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 dendrites grow in a stable manner at a lower undercooling than regular ones. In addition, the distribution and orientation of their side arms favors their growth at the expense of less developed regular dendrites. A mechanism to explain the lateral multiplication of twin planes is also proposed in this work. In terms of stability, observations after partial remelting of twinned DS specimens, then re-solidification in a Bridgman furnace, have shown that even if twin planes remain stable during partial remelting, independent regular non-twinned dendrites issued from the twinned and untwinned parts of the seed grow during solidification. This implies that the formation of twinned dendrites is not only related to the ability to nucleate a stacking fault, but also to the imposed solidification conditions. The favorable growth kinetics of twinned dendrites is also explained by their tip morphology. Three hypotheses have been evaluated in this work: i) the grooved tip [7], stabilized by the Young-Laplace equilibrium condition; ii) the doublon [5], i.e., a double tip dendrite that grows with a narrow liquid channel in its center that solidifies at a composition close to C0; and iii) the pointed (or edgy) tip [8], equilibrated by torque terms of γsl when it is too anisotropic. Observations performed at the twinned dendrite tip have revealed the presence of a small groove, which eliminates definitively the hypothesis of the edgy tip. Further examination of the stable growth morphology of twinned dendrites has been done by using a phase field model that reproduces the presence of a twin plane through an appropriate boundary condition. The results of these simulations show that twinned dendrites are doublons that grow with a narrow liquid channel (0.2 to 3 µm width) whose depth depends strongly on the alloy composition and the solidification conditions. In order to validate these simulations, Focused Ion Beam (FIB)-microtomography and X-ray chemical analysis (EDS) in a Scanning Transmission Electron Microscope (STEM) were performed on small specimens extracted from twinned dendrite trunks. These have shown the existence of a positive solute gradient in a region localized within 2 µm around the twin plane, i.e., as expected for a doublon morphology. Additionally, the presence of small particles aligned within the twin plane is in agreement with the formation of small liquid pockets below the doublon root as predicted by the numerical model, but further work is required to explain this remarkable feature. Finally, composition measurements performed after quenching a partially remelted specimen also seem to confirm the existence of a doublon. This work contributes to the understanding on twinned dendrite growth kinetics in several aspects, principally the effect of alloying elements on twin formation, their growth kinetic advantage, their stability and their stable tip morphology. However, further work must be carried out to overcome the limited knowledge on the mechanisms leading to twinned dendrite nucleation. In the same manner, the growth mechanisms of the doublon morphology should also be further investigated in future works.