃₅ outperformed Cr₁₈Fe₂₇Mn₂₇Ni₂₈ at various doses and temperatures. The phase stability and mechanical properties of the CCA compositions are also evaluated. The aging experiment showed that both alloys experience phase separation at 700 ºC. The alloys exhibited good mechanical properties, with Cr₁₅Fe₃₅Mn₁₅Ni₃₅ slightly less ductile than Cr₁₈Fe₂₇Mn₂₇Ni₂₈ with similar strength. Results demonstrate the opportunity to improve irradiation resistance using a multiple-principal-element base matrix and highlight the effect of Mn-content on stacking-fault energy and vacancy mobility.