Regular Dendrites Research Articles

Twinned Feathery Grains and Related Morphologies in Aluminum Alloys

Al-Cu, Al-Mg and Al-Si alloys without grain refiner addition have been directionally solidified under well-controlled thermal and convection conditions. For relatively high solidification rates, several changes in the dendrite growth morphology were observed. One of the most common structure that may appear under such conditions is called feathery grains: it was demonstrated recently that this lamellar structure is constituted of dendrites whose trunks are cut through by {111} twin planes. These grains undergo a selection mechanism which is similar to that occurring for regular dendritic grains. The transition between and dendrite forms is progressive and morphologies that develop arms in both type of crystallographic directions can be observed. Moreover, in the case of a 5182 Al-Mg alloy, regular dendrites, without twins, could also be obtained. This dendritic growth along crystallographic directions is supposed to be due to a change in the anisotropy of certain properties of the alloy, such as the solid/liquid interfacial energy and/or the atom attachment kinetics. The possibility of growth along other directions than the usual was further demonstrated by the observation of dendrites developing in an Al-Cu-Mg solidified in a Bridgman device. In this case, trunks form and progressively change their growth direction, thus showing curved shapes.

Growth of noncrystallographic dendrites

First time experimental evidence is presented for the steady state growth of dendrites away from their normal crystallographic growth directions. The evidence is shown for transparent dendrites in the succinonitrile system, which are induced to grow along wetting surfaces. A method for the quantitative characterization of the tips of such dendrites is described and the differences in the secondary arm formation between off-axis and regular dendrites are recorded. The scientific and technological implications of this finding are examined.

Simulation of oriented dendritic microstructures using the concept of dendritic lattice

The microstructures of dendritic single-crystals produced by directional solidification exhibit composition heterogeneties with a quite regular space periodicity. In the most commonly observed dendritic arangement, the [001] trunks are stacked along a [100] principal direction, thus producing (010) layered structures. This arrangement is described using the concept of a periodic dendritic lattice. Such a model permits to simulate the microstructures in three dimensions. The distribution of the microstructural heterogeneties is correlated with the crystallographic structure. Formation of regular dendrite arrangements is explained in terms of crystal growth and branching mechanisms.

Nigrothalamic pathway in the cat demonstrated by autoradiography and electron microscopy.

Light microscopic autoradiography and electron microscopy were used to trace the nigrothalamic projections and to study the sites of termination of this pathway in the cat. Injections of tritiated amino acids or electrolytic lesions were placed in the substantia nigra pars reticularis (SNr). An accumulation of radioactivity was found in the ventral medial nucleus and in the ventromedial part of the ventral anterior nucleus. At the ultrastructural level degenerating medium size synaptic boutons and medium size myelinated fibers were observed in these nuclei. The boutons contained clear pleomorphic veiscles and formed symmetrical type synaptic contacts with regular type dendrites and vesicel-containing dendrites. The present findings indicate that the ventral medial nucleus is the principal site of termination of nigrothalamic projections in the cat.

The nucleation of polyethylene dendrites from solution

This paper is a study of nucleation mechanisms in the formation of polyethylene dendrites from dilute solutions. Polyethylene dendrites may be either of regular morphology or of irregular morphology (“hedgehog” dendrites). By varying the temperatures of dissolution and crystallisation, as well as the general impurity level of the system, it was possible to alter systematically the relative ratio of regular to hedgehog dendrites. It was found that regular dendrites could be nucleated through a “self-seeded” process, as well as through homogeneous nucleation. The irregular hedgehog dendrites were observed to be nucleated heterogeneously. The degree of super-cooling necessary for dilute solution homogeneous nucleation was established (∼30° C).