# Unraveling Anomalous Eutectic Formation in Ni-Sn Alloys During Directional Solidification with Transition Variable Speed

## TL;DR

This paper explores how changing solidification speeds affects eutectic structures in Ni-Sn alloys, revealing new morphologies and properties.

## Contribution

The study identifies a specific processing window for anomalous eutectic formation during velocity transitions in directional solidification.

## Key findings

- Velocity jumps trigger anomalous eutectic morphologies in Ni-Sn alloys.
- Lamellar spacing decreases and microhardness increases with higher drawing speeds.
- CA simulations align with experiments, showing destabilization and decoupled nucleation during transitions.

## Abstract

This study investigates eutectic morphology transitions in Ni-Sn alloys using Bridgman directional solidification with a transition variable speed coupled with cellular automaton (CA) simulations. Steady-state solidification (0.1–2000 μm/s) produced only regular lamellar/rod-like eutectics, while velocity jumps triggered anomalous eutectic formation. As the drawing speed increased, the lamellar spacing decreased from ~3 μm to 0.4 μm, while the microhardness increased from ~426 HV to 500 HV. The experiments on Ni-Sn alloys revealed that anomalous eutectic morphologies form specifically at velocity transition interfaces (0.1–1000 μm/s), consistent with CA simulations showing destabilization of the lamellae, epitaxial growth of the Ni3Sn phase, and decoupled nucleation of the α-Ni phase for the formation. The work defines a processing window for anomalous eutectic formation and provides mechanistic insights bridging undercooling and directional solidification regimes.