Els Nagels

Columnar to Equiaxed Transition in Al-Cu-Ag

In this work the transition from columnar to equiaxed growth is studied during the solidification of the univariant eutectic L=> α(Al) + θ-Al2Cu in the ternary Al-Cu-Ag system. The columnar to equiaxed transition (CET) in a ternary system behaves very similar to the CET observed in binary systems [1]. It is observed that the CET occurs at lower temperature gradients when the amount of the third alloying element, in this case Ag, is increased. Another important solidification parameter is the cooling rate of the furnace. When the cooling rate is increased, the CET will occur at lower positions in the sample where the temperature gradient is lower.

Unconstrained solidification and characterisation of near-eutectic Al–Cu–Ag alloys

Abstract This study focuses on the microstructure formation in two Al – Cu – Ag alloys with near-eutectic composition on either side of the α(Al)/θ-Al2Cu groove. The alloys solidified equiaxially with two different cooling rates for each composition of alloy. The primary phases formed are α(Al) or θ-Al2Cu, but univariant and invariant eutectic reactions are common. In hypoeutectic samples macrosegregation of the α(Al) phase occurred. The univariant α(Al)/θ-Al2Cu eutectic in these samples is formed by coupled two-phase structures. The univariant eutectic in the samples, which exhibit primary θ-Al2Cu, grew partially competitively due to the α(Al) single phase instability. It is suggested that the difference in solubility of the segregating element Ag in α(Al) and θ-Al2Cu phases and processing parameters such as cooling rates determine the resulting microstructure.

Columnar to equiaxed transition during growth of a univariant eutectic

Abstract In this work the transition from columnar to equiaxed growth is studied during the solidification of the univariant eutectic L→ α(Al) + θ-Al2Cu in the ternary Al–Cu–Ag system. The experimental technique used is upwards solidification with a controlled cooling rate. In the lower part of the sample where the temperature gradient is high, a cellular eutectic is observed. When the temperature gradient decreases, the morphology first changes into two-phase dendrites and finally into an equiaxed eutectic. When the amount of Ag is increased, the formation of eutectic colonies becomes easier. Also, a higher cooling rate facilitates the formation of eutectic colonies. The addition of TiB2 does not influence the nucleation of eutectic colonies. Although the composition is on the eutectic groove, primary α(Al)-dendrites are found in the samples with TiB2. This leads to segregation effects due to the density difference between α(Al) and the liquid.

Effect of TiB2 on univariant eutectic growth in a directionally solidified Al-alloy

Abstract During directional solidification of a near-eutectic Al–Mn–Si alloy the univariant eutectic is formed, following the reaction: L→α-Al + α(AlMnSi) + L′ and reveals a non-faceted α-Al/faceted α(AlMnSi) structure. The addition of titanium diboride (TiB2) promotes the nucleation of intermetallic α(AlMnSi) silicide phase at solid/liquid interface as particles, which are pushed and periodically engulfed by the advancing planar interface. This type of growth, called symbiotic growth, leads to a layered microstructure. To study the influence of TiB2 upon the morphology of α(AlMnSi) phase various amounts (from 0·05 up to 2·00 wt-%) of TiB2 were added to alloys with identical composition. The results show that the behaviour and the influence of the inoculant is not trivial. Both the presence of titanium diboride at the solid/liquid interface and its inoculation effect determine the final morphology of the silicide phase.