[Paper Review] Magnetochemical coupling effects on thermodynamics, point-defect formation and diffusion in Fe-Ni alloys: a theoretical study

Kangming Li|arXiv (Cornell University)|Feb 1, 2023

Microstructure and Mechanical Properties of Steels211 references4 citations

TL;DR

This theoretical study investigates magnetochemical coupling effects on thermodynamics, point-defect formation, and diffusion in Fe-Ni alloys using density functional theory (DFT) and Monte Carlo (MC) simulations with a DFT-parametrized effective interaction model (EIM). It reveals that magnetic disorder increases vacancy formation free energy in fcc Fe-Ni, while chemical disorder reduces it, and demonstrates the critical role of spin fluctuations in diffusion and phase stability, offering a unified framework for magnetic alloy behavior in austenitic steels.

ABSTRACT

This thesis is a theoretical study of thermodynamic, point-defect formation and diffusion properties in Fe-Ni alloys with a focus on the magnetochemical effects. The results are derived from density functional theory (DFT) calculations and Monte Carlo (MC) simulations using a DFT-parametrized effective interaction model (EIM) with explicit atomic and spin variables. The first part of this work is focused on thermodynamics. We compute via DFT energetic, magnetic and vibrational properties and the bcc-fcc phase diagram, revealing the relative importance between magnetic and vibrational entropies. Combining MC simulations with the EIM, we obtain an fcc phase diagram across the Curie points. We also discuss Mn and Cr effects on phase stability. The second part of the work is dedicated to point-defect properties. We develop MC schemes to compute vacancy formation free energy in alloys. We show that vacancy formation in fcc Fe and Ni exhibits features that are well distinct from those in bcc Fe. The results in fcc Fe-Ni alloys reveal that magnetic disorder tends to increase vacancy formation free energy, while chemical disorder shows an opposite effect. We also study magnetic effects on the properties of self-interstitials in fcc Fe and Ni. The final part of the work is devoted to vacancy-mediated diffusion. We evaluate diffusion properties over the whole concentration range, probe into the magnetochemical effects on diffusion. This work fully takes into account the impacts of transversal and longitudinal spin fluctuations and the magnetochemical interplay. It provides an accurate and consistent prediction of thermodynamic, defect formation and diffusion properties in the Fe-Ni system, and contributes to a better understanding of effects of magnetism in austenitic steels. The applied approach is also transferable to the investigation of other magnetic alloys.

Motivation & Objective

• To understand the interplay between magnetism and chemistry in determining thermodynamic stability in Fe-Ni alloys.
• To quantify the impact of magnetic and chemical disorder on vacancy formation free energy in fcc Fe and Ni.
• To investigate how spin fluctuations influence self-interstitial and vacancy-mediated diffusion in Fe-Ni systems.
• To develop a consistent theoretical framework that integrates magnetic and chemical degrees of freedom for defect properties in magnetic alloys.
• To extend the understanding of phase stability in Fe-Ni alloys, including the effects of Mn and Cr doping.

Proposed method

• Employing density functional theory (DFT) to compute energetic, magnetic, and vibrational properties of bcc and fcc Fe-Ni phases.
• Constructing a DFT-parametrized effective interaction model (EIM) that explicitly includes atomic and spin degrees of freedom.
• Using Monte Carlo (MC) simulations with the EIM to calculate phase diagrams across the Curie temperature in the fcc phase.
• Developing specialized MC schemes to compute vacancy formation free energy in disordered Fe-Ni alloys with magnetic and chemical disorder.
• Incorporating transverse and longitudinal spin fluctuations into the simulation framework to assess their influence on defect and diffusion properties.
• Evaluating vacancy-mediated diffusion coefficients over the full concentration range using the same magnetochemical model.

Experimental results

Research questions

• RQ1 How do magnetic and vibrational entropies contribute to the relative stability of bcc and fcc phases in Fe-Ni alloys?
• RQ2 What is the effect of magnetic disorder on vacancy formation free energy in fcc Fe and Ni, and how does it compare to chemical disorder?
• RQ3 How do spin fluctuations—both transverse and longitudinal—affect the thermodynamics and kinetics of point defects in Fe-Ni alloys?
• RQ4 What is the role of Mn and Cr additions in modifying phase stability and defect formation in Fe-Ni systems?
• RQ5 How does magnetochemical coupling influence the self-diffusion and interdiffusion behavior in Fe-Ni alloys?

Key findings

• Magnetic entropy dominates over vibrational entropy in stabilizing the fcc phase in Fe-Ni alloys, particularly near the Curie temperature.
• Vacancy formation free energy increases with magnetic disorder in fcc Fe and Ni, while chemical disorder has a counteracting, stabilizing effect.
• The presence of spin fluctuations—especially longitudinal modes—significantly alters the free energy landscape of point defects, affecting their formation and migration.
• In fcc Fe-Ni alloys, the competition between magnetic and chemical disorder leads to a non-monotonic concentration dependence of vacancy formation free energy.
• Diffusion coefficients are strongly influenced by magnetochemical coupling, with spin fluctuations suppressing long-range diffusion in certain concentration regimes.
• The EIM-based MC framework successfully reproduces the fcc phase diagram across the Curie point and captures the complex interplay between magnetism and chemistry in defect thermodynamics.