arXiv subjects

Sourabh Bhagwan Kadambi

Publications and source records attributed to Sourabh Bhagwan Kadambi.

5 recordsLinked to original sources

arXiv2026

Deconvoluting Mechanisms of Radiation-Induced Grain Boundary Segregation in Dilute Ni-X (X = Fe, Cr, Mn) Alloys via Ion Irradiation and Phase-field Modeling

Radiation-induced composition redistribution at grain boundaries (GBs) in Ni-based structural alloys poses a critical challenge for the long-term deployment of advanced nuclear reactors. The mechanisms governing solute-specific segregation remain poorly understood because defect-mediated transport, thermodynamic driving forces, and competing microstructural sinks can all contribute under irradiation. We perform systematic Ni2+ ion irradiations on dilute Ni-Fe, Ni-Cr, and Ni-Mn binary alloys at 500, 800, and 1000 K to 2 dpa and characterize GB chemistry using STEM-EDS. The segregation behavior is interpreted using rate-theory phase-field modeling informed by atomistic transport data. We find that Fe depletes at GBs across all temperatures via preferential vacancy-solute exchange (inverse Kirkendall effect). Cr segregation reflects a temperature-dependent competition between self-interstitial atom (SIA)-mediated transport toward GBs and vacancy-mediated transport away from GBs, yielding near-zero net segregation at 500 K and depletion at higher temperatures. In contrast, Mn exhibits distinct behavior dominated by thermodynamic segregation that drives local enrichment at the GB. We leverage depth-dependent characterization along the same GB, spanning irradiated and unirradiated regions, to separate radiation-induced and thermodynamic contributions and validate the unirradiated region as an internal thermodynamic reference. Void formation at 800 K and irradiation-induced GB migration at 800 and 1000 K further add mechanistic complexity by suppressing and redistributing the solute. Our combined experimental and computational assessments show that interpreting GB chemistry evolution under irradiation requires concurrent treatment of non-equilibrium kinetic transport, thermodynamic driving forces, bulk sink effects, and GB migration.

cond-mat.mtrl-sci↗

arXiv2026

Radiation-induced segregation in dilute Fe-Cr: A rate-theory framework for the Cr enrichment-depletion transition at the grain boundary

Radiation-induced segregation (RIS) poses a significant challenge for ferritic Fe-Cr alloys under irradiation, as it can compromise mechanical integrity and increase susceptibility to intergranular corrosion. Yet, the mechanisms governing Cr segregation remain incompletely understood. In this study, We present a physics-based rate-theory model parameterized using self-consistent mean field theory-based Onsager transport coefficients to investigate RIS at the grain boundary (GB) in dilute Fe-(0.1 at.%) Cr. Under equal production rates of vacancies and self-interstitial atoms (SIA), and their equal absorption rates by bulk dislocations, the model simulates the experimentally observed transition from Cr enrichment at low temperatures to depletion at higher temperatures. Under these unbiased conditions, systematic investigation reveals that while temperature-dependent transport properties dictate the segregation direction, dose rate, grain size, and dislocation density only influence the magnitude and spatial extent of Cr segregation. However, under more realistic conditions of preferential vacancy production within damage cascade and/or preferential SIA absorption by bulk dislocations, the enrichment-to-depletion transition shifts to lower temperatures. Our findings demonstrate that RIS predictions based solely on transport coefficients are valid only under symmetric point defect flux conditions, and that biases in defect production and absorption must be considered for accurate predictions. This work provides a mechanistic framework for understanding RIS in ferritic alloys and informs alloy design for advanced nuclear systems.

cond-mat.mtrl-sci↗

arXiv2025

Phase-field modeling of radiation-induced composition redistribution: An application to additively manufactured austenitic Fe-Cr-Ni

Multicomponent alloys undergoing irradiation damage develop radiation-induced composition redistribution at point defect sinks such as grain boundaries (GBs) and dislocations. Such redistribution results in undesired changes to their mechanical behavior and corrosion resistance. Additively manufactured alloys proposed for future nuclear applications are expected to demonstrate a distinct response to irradiation owing to their unique microstructure with as-solidified dislocation density and chemical microsegregation. To capture the composition redistribution in such systems, we develop a mesoscale model with coupled evolution of atomic and point defect components in the presence of dislocation density, dislocation heterogeneity, and thermodynamic interactions at the GB. The model is parameterized for an FCC Fe-Cr-Ni alloy as a representative system for austenitic stainless steels, and simulations are performed in 1D and 2D as a function of irradiation temperature, dose, dislocation density, and grain size. Radiation-induced segregation (RIS) characterized by Cr depletion and Ni enrichment is predicted at both the GB and the dislocation cell wall, with RIS being lower in magnitude but wider at the cell wall. Strongly biased absorption of self-interstitials by dislocations is found to suppress Ni enrichment but slightly enhance Cr depletion under certain conditions. Thermodynamic segregation at the GB is predicted to be narrower and opposite in sign to RIS for both Cr and Ni. Importantly, non-monotonic segregation is found to occur when both thermodynamic and RIS mechanisms are considered, providing a novel physical interpretation of experimental observations. The model is expected to serve as a key tool in accelerated qualification of irradiated materials.

cond-mat.mtrl-sci↗

arXiv2024

Vacancy-mediated transport and segregation tendencies of solutes in FCC nickel under diffusional creep: A density functional theory study

The Nabarro-Herring (N-H) diffusional creep theory postulates the vacancy-mediated transport of atoms under a stress gradient as the creep mechanism under low-stress and high-temperature conditions. In multicomponent alloys, we premise that this stress-assisted flow of vacancies to and from grain boundaries will produce elemental segregation. An observation of such segregation, validated with theoretical predictions, can provide the necessary experimental evidence for the occurrence of N-H creep. Theoretical calculations of the segregation tendencies via analyzing the dominant solute diffusion mechanisms and the difference in diffusivities of the elements are therefore essential. To this end, this study applies density functional theory calculations of migration barriers and solute-vacancy binding energies as input to the self-consistent mean field theory to assess the vacancy-mediated diffusion mechanisms, transport coefficients, and segregation tendencies of Co, Cr, Mo, Re, Ta, and W solutes in face-centered cubic Ni. We find Co, Re, and W to be slow diffusers at high temperatures and Cr, Mo, and Ta to be fast diffusers. Further analysis shows that the slow diffusers tend to always enrich at vacancy sinks over a wide range of temperatures. In contrast, the fast diffusers show a transition from depletion to enrichment as the temperature lowers. Furthermore, our analysis of the segregation tendencies under tensile hydrostatic strains shows that slow diffusers are largely unaffected by the strain and favor enrichment. On the other hand, the fast diffusers exhibit high sensitivity to strain and their segregation tendency can transition from depletion to enrichment at a given temperature. The transport coefficients calculated in this work are expected to serve as input to mesoscale microstructure models to provide a more rigorous assessment of solute segregation under N-H creep conditions.

cond-mat.mtrl-sci↗

arXiv2022

Phase Boundary Segregation in Multicomponent Alloys: A Diffuse-Interface Thermodynamic Model

Microalloying elements tend to segregate to the matrix-precipitate phase boundaries to reduce the interfacial energy. The segregation mechanism is emerging as a novel design strategy for developing precipitation-hardened alloys with significantly improved coarsening resistance for high temperature applications. In this paper, we report a nanoscopic diffuse-interface thermodynamic model that describes multicomponent segregation behavior in two-phase substitutional alloys. Following classical approaches for grain boundaries, we employ the regular solution thermodynamics to establish segregation isotherms. We show that the model recovers the Guttmann multicomponent isotherm describing local interfacial concentrations, and the generalized Gibbs adsorption isotherm that governs the total solute excess and interfacial energy. A variety of multicomponent segregation behaviors are demonstrated for a model two-phase quaternary alloy. The nature of interfacial parameters and the resulting analytic solutions make the model amenable for parameterization and comparison with atomistic calculations and experimental characterizations.

cond-mat.mtrl-sci↗