In Situ Studies of Pattern Formation and Evolution in Complex, Multi-Phase Alloys

Abstract

This dissertation delves into the growth and coarsening of three-phase eutectics and off-peritectics, employing Al-Ag2Al-Al2Cu and Zn-AgZn3 alloys as representative systems. The investigation seeks a comprehensive understanding of pattern formation, exploring the interplay among local composition, crystalline anisotropy, and growth velocity. Through state-of-the-art experimental methods, the study conducts a thorough microstructural analysis during solidification and coarsening, leveraging recent advancements in X-ray transmission microscopy and machine learning tools. The research provides new insights into the four-dimensional (i.e., three-dimensional space- and time-resolved) morphological and crystallographic evolution of these materials. The outcomes lay the foundation for theoretical development and refinement, offering, e.g., real-time perspectives on the formation of a three-phase eutectic microstructure along the surface of a primary phase; a 4D view of the morphological and crystallographic evolution of three-phase eutectic; and in situ observations of the directional solidification of a metallic off-peritectic alloy. Broadly, these studies bring to light the importance of growth competition in shaping multi-phase patterns. For example, we observed pseudo-2D dendritic ‘finger’ formations on primary Al2Cu rods due to local microsegregation of Al and Ag, in turn influencing pattern formation in the Al-Ag-Cu eutectic. Similar effects were found in Zn-AgZn3 alloys, leading to a distinct partially banded morphology. Crystallographic anisotropy also played a significant role in pattern formation and evolution, with primary Al2Cu rods dominating the surrounding morphology and crystallography of the Al-Ag-Cu eutectic. Likewise, the interplay of multiple phases and their associated phase-interfaces led to a remarkable complexity in the patterns that formed. Take for instance the Al-Al2Cu and Ag2Al-Al2Cu interphase boundaries, which influenced eutectic morphology and suppressed the coarsening rate of Ag2Al. Additionally, our work highlights the impact of alloy composition and growth conditions on pattern stability, demonstrated by a composition-velocity eutectic stability field for the AlAg-Cu ternary system. In directionally solidified Zn-AgZn3, the solidification time emerged as a critical parameter influencing pattern selection, particularly with reference to solid-state coarsening of primary AgZn3 columns, which eliminated trapped liquid channels within which the peritectic could nucleate and grow. By focusing on the model alloy systems of Al-Ag2Al-Al2Cu and Zn-AgZn3, this dissertation aims to advance our understanding of pattern formation and evolution in three-phase eutectics and off-peritectic materials. The insights gained may hold potential implications for the manufacture of natural composite materials with various commercial and industrial applications.