Ab initio study of phosphorus effect on vacancy-mediated process in nickel alloys – An insight into Ni2Cr ordering

Abstract

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 2 Cr 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 2 Cr precipitation.

A First-Principles Study of the Vacancy-Mediated Silver Impurity Diffusion in fcc Nickel

“…Parameter-free first-principles approaches based on density functional theory (DFT) provide a powerful method for obtaining detailed point defect energetics. They are increasingly used to predict selfand impurity diffusion of elements in pure metals and multi-component alloys [1,[2][3][4]. In this report, we demonstrate the feasibility of combining a thermodynamics integration method with ab initio total energy calculations in order to evaluate thermodynamics properties, namely the impurity coefficient.…”

On the Formation of AlNiCo Nano-Quasicrystalline Phase during Mechanical Alloying through Electroless Ni-P Plating of Starting Particles

“…However, there is no report in the literature investigating the influence of phosphorus on the prohibition or promotion of QC formation. On the other hand, the role of this element in the formation of other phases is available [14,15]. As an example, a combined computational and experimental approach was applied by Ke et al [15] to explain how minute amounts of phosphorus accelerate the ordering rate of stoichiometric Ni 2 Cr in Ni-Cr alloys.…”

“…On the other hand, the role of this element in the formation of other phases is available [14,15]. As an example, a combined computational and experimental approach was applied by Ke et al [15] to explain how minute amounts of phosphorus accelerate the ordering rate of stoichiometric Ni 2 Cr in Ni-Cr alloys. It was declared that the acceleration is caused by strong binding between phosphorus and vacancies in the applied alloy, which can increase the rate of phase transformation in a significant manner.…”

Influence of mechanical deflector assisted electroless nickel deposits for wear resistance

“…In the process, the hardness of the alloy reaches a maximum and then decreases in a manner typical of precipitation hardening processes. The amount of nickel phosphide formed depends on the phosphorous content of the alloy and the time-temperature history of heat treatment [6,7].…”

“…The phosphorous content of deposits may range from 1 to 13 percent, and their structure may vary from pure crystalline beta phase to totally amorphous gamma phase. Their relative proportions have a marked influence on many of the properties of the coatings [6]. The degree of hardening that could be expected from EN deposits on aging at a given temperature depends on the phosphorus content and the aging time employed [8,9].…”