Study of the twinned dendrite tip shape II: Experimental assessment

Abstract

The favorable growth kinetics of twinned dendrites can be explained by their complex morphology, multiple side branching mechanisms, growth undercooling and tip morphology. Three models were proposed for the twinned dendrite tip shape: (i) a grooved tip [1] satisfying the Smith condition at the triple line; (ii) a doublon [2], i.e. a double-tip dendrite that grows with a narrow and deep liquid channel in its center; and (iii) a pointed (or edgy) tip [3], with consideration of the solid-liquid interfacial energy anisotropy. In the first part of this work, phase field simulations of half a twinned dendrite with an appropriate boundary condition to reproduce the Smith condition supported the doublon conjecture, with a narrow liquid channel ending its solidification with the formation of small liquid droplets. In this part, experimental observations of twinned dendrite tips reveal the presence of a small, but well-defined, groove, thus definitely eliminating the edged tip hypothesis. Focused ion beam nanotomography and energy-dispersive spectroscopy chemical analysis in a transmission electron microscope reveal the existence of a positive solute gradient in a region localized within 2 lm around the twin plane. In Al-Zn specimens, small particles aligned within the twin plane further support the doublon conjecture and the predicted formation of small liquid droplets below the doublon root.

Morphology and Growth Kinetic Advantage of Quenched Twinned Dendrites in Al-Zn Alloys

“…Henry et al [10,11] made a conjecture that the twinned dendrite tip is in fact a doublon, i.e., a double-tip dendrite growing with a thin liquid channel in the center, the root of which contains the twin plane that solidifies at a solid composition close to C 0 . Recent phase-field simulations [12][13][14] as well as FIB observations [15] made by the present authors also strongly support this conjecture. On the other hand, if the propagation of twinned dendrites along the twin plane can be easily understood by branching mechanisms similar to those of regular columnar dendrites, their extension perpendicular to the twin plane, in particular the formation of new twins, remains unclear.…”

“…[10] D. Competition Mechanisms While the competition of regular dendrites and the formation of solidification textures are well known since the work of Chalmers, [25][26][27] the growth advantage that twinned aluminum dendrites can have over regular ones under certain conditions remains unclear. The weak anisotropy of aluminum, [28] the reduced solute segregation associated with a doublon-type twinned dendrite tip, [14][15][16] and the solidification conditions (high thermal gradient, presence of convection, etc.) all play an important role.…”

“…This discrepancy can be due to (1) The morphology of secondary arms which is much more complex in the case of twinned dendrites (presence of both h100i and h110i arms); (2) Unsteady-state effects associated with the change of the section near the chill device; and (3) Microsegregation at the scale of primary trunks if one considers a doublon-type morphology. [14,15] The small white arrows in Figure 2a indicate regions where the continuity of the coherent twin planes is interrupted by the extension of an untwinned lamellae into a twinned region. This phenomenon had already been observed in Al-Cu alloys, [7] but not previously in an Al-Zn specimen.…”

Study of the twinned dendrite tip shape I: Phase-field modeling

“…Such a condition implicitly assumes that the solid-liquid interfacial energy c s' (n) is isotropic, which is a reasonable assumption for the very weak anisotropy exhibited by aluminum alloys [17]. 1 The angle h 0 remained constant throughout the calculation and was measured in the solid; for example, an angle larger than 90°corresponds to a grooved dendrite tip. This "wetting" angle of the solid on the yz-plane is equal to the complement of half the dihedral angle measured at the triple line between the twinned solid, untwinned solid and liquid.…”

Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al–Zn–Cr alloys: Can icosahedral solid clusters play a role?

“…More confusion has been brought recently by Li et al [13], who observed that the application of a static magnetic field of only 0.2 T, which is supposed to weaken convection, induces the formation of twinned dendrites in Al-Fe and Al-Zn alloys in small DC cast ingots. Finally, although twinned dendrites have a growth advantage over regular dendrites under the above-mentioned conditions, there is still some uncertainty about their tip morphology [3,12,[14][15][16][17][18]. More important, nothing is known about twin nucleation.…”