Phase equilibria and solidification of Mg-rich Al-Mg-Si alloys

The solidification and solid-state phase equilibria of four Al-Mg-Si alloys containing 30-70%Mg and 0.5-3.5%Si, selected on the basis of an isothermal section of the Al-Mg-Si system calculated at 300 °C, have been investigated. Solidification paths of Mg-rich Al-Mg-Si alloys finish on ternary eutectics and the temperatures of two of these eutectic reactions, i.e. L↔(Al)+β+ Mg2Si and L↔(Mg)+γ+ Mg2Si, have been determined to be at ~ 448 °C and ~ 436 °C respectively by DTA. The characteristic temperatures recorded on the DTA curves are analysed and a linear relationship is found between the peak temperature and the square root of the scanning rate

Modelling the corrosion behaviour of Al2CuMg coarse particles in copper-rich aluminium alloys

The corrosion behaviour of 2024 aluminium alloy in sulphate solutions was studied; attention was focused on the influence of coarse intermetallic Al2CuMg particles on the corrosion resistance of the alloy. Model alloys representative of the aluminium matrix and of Al2CuMg coarse intermetallics were synthesized by magnetron sputtering. Open-circuit potential measurements, current–potential curve plotting and galvanic coupling tests were performed in sulphate solutions with or without chlorides. Further explanations were deduced from the study of the passive films grown on model alloys in sulphate solutions. The results showed that model alloys are a powerful tool to study the corrosion behaviour of aluminium alloys

Characterisation of rosette formation in an aluminium–silicon alloy

Differential thermal analysis has been used to investigate the effect of cooling rate on rosette formation during solidification of a synthetic Al–Fe–Si alloy. Rosettes can be characterised as a very fine multiphase structure within more or less convex areas dispersed in the matrix. Their formation during solidification is related with liquid entrapment and high solidification undercooling associated with the need of independent nucleation events of second phases. It is here shown that their density and internal coarseness depend on cooling rate. Further, metallographic observation of rosettes in contact with large precipitate of b-Al9Fe2Si2 phase allowed to conclude that this latter phase does not help silicon nucleation

Solidification and Room Temperature Microstructure of a Fully Pearlitic Compacted Graphite Cast Iron

Compacted graphite cast irons are rapidly developing for they have better mechanical properties than lamellar graphite cast irons and present less porosity than spheroidal graphite cast irons. For many applications, an as-cast fully pearlitic matrix would be desired which can hardly be achieved when graphite is compacted. Addition of manganese, copper and tin are thus made as these elements are known to be pearlite promoters. However, their amount should be limited so as to avoid detrimental effects amongst which are heterogeneities in the matrix properties which impede easy machining. In the present work, a compacted graphite cast iron containing 0.3 wt% Mn, 0.8 wt% Cu and 0.1 wt% Sn was cast in sand mould and in standard thermal analysis cup. The cup sample showed a nearly fully pearlitic matrix and was selected for further study. The characterization consisted of measuring and correlating the distributions of pearlite interlamellar spacings and microhardness values. An attempt was made to look for the effect of solidification microsegregation on microhardness which did not reveal any trend

Effects of the Cr-depletion on the stress state of the sublayer of ni-base alloys oxidized in high temperature water

The exposure in high-temperature water (300–360°C) of Alloy 600 is known to induce not only generalized corrosion, but also degradation at the underlying base metal such as intergranular penetrations of oxygen and\or oxides and Cr-depletion. Possible consequences of Cr-depletion are the creation of local stresses in the underlying metal due to local variations of the lattice parameter, and the formation of physicals properties gradient, like the elastic modulus or thermal expansion coefficient. In order to assess the effects caused by the Cr-depletion on an exposed Alloy 600, finite element (FE) calculations and physical properties characterisations on synthetic alloys with a chemical composition representative of different Cr-depleted layers, were performed. The levels of the calculated stresses were then discussed regarding to the other features of oxidation involved in the high temperature water stress-corrosion cracking mechanism

Solid-state phase transformation in a lithium disilicate-based glass-ceramic

The solid-state phase transformation in a lithium disilicate-based glass-ceramic (IPS e.max® CAD) was revisited on the basis of quantitative data. IPS e.max® CAD is widely used as material for the dental restoration in the dental industry. In-situ X-ray diffraction and differential scanning calorimetry accompanied by scanning electron microscopy observations were applied to understand phase transformation during heat treatment in a dental ceramic. In-situ X-ray diffraction evidences the concomitant formation of cristobalite and lithium orthophosphate at 770 °C. Then, the formation of lithium disilicate occurred at the expense of a complete dissolution of cristobalite and lithium metasilicate. No phase transformation occurred during cooling. The quantitative results of microstructural features (amount of each phase, morphology, and number density of lithium disilicate and lithium metasilicate) indicate that lithium disilicate is probably formed by diffusional process at the lithium metasilicate/cristobalite interface, which acts as favorable nucleation sites. The energy barrier is probably too high for lithium disilicate nucleation in the amorphous matrix. The quantitative results will provide the background for further modeling of phase transformation kinetics, which may have potential industrial benefits

Effect of Cooling Rate on the Eutectoid Transformation in Compacted Graphite Cast Iron

Differential thermal analysis has been used to characterize the effect of cooling rate on the eutectoid transformation of a compacted graphite iron. The samples were machined out from an as-cast thermal cup, austenitized at 950°C and then cooled to room temperature at various rates within the range 1−55.5°C/min. It was found that even at the highest investigated cooling rate, significant amounts of ferrite could be observed. When comparing the microstructure before and after Nital etching on samples cooled at intermediate cooling rates, it appeared that ferrite formed preferentially along the worms. This is discussed in terms of graphite shape and microsegregation and this latter seems prevalent. Finally, analysis of the thermal records was performed to characterize the temperatures for the start of the stable and metastable eutectoid reactions which confirms the eutectoid transformation sets up in compacted graphite irons as in lamellar and spheroidal graphite irons

Effect of cast iron microstructure on adherence of an epoxy protection

To study the influence of the microstructure of cast iron on the adhesion of an epoxy coating, ferritic, pearlitic and austempered samples were prepared in as-received, polished and oxidised states. A pull-off test (dry adhesion) was performed before immersing in water while the crosscut test was made after 24 days of exposition in distilled water (wet adhesion). X-rays were combined with optical microscopy (LOM) and scanning electron microscopy (SEM) for surface analysis. The adhesion of the epoxy coating on the cast iron surface firstly depends on the roughness of the surface; polished samples showed high adherence by comparison with asreceived samples. On the oxidised samples, the surface oxide significantly improves the adhesion of the coating in both dry and wet states for all three sample microstructure. The presence of carbide in the structure was observed to decrease adherenc

Microstructure and thermal analysis of Cu-Cu2O eutectic – Can we mimic archeological remains?

An attempt was performed to reproduce in laboratory the microstructure observed in ancient artefacts and metallurgical remains made of nearly pure copper cast under ambient atmosphere. Two samples of pure Cu were prepared for thermal analysis, then heated and held for various times in the liquid state under ambient air for oxidizing, and finally cooled down at given rate. It was observed that a thick Cu2O oxide layer developed around the samples which gives the metal the same outer appearance than reported for metallurgical remains, namely a highly oxidized surface with large voids. Further, the microstructure of the metal consisted of (Cu) or Cu2O dendrites and a (Cu)-Cu2O eutectic showing strong similarity with the observations made on remains and artefacts

Study of the Eutectoid Transformation in Nodular Cast Irons in Relation to Solidification Microsegregation

Eutectoid transformation in cast irons may proceed in the stable or the metastable systems giving ferrite and graphite for the former and pearlite for the latter. The present work demonstrates that composition profiles across ferrite/pearlite boundaries are smooth and similar to those issued from the solidification step. No trace of long-range diffusion of substitutional solutes due to austenite decomposition could be observed. In turn, this ascertains that both stable and metastable transformations proceed with the product matrix—either ferrite opearlite—inheriting the parent austenite content in substitutional solutes. This result sustains a physical model for eutectoid transformation based on the so-called local para-equilibrium which is commonly used for describing solid-state transformation in steels