Plate-like precipitate effects on plasticity of Al-Cu alloys at micrometer to sub-micrometer scales

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 endangering structural stability. In this paper, we studied the effects of plate-like θ′-Al₂Cu precipitates on the strength and plastic fluctuations of Al-Cu alloys from micro-pillar compression testing. The particularity of our samples is that 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 through the Orowan mechanism at large sample size. However, the breakdown of the mean-field pinning landscape weakens its suppression effect on plastic intermittency. On the other hand, the plastic intermittency can be inhibited efficiently at small sample size by using the plate-like θ′-Al₂Cu precipitates penetrating the entire micro-pillar, in association with the presence of {100}-slip traces along the coherent precipitate/Al matrix interface and enhanced strain isotropy. Besides these experimental observations, we also aim to extract the laws common in different Al alloys, and address the specific plastic mechanisms for Al-Cu micro-crystals.