Cell invasion during competitive growth of polycrystalline solidification patterns

Area of Science:

• Materials Science
• Solidification Science
• Crystallography

Background:

• Polycrystalline structures, including cellular and dendritic arrays, form during alloy solidification processes like casting, welding, and additive manufacturing.
• Both intra-grain array structures and macro-scale grain structures significantly influence the performance of structural alloys.
• The coevolution of these intra-grain and macro-scale structures during solidification is not well understood.

Purpose of the Study:

• To investigate the dynamic evolution of grain structures during alloy solidification.
• To understand the mechanisms governing the interaction and growth of adjacent grains.
• To challenge and refine existing models of grain formation and boundary development.

Main Methods:

• In situ observation of alloy solidification experiments conducted under microgravity conditions on the International Space Station.
• Utilizing phase-field simulations to model and reproduce the observed solidification phenomena.
• Analyzing the invasion process of cells between grains with varying misorientations.

Main Results:

• Discovery of unexpected invasion of individual cells or cell rows from one grain into adjacent grains with different misorientations.
• Observation that this invasion leads to grain interpenetration and highly convoluted grain boundary shapes.
• Phase-field simulations successfully reproduced these invasion phenomena across a wide range of misorientations.

Conclusions:

• The findings fundamentally alter the traditional understanding of grains as discrete, non-interpenetrating regions in three-dimensional space.
• Grain boundaries are not always simple interfaces but can be highly complex and convoluted due to cell invasion.
• This research provides new insights into the microstructural evolution during solidification with implications for materials design and manufacturing.