Characterization and modeling of twinned dendrite growth

The formation of feathery grains during semi-continuous casting of Al-alloys [1, 2] is an interesting problem from both practical and theoretical points of view. These structures are formed by a lamellar sequence of twinned and untwinned regions separated by straight and wavy-like boundaries. Each pair of lamellae contains twinned dendrites split in their trunk center by a coherent {111} twin plane, while lateral arms meet at an incoherent {111} boundary. In practice, feathery grains are considered as defects which reduce the mechanical properties of a solidified ingot. In theory, an understanding of twinned dendrite growth includes different solidification phenomena, e.g., interfacial energy anisotropy, crystallographic growth directions, twinning, growth competition mechanisms, etc. Although several studies have been performed in order to understand the physics leading to the nucleation and growth of twinned dendrites, various questions remain unanswered. In this work, a comprehensive study of twinned dendrite growth has been undertaken, with the main objectives being: i) to study the effect of different alloying elements and solidification conditions on twin formation in binary Al-alloys; ii) to establish a better understanding on the stability of twinned dendrites and the growth kinetic advantage that they exhibit over regular ones; and iii) to elucidate the stable shape of the twinned dendrite tip. In order to study alloying-element effects, binary Al-X alloys (where X = Zn, Mg, Cu and Ni), were produced under Directional Solidification (DS) conditions in the presence of a slight natural convection in the melt. Analysis of these castings has shown that feathery grains can form for all solute elements of interest, but not for all compositions. The probability of forming feathery grains is relatively high when the alloying elements are hcp (Zn or Mg), but decreases for fcc solute elements (Cu or Ni). A study on the effect of forced convection in the melt, performed using different experimental set-ups, confirms previous observations suggesting that it is the shearing components of the liquid slow which induce twin nucleation [3, 4]. However, the poor reproducibility of these experiments and the variable rate of feathery grains formation indicate that twin nucleation is governed by a highly stochastic behavior. The probability of such an event decreases as the melt slow is less complex and the associated Stacking Fault Energy (SFE) of the alloying element is increased. In terms of growth kinetics advantage, a characterization using various metallography techniques and X-ray synchroton tomography has shown that this is in part due to the complex morphology of twinned dendrites. Indeed, it has been confirmed that these dendrites grow along ‹110› directions with ‹110›, and also sometimes ‹100› secondary arms, the primary trunk spacing of these dendrites being much less anisotropic than previously thought [5, 6]. It has been shown that twinned dend