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.