Molecular dynamics simulation of phase competition in terbium

Area of Science:

• Materials Science
• Solidification Science
• Computational Materials Science

Background:

• Material microstructure is determined by solid phase competition during solidification.
• This competition can begin at the initial stages of solidification.

Purpose of the Study:

• To investigate the phase competition between hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc) phases during pure terbium (Tb) solidification.
• To understand the mechanisms governing the formation of different solid phases under varying supercooling conditions.

Main Methods:

• Molecular dynamics simulations were employed to model the solidification process.
• Simulations focused on the competitive nucleation and growth of hcp, fcc, and bcc phases in pure Tb.

Main Results:

• Liquid supercooled below the hcp melting temperature exhibited both bcc and hcp/fcc nuclei.
• Only bcc nuclei grew, resulting in solidification into the bcc phase, irrespective of thermodynamic stability.
• The hcp phase formed in residual liquid droplets or at bcc grain boundaries, undergoing massive transformation or martensitic transformation.
• High supercooling led to ultra-fine hcp and bcc grains, with the bcc phase eventually disappearing.

Conclusions:

• The solidification pathway of Tb is dominated by kinetic factors, favoring bcc phase formation over thermodynamically stable hcp.
• Solid-state transformations, including massive and martensitic transformations, play a crucial role in the evolution of microstructures.
• The final microstructure is highly sensitive to the degree of supercooling and grain boundary interactions.