The impact of twin boundary migration on mechanical performance of magnesium

While there have been many numerical and experimental studies of various aspects of mechanical twinning in polycrystalline magnesium, little has been done to understand twin boundary migration and its influence on concurrent dislocation plasticity. Here, we investigate the migration of a single twin boundary in pure Mg and in Mg-4wt%Gd, a solution strengthened Mg alloy known to display interesting twinning behavior (Fig. 1). By utilizing a micromechanical approach along with EBSD and TEM characterization, we are able to isolate a single twin boundary of chosen orientation and analyze the structural changes and stress-strain response resultant from uniaxial loading. Microcompression experiments were carried out on microcolumns fabricated with a single twin boundary, as well as single crystals of the parent orientation as a reference. Interestingly, compression-induced detwinning was found to lead to hardening. Results will be discussed in terms of twinning and detwinning mechanisms, the influence of such mechanisms on the strength and hardening response, and the impact of Gd solutes on the twinning response of Mg at the micron scale. Please click Additional Files below to see the full abstract

Effect of pre‐existing dislocations on the strength of gold at very small scales

This work aims at understanding the effect of pre-existing dislocations on the strength of gold at very small scales. Previous studies have investigated this topic by correlating values of site-specific strength with average values of pre-strain or dislocation density of the parent bulk material. However, the mechanical response at the nano-scale is rather governed by the local microstructure of the small volumes being tested, which might not be representative of the average microstructure of the bulk material. Therefore, we propose a new method to correlate site-specific strength, as characterized by the hardness at the elastic-to-plastic transition, with the respective values of local deformation states, characterized non-destructively prior to nanoindentation using a new EBSD strain parameter derived from uncorrelated misorientation angle distributions. The analysis is performed on polycrystalline gold samples submitted to different degrees of bulk pre-straining. Consistent with other works, our results show that pre-existing dislocations decrease the strength of gold at the nano-scale; the Taylor relation is shown not to hold in this regime. We discuss this in view of a model proposed by Johnson and Ashby to predict the strength of a metal as a function of dislocation density, and a similar one applied to nanoindentation by Lilleodden and Nix. In addition, a conceptual model similar to that of Hall-Petch is used to analyze the results in the context of size effects in the strength of metals at small scales

On the mechanistic origin of the enhanced strength and ductility in rare earth-based Mg alloys

The applicability of classical wrought Mg alloys is limited by their comparatively poor room temperature ductility and low yield strength. Conversely, various experimental and computational efforts do confirm that low concentrations of rare earth (RE) in Mg significantly improves these properties. However, the mechanistic origin of these improvements are still been debated. In order to contribute to the discourse, we carried out in-depth comparison of deformation modes in single crystals of pure Mg and a homogenized Mg–0.75 at.% Gd alloy oriented for twinning, pyramidal- and basal-slip using a combination of microcompression testing, scanning transmission electron microscopy and small angle x-ray scattering technique. We observed a fivefold increase in basal CRSS and a fourfold decrease of the pyramidal/basal CRSS (P/B) ratio as a result of Gd addition. We also observed that slip was planar in the basal orientation of the alloy but wavy in pure Mg. Pyramidal slip and twinning activity in the two systems were however similar; an indication that the same mechanisms underlie deformation in these orientation. We show that the observed planar slip, increase in basal CRSS and decrease in P/B ratio are consequence of Gd-rich short-range ordered (SRO) clusters in the alloy. Our analysis show that these SRO clusters lead to significantly high strengths in the basal orientation since additional stress is required to destroy the ordering therein. This not only leads to a dramatic increase in yield strength, given the drastic reduction in P/B CRSS ratio, it should also significantly promote pyramidal slip activities in polycrystals and by extension ductility improvements

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

On the consequences of intrinsic and extrinsic size effects on the mechanical response of nanoporous Au

In this study, the consequence of intrinsic and extrinsic size effects on mechanical responses of nanoporous gold is investigated via microcompression testing. By varying the micropillar diameter (D) between 1 µm and 20 µm and the ligament size (L), 50 nm and 350 nm, a critical ratio (α = D/L = 20) was found, above which the test structure can be considered a representative volume element, resulting in identical mechanical response and uniform deformation. Below that value, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a measurement series with a fixed ligament size of 350 nm where the flow stress decreased by more than 50% (from approximately 5 to 2.5 MPa) and the elastic modulus was reduced from approximately 0.5 GPa to almost zero. Stochastic behavior along with non-uniform deformation and failure is observed for α < 10, suggesting that the size of the load-bearing units in this material is about 10 times the corresponding ligament size

Mechanical behavior of nanoscale Cu/PdSi multilayers

International audienceBerkovich nanoindentation and uniaxial microcompression tests have been performed on sputter-deposited crystalline Cu/amorphous Pd0.77Si0.23 multilayered films with individual layer thicknesses ranging from 10 to 120 nm. Elastic moduli, strengths and deformation morphologies have been compared for all samples to identify trends with layer thicknesses and volume fractions. The multilayer films have strengths on the order of 2 GPa, from which Cu layer strengths on the order of 2 GPa can be inferred. The high strength is attributed to extraordinarily high strain hardening in the polycrystalline Cu layers through the inhibition of dislocation annihilation or transmission at the crystalline/amorphous interfaces. Cross-sectional microscopy shows uniform deformation within the layers, the absence of delamination at the interfaces, and folding and rotation of layers to form interlayer shear bands. Shear bands form where shear stresses are present parallel to the interfaces and involve tensile plastic strains as large as 85% without rupture of the layers. The homogeneous deformation and high strains to failure are attributed to load sharing between the amorphous and polycrystalline layers and the inhibition of strain localization within the layers

Deformation at the nanometer and micrometer length scales: Effects of strain gradients and dislocation starvation

Nanomechanical devices are certain to play an important role in future technologies. Already sensors and actuators based on MEMS technologies are common and new devices based on NEMS are just around the corner. These developments are part of a decade-long trend to build useful engineering devices and structures on a smaller and smaller scale. The creation of structures and devices calls for an understanding of the mechanical properties of materials at these small length scales. Here we examine some of the effects that arise when crystalline materials are mechanically deformed in small volumes. We show that indentation size effects at the micrometer scale can be understood in terms of the hardening associated with strain gradients and geometrically necessary dislocations, while indentation size effects at the nanometer scale involve the concepts of dislocation starvation and the nucleation of dislocations. We also describe uniaxial compression experiments on micrometer size pillars of single crystal gold and find surprisingly strong size effects, even though no significant strain gradients are present and the crystals are not initially dislocation free. We argue that these size effects are caused by dislocation starvation hardening, with dislocations leaving the crystal more quickly than they multiply and leading to the requirement of continual dislocation nucleation during the course of deformation. A new length scale for plasticity, the distance a dislocation travels before it creates another, arises naturally in this treatment. Hardening of crystals smaller than this characteristic size is expected to be dominated by dislocation starvation while crystals much larger than this size should exhibit conventional dislocation plasticity

Mechanisms of plastic deformation of magnesium matrix nanocomposites developed 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 alloys. The study focuses on Mg reinforced by Y2O3 nanoparticles. The aim of the work is to investigate the typical plastic behavior of Mg single crystal strengthened by oxide dispersed particles. One challenge is to elaborate samples with a homogeneous distribution of particles. First, yttrium oxide reinforced magnesium matrix nanocomposites were prepared by Friction Stir Processing (FSP). This technique shows to be an efficient method to elaborate metal-based composites. As shown in Fig.1(a), the initial 3 μm particles were milled during the process and their size was reduced to few hundreds of nanometers. The three-dimensional dispersion was confirmed by nano-holotomography. A subsequent heat treatment was performed to enable abnormal grain growth. Microcolumns were machined inside a single grain using Focus Ion Beam (Fig.1(b)). Microcompression experiments were then conducted (Fig. 1(c-d)) to obtain the stress-strain response for different single crystal orientations. Three-dimensional discrete dislocation dynamics (3D DDD) simulations were compared with the experimental results. The simulations provide relevant insights on the role of nanoparticles on the onset of plastic deformation as well as twinning nucleation in Mg nanocomposites