Date of Award
Doctor of Philosophy
Materials Science and Engineering
John D. Verhoeven
The main objective of this study was to examine the influence of four impurities S, Te, O, and H on structure control in the directional solidification of Fe-C-Si alloys. In the course of the solidification experiments, it was also observed that control of the solidification atmosphere by either H[subscript]2 or O[subscript]2 gas had a substantial influence on the microstructure. Both the S and H had similar effects on the gray solidification. Their effects were demonstrated by coarsening of the type A flake graphite structures and also by the promotion of the type A growth at higher velocities, compared to the results obtained from the solidification of high purity alloys under pure Ar atmosphere. The S effect was different from the H effect in that the former induced fine type D undercooled graphite and the latter coral graphite when the type A growth was terminated upon increasing the solidification velocity. In contrast to the S and H, it was found that the O suppressed the type A growth in high purity alloys, which was replaced by the growth of coral graphite at an extremely low solidification rate;The Auger analysis in Part II showed that the iron matrix/graphite interface regions were contaminated by the adsorbed elements S or O when the graphite solidified in flake morphology, type A or D, in both high purity and S-added alloys. The O presence was, however, observed only in coarse type A structure. The experiments in Part III produced evidence supporting that both the type A and D graphite grow by the extension of basal growth layers. Part IV of this study produced evidence that the interaction of the H with Te, as opposed to independent action of H, is primarily responsible for the enhancement of Te effectiveness in promoting white iron growth. The experimental results of the present study lead to the conclusion that the presence of impurity elements, either by doping addition (S and Te) or by atmosphere control (H and O), controls the structure of cast iron through the adsorption at the growth front (S and Te) or through the interaction with O (H). (Abstract shortened with permission of author.)
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Park, Jang-Sik, "Microstructural evolution and control in the directional solidification of Fe-C-Si alloys " (1990). Retrospective Theses and Dissertations. 9404.