Characterization of the spacing selection in AlCu alloys

We study the relationship between growth conditions relevant to strip casting of binary alloys and the resulting microstructure. We report new experiments of directionally solidified Al-Cu alloys under upward unsteady-state conditions in a range of cooling rates and present new phase-field simulations of directional solidification wherein we vary growth conditions dynamically, focusing on transient behavior and growth history. Using power spectral analysis to characterize sparing in a statistical sense, we find that for slow enough cooling rates the resulting primary spacing falls within a fairly tight, band that decreases monotonically with the front velocity, consistent with Greenwood et. al [6]. Under rapidly changing cooling conditions the spacing appears not to be uniquely determined by the cooling conditions but still falls within bands of accessible spacings, consistent with Warren and Langer [2]. Trivedi et al [1] and Huang et al [3]. This study serves as the first step to consolidate the notion that primary spacing can be both predictable and non-unique depending on growth conditions.