Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Abstract

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 μm. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200 nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale.

Laue microdiffraction characterisation of as-cast and tensile deformed Al microwires

“…Cast microwires also have a low initial dislocation density, and their high aspect ratio (≈50) makes them suitable for tensile testing. As is reported in [7], these microcast crystals show a size effect in plasticity. This is seemingly because the small size of the crystals leads to source truncation, that is, typical double-pinned Frank-Read sources becoming single-ended (spiral) sources with higher activation stress, as predicted by statistical models [8] and dislocation dynamics simulations [9,10], and evidenced by [11][12][13] in another class of microcrystals extracted from solidified specimens.…”

“…Aluminium (99.99%) single-crystalline microwires with a diameter below 20 mm and an aspect ratio (length divided by diameter) above 30 were produced and tested according to the procedure described in [7], to which was added a series of individual, force-triggered, stress relaxation tests lasting 60 s each (found not to affect the tensile behaviour of these microwires). A total of four samples 15 mm in diameter were characterised, namely one as-cast sample (not tested in tension) and three samples that were tested in tension up to failure and then characterised 'post-mortem' by Laue X-ray microdiffraction.…”

“…It was shown in a recent publication that single-crystalline net-shape Al microwires, with a diameter in the range of 7-100 mm, can be produced by a new microcasting process [7]. As compared to other methods used for the fabrication of microtest specimens, specific characteristics of the microcast wires are that they are generally single-crystalline and free of the artefacts that are introduced by other preparation methods such as ion-milling.…”

“…The initial dislocation distribution in such microcrystals, and its evolution during deformation, are important parameters and signatures of their plastic deformation [10,14,15]. In [7], these parameters were estimated by means of a few transmission electron microscopy images; given the long length of microcrystals that can be produced and tested by the process of [7], alternative measurements giving access to the dislocation distribution in such samples, before and after deformation, are of clear interest to further understand their nature and deformation mechanisms. We provide here such data, gleaned by means of synchrotron X-ray Laue microdiffraction characterisation of as-cast and deformed aluminium microcrystals.…”

“…Larger pillars (10 mm diameter) on the other hand were found to deform more conventionally, that is, by slip on the predicted system and with unconstrained rotation of the crystal. In the work presented here, we characterise microcast samples, which are free of ion-milling artefacts and show strong size effects already in 10 mm diameter samples [7].…”