[Paper Review] Segregation competition and complexion coexistence within a polycrystalline grain boundary network

Pulkit Garg, Zhiliang Pan+2 more|arXiv (Cornell University)|Mar 30, 2021

Microstructure and mechanical properties94 references4 citations

TL;DR

This study investigates interfacial segregation and complexion transitions in polycrystalline Cu-Zr alloys using hybrid Monte Carlo/molecular dynamics simulations. It reveals inhomogeneous Zr segregation across grain boundaries, with local concentrations up to an order of magnitude above the global average, and demonstrates coexistence of ordered and amorphous complexions at elevated temperatures, highlighting the structural and compositional diversity in polycrystalline networks.

ABSTRACT

Interfacial segregation can stabilize grain structures and even lead to grain boundary complexion transitions. However, understanding of the complexity of such phenomena in polycrystalline materials is limited, as most studies focus on bicrystal geometries. In this work, we investigate interfacial segregation and subsequent complexion transitions in polycrystalline Cu-Zr alloys using hybrid Monte Carlo/molecular dynamics simulations. No significant change in the grain size or structure is observed upon Zr dopant addition to a pure Cu polycrystal at moderate temperature, where grain boundary segregation is the dominant behavior. Segregation within the boundary network is inhomogeneous, with some boundaries having local concentrations that are an order of magnitude larger than the global value and others having almost no segregation, and changes to physical parameters such as boundary free volume and energy are found to correlate with dopant concentration. Further, another alloy sample is investigated at a higher temperature to probe the occurrence of widespread transitions in interfacial structure, where a significant fraction of the originally ordered boundaries transition to amorphous complexions, demonstrating the coexistence of multiple complexion types, each with their own distribution of boundary chemical composition. Overall, this work highlights that interfacial segregation and complexion structure can be diverse in a polycrystalline network. The findings shown here complement existing computational and experimental studies of individual interfaces and help pave the way for unraveling the complexity of interfacial structure in realistic microstructures.

Motivation & Objective

• To understand interfacial segregation and complexion transitions in polycrystalline materials beyond simplified bicrystal models.
• To investigate how dopant concentration and boundary-specific properties influence segregation behavior in real microstructures.
• To explore the coexistence of multiple complexion types—ordered and amorphous—within a single polycrystalline network.
• To link variations in boundary free volume and energy to local dopant segregation and structural transitions.

Proposed method

• Hybrid Monte Carlo/molecular dynamics simulations were employed to model atomic-scale dynamics and thermodynamics in polycrystalline Cu-Zr systems.
• Simulations were conducted at moderate and elevated temperatures to probe segregation and phase transitions.
• Grain boundary networks were analyzed for local dopant concentrations, free volume, and energy to correlate with structural states.
• The evolution of interfacial structure was tracked to detect transitions from ordered to amorphous complexions.
• Statistical analysis of boundary-specific properties was used to identify correlations with segregation levels and structural states.
• The model incorporated realistic polycrystalline microstructures with varying boundary types and orientations.

Experimental results

Research questions

• RQ1 How does Zr segregation distribute across grain boundaries in a polycrystalline Cu matrix at moderate temperatures?
• RQ2 What role do local boundary properties—such as free volume and energy—play in determining segregation behavior?
• RQ3 Can multiple complexion types coexist in a single polycrystalline network under thermal activation?
• RQ4 To what extent do structural transitions from ordered to amorphous complexions occur, and how are they linked to local composition?
• RQ5 How do the distributions of chemical composition and interfacial structure vary across different grain boundaries in a polycrystal?

Key findings

• Zr segregation in polycrystalline Cu is highly inhomogeneous, with local concentrations up to an order of magnitude higher than the global average.
• Boundaries with higher free volume and energy exhibit stronger Zr segregation, indicating a direct correlation between structural and chemical properties.
• At elevated temperatures, a significant fraction of initially ordered grain boundaries transitioned to amorphous complexions.
• Coexistence of ordered and amorphous complexions was observed, each with distinct and non-uniform chemical compositions.
• No significant grain growth or structural reorganization occurred at moderate temperatures, indicating segregation dominates over microstructural evolution.
• The findings demonstrate that interfacial behavior in polycrystalline systems is far more heterogeneous than predicted by bulk-averaged models.