α-Melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl- sn -glycerol

The α-melt-mediated crystallization of 1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol (POS) has been investigated by differential scanning calorimetry (DSC), combined with polarized-light microscopy. Starting from a completely liquid state, the melt was first cooled down and maintained at a temperature, T 1, during a time, t 1, where the α-phase formed. Then it was heated to a temperature, T 2, above the melting point of α for isothermal solidification into a solid phase, which was identified as δ. Based upon DSC solidification peaks, the time-temperature-transformation (TTT) diagram of POS was constructed for these solidification conditions and was compared with the TTT diagram of direct crystallization from the melt. The α-melt-mediated solidification showed accelerated kinetics of the δ-phase. The effects of T 1 and t 1 were also studied: at short t 1, crystallization was faster with a decreasing value of T 1, whereas the opposite trend was observed for a longer plateau at T 1. These tendencies were interpreted in terms of three competing phenomena: the density of δ-nuclei that can form during the plateau at T 1, α-δ solid-state transformation, and memory effects of molecule arrangements in the α-remelted phas

Simulation of Semi-Solid Material Mechanical Behavior Using a Combined Discrete/Finite Element Method

As a necessary step toward the quantitative prediction of hot tearing defects, a three-dimensional stress-strain simulation based on a combined finite element (FE)/discrete element method (DEM) has been developed that is capable of predicting the mechanical behavior of semisolid metallic alloys during solidification. The solidification model used for generating the initial solid-liquid structure is based on a Voronoi tessellation of randomly distributed nucleation centers and a solute diffusion model for each element of this tessellation. At a given fraction of solid, the deformation is then simulated with the solid grains being modeled using an elastoviscoplastic constitutive law, whereas the remaining liquid layers at grain boundaries are approximated by flexible connectors, each consisting of a spring element and a damper element acting in parallel. The model predictions have been validated against Al-Cu alloy experimental data from the literature. The results show that a combined FE/DEM approach is able to express the overall mechanical behavior of semisolid alloys at the macroscale based on the morphology of the grain structure. For the first time, the localization of strain in the intergranular regions is taken into account. Thus, this approach constitutes an indispensible step towards the development of a comprehensive model of hot tearin

A New Tensile Test for Aluminum Alloys in the Mushy State: Experimental Method and Numerical Modeling

A fairly simple experimental setup has been designed for testing the resistance of the mushy zone of alloys during solidification under tensile conditions. It has been used to study the effect of coalescence among the solid grains at a late stage of solidification. The experimental approach involves both tensile-strength measurements and scanning electron microscope (SEM) observations of fracture surfaces. Complementary information can be obtained by numerical modeling of this solidification process. The latter takes into account heat flow in the sample, rheology of the mushy alloy, liquid feeding, and porosity formation. All of the available information indicates that the transition from a granular mushy alloy to a coalesced solid-skeleton behavior starts for a solid fraction of approximately 92pc

Modeling and characterization of grain structures and defects in solidification

The paper by Karma and Tourret (this volume) in this special issue focuses on multiscale modeling approaches ranging from atoms to microstructure. In the present one, the most recent and significant modeling contributions dealing with the scale of solidification from microstructure to grain structure are briefly reviewed. The paper also covers modeling of defect formation during the last stage of solidification, namely porosity and hot tearing. As will be shown, the field of solidification has taken advantage of several simulation and experimental tools which have become increasingly powerful and accessible over the past decade. The emphasis will be put on complex 2D and 3D models for which correlations with in situ observations using synchrotron radiation and/or combined orientation and metallography imaging have been made. (C) 2015 Elsevier Ltd. All rights reserved

Two-Phase Modeling of Hot Tearing in Aluminum Alloys: Applications of a Semicoupled Method

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 tearin

Connectivity of Phases and Growth Mechanisms in Peritectic Alloys Solidified at Low Speed: an X-Ray Tomography Study of Cu-Sn

The variety of microstructures that form at low solidification speed in peritectic alloys, bands, and islands, or even coupled (or cooperative) growth of the primary α and peritectic β phases, have been previously explained by nucleation-growth mechanisms. In a recent investigation on Cu-Sn, a new growth mechanism was conjectured on the basis of two-dimensional (2-D) optical microscopy and electron backscattered diffraction (EBSD) observations made in longitudinal sections. In the present contribution, synchrotron-based tomographic microscopy has been used to confirm this mechanism: α and β phases totally interconnected in three dimensions and bands (or islands) can result from an overlay mechanism, rather than from a nucleation events sequence. When the lateral growth of a new layer is too fast, an instability can lead to the formation of a lamellar structure as for eutectic alloy

Some Mathematical and Numerical Aspects inAluminum Production

In this paper, we present a mathematical modeling of some magnetohydrodynamic effects arising in an aluminum production cell as well as its numerical approximation by a finite element method. We put the emphasis on the magnetic effects which live in the whole three dimensional space and which are solved numerically with a domain decomposition metho

Melting at dislocations and grain boundaries: A Phase Field Crystal study

Dislocation and grain boundary melting are studied in three dimensions using the Phase Field Crystal method. Isolated dislocations are found to melt radially outward from their core, as the localized excess elastic energy drives a power law divergence in the melt radius. Dislocations within low-to-mid angle grain boundaries melt similarly until an angle-dependent first order wetting transition occurs when neighboring melted regions coalesce. High angle boundaries are treated within a screening approximation, and issues related to ensembles, metastability, and grain size are discussed.Comment: 4 pages, 3 figure

Modeling of ingot distortions during direct chill casting of aluminum alloys

A comprehensive three-dimensional mathematical model based upon the Abaqus software has been developed for the computation of the thermomechanical state of the solidifying strand during direct chill (DC) casting of rolling sheet ingots and during subsequent cooling. Based upon a finite element formulation, the model determines the temperature distribution, the stresses and the associated deformations in the metal. For that purpose, the thermomechanical properties of the alloy have been measured up to the coherency temperature using creep and indentation tests. The thermophysical properties as well as the boundary conditions associated with the lateral water spray have been determined using inverse modeling. The predicted ingot distortions, mainly butt curl, butt swell and lateral faces pull-in are compared with experimental measurements performed during solidification and after complete cooling of the ingot. Particular emphasis is placed on the non-uniform contraction of the lateral faces. The influence of the mold shape and the contributions to this contraction are assessed as a function of the casting conditions

Influence of anisotropy on heterogeneous nucleation

Heterogeneous nucleation is governed by the interplay of interfacial energies between a substrate, a solid and a liquid. Although the intensity of these energies can strongly change with the orientation of the nucleus for anisotropic media, this parameter is not taken into account in the available nucleation theories. In this paper, the Gibbs free energy barrier for nucleation is computed for an arbitrary solid liquid interface energy. It is shown that anisotropy favors particular orientations of the nucleus on the substrate. Experimental evidence from the zinc aluminum system is given as an application of this extended nucleation theory. It also sheds new light on the texture of galvanized steel sheets. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved