An extended Rice model for intergranular fracture

TL;DR

This paper extends the Rice model to better predict intergranular fracture in metals by incorporating grain boundary structural transformations, validated through molecular dynamics simulations, revealing rate-dependent critical stress intensity factors.

Contribution

The paper introduces a semi-analytical, transition-state-theory-based extension of the Rice model that accounts for grain boundary structural changes during fracture.

Findings

Grain boundaries significantly alter the stress intensity needed for plastic events.

The critical dynamic SIF increases with loading rate.

Classical Rice model underestimates the critical SIF due to neglecting localized effects.

Abstract

The plastic events occurring during the process of intergranular fracture in metals is still not well understood due to the complexity of grain boundary (GB) structures and their interactions with crack-tip dislocation plasticity. By considering the local GB structural transformation after dislocation emission from a GB in the Peierls-type Rice-Beltz model, herein we established a semi-analytical transition-state-theory-based framework to predict the most probable Mode-I stress intensity factor (SIF) for dislocation emission from a cracked GB. Using large-scale molecular dynamics (MD) simulations, we studied the fracture behaviors of bi-crystalline Fe samples with 12 different symmetric tilt GBs inside. The MD results demonstrate that the presence of GB could significantly change the SIF required for the activation of plastic events, confirming the theoretical predictions that attributes this to the energy change caused by the transformation of GB structure. Both the atomistic simulation and the theoretical model consistently indicate that, the critical dynamic SIF ($K_{I}^{c}(t)$) at which the dynamic SIF $K_{I}(t)$ deviates from the linearity with respect to the strain ${\epsilon}$, increases with the increasing loading rate. However, the classical Rice model underestimates the $K_{I}^{c}(t)$ due to its failure to consider the effects of localized fields. The present theoretical model provides a mechanism-based framework for the application of grain boundary engineering in the design and fabrication of nano-grained metals.