| The objective of this study was to experimentally control directional solidification processing parameters such as solidification front velocity (R) and liquid temperature gradient (G), in an effort to accurately determine and better understand microstructural transitions in Al-In immiscible alloys of approximately monotectic composition.; Since tailoring of mechanical, chemical, magnetic, and electronic properties of selected alloy systems may theoretically be achieved through appropriate processing conditions, control of microstructures through directional solidification of immiscible alloys has been a subject of interest over the past several decades. Depending on the application, aligned fibrous, doubly oriented droplet arrays, or dispersed microstructures are desirable. Therefore, understanding the processing conditions under which such microstructural morphologies can be produced is crucial for various applications of immiscible alloys.; Numerous investigations have shown that depending on experimental conditions, aligned fibrous, doubly oriented droplet arrays or dispersed microstructures are observed in Al-In immiscible systems. However, there has been limited agreement regarding alloy composition and directional solidification processing conditions under which these different microstructure morphologies are obtained. These differences may be due to the slightly different compositions utilized in different studies, the different solidification facilities, differences in thermal gradient measurement procedures, or assumptions that solidification front velocities were the same as furnace translation velocities.; Through special interrupted directional solidification procedures, solidification front velocity as a function of furnace translation rate was determined in this study. The liquid temperature gradient as a function of furnace hot zone temperature was determined by lowering a thermocouple into partially solidified samples and subsequently raising it in 1 mm intervals. Following processing parameter determination, a series of alloys of Al-17.1 wt.%In, Al-17.3 wt.%In, and Al-17.5wt.%In composition were directionally solidified. Microstructural transitions were determined as a function of G and R.; The results showed that unlike theoretical predictions, where aligned fibrous microstructures for all samples processed was anticipated, microstructural transitions from aligned fibrous, to doubly oriented droplet arrays, to dispersed microstructures were observed as G/R decreased for all alloy compositions processed. However, a higher G/R value was necessary to maintain coupled-growth and produce aligned fibrous microstructures for alloys of Al-17.5wt.%In composition. The presence of microstructural transitions observed for all alloy compositions indicated that a solute boundary layer was present in the liquid adjacent to the solidification front of all alloy compositions. An investigation of the solute boundary layer composition indicated that a shift occurs in the monotectic invariant composition during processing of near-monotectic composition alloys. |
