Gilles L’Espérance

Interfacial Reactions in Al-Mg (5083)/SiCp Composites during Fabrication and Remelting

The interfaces of aluminum alloy composites (5083) reinforced by SiC particles (as-received, oxidized 3.04 wt pct and 14.06 wt pct) were studied. The composites were fabricated by compocasting and certain samples were also remelted at 800 °C for 30 minutes. The reaction mechanisms between SiCp and liquid Al and between the SiO2 layer and Al(Mg) are discussed. The crystal boundaries of the MgO (or MgAl2O4) reaction products are believed to be the diffusion paths (or channels) during the interfacial reactions. A SiO2 layer, formed by oxidation of the SiC particles prior to their incorporation into the melt, plays an important role in preventing the SiCp from being attacked by the matrix. The interfacial reaction products are affected by both the alloy composition and the thickness of the initial SiO2 layer.

Tensile properties of a 2014 aluminum alloy in the temperature range 250° to 500 °C

The effect of deformation temperature in the range 250° to 500 °C on the tensile ductility of a 2014 aluminum alloy was measured in constant extension-rate tensile tests, generally with an initial strain rate of 2.6 X 10-3 s-1. Elongation at fracture increased with increasing temperature in the range 250° to 450 °C, reaching a maximum value of ~180 pct, but in stretching above ~450 °C ductility was reduced by void growth. Below ~350 °C the limiting elongations of solution-treated sheets were much inferior to those of annealed samples. Relatively poor performance in the solution-treated condition was associated with rapid hardening during the first few percent extension followed by overaging at higher strains. When solution-treated sheets were stretched above 400 °C, recovered structures and precipitation on a coarse scale developed at an early stage of straining and the stretching limits were not much inferior to those of annealed sheets at temperatures above 400 °C. The results are discussed in terms of the contributions made to necking resistance by rate-dependent and strain-dependent components of flow strength.

Solvent-cast based metal 3D printing and secondary metallic infiltration

Affordable 3D printing methods are needed for the development of high performance metallic structures and devices. We develop a method to fabricate dense metallic structures by combining a room temperature 3D printing and subsequent heat-treatments: sintering and secondary metallic infiltration. The high flexibility of this method enables the fabrication of customized 3D structures, such as fully-filled, porous, interlocked and overhung structures. These geometries are printed using a highly concentrated metallic ink (metallic load up to 98 wt%) consisting of highly alloyed steel (HAS) microparticles, polylactic acid (PLA) and dichloromethane (DCM). In order to improve the mechanical properties and the electrical conductivity, the as-printed structures are sintered and infiltrated by copper in a furnace protected by a mixture of H2 and Ar. The filament porosity of the copper infiltrated samples is as low as 0.2%. Mechanical testing and electrical conductivity measurement on the copper infiltrated structures reveal that the Youngs modulus reaches up to ∼195 GPa and the electrical conductivity is as high as 1.42 × 106 S m−1. Our method enables the simple fabrication of high performance metallic structures which could open up new technological applications where cost is an important factor.

Assessment of the Contribution of Electron Microscopy to Nanoparticle Characterization Sampled with Two Cascade Impactors

This study assessed the contribution of electron microscopy to the characterization of nanoparticles and compared the degree of variability in sizes observed within each stage when sampled by two cascade impactors: an Electrical Low Pressure Impactor (ELPI) and a Micro-Orifice Uniform Deposit Impactor (MOUDI). A TiO2 nanoparticle (5 nm) suspension was aerosolized in an inhalation chamber. Nanoparticles sampled by the impactors were collected on aluminum substrates or TEM carbon-coated copper grids using templates, specifically designed in our laboratories, for scanning and transmission electron microscopy (SEM, TEM) analysis, respectively. Nanoparticles were characterized using both SEM and TEM. Three different types of diameters (inner, outer, and circular) were measured by image analysis based on count and volume, for each impactor stage. Electron microscopy, especially TEM, is well suited for the characterization of nanoparticles. The MOUDI, probably because of the rotation of its collection stages, which can minimize the resuspension of particles, gave more stable results and smaller geometric standard deviations per stage. Our data suggest that the best approach to estimate particle size by electron microscopy would rely on geometric means of measured circular diameters. Overall, the most reliable data were provided by the MOUDI and the TEM sampling technique on carbon-coated copper grids for this specific experiment. This study indicates interesting findings related to the assessment of impactors combined with electron microscopy for nanoparticle characterization. For future research, since cascade impactors are extensively used to characterize nano-aerosol exposure scenarios, high-performance field emission scanning electron microscopy (FESEM) should also be considered.

Effect of temperature on beam damage of asbestos fibers in the transmission electron microscope (TEM) at 100 kV

Damage to asbestos fibers by the transmission electron microscope (TEM) electron beam is a known limitation of this powerful method of analysis. Although it is often considered only in terms of loss of crystallinity, recent studies have shown that the damage may also change the elemental composition of fibers, thus causing significant identification errors. In this study, the main objective was to assess whether temperature is a factor influencing damage to asbestos fibers and, if so, how it can be used to minimize damage. It was found that lowering the temperature to 123K can inhibit, for a given time, the manifestation of the damage. The significant decrease of atom diffusion at low temperature momentarily prevents mass loss, greatly reducing the possibility of misidentification of anthophyllite asbestos fibers. The results obtained in this study strongly suggest that the predominant mechanism damage is probably related to the induced-electric-field model relegating radiolysis to the status of a subsidiary damage mechanism.

Exposure Assessment in a Single-Walled Carbon Nanotube Primary Manufacturer

Objectives This study was aimed at documenting and characterizing occupational exposure to single-walled carbon nanotubes (SWCNTs) generated in a primary manufacturing plant. It also compared various strategies of exposure monitoring. Methods A 6-day measurement protocol was scheduled (D1-D6) including both (i) quasi-personal monitoring with an array of direct reading instruments (DRIs) and (ii) offline electron microscopy analyses of surface and breathing zone filter-based samples. The first step (D1 and D2) consisted of contamination screenings resulting from the various SWCNT production tasks using a multimetric approach. Surface sampling was also carried out to assess workplace cross-contamination. The second step (D3-D6) focused on the exposure monitoring during recovery/cleaning task, by comparing three personal elemental carbon (EC) measurements [respirable EC using a cyclone following the NIOSH 5040 method (REC-CYC), respirable and thoracic EC using parallel particle impactors [REC-PPI and TEC-PPI, respectively)] and gravimetric mass concentration measurements. Results DustTrak DRX and electrical low-pressure impactor measurements indicated that particles were released during weighing, transferring, and recovery/cleaning tasks of the manufacturing process. Electron microscopy revealed the presence of agglomerated SWCNTs only during the recovery/cleaning task. REC-CYC concentrations remained under the limits of quantification; REC-PPI showed levels up to 58 µg m-3; and TEC-PPI ranged from 40 to 70 µg m-3. Ratios calculated between gravimetric measurements and estimated DustTrak mass concentrations ranged from 2.8 to 4.9. Cross-contamination appeared to be limited since SWCNTs was only found on surface samples collected close to the reactor in the production room. Conclusions This case study showed that the DustTrak DRX should be the preferred device among DRIs to identify potential exposure to SWCNTs. However, there is a risk of false positive since it is a non-specific instrument; therefore, the actual release of SWCNTs must be confirmed with scanning electron microscopy/transmission electron microscopy analyses. Besides, using EC measurements as a proxy for SWCNT exposure assessments, as suggested by the NIOSH, is still challenging since interferences can occur with other EC sources such as carbon black, which is also present in the workplace.

Punch-stretching behavior of a 2014 aluminum alloy in the temperature range 250° to 500 °C

The influence of forming temperatures in the range 250° to 500 °C on the performance of a 2014 aluminum alloy in punch stretching has been investigated. In tests at moderate strain rates within the 250° to 450 °C range, the biaxial stretching limits of annealed sheets were greatly superior to the room-temperature values. Limiting depths of pressing increased with increasing temperature in the range 250° to 400 °C as a result of improvement in both limit strains and strain distribution, but increasing deformation temperature above ~400 °C caused the limit strains to decrease as a result of cavitation at high strains. Under comparable conditions of temperature and extension rate, such cavity growth was more rapid in equibiaxial stretching than in uniaxial or plane-strain stretching. At 500 °C with a punch speed of 0.083 mm s-1, the thickness strain which could be applied in biaxial stretching without significant cavitation damage was less than 0.4. Thus, susceptibility to cavitation imposes an important restriction on opportunities for combining solution treatment with a hot stretch-forming operation when using an alloy based on Al-4 pct Cu.

Effect of accelerating voltage on beam damage of asbestos fibers in the transmission electron microscope (TEM)

Transmission electron microscopy (TEM) is a powerful and efficient tool for the analysis of asbestos fibers. Although this analysis technique is common and several standard methods exist for asbestos analysis, questions remain about the optimal conditions to be used. Because asbestos fibers are relatively sensitive to the electron beam, it is important to better understand the phenomena of damage in order to avoid them. This study specifically investigates the effect of the acceleration voltage on damage to four different types of asbestos fibers: chrysotile, amosite, crocidolite and anthophyllite. The results support the conclusion that, contrary to what is usually recommended, it is best to use an acceleration voltage of 200kV rather than 100kV in order to avoid damage. The findings shed light on possible damage mechanisms, the most predominant of which seems to be caused by an induced electric field.

Design of a HAADF Detector for Z Contrast in SEM

In (S)TEM, the use of HAADF detectors to acquire images of thin foils for which the contrast arises from differences in chemical composition (Z contrast) is well known [1]. In the SEM, however, such Z contrast detectors, capable of precise angular adjustment and quantitative detection, are not available to our knowledge. The objective of the work presented here is to design such a HAADF detector that can be used in the SEM to observe Z contrast in a thin foil. In the case of crystalline materials, the design of the detector must be such that strongly diffracted beams are not collected.

Observation of Nanometric Silicon Oxide Bifilms in a Water-Atomized Hypereutectic Cast Iron Powder

This study investigated the reasons for the irregular structure of primary graphite nodules that were formed in a hypereutectic cast iron powder during water atomization. The graphite nodules contain a significant amount of micron-sized pores and multiple nanometric voids that formed from silicon oxide bifilms. The bifilms theory is often used to explain the mechanisms responsible for the presence of pores in castings. However, even if many results presented in the literature tend to corroborate the existence of bifilms, to this date, only indirect evidences of their existence were presented. The observations presented in this paper are the first to show the double-sided nature of these defects. These observations support the bifilms theory and give an explanation for the presence of porosities in castings. The bifilms were used as substrate for graphite growth during solidification. The irregular structure of the graphite nodules is a consequence of the rather random structure of the bifilms that were introduced in the melt as a result of turbulences on the surface of the melt during pouring. The confirmation of the existence of bifilms can contribute to the understanding of the mechanical properties of various metallic parts.