Competitive growth during directional solidification experiments of 〈111〉 Dendrites

Competitive growth is a key topic in directional solidification, however, to the authors' best knowledge, there are no experimental or numerical studies on competitive growth between neighbouring 〈111〉 dendrites. The secondary arms of 〈111〉 dendrites can grow with more favourable orientation than those of 〈100〉 dendrites and this has implications for the competitive growth mechanism. An experimental apparatus was developed for in-situ directional solidification with a transparent model alloy, Neopentyl Glycol-35 wt% (D)Camphor (NPG-35wt.%DC), which has 〈111〉 dendrite orientation. Seven tests were conducted with different temperature gradients and isotherm speeds to investigate competitive growth behaviour. Multiple grains were identified in each test with examples of competition at converging and diverging grain boundaries as well as obedience and disobedience with the well-known Walton-Chalmers rule. Unfavourably oriented crystal colonies at diverging grain boundaries were able to outgrow favourably oriented colonies via tertiary branching. Overgrowth by the unfavourably oriented crystal was linked to the advantageous growth directions of secondary arms in 〈111〉 dendrites. Automated dendrite tip velocity tracking further elucidated the tertiary branching mechanism that allowed the unfavourably oriented crystal colonies to stabilise along the growth direction. It is proposed that the deviation from the Walton-Chalmers rule of competitive growth is probabilistic and depends on the initial seeding as well as the 3D nature of the experiments.