Thermal Activation, Intermittency and Size Effects in the Plastic Deformation of Cast Aluminium Microwires

The microcasting process is a scaled-down investment casting process in which molten metal is pressure-infiltrated and directionally solidified in water-soluble moulds. It was previously developed to produce metallic microwires with a diameter (D) between 7 and 120 µm, characterized by a high surface quality and aspect ratio. In tension, the yield stress of aluminium (99.99%) microwires (initial dislocation density ~8*10^11 m-2) scales with the inverse of the microwire diameter and becomes highly stochastic at the smallest diameters. The plastic deformation behaviour is intermittent, suggesting that deformation progresses through the repeated activation and blockage of single-arm sources. The objective of this work is to contribute to the general understanding of plasticity size effects, using monocrystalline microcast wires. The focus is placed here on the contribution of thermal activation and initial dislocation content on the overall plastic deformation behaviour. In this frame aluminium (99.99% and 99.999%) and Al-2wt%Mg microwires with a diameter between 12-125 µm were produced and tested. Thermal activation was studied through (60 s) stress relaxations applied at different load levels throughout a tensile test. Single slip microwires (D

The thermally activated deformation behaviour of single-crystalline microcast aluminium wires

Léa Deillon, Andreas Mortensen, Suzanne Godelieve Alphonsine Verheyden

A microcasting process is used to produce high aspect ratio ( > 30) monocrystalline pure aluminium wires with a diameter between 14 and 115 mu m. The role of thermal activation in the plastic deformation of these microwires is measured by means of (single) tensile stress relaxation tests. The microwires deform largely in an intermittent fashion, i.e., through repeated sudden displacement bursts, also during stress relaxation, implying that the bursts can be triggered through thermal activation. By separating the intermittent and the continuous parts of the stress relaxation load vs . time signal, we measure an activation area characteristic of the continuous relaxation mechanism. Haasen plots of the continuous relaxation data suggest the presence of a back-stress on the order of 2.5 MPa, which can be attributed to the thin layer of oxide covering the metal. Smaller (14-25 mu m) diameter crystals oriented for single slip show highly scattered activation area values and a steeper rate of increase of data in the Haasen plot than all other samples, which conform, save for the back-stress, with what is observed in bulk aluminium. Present findings are consistent with data from commensurate aluminium crystals in replicated micro cellular structures. Data of this work show that, in single slip within aluminium crystals, the coupling between the activation area and the flow stress is altered when crystals are below 30 mu m in diameter. The absence of a systematic difference in data between samples produced of 4N or 5N aluminium suggests that this conclusion is characteristic of the pure metal.(c) 2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

PERGAMON-ELSEVIER SCIENCE LTD2022

Cast aluminium microwires

Jérôme Krebs

Plasticity size effects have been the focus of strong recent interest due to the miniaturization of devices such as actuators and different medical apparatuses. Devices with a sub-micrometre dimension have a high market potential and therefore a thorough understanding of their mechanical properties is required, both for fundamental reasons and to increase the reliability of such devices. The development of new sample preparation techniques and the continuous improvement of testing methods for the characterization of samples with a submillimetre dimension, have allowed to gain new insights in the underlying phenomena governing the plastic deformation at the scale of a few micrometres or less. Nevertheless, the precise phenomena remain elusive and some data are debatable, notably due to constraints of the used testing methods and the damage present in samples produced through focused ion beam milling, which is the most used fabrication process for specimens in the micrometre scale. This thesis has two distinctive objectives. On one hand, we develop a novel process for the production of samples having a submillimetre dimension based on casting. This process consists in pressure infiltration and directional solidification of a liquid metal in monocrystalline moulds containing submillimetre cavities. On the other hand, we use the wires prepared by this novel microcasting process to study dislocational plastic deformation at small length scales, in tension, in order to contribute to our understanding of plasticity size effects and the underlying plasticity mechanisms at the micro scale. Monocrystalline 99.99% purity aluminium wires, with a random crystallographic axis orientation and a diameter between 6 um and 110 um were fabricated using the developed microcasting process. These wires have a surface roughness on the order of 10 nm and are between 1 mm and 3 mm long. Furthermore, they are monocrystalline and contain a low density of dislocations. Results from tensile tests indicate that the critical resolved shear stress increases with decreasing wire diameter below 20 um. The observed increase is inversely proportional to the wire diameter, indicating that the plastic deformation is governed by the dislocation source length. Critical resolved shear stress values calculated from a model based on the stress required to activate a single arm and the additional stress necessary to produce slip step on both sides of the oxide, follow the same trend than the experimental data but with lower stress values. Unlike bulk aluminium, the wires exhibit a highly intermittent flow stress corresponding to the abrupt emission of discrete dislocation avalanches. The complementary cumulative size distribution function of events of size greater than 250 nm follows an exponential distribution. This suggests that the plastic deformation of the wires is governed by a random event by which source activation is stopped, the probability of this event remaining roughly constant as plastic deformation progresses. Variations in the exponent of the exponential distribution of the avalanche sizes shows that the probability that the event by which a given dislocation emission event is stopped depends on the monocrystalline wire axis orientation but not on its diameter. This suggests that the blockage of an emitting dislocation sources is not due to obstacles in its glide plane or along the dislocation length, but to unpinning of its blocked segment.

EPFL2015

The effect of size on the plastic deformation of annealed cast aluminium microwires

Léa Deillon, Andreas Mortensen, Suzanne Godelieve Alphonsine Verheyden

Monocrystalline, cast aluminium microwires of diameter (D) below 25 mu m free of micromilling artefacts are annealed under protective atmosphere. Annealing increases the flow stress of all tested microwires, by an amount that is found, in a Haasen plot of measured activation area values, to be an athermal contribution to the flow stress that is attributed to surface oxide layer thickening. Microwires oriented for single slip with D similar to 14 mu m behave as do as-cast microwires with D similar to 7 mu m, showing a transition to higher and more stochastic flow stress values; this effect is attributed to a greater scarcity of dislocation sources after annealing. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd.

2019

Cast aluminium microwires

Jérôme Krebs

EPFL2015