Complex precipitation pathways in multicomponent alloys

One usual way to strengthen a metal is to add alloying elements and to control the size and the density of the precipitates obtained. However, precipitation in multicomponent alloys can take complex pathways depending on the relative diffusivity of solute atoms and on the relative driving forces involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle calculations combined with various complementary experimental approaches working at different scales reveal a strongly inhomogeneous structure of the precipitates: owing to the much faster diffusivity of Sc compared with Zr in the solid solution, and to the absence of Zr and Sc diffusion inside the precipitates, the precipitate core is mostly Sc-rich, whereas the external shell is Zr-rich. This explains previous observations of an enhanced nucleation rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much higher resistance to Ostwald ripening, two features of the utmost importance in the field of light high-strength materials.     

Acoustic decoupling of longitudinal modes in generic combustion systems

Conditions are examined under which acoustic modes of a chamber filled with hot combustion products can be considered to be decoupled from the plenum acoustics supplying the fresh reactants through a feeding manifold. It is shown that this is controlled by a coupling index Ξ = (ρ_bc_b)/(ρ_uc_u)S₁/S₂ ? (T_u/T_b)^1/2(S₁/S₂), where T_u and T_b are the fresh and burned gases temperatures and S₂/S₁ is the expansion ratio between the chamber and injection unit cross sections. It is demonstrated that the acoustic response of a coupled system can be analyzed by considering the plenum and the chamber acoustics separately for small values of the coupling parameter Ξ. Longitudinal self-sustained combustion oscillations may then lock on (i) the plenum resonant frequencies, thus becoming independent of downstream modifications of the combustion chamber acoustics, or on (ii) the combustion chamber modes, thus becoming essentially indifferent to the plenum acoustics. The case of a plenum featuring a Helmholtz resonance is investigated in further detail when the chamber exhaust impedance is varied. Exact relations under which the plenum and the chamber modes are decoupled are derived when the chamber is open to atmospheric conditions or when it is equipped with a sonic nozzle. Predictions are compared to measurements for a generic system equipped with a swirl injector, a compact chamber and terminated by an open atmospheric pressure exhaust. It is shown that in this case, self-sustained longitudinal combustion-instabilities develop preferentially near the plenum mode frequencies and are weakly sensitive to modifications in the chamber geometry.     

Crater-formation-induced metastable structure in an AISI D2 steel treated with a pulsed electron beam

A detailed investigation concerning the microstructure and phase formations in the crater-erupted areas formed under low energy high current pulsed electron beam bombardment has been carried out for a treated AISI D2 steel. The crater formations are initiated at carbides. Such carbide eruption events change the composition, phase selection and grain growth behaviour inside the craters. Under these extremely non-equilibrium conditions, amorphous patches and feathery type structures containing twinned nano-grains are retained.     

Effect of aging on microstructural development in an Al–Mg–Si alloy processed by high-pressure torsion

Experiments were conducted to evaluate the microstructural evolution in a commercial Al-0.6 % Mg-0.4 % Si alloy processed using high-pressure torsion for up to 20 turns. Disks of the alloy were tested in two different conditions: in a solution-treated condition and after a short aging treatment at 523 K. The results show that HPT processing introduces significant grain refinement through HPT processing including a reduction in grain size from ~150 μm to ~720 nm in 1 turn of HPT. The final grain size in this alloy was ~250 nm after 20 turns. Some tensile tests were conducted to evaluate the mechanical properties of the alloy at the solution treatment temperature. The results from these tests show that aging at 523 K leads to a small increase in ductility for all tensile samples with a maximum recorded elongation of ~230 %.     

Frequency Methods Applied to the Characterization of the Thermophysical Properties of a Granular Material with a Cylindrical Probe

In many fields, such as in the agri-food industry or in the building industry, it is important to be able to monitor the thermophysical properties of granular materials. Regular thermal probes allow for the determination of one or several thermophysical factors. The success of the method used depends in part on the nature of the signal sent, on the type of physical model applied and eventually on the type of probe used and its implantation in the material. Although efficacious for most applications, regular thermal probes do present some limitations. It is the case, for example, when one has to know precisely the thermal contact resistance or the nature of the signal sent. In this article is presented a characterization method based on thermal impedance formalism. This method allows for the determination of the thermal conductivity, the thermal diffusivity, and the contact thermal resistance in one single test. The application of this method requires the use of a specific probe developed to enable measurement of heat flux and temperature at the interface of the probe and the studied material. Its practical application is presented for dry sand.     

The predominant role of collagen in the nucleation, growth, structure and orientation of bone apatite

The involvement of collagen in bone biomineralization is commonly admitted, yet its role remains unclear. Here we show that type I collagen in?vitro can initiate and orientate the growth of carbonated apatite mineral in the absence of any other vertebrate extracellular matrix molecules of calcifying tissues. We also show that the collagen matrix influences the structural characteristics on the atomic scale, and controls the size and the three-dimensional distribution of apatite at larger length scales. These results call into question recent consensus in the literature on the need for Ca-rich non-collagenous proteins for collagen mineralization to occur in vivo. Our model is based on a collagen/apatite self-assembly process that combines the ability to mimic the in vivo extracellular fluid with three major features inherent to living bone tissue, that is, high fibrillar density, monodispersed fibrils and long-range hierarchical organization.     

Influence of Laser Glazing on the Characterization of Plasma-Sprayed YSZ Coatings

In this study, 8 wt.% yttria-stabilized zirconia powder was deposited on the substrates by atmospheric plasma spray. The coatings were post-treated by laser glazing under different parameters in order to densify them. The characterization of the laser molten pools under different laser treatment conditions was studied. Preheating processes were also employed. Scanning electron microscopy observations of the surface and cross section of as-sprayed and laser-glazed coatings were carried out to investigate the influence of laser glazing on the microstructure on laser-glazed coatings. The results show that preheating processes improve the coating in terms of deepening the laser-glazed layer, reducing the number of vertical cracks and surface density of cracks and widening the molten pool. Finally, the influences of linear energy density on the characterization of the glazed layer are discussed.