Segregation-Controlled Diffusion-Induced Grain Boundary Migration in Alloy 690

TL;DR

This study uses simulations to reveal how chromium segregation and diffusion drive grain boundary migration and Cr depletion in Alloy 690, explaining experimental observations and providing a unified mechanistic framework.

Contribution

It uncovers the fundamental driving force behind GB migration linked to Cr diffusion and segregation, integrating solute-GB interactions into a comprehensive model.

Findings

Cr segregation can facilitate GB migration and Cr depletion.

Cr diffusion creates depletion zones that drive GB migration.

Migration is controlled by solute diffusivity and segregation strength.

Abstract

Grain boundary (GB) migration accompanied by Cr depletion is widely observed in Alloy 690 and is closely linked to intergranular degradation and stress corrosion cracking. However, the fundamental driving force for GB migration and its link with Cr depletion remains unclear. In this work, hybrid molecular dynamics and semi-grand canonical Monte Carlo simulations were employed to investigate GB migration in Alloy 690 under coupled solute diffusion and segregation effects across a range of GB characters. The results show that Cr segregation at GBs, while generally considered favorable for GB stability, can facilitate diffusion-induced GB migration and Cr depletion. Cr diffusion along GBs produces localized Cr depletion zones that are energetically incompatible with positively segregating GBs, generating a chemical driving force that drives GB migration toward the Cr-rich matrix, which ultimately results in persistent GB migration accompanied by a Cr depletion. By quantifying solute-GB interaction energetics, we demonstrate that GB migration is quantitively controlled by the coupled effects of solute diffusivity and segregation strength. These mechanistic insights provide a unified framework that rationalizes experimentally observed correlations between GB character, Cr depletion, and GB migration in Cr-containing alloys.