![Atomic-scale insights into the strengthening effect of Cu segregation on Al Σ9 (221)[11̄0] grain boundary](https://lh3.googleusercontent.com/d/1LxK6CBvbdFqFG6g2X9WXCi7eBBGZ-CJx)
Key Takeaways & Executive Findings
- •• • Cu segregation at Al Σ9 (221)[11̄0] GB yields a negative segregation energy, indicating strong thermodynamic driving force for segregation; this improves GB stability and strength, with tensile strength enhancement attributed to reduced free volume and restricted atomic displacement. • • Dislocation nucleation shifts from a shuffling-assisted regime to a collective-migration regime upon Cu doping, requiring higher critical stress; this delays dislocation nucleation and increases tensile strength of the GB. • • Cu segregation stabilizes E structures at the GB, preserving their kite shape against structural transition during straining, which contributes to exceptional stability and reduced atomic free volume. • • Under shear deformation, Cu doping elevates GB shear strength by blocking shear-coupled GB migration; the enhanced resistance is linked to stabilized E structures with reduced atomic free volume.
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Abstract
Nanoscale segregation of alien solute atoms at grain boundaries (GBs) can enhance the stability and mechanical properties of the GB. Systematic molecular dynamics simulations were conducted to clarify the strengthening effect of Cu segregation on Al Σ9 (221)[11̄0] GB. The predicted negative segregation energy indicates a strong driving force for Cu segregation at Al GBs, which is expected to improve GB stability and strength. Detailed structural analysis during uniaxial tensile testing reveals that Cu segregation reduces the free volume of GB atoms and restricts GB atomic displacement, thereby retarding dislocation nucleation and increasing the tensile strength of the GB. The suppressed atomic migrations by Cu doping also give rise to exceptional stability of E structures at the GB, which retain their kite shape against structural transition during straining. With Cu segregation, the pattern of dislocation nucleation from the GB shifts from a shuffling-assisted regime to a collective-migration regime, the latter necessitating higher critical stress. Furthermore, Cu doping elevates the GB shear strength by blocking shear-coupled GB migration under shear deformation. The enhanced GB resistance against shear straining is attributed to the stabilized E structures with Cu segregation featuring reduced atomic free volume. This study provides atomic-scale insights into the stabilizing and strengthening effect of Cu segregation on Al GBs.
1. Introduction
Grain boundaries (GBs) in polycrystalline materials possess excess energy and are inherently thermodynamically unstable even at room temperature, making them susceptible to sliding or migration. These GBs are typically mechanically weak regions, exhibiting reduced fracture strength either at the GB or within adjacent grains. Such reduced thermodynamic stability and mechanical properties pose significant challenges for structural applications, adversely affecting load-bearing capacity and overall reliability. Consequently, understanding and improving GB stability and mechanical properties is crucial for advancing polycrystalline material performance in engineering applications.
GB segregation engineering (GBSE) has emerged as an effective strategy for manipulating GB characteristics via solute doping. The decoration of GBs with alien solute atoms facilitates improved GB properties, thereby ameliorating polycrystalline material performance, including tensile strength, fatigue resistance, fracture toughness, and strain hardening. Solute atoms exhibit a large driving force to segregate at GBs due to high excess energy and large excess free volume. For instance, Mg atoms preferentially segregate to GB regions in Al-1% Mg alloy, and Cu segregation in Zr alloy reduces Coble creep rate. Such spontaneous segregation not only improves thermodynamic stability but also affects mechanical properties such as cohesion strength, mobility, and dislocation nucleation. This study specifically addresses the bottleneck of GB weakness by employing Cu segregation to strengthen Al Σ9 (221)[11̄0] GB, providing atomic-scale insights into the underlying mechanisms.
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WANG Jianteng, WANG Xinru, RONG Xudong, LIU Enzuo, SHI Chunsheng, ZHAO Dongdong, HE Chunnian, ZHAO Naiqin (2025). Atomic-scale insights into the strengthening effect of Cu segregation on Al Σ9 (221)[11̄0] grain boundary. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3561-3
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Frequently Asked Questions
What is the quantitative evidence for Cu segregation at the Al Σ9 (221)[11̄0] grain boundary?
The predicted segregation energy is negative, indicating a strong thermodynamic driving force for Cu segregation at the Al GB. This negative segregation energy suggests that Cu atoms preferentially segregate to the GB, which is expected to improve GB stability and strength.
How does Cu segregation affect dislocation nucleation from the grain boundary under tensile loading?
Cu segregation shifts the dislocation nucleation pattern from a shuffling-assisted regime to a collective-migration regime. The latter necessitates higher critical stress, thereby retarding dislocation nucleation and increasing the tensile strength of the GB. This shift is attributed to reduced free volume and restricted atomic displacement caused by Cu doping.
What is the mechanism by which Cu segregation enhances shear strength of the Al grain boundary?
Cu doping elevates the GB shear strength by blocking shear-coupled GB migration under shear deformation. The enhanced resistance is attributed to stabilized E structures with Cu segregation featuring reduced atomic free volume, which impede atomic migration and structural transitions.
What role do E structures play in the strengthening effect of Cu segregation?
Cu segregation stabilizes E structures at the GB, allowing them to retain their kite shape against structural transition during straining. This exceptional stability, combined with reduced atomic free volume, contributes to the overall strengthening by suppressing atomic migrations and dislocation nucleation.
What are the implications of this study for the design of polycrystalline Al alloys with improved mechanical properties?
The findings provide atomic-scale insights into how Cu segregation can stabilize and strengthen Al GBs by reducing free volume, restricting atomic displacement, and altering dislocation nucleation mechanisms. This suggests that controlled Cu doping could be a viable strategy for enhancing the mechanical performance of polycrystalline Al alloys, particularly in applications requiring high tensile and shear strength.
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