Abstract
Ab initio molecular dynamic simulations of liquid Al93Cr7 and Al83Zn10Cr7 alloy have evidenced the presence of an icosahedral short range order (iSRO) which develops into icosahedral medium ...range order (iMRO) as the melt is undercooled. This atomic arrangement accounts for the presence of Dynamic Heterogeneities characterized by Al fast-dynamics regions and Cr-rich slow-dynamics regions. Characterisation of the medium range order was carried out by a direct connectivity approach. However, given the small size of the simulation (256 atoms), such characterisation remains partial. In order to better describe both iMRO formation and more dilute alloys closer to industrial compositions, a new modelling strategy has been initiated to allow in the long term for large-scale atomic-level simulations. Molecular Dynamics (MD) of million to billion atoms may indeed lead to meaningful results. Exploitation of such large amounts of MD-generated big data can be carried out by means of Machine Learning (ML) tools which provide relevant and powerful analysis methods. An unsupervised ML approach based on topological descriptors using persistent homology concepts is proposed to reveal the structural features of atomic arrangements without
a priori
knowledge on the studied system. This approach has been applied so far to pure Al melts. Both translational and orientational orderings are thus evidenced together with nucleation pathways, whose revealed features are beyond the hypotheses of the Classical Nucleation Theory.
A new family of all-inductive dual-mode filters Guglielmi, M.; Jarry, P.; Kerherve, E. ...
IEEE transactions on microwave theory and techniques,
10/2001, Letnik:
49, Številka:
10
Journal Article
Recenzirano
In this paper, we describe a new family of dual-mode filters that is based on the use of simple inductive discontinuities in a rectangular waveguide environment. The proposed filter structure can be ...analyzed and optimized very efficiently using multimode equivalent network representations, thus leading to a simple and rapid development procedure. In addition to theory, the measured performance of a number of filter structures is also presented, thereby fully validating the proposed filter concept.
Hot tearing formation in both a classical tensile test and during direct chill (DC) casting of aluminum alloys has been modeled using a semicoupled, two-phase approach. Following a thermal ...calculation, the deformation of the mushy solid is computed using a compressive rheological model that neglects the pressure of the intergranular liquid. The nonzero expansion/compression of the solid and the solidification shrinkage are then introduced as source terms for the calculation of the pressure drop and pore formation in the liquid phase. A comparison between the simulation results and experimental data permits a detailed understanding of the specific conditions under which hot tears form under given conditions. It is shown that the failure modes can be quite different for these two experiments and that, as a consequence, the appropriate hot tearing criterion may differ. It is foreseen that a fully predictive theoretical tool could be obtained by coupling such a model with a granular approach. These two techniques do, indeed, permit coverage of the range of the length scales and the physical phenomena involved in hot tearing.
In 1952, Frank (Proc R Soc Lond Ser-Math Phys Sci 215:43–46, 1952) already postulated that Icosahedral Short Range Order (ISRO) of atoms in the liquid could possibly explain the large nucleation ...undercoolings measured in metallic alloys by Turnbull and Fisher (J Chem Phys 17:71–73, 1949). About thirty years later, this conjecture was proven to be key for the understanding of Quasicrystals (QC) formation (Shechtman
et al.
in Phys Rev Lett 53:20, 1951–3, 1984). More recently, it has been found that a few tens to thousand ppm of solute elements in Al-base and Au-base alloys can influence the nucleation and growth of the primary fcc phase via mechanisms involving ISRO and QC formation. ISRO has also been found to limit the mobility, and thus diffusion, of atoms in the liquid. This can lead to out-of-equilibrium conditions, e.g., the formation of metastable phases or supersaturated solid solution, at reduced velocity compared to alloys where ISRO is not predominantly present. Finally, there are several experimental evidences that ISRO is also responsible for twinned dendrites formation in Al alloys. The present contribution summarizes these recent findings and points out the implications that these might have in the field of solidification and additive manufacturing.
Raman spectra of anorthite crystal and glass have been obtained on a wide frequency range from 5 to 1200 cm-1. Medium-range order has been pointed out in this study: numerous low frequency peaks in ...anorthite have been observed and a comparison with the well known band around 60 cm-1 in the Raman spectrum of anorthite glass is made. In order to probe the medium-range structure of CaAl2Si2O8 glass, a comparison between neutron inelastic scattering, Raman low frequency vibrations and heat capacity data is done for both glass and crystal. 16 refs.
110〉 dendrite growth in aluminum feathery grains Henry, S.; Rappaz, M.; Jarry, P.
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
11/1998, Letnik:
29, Številka:
11
Journal Article
< 110 > dendrite growth in aluminum feathery grains Henry, S; Jarry, P; Rappaz, M
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
11/1998, Letnik:
29A, Številka:
11
Journal Article
Recenzirano
Automatic indexing of electron backscattered diffraction patterns, scanning electron microscopy, and optical microscopy observations have been carried out on aluminum-magnesium-silicon, ...aluminum-copper, and aluminum-silicon alloys directionally solidified or semicontinuously cast using the direct chill casting process. From these combined observations, it is shown that the feathery grains are made of < 110 > primary dendrite trunks (e.g.,011) split in their centers by a coherent (111) twin plane. The average spacing of the dendrite trunks in the twin plane (approx10-20 mu m) is typically one order of magnitude smaller than that separating successive rows of trunks (or twin planes). The 011 orientation of these trunks is close to the thermal gradient direction (typically within 15 deg ), a feature probably resulting from a growth competition mechanism similar to that occurring during normal < 100 > columnar dendrite growth. On both sides of these trunks, secondary dendrite arms also grow along < 110 > directions. Their impingement creates wavy noncoherent twin boundaries between the coherent twin planes. In the twin plane, evidence is shown that < 110 > branching mechanisms lead to the propagation of the twinned regions, to the regular arrangement of the primary dendrite trunks along a 211 direction, and to coherent planar twin boundaries. From these observations, it is concluded that the feathery grains are probably the result of a change from a normal < 100 > to a < 110 > surface tension/attachment kinetics anisotropy growth mode. This change might be induced by the added solute elements, by the local solidification conditions (thermal gradient, growth rate, and melt convection), and possibly by the help of the twin plane itself. Convection in the melt could also in the symmetrization of the < 110 > growth directions of the side arms. Finally, the proposed mechanisms of feathery grain growth are further supported by the observation of < 110 > dendrite growth morphologies in thin aluminum-zinc coatings.
Design of FBAR filters at high frequency bands Shirakawa, A. A.; Pham, J.-M.; Jarry, P. ...
International journal of RF and microwave computer-aided engineering,
01/2007, Letnik:
17, Številka:
1
Journal Article
The gradual transformation of a mushy zone during alloy solidification, from a continuous liquid film network to a fully coherent solid, has been simulated using a granular model. Based on a Voronoi ...tessellation of a random set of nucleation centres, solidification within each polyhedron is computed considering back-diffusion and coalescence. In the network of connected liquid films, a pressure drop calculation is performed assuming a Poiseuille flow in each channel, Kirchhoff’s conservation of flow at nodal points and flow losses compensating solidification shrinkage (KPL model). In addition to intergranular liquid pressure maps, the model shows the progressive formation of grain clusters, the localisation of the flow at very high solid fraction and thus natural transitions of the mushy zone.
The grain structure formation in direct chill (DC) casting is directly linked to nucleation, which is generally promoted by inoculation. Inoculation prevents defects, but also modifies the physical ...properties by changing the microstructure. We studied the coupling of the nucleation on inoculant particles and the grain growth in the presence of melt flow induced by thermosolutal convection and of the transport of free-floating equiaxed grains. We used a volume-averaged two-phase multiscale model with a fully coupled description of phenomena on the grain scale (nucleation on grain refiner particles and grain growth) and on the product scale (macroscopic transport). The transport of inoculant particles is also modeled, which accounts for the inhomogeneous distribution of inoculant particles in the melt. The model was applied to an industrial sized (350mm thick) DC cast aluminium alloy ingot. A discretised nuclei size distribution was defined and the impact of different macroscopic phenomena on the grain structure formation was studied: the zone and intensity of nucleation and the resulting grain size distribution. It is shown that nucleation in the presence of macroscopic transport cannot be explained only in terms of cooling rate, but variations of composition, nuclei density and grain density, all affected by transport, must be accounted for.
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