In this article, for the first time, in situ and real-time experimental observations of changes in solid/liquid (s/l) interface shape during interactions with a particle or void are reported for metallic systems. Real-time interface shape evolution for both stationary and growing interfaces was observed by use of a state-of-the-art X-ray transmission microscope. Localized interfacial perturbations were studied as a function of the particle or void diameter, the distance between the s/l interface and the particle or void, and the thermal conductivity ratio between the matrix and the particle or void. In particular, the sensitivity of interfacial perturbation to the thermal conductivity ratio is critically analyzed. Analytical predictions of interface shape are compared to the real-time, in situ experimental data. A good agreement between the experimentally observed and predicted interface shapes was found for stationary interfaces. Based on the differences in experimental observations, between a moving and a stationary interface, an alternate hypothesis is suggested to explain the observed kinetics of particle engulfment by a growing interface.
In this article, an investigation of the interaction between gas porosity and a planar solidniquid (SL) interface is reported. A two-dimensional numerical model able to accurately track sharp SL interfaces during solidification of pure metals and alloys is proposed. The finite-difference method and a rectangular undeformed grid are used for computation. The SL interface is described through the points of intersection with the grid lines. Its motion is determined by the thermal and solute gradients at each particular point. Changes of the interface temperature because of capillarity or solute redistribution as well as any perturbation of the thermal and solute field produced by the presence of non-metallic inclusions can be computed. To validate the model, the dynamics of the interaction between a gas pore and a solidification front in metal alloys was observed using a state of the art X-ray transmission microscope (XTM). The experiments included observation of the distortion of the SL interface near a pore, real-time measurements of the growth rate, and the change in shape of the porosity during interaction with the SL interface in pure AI and Al-0.25 wt pct Au alloy. In addition, porosity-induced solute segregation patterns surrounding a pore were also quantified.
Real-time observations of transparent analog materials have provided insight, yet the results of these observations are not necessarily representative of opaque metallic systems. In order to study the detailed dynamics of the solidification process, we develop the technologies needed for real-time Xray microscopy of solidifying metallic systems, which has not previously been feasible with the necessary resolution, speed, and contrast. In initial studies of Al-In monotectic alloys unidirectionally solidified in an X-ray transparent furnace, in situ records of the evolution of interface morphologies, interfacial solute accumulation, and formation of the monotectic droplets were obtained for the first time: A radiomicrograph of AI-30In grown during aircraft parabolic maneuvers is presented, showing the volumetric phase distribution in this specimen. The benefits of using X-ray microscopy for postsolidification metallography include ease of specimen preparation, increased sensitivity, and three-dimensional analysis of phase distribution. Imaging of the solute boundary layer revealed that the isoconcentration lines are not parallel (as is often assumed) to the growth interface. Striations in the solidified crystal did not accurately decorate the interface position and shape. The monotectic composition alloy under some conditions grew in an uncoupled manner.
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