The development of long range order in nickel-chromium alloys is of great technological interest but the kinetics and mechanisms of the transformation are poorly understood. The present research utilizes a combined computational and experimental approach to elucidate the mechanism by which phosphorus accelerates the ordering rate of stoichiometric Ni_2Cr in Ni-Cr alloys. A series of Ni-33%Cr-x%P samples (in atomic percent) were fabricated with phosphorus concentrations, x = <0.005-0.1 at.% and aged between 373 and 470°C for times up to 3000 h. The first-principles modeling considers fcc Ni with dilute P as a reasonable approximation for the complex Ni-Cr-P alloy system. Calculation results show a pronounced enhancement of vacancy transport by vacancy-solute pair diffusion via consecutive exchange and rotation jumps of vacancies associated with the phosphorus atom. The energy barriers of these two migration paths are at least 0.35 eV lower than that of vacancy-atom exchange in pure Ni solvent. The analytical diffusion model predicts enhanced solvent diffusion by 2 orders of magnitude for 0.1 at.% P at 400-500°C. The model prediction is in good agreement with the evolution of micro-hardness. We characterize the micro-hardness result by a kinetic ordering model, showing a significant decrease of the activation energy of ordering transformation. These results help gauge the risk of industrial alloys developing long range order which increases strength but degrades ductility and toughness. Specifically, minor alloying additions that bind with excess vacancies and lower the vacancy migration barrier can greatly accelerate hardening via Ni_2Cr precipitation.
Incorporation of surface correction and anharmonic correction to vacancy formation energy for nickel and copper: bridging the gap between Density Functional Theory and experiment
Synergistic Effects of Phosphorus Doping and Oxygen Vacancies on Formaldehyde Oxidation over CeO$_2$(111): A First Principles Investigation
Université de Lorraine（洛林大学）；CNRS（法国国家科学研究中心）；LEMTA（材料力学与转化实验室）；Université de Monastir（莫纳斯特尔大学）；Faculté des Sciences de Monastir（莫纳斯特尔理学院）；Laboratoire de la matière condensée et nanosciences LR11ES40（凝聚态物质与纳米科学实验室 LR11ES40）；Laboratoire Lorrain de Chimie Moléculaire（洛林分子化学实验室）；Université Lyon 1（里昂第一大学）；IRCELYON（催化、材料和表面研究所）；UMR 5256（联合研究单位 5256）
