Columnar to equiaxed transition of eutectic in hypoeutectic aluminium–silicon alloys

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

“…3 with G = 18.4 K/mm (°C/mm), v s = 10.5 lm/s, and Dt = 140 ms. Times given are relative to the eutectic nucleation event occurring at t = 0 s. modified eutectic microstructure similar to those found in castings with Sr-modified binaries or industrial variants. [10][11][12][13] Finally, with the presence of a-Al dendrites as well as other eutectic colonies, modified eutectic growth may be restricted by other factors in addition to interface capillarity and free diffusive transport into the nearand far-field nominal melts. Prior to complete solidification of the remaining Cu-enriched liquid, continuous change with temperature in the a-Al Cu solubility, combined with dendrite coarsening, will cause local constitutional variations in the liquid, and possible dendritic-eutectic and eutectic-eutectic mass transfer, where the difference in melting and freezing kinetics between the different constituents also could be of importance.…”

“…Yet, assuming so would be in accordance with the present hypotheses of Sr preventing or retarding Si nucleation either by impurity-induced twinning [10] or by promoting precipitation of intermetallics with low potency as Si inoculants. [11][12][13][14][15][16] Furthermore, it has been possible to find supporting evidence from one of the G || -g DS sequences collected with the unmodified alloys, where two Si crystals were observed to precipitate from the bulk melt. These crystals could be followed as they moved and grew, initially in swirling motion inside the eutectic mush.…”

“…[10][11][12][13][14][15][16] With eutectic nucleation observed to occur exclusively in the vicinity of the a-Al dendrite surface, it therefore at a first glance seems reasonable to assume some form of correlation to exist between the Si nucleation mechanism and the presence of a-Al. Such a correlation would be in favor of Si nucleation on chemically potent sites situated at the a-Al surface, or alternatively on potent intermetallic particles formed in the impurity-element enriched melt very close to the dendrite surface.…”

“…Such a correlation would be in favor of Si nucleation on chemically potent sites situated at the a-Al surface, or alternatively on potent intermetallic particles formed in the impurity-element enriched melt very close to the dendrite surface. [11][12][13][14][15][16] To shed more light on the relationship between formation of the eutectic, presence of a-Al, and crystallization of the first Si, the video sequence illustrated in Figure 3 was reconstructed from images subjected to additional contrast enhancement operations. In this video, it was possible to locate and follow the motion of a~10-lm size particle over a sequence of 70 frames or 9.8-second duration.…”

“…[10][11][12] Evidently, chemical modification leads to a refinement of the eutectic by converting the relatively coarse and platelike Si crystals into a much finer and fibrous network. [11,13] However, there are still unsettled issues concerning how these chemical modifiers actually work in combination with other minor constituents to affect the relevant nucleation and growth mechanisms. [10][11][12][13][14][15][16] The development of theory and models to describe pattern selection in regular eutectics [4][5][6] has heavily relied on in-situ experimental observations with transparent alloys, [5,[17][18][19] which was decisive for identifying and characterizing dynamic instabilities that limit the range of stable growth.…”