Interaction between liquid aluminium and solid iron Al-rich intermetallics formation

Intermetallic phases formation is one of the phenomena, which when coupled to thermal and mechanical strains leads to the premature wear of aluminium die casting tools made out of steel. This work focuses on two aspects of this wear problem. The first and major part investigates a simplified chemical system Fe(s)–Al(l) at 700 °C in order to study the formation of the intermetallic compounds. The second part focuses on the limitations of the tool wear by testing different coatings on the steel currently used for die casting. Experiments are performed on two sample types, differentiated by their interface orientation between the two initial phases (vertical and horizontal). Observation are also of two types. Post mortem observations are performed by light microscopy and both scanning and transmission electron microscopy. In situ observation are performed by X-ray tomography, that allows to follow the evolution of the intermetallic layer with time. In the Fe(s)–Al(l) system, one main phase, namely Fe2Al5 , forms with a particular morphology called tongue-like feature in the iron matrix. Tongue tips are generally single crystals. It is also observed that these exhibit a periodical contrast at the nanometre scale. This contrast is found to be due to a slight chemical variation within the existence domain of Fe2Al5. This splitting is linked to the spinodal decomposition of the phase observed at higher temperature. At the interface between tongues and the iron matrix, a layer of about hundred nanometres that surrounds the tongues is observed. It corresponds to the Æ2 FeAl phase. Iron matrix, due to the tongue growth, suffers a plastic deformation that is observed by recrystallisation of the direct tongue surrounding. The induced strain field also leads to porosity formation at the interface between the tongues and the matrix. Pores are then trapped in the Fe2Al5 phase and partially disappear near from the liquid interface. At this interface, a layer of the order of 10μm of Fe4Al13 is observed. The thickness of this layer is stable in the explored time range of 0-4 h. However, intermetallic blocks that detach from the layer are observed in the liquid. This is interpreted as mainly due to the dissolution of the formed layer in the bath to achieve its saturation in Fe. Intermetallic thicknesses and tongue density measurements as functions of time allow to highlight the presence of three main growth regimes. The first at the beginning of the contact corresponds to rapid emergence of the tongues in the matrix. The second is linked to their progressive slow down while they start to thicken. Both thickening and tip growth are in competition from the beginning of this regime. A third regime is however defined and corresponds to a more stable growth during which thickening and tip growth still act in parallel is but their effect is less visible as they vary with the square root of time. The tongue growth observed at the beginning of the reaction is explained by a growth front destabilisation, similarly to the destabilisation of a planar front that leads to dendritic solidification described by Mullins and Sekerka. A calculation based on their model is proposed and confirms the effect of the law aluminium diffusion in iron but is limited by assumptions made for its development. Technological investigation proceeds by modification of the studied system and comparison with the fundamental one. Two ferrous materials and one aluminium alloy are studied. In addition, different coatings deposited by electroless plating on one of the ferrous substrates are tested. A quality coefficient is proposed in order to compare the result of the contact between coating and liquid aluminium. One Co-based coating is revealed as the most promising material that allows to limit the wear of the ferrous part by liquid aluminium.

Characterization and modeling of twinned dendrite growth

Mario Alberto Salgado Ordorica

The formation of feathery grains during semi-continuous casting of Al-alloys [1, 2] is an interesting problem from both practical and theoretical points of view. These structures are formed by a lamellar sequence of twinned and untwinned regions separated by straight and wavy-like boundaries. Each pair of lamellae contains twinned dendrites split in their trunk center by a coherent {111} twin plane, while lateral arms meet at an incoherent {111} boundary. In practice, feathery grains are considered as defects which reduce the mechanical properties of a solidified ingot. In theory, an understanding of twinned dendrite growth includes different solidification phenomena, e.g., interfacial energy anisotropy, crystallographic growth directions, twinning, growth competition mechanisms, etc. Although several studies have been performed in order to understand the physics leading to the nucleation and growth of twinned dendrites, various questions remain unanswered. In this work, a comprehensive study of twinned dendrite growth has been undertaken, with the main objectives being: i) to study the effect of different alloying elements and solidification conditions on twin formation in binary Al-alloys; ii) to establish a better understanding on the stability of twinned dendrites and the growth kinetic advantage that they exhibit over regular ones; and iii) to elucidate the stable shape of the twinned dendrite tip. In order to study alloying-element effects, binary Al-X alloys (where X = Zn, Mg, Cu and Ni), were produced under Directional Solidification (DS) conditions in the presence of a slight natural convection in the melt. Analysis of these castings has shown that feathery grains can form for all solute elements of interest, but not for all compositions. The probability of forming feathery grains is relatively high when the alloying elements are hcp (Zn or Mg), but decreases for fcc solute elements (Cu or Ni). A study on the effect of forced convection in the melt, performed using different experimental set-ups, confirms previous observations suggesting that it is the shearing components of the liquid slow which induce twin nucleation [3, 4]. However, the poor reproducibility of these experiments and the variable rate of feathery grains formation indicate that twin nucleation is governed by a highly stochastic behavior. The probability of such an event decreases as the melt slow is less complex and the associated Stacking Fault Energy (SFE) of the alloying element is increased. In terms of growth kinetics advantage, a characterization using various metallography techniques and X-ray synchroton tomography has shown that this is in part due to the complex morphology of twinned dendrites. Indeed, it has been confirmed that these dendrites grow along ‹110› directions with ‹110›, and also sometimes ‹100› secondary arms, the primary trunk spacing of these dendrites being much less anisotropic than previously thought [5, 6]. It has been shown that twinned dendrites grow in a stable manner at a lower undercooling than regular ones. In addition, the distribution and orientation of their side arms favors their growth at the expense of less developed regular dendrites. A mechanism to explain the lateral multiplication of twin planes is also proposed in this work. In terms of stability, observations after partial remelting of twinned DS specimens, then re-solidification in a Bridgman furnace, have shown that even if twin planes remain stable during partial remelting, independent regular non-twinned dendrites issued from the twinned and untwinned parts of the seed grow during solidification. This implies that the formation of twinned dendrites is not only related to the ability to nucleate a stacking fault, but also to the imposed solidification conditions. The favorable growth kinetics of twinned dendrites is also explained by their tip morphology. Three hypotheses have been evaluated in this work: i) the grooved tip [7], stabilized by the Young-Laplace equilibrium condition; ii) the doublon [5], i.e., a double tip dendrite that grows with a narrow liquid channel in its center that solidifies at a composition close to C0; and iii) the pointed (or edgy) tip [8], equilibrated by torque terms of γsl when it is too anisotropic. Observations performed at the twinned dendrite tip have revealed the presence of a small groove, which eliminates definitively the hypothesis of the edgy tip. Further examination of the stable growth morphology of twinned dendrites has been done by using a phase field model that reproduces the presence of a twin plane through an appropriate boundary condition. The results of these simulations show that twinned dendrites are doublons that grow with a narrow liquid channel (0.2 to 3 µm width) whose depth depends strongly on the alloy composition and the solidification conditions. In order to validate these simulations, Focused Ion Beam (FIB)-microtomography and X-ray chemical analysis (EDS) in a Scanning Transmission Electron Microscope (STEM) were performed on small specimens extracted from twinned dendrite trunks. These have shown the existence of a positive solute gradient in a region localized within 2 µm around the twin plane, i.e., as expected for a doublon morphology. Additionally, the presence of small particles aligned within the twin plane is in agreement with the formation of small liquid pockets below the doublon root as predicted by the numerical model, but further work is required to explain this remarkable feature. Finally, composition measurements performed after quenching a partially remelted specimen also seem to confirm the existence of a doublon. This work contributes to the understanding on twinned dendrite growth kinetics in several aspects, principally the effect of alloying elements on twin formation, their growth kinetic advantage, their stability and their stable tip morphology. However, further work must be carried out to overcome the limited knowledge on the mechanisms leading to twinned dendrite nucleation. In the same manner, the growth mechanisms of the doublon morphology should also be further investigated in future works.

EPFL2009

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

The understanding of material self-assembling and self-organization mechanisms, at mesoscale, is crucial for nanotechnologies development. Such structures, hierarchically organized in superlattices or colloïdal crystals, are observed in inorganic or organo-metallic precipitates. The classical characterization, using electron microscopy and X-ray diffraction, of a synthesized powder is inadequate to describe the hierarchically organized polycrystalline structure of the final particles. Here, we have highlighted the successive steps that lead to the spontaneous formation of a cobalt oxalate dihydrate (COD) colloïdal crystals that precipitate in aqueous media. The characterization of the final particles, made by X-ray and electronic diffraction, has allowed us to determine the controversial crystalline phase of the precipitated COD. The β and γ COD phases has been discriminated by comparing experimental results with simulations of X-ray diffractogram and electron diffraction patterns for these both structures. The precipitated COD corresponds to the γ phase indicating that it comes from solid dehydration of cobalt oxalate tetrahydrate. The final powder characterization, performed by X-ray diffraction (XRD), atomic force microscopy (AFM), low voltage high resolution scanning electron microscopy (LVHRSEM) and transmission electron microscopy (TEM), gives some diverging results about monocrystalline or polycrystalline nature of the precipitates. XRD and AFM analysis show a polycrystalline structure of particles whereas LVHRSEM, TEM and electron diffraction observations indicate a monocrystalline structure. This apparent contradiction has led us to elaborate a new characterization method allowing to follow the growth and the structural evolution of the precipitate as a function of time. The cryo- preparation techniques are well adapted as they allow freezing and direct observation of solid state suspension by electron microscopy. The freeze/drying method has been modified and used for both SEM and TEM, and has allowed us to study the morphological and structural evolution of the COD precipitate from nanoscale to mesoscale. In addition, TEM analysis of samples prepared by classical techniques has been performed to complement cryo-electron microscopy observations. The combination of these two methods show that COD precipitation is a complex multi-step process leading to the formation of a core/shell heterostructure. The anisotropic core is porous and partially crystalline. It is formed by agglomeration of isolated primary particles and agregated ones (15-20 nm sized secondary particles). The crystalline shell corresponds to the final particles faces and is made up by the layer by layer self-assembling and alignment of 5-7 nm sized crystalline primary particles. These nanoparticles are aligned and perfectly ordered in strings of nanograins in the layer structure. The self-assembling occurs in the last step of growth where we have a lower ionic strength and supersaturation inducing slower kinetics of growth and aggregation. Moreover, the crystalline order in the primary and secondary particles increase with the time of reaction consistent with a continuous process of primary particle nucleation and growth. Our results show that self-assembling occurs layer by layer with terraces, kinks and steps that are present on the faces. This reminds the layer by layer crystalline growth models but in this case, the buiding blocks are colloïdal instead of atomic or molecular. Based on these different results, we propose an original model describing the COD precipitate growth. In addition, we have studied the poly(methacrylic acid-sodium salt) effect on the COD precipitation. This additive (PMMA) appeared to be a very good growth inhibitor. The PMMA effect on the final particles morphology is dramatic. Its concentration variation in solution slows down the nanoparticles aggregation rates along a [101] preferential direction. The crystal habit can then be controlled and we have synthesized platelets, cubes and rods with tailored length. The use of cryogenic electron microscopy has been shown to be a powerful tool for the understanding of time dependent aspects of particle growth. The use of such techniques for the study of other systems, where the final precipitate appearance may hide particle substructures, will help to elucidate growth mechanisms and allow finer control of nanostructured materials for many applications. The complex self-assembling and self-organization processes could then be highlighted and studied on a nanometer to micrometer scale.

EPFL2004

Spatial and Morphological Control of Cyanine Dye Thin Films

Nicolas Alexandre Serge Leclaire

Cyanine dyes are organic semiconductor compounds with light absorption and emission properties useful for emerging technologies such as solar cells and light-emitting devices. The characteristics of these materials in the solid state depend on their organization of the constituting building blocks. This thesis focuses on controlling the morphology of cyanine dye thin films at different length scales and clarifying the resulting properties. When microstructures present features whose size matches visible light wavelengths, new properties may arise from light-matter interactions. Here the properties resulting from the light-matter interactions of cyanine droplet films cast from solution are studied. Based on experimental evidence, it is shown that dye droplet ensembles scatter light with different efficiencies and wavelength ranges depending on their dimensions. FDTD simulations are used to show that this effect results fromscattering enhancement at the absorption edge of the dye where the refractive index varies considerably. Simulations also provide a better understanding of individual dropletsâ interaction with light. While earlier work had hypothesized that the observed scattering phenomenon were due to crystalline clusters within the droplets, this work highlights the contribution of the dye filmmorphology. Cyanines also form single crystals whose fabrication induces molecular-scale order in the material. Previous work demonstrated that thin single crystals could be grown by solvent vapor annealing of dye droplets. Here it is shown that in uncontrolled conditions, cyanine single crystals destabilize to formdendritic crystals. In-situ microscopy observations highlight the solute reservoir role of the droplet distribution surrounding a growing crystal; when the distance between droplets and the crystal front is large, the solute supply is diffusion-limited. Moreover variations in local pressure equilibrium between the droplets and crystal front lead to advection fluxes which perturb the crystal growth. These observations help design configurations to either prevent crystal destabilization or take advantage of the dendritic growth in a controlled manner. In addition, the patterning of crystals on a substrate is relevant for their application in devices. A practical challenge is to induce single crystal growth at specific locations. Here, surfaces patterned with SAMs of hydrophilic and hydrophobic thiols are used to create dye droplet arrays from which crystals can be grown. This method is shown to yield local crystallization of the dye and to prevent crystal destabilization through better control of the droplet distribution. By varying the dye solution concentration, partial control over crystal density is achieved, however it proved difficult to control the number of nuclei per droplet. A more controlled evaporation and solvent vapor annealing system might be necessary to master the nucleation process. Finally the structure and optical properties of cyanine single crystals are addressed. The crystal structure was determined by X-ray diffraction. Structural aspects are shown to lead to excitonic couplings, which are evidenced by orientation-dependent spectroscopic measurements of single crystals. Although further investigation of the absorption band structure is necessary, the results are promising for photovoltaic devices as they might improve exciton transport compared to amorphous layers.

EPFL2018