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We study microstructure selection during during directional solidification of a thin metallic sample. We combine in situ X-ray radiography of a dilute Al-Cu alloy solidification experiments with three-dimensional phase-field simulations. We explore a range of temperature gradient G and growth velocity V and build a microstructure selection map for this alloy. We investigate the selection of the primary dendritic spacing ⇤ and tip radius ⇢. While ⇢ shows a good agreement between experimental measurements and dendrite growth theory, with ⇢ ⇠ V 1/2 , ⇤ is observed to increase with V (@⇤/@V > 0), in apparent disagreement with classical scaling laws for primary dendritic spacing, which predict that @⇤/@V < 0. We show through simulations that this trend inversion for ⇤(V ) is due to liquid convection in our experiments, despite the thin sample configuration. We use a classical di↵usion boundary-layer approximation to semi-quantitatively incorporate the e↵ect of liquid convection into phase-field simulations. This approximation is implemented by assuming complete solute mixing outside a purely di↵usive zone of constant thickness that surrounds the solid-liquid interface. This simple method enables us to quantitatively match experimental measurements of the planar morphological instability threshold and primary spacings over an order of magnitude in V . We explain the observed inversion of @⇤/@V by a combination of slow transient dynamics of microstructural homogenization and the influence of the sample thickness.

The initiation and propagation of shear bands is an important mode of localized inhomogeneous deformation that occurs in a wide range of materials. In metallic glasses, shear band development is considered to center on a structural heterogeneity, a shear transformation zone that evolves into a rapidly propagating shear band under a shear stress above a threshold. Deformation by shear bands is a nucleation-controlled process, but the initiation process is unclear. Here we use nanoindentation to probe shear band nucleation during loading by measuring the first pop-in event in the load-depth curve which is demonstrated to be associated with shear band formation. We analyze a large number of independent measurements on four different bulk metallic glasses (BMGs) alloys and reveal the operation of a bimodal distribution of the first pop-in loads that are associated with different shear band nucleation sites that operate at different stress levels below the glass transition temperature, T g . The nucleation kinetics, the nucleation barriers, and the density for each site type have been determined. The discovery of multiple shear band nucleation sites challenges the current view of nucleation at a single type of site and offers opportunities for controlling the ductility of BMG alloys.plastic deformation | stochastic analysis D eformation by shear bands occurs in both crystalline and amorphous phases over a wide range of size scales. In geological materials with a granular nature such as sand (1, 2) or within rocks, shear bands are of macroscopic size (3); in polymers and crystalline metals, shear bands are several micrometers in size; in metallic glasses (MGs), shear bands are of the order of 10-20 nm in thickness (4). The onset of shear bands in metallic glasses represents initial plastic yielding and induces a highly localized plastic flow that limits ductility and is responsible for strain softening (4, 5). Because shear bands directly determine the capability of an amorphous phase to sustain plastic flow and exhibit ductile behavior, there have been numerous studies on the propagation of shear bands to promote shear band branching and additional nucleation to achieve useful ductility (6-9). On a microscopic scale, the initiation of shear bands in MGs is considered to be controlled by the activation of a shear transformation zone (STZ) (10, 11) that represents a localized atomic arrangement. The STZs are activated by a two-stage process starting with a flow-induced dilatation that broadens homogeneously in the initial stage, but subsequently narrows into a shear band autocatalytically at the expense of shear flow in the surrounding material (10, 12). The localization of plastic flow would become predominant at low temperatures because the diffusive atomic rearrangements are too slow to disperse the dilatation effect quickly enough. Clearly, the nucleation of shear bands is of critical importance in the deformation behavior, but the detailed examination of the nucleation kinetics behavior has received only limited s...

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We present a three-dimensional extension of the multiscale dendritic needle network (DNN) model. This approach enables quantitative simulations of the unsteady dynamics of complex hierarchical networks in spatially extended dendritic arrays. We apply the model to directional solidification of Al-9.8 wt.%Si alloy and directly compare the model predictions with measurements from experiments with in situ x-ray imaging. We focus on the dynamical selection of primary spacings over a range of growth velocities, and the influence of sample geometry on the selection of spacings. Simulation results show good agreement with experiments. The computationally efficient DNN model opens new avenues for investigating the dynamics of large dendritic arrays at scales relevant to solidification experiments and processes.

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