Date of Award
Doctor of Philosophy
Materials Science and Engineering
Directional solidification experiments have been carried out in the succinonitrile-acetone system to study transient and steady-state pattern formation of the interface. Initial breakup of a planar interface is studied by driving the system beyond the limit of planar interface stability, and it is found that the interface breaks up into two characteristic wavelengths, (lamda)(,1) and (lamda)(,2). Two pattern propagation modes are found, one leading to a cellular structure and the other to a dendritic structure;A systematic experimental study is carried out to quantitatively study the effect of experimental variables on the steady-state length characteristics of the interface pattern. Four steady-state lengths are studied: primary spacing, secondary spacing, dendrite tip radius and the distance between the dendrite tip and the first sidebranch perturbation. These results are compared with the existing theoretical models and it is found that the marginal stability criterion plays a key role in establishing the various length scales of the dendritic structure. This conclusion was also established by dynamic studies in which a steady-state was perturbed and the process of restabilization was examined quantitatively;Important scaling laws have also been observed among various steady-state length. The primary spacings and secondary spacings have been found to be precisely the same as the wavelengths (lamda)(,1) and (lamda)(,2) observed during the planar interface breakup. Furthermore, the dendrite tip radius is found to be equal to (lamda)(,2)/2 for all experimental conditions. The distance between the dendrite tip and the first sidebranch perturbation has been found to be equal to 0.5 ((lamda)(,1)(lamda)(,2))(' 1/2);The coarsening phenomenon by which the secondary branches change their spacing with time has been investigated, as the effect of velocity, temperature gradient and composition and the coarsening rate is measured. The conditions for the formation of tertiary branches have also been investigated and the mechanism by which tertiary branches become primary branches is proposed. These results have been applied to understand the mechanism by which a chill crystal zone in a casting transforms to a columnar zone.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Somboonsuk, Khanchai, "Microstructural characteristics during the controlled solidification of a binary system " (1984). Retrospective Theses and Dissertations. 7798.