Influence of growth velocity on degenerate pattern evolution in Al-4.5 wt% Cu during non-axially oriented directional solidification

Step-increasing directional solidification experiments are used to reveal the microstructural evolution of the degenerate pattern in Al-4.5 wt% Cu. It is found that the degenerate pattern could remain stable for abrupt growth velocity increases from 15 µm/s to 100 µm/s and the texture dimensionless intensity of the degenerate structure remains unchanged. A strong dependence of the tip-splitting spacing (λ) and frequency (ƒ) of the degenerate pattern on growth velocity was observed and fitted to the power law as λ = 134.4 × V−0.46 and ƒ = 0.005 × V1.45, suggesting that degenerate pattern growth is diffusion-limited and its tip-splitting instability dynamics are related to the Mullins-Sekerka (M-S) instability. Further characterization by electron backscattered diffraction (EBSD) analysis showed the transition from the degenerate pattern to tilted dendrites can prevail through an orientation deviation in the solidification direction.