Eutectic nucleation and growth in hypoeutectic Al-Si alloys at different strontium levels

Abstract

The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents, the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic solidification.

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.…”

“…[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.…”

“…In addition, further improvements in cast component performance are available through modification of the eutectic microstructure, either by employing a high cooling rate or by relatively modest melt additions of certain elements such as Sr, Na, Sb, or Ca. [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] 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.…”