Proceedings of the 12th International Conference on Aluminium Alloys, September 5-9, 2010, Yokohama, Japan 2010 The Japan Institute of Light Metals In-situ X-ray video microscopy studies of Al-Si eutectic solidification Ragnvald H. Mathiesen 1 , Lars Arnberg 2 , Yanjun Li 3 , Arne K. Dahle 4 1 Department of Physics, NTNU, N-7491 Trondheim, Norway 2 Department of Material Science, NTNU, N-7491 Trondheim, Norway 3 SINTEF Materials and Chemistry, N-7465 Trondheim, Norway 4 ARC CoE for Design in Light Metals, Materials Engineering, Univ. Queensland, Brisbane Qld 4072, Australia In-situ studies of Al-Si eutectic growth has been carried out for the first time by X-ray video microscopy during directional solidification of Al-Si-Cu alloys with and without Sr-addtions. The unmodified eutectics showed distinctive non-isothermal growth dynamics, where Si-crystals attained needle-like tip morphologies and progressed under significantly higher undercooling than Al, leading to formation of an irregular eutectic with Si as the leading phase and subsequent nucleation of Al on the Si-surfaces. In the Sr-modified alloys, the eutectic reaction was found to be strongly suppressed, occurring with low nucleation frequencies at undercoolings in the range 10-18 K. In the Cu-enriched melt, the eutectic front was found to attain meso-scale interface perturbations evolving into equiaxed cellular rosettes in order to accommodate to the long-range redistribution of Cu from the composite eutectic interface. The eutectic front also attained short-range microscale interface perturbations consistent with characteristics of a fibrous Si growth, however further improvements in spatial resolution is required in order to study microscale structure formation in greater detail. Evidence was found in support of Si-nucleation occurring on potent particles suspended in the melt. Yet, both with Sr- modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form on the Si-particles. To what extent nucleation mechanisms in the Cu-enriched systems are transferable to binary or commercial Al-Si alloys remains uncertain. Keywords: Aluminium alloys, Al-Si eutectic solidification, radiography 1. Introduction Eutectic alloy systems offer specific advantages in metals such as ease of processing, isothermal or near-isothermal freezing and beneficial mechanical properties of a self-assembling composite material. Our understanding of regular eutectic solidification microstructure formation is quite far progressed. Within the operation limits of quasi-planar near-isothermal interface propagation, the pattern selection that defines the eutectic growth morphologies is fairly well described by the Jackson-Hunt model [1], and more recent extensions to this, e.g. by phase field simulations and experiments with transparent analogs, for growth behavior beyond the basic state interface stability limits [2-5]. Our understanding of irregular eutectic solidification, however, has not progressed to the same extent [6]. Presumably this can be ascribed to their substantially more complex growth behavior and conditions, which by the simultaneous progress of a faceted and a non-faceted phase, is bound to yield distinctively non-isothermal eutectic fronts [7-9], where the faceted phase tends to lead the growth [7]. Furthermore, since changes in growth direction or side branching generally is more difficult for a strongly anisotropic faceted phase compared to the typical weakly anisotropic non-faceted component, filling of space by a progressing irregular eutectic is considerably more cumbersome than for the regular systems [7]. 1267 Proceedings of the 12th International Conference on Aluminium Alloys, September 5-9, 2010, Yokohama, Japan ©2010 The Japan Institute of Light Metals pp. 1267-1272