Dislocation multiplication rate in the early stage of germanium plasticity

A set of constitutive equations describing the plasticity of semiconductors is compared with the results of mechanical tests. Constant strain-rate compression tests are interrupted before the peak of the initial multiplication yield point, i.e. at the moment of intense dislocation multiplication, by transient tests (stress-relaxations and creep tests). Various laws for dislocation multiplication are used in constitutive modelling and their predictions are compared with experimental transient curves. A generalized law is proposed, which perfectly fits all the transient tests data, provided each sample is considered separately. It seems necessary to account for the properties of dislocation sources in the multiplication law, at least at the early stages of plasticity.

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

Suzanne Godelieve Alphonsine Verheyden

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

EPFL2018

Thermally activated deformation of two- and three-variant nanotwinned L10 Au–Cu–Pt

Frédéric Diologent, Alexandre Durussel, Edwina Klay, Andreas Mortensen

Activation volumes characteristic of room temperature compressive plastic deformation in Au- 45.2 at.% Cu- 1.7 at.% Pt, a near-AuCu alloy that orders to a fully L1(0) structure, are measured using the repeated stress-relaxation method. Four different ordering treatments (540 and 10(5)s at either 250 or 400 degrees C) are conducted on samples prior to deformation, leading to a classical two-variant polytwin structure after ordering at 400 degrees C, or alternatively a three-variant highly twinned structure at 250 degrees C. In both structures, {110} twin boundaries only a few tens of nanometres apart separate variants and constitute the main barriers to dislocation glide. All four ordered nanotwinned microstructures have an initial effective activation volume V-eff of 100-120 b(3); however, the two structures differ strongly as they work harden. The two-variant polytwin structure yields a straight line on a Haasen plot, showing that work hardening is associated with an increase in slip obstacle density, most likely dislocation debris at intervariant boundaries. The three-variant structure on the other hand shows a nearly constant (slightly decreasing) activation volume. This suggests that there is little obstacle accumulation when three variants are present, likely because plastic deformation occurs by slip of superdislocation only. (C) 2011 Elsevier Ltd. All rights reserved.

Elsevier2011

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

Cast aluminium microwires

Jérôme Krebs

EPFL2015