Probing the degenerate pattern growth of {100} orientation in a directionally solidified Al-4.5 wt% Cu alloy

Abstract Degenerate pattern is a seemingly disordered morphology but it exhibits the inherently ordered crystal connected with tip-splitting and limited stability which makes it difficult to observe in the metallic system. Here we employ (100)[011] orientated planar-front seeds using directional solidification and reveal the fundamental origins of the degenerate pattern growth in an Al-4.5 wt% Cu alloy. We find that the spacing of the tip-splitting ( λ ) in the degenerate of the alloys followed a power law, λ ∝ V −0.5 , and the frequency ( f ) of the splitting was related to the growth velocity ( V ) by ƒ∝ V 1.5 . The dimensionless growth direction ( θ / θ 0 ) increased monotonously and approached 0.6 with faster velocity, attributed to its anisotropy in the interface kinetics. Once growth velocity exceeded a threshold, two types of pattern transitions from degenerate to regular dendrites were proposed. One of them exhibited a random and chaotic mode and the other underwent a rotation in growth direction.