[Paper Review] Plate-like precipitate effects on plasticity of Al-Cu alloys at micrometer to submicrometer scales

Peng Zhang, Jianjun Bian+5 more|arXiv (Cornell University)|Jul 23, 2019

Microstructure and mechanical properties85 references5 citations

TL;DR

This study investigates how plate-like θ′-Al₂Cu precipitates influence the plasticity of Al-Cu alloys at micrometer to submicrometer scales using micro-pillar compression testing. It reveals that these precipitates enhance strength and suppress strain fluctuations at larger scales, but their effectiveness diminishes at smaller sizes due to breakdown of mean-field pinning, with optimal performance observed when precipitates span the entire pillar, leading to reduced jerkiness and enhanced strain hardening via coherent interface slip and precipitate shearing.

ABSTRACT

The continuous miniaturization of modern electromechanical systems calls for a comprehensive understanding of the mechanical properties of metallic materials specific to micrometer and sub-micrometer scales. At these scales, the nature of dislocation-mediated plasticity changes radically: sub-micrometer metallic samples exhibit high yield strengths, however accompanied by detrimental intermittent strain fluctuations compromising forming processes and endangering structural stability. In this paper, we studied the effects of plate-like $θ^\prime$-Al$_2$Cu precipitates on the strength, plastic fluctuations and deformation mechanisms of Al-Cu alloys from micro-pillar compression testing. The plate-like precipitates have diameters commensurate with the external size of the Al-Cu micro-pillars. Our results show that these plate-like precipitates can strengthen the materials and suppress plastic fluctuations efficiently at large sample sizes ($\geq 3 μm$). However, the breakdown of the mean-field pinning landscape at smaller scales weakens its taming effect on intermittency. Over an intermediate range of sample sizes allowing the precipitates to cross the entire pillar, an enhanced apparent strain hardening and a sharp decrease of jerkiness are observed, in association with the presence of {100}-slip traces along the coherent $θ^\prime$-Al$_2$Cu precipitate/$α$-Al matrix interface and precipitate shearing. These complex effects of plate-like precipitates on plasticity are analyzed, experimentally and theoretically, in view of the interferences between external and internal sizes, and the related modifications of the underlying plastic mechanisms.

Motivation & Objective

• To understand the mechanical behavior of Al-Cu alloys at micrometer to submicrometer scales, where conventional plasticity theories break down.
• To investigate how plate-like θ′-Al₂Cu precipitates affect yield strength, plastic fluctuations, and deformation mechanisms in small-scale samples.
• To identify the role of precipitate size and spatial distribution relative to sample dimensions in modifying plasticity.
• To analyze the interplay between external sample size and internal precipitate structure in determining deformation stability.
• To link experimental observations of strain hardening and jerkiness with underlying dislocation-precipitate interactions at the nanoscale.

Proposed method

• Micro-pillar compression testing was performed on Al-Cu alloy samples with varying pillar diameters (from submicrometer to several micrometers).
• Plate-like θ′-Al₂Cu precipitates were characterized for size, morphology, and orientation relative to the pillar axis.
• Strain-hardening behavior and strain fluctuations (jerkiness) were quantified from load-displacement curves.
• Transmission electron microscopy (TEM) was used to identify {100}-slip traces along coherent θ′/α-Al interfaces and evidence of precipitate shearing.
• Theoretical analysis considered the breakdown of mean-field pinning models at small scales, where precipitate size becomes comparable to sample size.
• Interference between external sample size and internal precipitate dimensions was modeled to explain anomalous hardening and reduced intermittency.

Experimental results

Research questions

• RQ1 How do plate-like θ′-Al₂Cu precipitates affect the yield strength and plasticity of Al-Cu alloys at micrometer to submicrometer scales?
• RQ2 What is the role of precipitate size and spatial distribution relative to sample size in modulating strain fluctuations and deformation stability?
• RQ3 Why is there a sharp reduction in strain rate intermittency (jerkiness) when precipitates span the entire micro-pillar?
• RQ4 How do coherent interfaces and precipitate shearing contribute to enhanced strain hardening in the intermediate size regime?
• RQ5 To what extent does the breakdown of mean-field pinning models explain the loss of plasticity stabilization at submicrometer scales?

Key findings

• At sample sizes ≥3 µm, plate-like θ′-Al₂Cu precipitates effectively strengthen the material and suppress plastic strain fluctuations due to stable pinning of dislocations.
• In the intermediate size range where precipitates span the entire pillar, a sharp decrease in strain rate intermittency (jerkiness) is observed, indicating improved deformation stability.
• Enhanced apparent strain hardening is linked to the presence of {100}-slip traces along coherent θ′/α-Al interfaces and dislocation shearing of precipitates.
• At submicrometer scales, the breakdown of the mean-field pinning landscape reduces the effectiveness of precipitates in taming plasticity fluctuations, leading to increased instability.
• The interplay between external sample size and internal precipitate dimensions critically determines the dominant plasticity mechanisms and overall mechanical response.
• The study demonstrates that optimal mechanical performance is achieved when precipitate size matches the pillar diameter, enabling synergistic strengthening and stable deformation.