319 aluminum (6.5-8.5% Si, 3.0-4.0%Cu, 0.2-0.45%Mg, 0.8%max.Fe, 0.5%max.Mn) is utilized by the automobile industry for engine block and cylinder head applications. Detailed understanding of the solidification behavior is critical to optimizing the “as-cast” microstructure and the physical properties of this alloy. Samples were generated by allowing a 50 gram sample of molten alloy to partially solidify at a slow cooling rate (0.34 °C/sec). During cooling, the temperature of the alloy was monitored by a thermocouple in the center of the sample to generate a cooling curve (Fig. 1). At various locations along this cooling curve the normal cooling was interrupted by plunging the cup into an iced- brine quench. In addition to optical and SEM examination, detailed TEM analysis was performed on three quenched samples as well as one sample allowed to cool uninterrupted to room temperature. TEM sample preparation consisted of standard grinding and dimpling procedures followed by ion milling at liquid nitrogen temperature using a gun voltage of 4keV.