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

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 ′-Al2Cu 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 (≳ 3 µ ). 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 ′-Al2Cu precipitate/ -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.

Plate-like precipitate effects on plasticity of Al-Cu micro-pillar: {100}-interfacial slip

“…MD simulations indicate that 60° mixed dislocations can shear the -Al2Cu precipitate when the precipitate cross-over the entire crosssection. This is consistent with our TEM characterization presented in our recent work focusing on the mechanical behaviors [11], but cannot explain the non-trivial {100}-slips.…”

“…5d). This simulation result is in excellent agreement with our TEM observations of the shearing steps and highly disordered lattice structure in the precipitate after deformation (see a relevant paper [11]). However, this mechanism is found to be irrelevant to the observed {100}-slips and will not be discussed in detail.…”

“…Given an external stress , the shear stress component at the head of a dislocation pile-up reads [40,53,54] = ( − 1) = − 1 (7) where = is the number of dislocations piling up within a distance between the pinning point of a single-arm dislocation source (SAS) and the precipitate [40]; is the external shear stress on the plane of the pile-up, and simply obtained from the engineering stress at 2% here (~ 113 MPa, see Ref. [11]); G is the shear modulus; is the angle between the original {111}-slip plane and {100} plane of precipitate; = 1 for screw dislocations [28]. The parameter in Eq.…”

“…The significant strength improvement in Al-Cu alloy was believed to stem from the Orowan mechanism, based on the conventional knowledge that the ′-Al2Cu phase is shear-resistant to dislocations [3,[7][8][9]. However, in severely deformed Al-Cu alloys [5,10] and Al-Cu micro-pillars [11], a shearing of ′-Al2Cu precipitates has been observed, giving an indication that the underlying precipitate-dislocation interactions are more complex than the classical Orowan mechanism. Indeed, a recent micro-mechanical testing combined with transmission X-ray microscopy supports a transition of strengthening modes in Al-Cu alloy, evolving from dislocation bypassing to dislocation accumulation as increasing the precipitate diameter [12].…”

Twisting of pre-twinned α-Fe nanowires: from mild to wild avalanche dynamics

“…Our statistical analysis indicates that the deformation mode changes from a mild regime at small twist angles to a wild regime at higher angles. In addition, the associated wildness is controlled by both an internal length scale (twin boundary spacing) and the external length scale (wire diameter, while keeping the twin boundary spacing constant), much like in dislocation-mediated plasticity [8,37]. This suggests material design strategies to limit these detrimental wild fluctuations and the generation of complex patterns.…”