Processing magnesium at room temperature Proust, Gwénaëlle
Science (American Association for the Advancement of Science),
07/2019, Letnik:
365, Številka:
6448
Journal Article
Recenzirano
In situ microscopy experiments show how pyramidal slip increases magnesium's ductility
Aluminum (Al) alloys are often used for lightweight applications such as airframes, but with proper alloying ...elements and heat treatments, magnesium (Mg) alloys can reach strengths comparable to those of some Al alloys but be 35% lighter (
1
). Nonetheless, the use of Mg alloys has been limited by their poor processability at room temperature and low corrosion resistance (
2
). The latter can be improved either with additional alloying elements (
2
) or by refining their grain size (
3
). Improvements to the ductility of Mg, which controls its processability, will require better understanding of its plastic deformation mechanisms and the parameters controlling their activities. On page 73 of this issue, Liu
et al.
(
4
) offer new insight into how Mg deforms, and more specifically, on the role of slip along the pyramidal planes of its unit cell in accommodating large deformation (see the figure).
Hexagonal materials deform plastically by activating diverse slip and twinning modes. The activation of such modes depends on their relative critical stresses, and the orientation of the crystals ...with respect to the loading direction. To be reliable, a constitutive description of these materials has to account for texture evolution associated with reorientations due to both dislocation slip and twinning, and for the effect of the twin boundaries as barriers to dislocation propagation. We extend a previously introduced twin model, which accounts explicitly for the composite character of the grain formed by a matrix with embedded twin lamellae, to describe the influence of twinning on the mechanical behavior of the material. The role of the twins as barriers to dislocations is explicitly incorporated into the hardening description of slip deformation via a directional Hall–Petch mechanism. We introduce here an improved hardening law for twinning, which discriminates for specific twin/dislocation interactions, and a detwinning mechanism. We apply this model to the interpretation of compression and tension experiments done in rolled magnesium alloy AZ31B at room temperature. Particularly challenging cases involve strain-path changes that force strong interactions between twinning, detwinning, and slip mechanisms.
•A fabrication method transforms sawdust into semi-green and 3D printable WPC.•The morphology of wood particles affects mechanical properties of 3D printed WPC.•Rounder wood particles with smoother ...surfaces result in stronger 3D printed WPC.•Higher wood to plastic ratios result in stiffer and more sustainable 3D printed WPC.•The double extrusion improves the printability of WPC filaments effectively.
The material composition of 3D printed elements has enormous influence on the qualities of the final product – it is decisive in determining the mechanical performance, aesthetic appearance and sustainability of a product and as a result, on its marketability and the range of possible applications. This paper presents how sawdust, a by-product of the forest industry, can be valorised as a component of Wood-Plastic Composite (WPC) to manufacture 3D printed elements that are semi-green and suitable for architectural applications. Specifically, the study investigates two common types of Australian hardwood, Red Gum and Grey Box, and the effects that the variation of the wood compositions have on the mechanical properties of 3D printed WPCs in terms of wood species, particle size and shape distributions, and the wood to plastic ratio in the composite. WPC filaments were extruded from a mixture of wood flour, Acrylonitrile Butadiene Styrene (ABS) and maleic anhydride. Tensile specimens were 3D printed from the manufactured WPC filaments. The results show that wood particles with rounder shapes and smoother surfaces increase the strength, stiffness and density of 3D printed WPCs due to the better adhesion of these shaped wood particles with the polymer. A higher wood to plastic ratio creates a ‘greener’ product with improved stiffness. The ‘double extrusion’ process, used in this study to homogenise the filament fabricated via extrusion, has a profound effect on enhancing the quality of the WPC filaments for 3D printing.
Additive friction stir deposition (AFSD) is an emerging solid-state additive manufacturing process with a high deposition rate. Being a non-fusion additive manufacturing (AM) process, it ...significantly eliminates problems related to melting such as cracking or high residual stresses. Therefore, it is possible to process reactive materials or high-strength alloys with high susceptibility to cracking. Although the residual stresses are lower in this process than with the other AM processes, depending on the deposition path, geometry, and boundary conditions, residual stresses may lead to undesired deformations and deteriorate the dimensional accuracy. Thermal cycling during layer deposition, which also depends on the geometry of the manufactured component, is expected to affect mechanical properties. To this day, the influence of the deposit geometry on the residual stresses and mechanical properties is not well understood, which presents a barrier for industry uptake of this process for large-scale part manufacturing. In this study, a stepped structure with 4, 7, and 10 passes manufactured via AFSD is used to investigate changes in microstructure, residual stress, and mechanical property as a function of the number of passes. The microstructure and defects are assessed using scanning electron microscopy and electron backscatter diffraction. Hardness maps for each step are created. The residual stress distributions at the centreline of each step are acquired via non-destructive neutron diffraction. The valuable insights presented here are essential for the successful utilisation of AFSD in industrial applications.
Four dimensional printing enables the advanced manufacturing of smart objects that can morph and adapt shape over time in response to stimuli such as heat. This study presents a single-material 4D ...printing workflow which explores the residual stress and anisotropy arising from the fused deposition modelling (FDM) printing process to create heat-triggered self-morphing objects. In particular, the study first investigates the effect of printing patterns on the residual stress of FDM-printed acrylonitrile butadiene styrene (ABS) products. Through finite element analysis, the raster angle of printing patterns was identified as the key parameter influencing the distribution of residual stresses. Experimental investigations further reveal that the non-uniform distribution of residual stress results in the anisotropic thermal deformation of printed materials. Thus, through the design of printing patterns, FDM-printed materials can be programmed with desired built-in residual stresses and anisotropic behaviours for initiating and controlling the transformation of 4D-printed objects. Using the proposed approach, any desktop FDM printers can be turned into 4D printers to create smart objects that can self-morph into target geometries. A series of 4D printing prototypes manufactured from conventional ABS 3D printing feedstock are tested to illustrate the use and reliability of this new workflow. Additionally, the custom-made wood–plastic composite (WPC) feedstocks are explored in this study to demonstrate the transposability of the 4D printing approach.
Natural fiber-reinforced composite (NFRC) filaments for 3D printing were fabricated using polylactic acid (PLA) reinforced with 1–5 wt% henequen flour comprising particles with sizes between 90–250 ...μm. The flour was obtained from natural henequen fibers. NFRCs and pristine PLA specimens were printed with a 0° raster angle for tension tests. The results showed that the NFRCs’ measured density, porosity, and degree of crystallinity increased with flour content. The tensile tests showed that the NFRC Young’s modulus was lower than that of the printed pristine PLA. For 1 wt% flour content, the NFRCs’ maximum stress and strain to failure were higher than those of the printed PLA, which was attributed to the henequen fibers acting as reinforcement and delaying crack growth. However, for 2 wt% and higher flour contents, the NFRCs’ maximum stress was lower than that of the printed PLA. Microscopic characterization after testing showed an increase in voids and defects, with the increase in flour content attributed to particle agglomeration. For 1 wt% flour content, the NFRCs were also printed with raster angles of ±45° and 90° for comparison; the highest tensile properties were obtained with a 0° raster angle. Finally, adding 3 wt% content of maleic anhydride to the NFRC with 1 wt% flour content slightly increased the maximum stress. The results presented herein warrant further research to fully understand the mechanical properties of printed NFRCs made of PLA reinforced with natural henequen fibers.
The identification and quantification of the different ferrite microconstituents in steels has long been a major challenge for metallurgists. Manual point counting from images obtained by optical and ...scanning electron microscopy (SEM) is commonly used for this purpose. While classification systems exist, the complexity of steel microstructures means that identifying and quantifying these phases is still a great challenge. Moreover, point counting is extremely tedious, time consuming, and subject to operator bias. This paper presents a new automated identification and quantification technique for the characterisation of complex ferrite microstructures by electron backscatter diffraction (EBSD). This technique takes advantage of the fact that different classes of ferrite exhibit preferential grain boundary misorientations, aspect ratios and mean misorientation, all of which can be detected using current EBSD software. These characteristics are set as criteria for identification and linked to grain size to determine the area fractions. The results of this method were evaluated by comparing the new automated technique with point counting results. The technique could easily be applied to a range of other steel microstructures.
•New automated method to identify and quantify ferrite microconstituents in HSLA steels is presented.•Unique characteristics of the ferrite microconstituents are investigated using EBSD.•Characteristics of ferrite microconstituents are exploited to identify the type of ferrite grains within the steel's microstructures.•The identified ferrite grains are linked to their associated grain's size for area fraction calculations.
Clay minerals and clayey soils have been extensively researched over the last century leading to a rich and still evolving corpus of knowledge on clay chemistry, microstructure and macroscopic ...behaviour. Clay has the ability, under certain conditions, to spontaneously repair its cracks. However, despite ample evidence, clay self-repair remains understudied and under-theorised. For example, the majority of experimental studies discussing clay self-repair infer its existence from changes to macroscopic properties assumed to be caused by self-repair, and only a small number of studies have attempted to observe self-repair directly. This paper reviews the literature on clay self-repair. First, it situates clay self-repair within the broader context of self-repairing material. Next,
autogenous
self-repair of clay, under wet-dry cycles, freeze–thaw cycles and deep-ground consolidation, is presented focusing on evidence, driving mechanisms and key variables of influence. Next, theories of clay self-repair proposed in the literature are discussed, highlighting their scope and limitations, as well as the extent to which they have been validated by experimental observations. Key gaps in current knowledge of clay self-repair are highlighted and ways in which they can be addressed in future research are proposed. Finally, a nomenclature distinguishing between different kinds of clay self-repair is proposed based on eight different attributes.
Improvements to the wear performance of CD4MCu (a cast Fe-Cr-Ni-Cu-Mo duplex stainless steel grade) would be of great benefit in pump applications. This study demonstrates that CD4MCu stainless ...steels reinforced with (Nb
0.75
,Ti
0.25
)C particles can be fabricated by simply melting the elements required for the CD4MCu stainless steel together with the amounts of Nb, Ti and C required to achieve a specific volume fraction of (Nb
0.75
,Ti
0.25
)C particles in a miniature laboratory argon arc furnace. Their potential as erosion resistant materials was evaluated through microstructural characterization, hardness testing, and sliding wear tests. The formation of (Nb
0.75
,Ti
0.25
)C particles was found to improve the bulk hardness of the CD4MCu stainless steels without significantly altering its duplex microstructure, which then led to better wear performance. The results indicate that large-scale casting of the proposed materials should be possible and that they have potential as erosion resistant materials.
Austenitic 316L stainless steel can be used for orthopedic implants due to its biocompatibility and high corrosion resistance. Its range of applications in this field could be broadened by improving ...its wear and friction properties. Surface properties can be modified through surface hardening treatments. The effects of such treatments on the microstructure of the alloy were investigated here. Surface Mechanical Attrition Treatment (SMAT) is a surface treatment that enhances mechanical properties of the material surface by creating a thin nanocrystalline layer. After SMAT, some specimens underwent a plasma nitriding process to further enhance their surface properties. Using electron backscatter diffraction, transmission Kikuchi diffraction, energy dispersive spectroscopy, and transmission electron microscopy, the microstructural evolution of the stainless steel after these different surface treatments was characterized. Microstructural features investigated include thickness of the nanocrystalline layer, size of the grains within the nanocrystalline layer, and depth of diffusion of nitrogen atoms within the material.
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