Mechanisms of plastic deformation of magnesium matrix nanocomposites elaborated by friction stir processing

Magnesium based composites have attracted much attention over the past few years as a promising solution to lightweighting, energy saving and emission reduction, especially for automotive and aerospace applications. With a specific weight as low as 1.74 g.cm-3, magnesium is the lightest of all structural metals. However, the strength of Mg needs to be improved in order to compete with other light metals such as Al or Ti. The present study focuses on Mg reinforced by Y2O3 nanoparticles. The aim of the work is to investigate the single crystalline plastic behavior of Mg strengthened by oxide dispersed particles, in comparison to that of pure Mg. A major challenge is to elaborate single crystalline samples with a homogeneous distribution of particles. In the present work, yttrium oxide reinforced magnesium matrix nanocomposites were produced using friction stir processing (FSP). FSP is a novel solid-state processing technique based on the same principle of friction stir welding. It proves to be an efficient method to produce metal-based composites. As shown in Fig.1(a), the initial 3 µm particles were fragmented during the process and their size was reduced to less than 100 nm. The three-dimensional dispersion of nanoparticles was confirmed by synchrotron X-ray microtomography, as shown in Fig.1(b). Since the FSP sample presents fine grains (around 10 µm), a subsequent heat treatment was performed to enable abnormal grain growth. The increased grain size allows the subsequent fabrication of single crystalline micropillars for microcompression testing. The advantage of this method over traditional mechanical testing for studying the mechanisms of deformation is that the entire sample can be investigated post-mortem (Fig. 1(c)), and a variety of grain orientations can be tested for a single processing history. Micropillars were machined inside a single grain with a known orientation using focused ion beam (FIB) machining. Microcompression experiments were then conducted in a nanoindenter equipped with a diamond flat-ended conical indenter. The stress-strain response was measured for different single crystal orientations. In addition to experimental investigations, three-dimensional discrete dislocation dynamics (3D DDD) simulations (Fig. 1(d)) were carried out for comparison. The results provide relevant insights on the role of nanoparticles on the onset of plastic deformation in single crystals as well as twinning nucleation in Mg nanocomposites

Microstructure and optical appearance of anodized friction stir processed Al - Metal oxide surface composites

Multiple-pass friction stir processing (FSP) was employed to impregnate Ti, Y and Ce oxide powders into the surface of an Aluminium alloy. The FSP processed surface composite was subsequently anodized with an aim to develop optical effects in the anodized layer owing to the presence of incorporated oxide particles which will influence the scattering of light. This paper presents the investigations on relation between microstructure of the FSP zone and optical appearance of the anodized layer due to incorporation of metal oxide particles and modification of the oxide particles due to the anodizing process. The effect of anodizing parameters on the optical appearance of the anodized surface was studied. Characterization was performed using SEM, FIB-SEM, TEM and GI-XRD. The surface appearance was analysed using photospectrometry technique which measures the diffuse and total reflectance of the surface. The appearance of the anodized surface changed from dark to bright upon increasing the anodizing voltage. Particles in the FSP zone were partially or completely modified during the anodizing process and modified the morphology of the surrounding anodized Al matrix which has a clear influence on the mechanism of light interaction like scattering and absorption from the anodized surface

High Frequency Anodising of Aluminium-TiO₂ Surface Composites:Anodising Behaviour and Optical Appearance

High frequency anodising of Al–TiO2 surface composites using pulse reverse pulse technique was investigated with an aim to understand the effect of the anodising parameters on the optical appearance, microstructure, hardness and growth rate of the anodic layer. Friction stir processing was employed to prepare the Al–TiO2 surface composites, which were anodised in a 20 wt.% sulphuric acid bath at 10 °C as a function of pulse frequency, pulse duty cycle, and anodic cycle voltage amplitudes. The optical appearance of the films was characterized and quantified using an integrating sphere-spectrometer setup, which measures the total and diffuse reflectance from the surface. The change in optical reflectance spectra from the anodised layer was correlated to the applied anodising parameters and microstructure of the anodic layer as well as the Al–TiO2 substrate. Change in hardness of the anodised layer was also measured as a function of various anodising parameters. Anodic film growth, hardness, and total reflectance of the surface were found to be highly dependent on the anodising frequency and the anodic cycle potential. Longer exposure times to the anodising electrolyte at lower growth rates resulted in lowering of the reflectance due to TiO2 particle degradation and low hardness due to increased dissolution of the anodised layer during the process. [All rights reserved Elsevier]

Controlled precipitation in a new Al-Mg-Sc alloy for enhanced corrosion behavior while maintaining the mechanical performance

peer reviewedThe hot working of 5xxx series alloys with Mg ≥3.5 wt% is a concern due to the precipitation of β (Al3Mg2) phase at grain boundaries favoring Inter Granular Corrosion (IGC). The mechanical and corrosion properties of a new 5028-H116 Al-Mg-Sc alloy under various β precipitates distribution is analyzed by imposing different cooling rates from the hot forming temperature (i.e. 325 °C). The mechanical properties are maintained regardless of the heat treatment. However, the different nucleation sites and volume fractions of β precipitates for different cooling rates critically affect IGC. Controlled furnace cooling after the 325 °C heat treatment is ideal in 5028-H116 alloy to reduce susceptibility to IGC after sensitization

A multiscale multiphysics investigation of aluminum friction stir welds : from thermal modelling to mechanical properties through precipitation evolution and hardening/

The aim of the thesis is the understanding of the thermal, mechanical and metallurgical phenomena occurring during friction stir welding of a 6005A aluminum alloy and the determination of the mechanical properties of the welded joints. The forces and the torque needed for welding as well as the thermal cycles were measured in the various zones of the joint. A first model predicts the influence of the welding parameters on the thermal cycles, and especially on the asymmetry of the temperature evolution with respect to the weld centerline. A second model links the local microstructural evolutions, and in particular the precipitation kinetics, to the thermomechanical cycles. A third model relates the local microstructures to the local mechanical properties, based on a novel physics-based strain hardening model. Finally, the link between the mechanical properties of the joint in service, the local mechanical properties and the microstructures is modeled using a multi-scale approach including a micro-mechanics damage constitutive model. The models are calibrated and validated through in-depth microstructure characterization and mechanical tests on the base material, on heat treated samples and on the friction stir welds.(FSA 3)--UCL, 200