In this article, we present a review of observations during Al-3.5 wt pct Ni alloy solidification experiments performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble. These experiments provide direct access to dynamical phenomena during columnar growth (initial transient and breakdown of a planar solid-liquid interface), and for the first time to the transition from columnar-to-equiaxed microstructure (nucleation ahead of a columnar front and blocking of a columnar front by an equiaxed microstructure) and fully equiaxed growth (propagation of an effective front). Based on these experimental observations, critical parameters such as columnar growth velocity variation during the transition or equiaxed-grain diameter are measured and discussed.
Alloy solidification was investigated in situ and real time by using a unique experimental setup developed at the European Synchrotron Radiation Facility (ESRF) combining both synchrotron X-ray radiography and topography. Although synchrotron X-ray radiography enables the investigation of the solid-liquid interface of metallic alloys, white-beam synchrotron X-ray topography enables the investigation of the formation of strains and defects formation in the growing solid microstructure. In this article, we present results obtained during directional solidification experiments performed with Al-3.5 wt pct Ni samples. First, the initial state after thermal stabilization is characterized. Next, the interface morphological instability and the transition to the columnar growth regime are thoroughly investigated. Topography observation shows that several parts of each dendrite become disoriented while the microstructure is developing. Disorientations are quantified and the aluminum yield stress at the melting point is estimated from the bending of secondary arms. Last, coupled growth of eutectic and dendrites settles with the formation of the eutectic phase. The eutectic grains grow strained and the dendrites concomitantly undergo additional stress.
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