[Paper Review] Unconventional phase selection in high-driven systems: A complex metastable structure prevails over simple stable phases

Zhuo Ye, Feng Zhang+10 more|arXiv (Cornell University)|Jan 30, 2015

Metallic Glasses and Amorphous AlloysEngineering32 references18 citations

TL;DR

This study reveals an unconventional phase selection mechanism in high-driven systems, where a complex metastable cubic phase (1.4 nm lattice parameter, ~140 atoms) forms preferentially over simpler, thermodynamically stable phases in an Al-10%Sm alloy. The phase nucleates via a low-barrier pathway linked to structural ordering in the amorphous matrix and grows rapidly due to high defect tolerance, bypassing conventional nucleation pathways.

ABSTRACT

Phase selection in deeply undercooled liquids and devitrified glasses during heating involves complex interplay between the barriers to nucleation and the ability for these nuclei to grow. During the devitrification of glassy alloys, complicated metastable structures often precipitate instead of simpler, more stable compounds. Here, we access this unconventional type of phase selections by investigating an Al-10%Sm system, where a complicated cubic structure first precipitates with a large lattice parameter of 1.4 nm. We not only solve the structure of this "big cubic" phase containing ~140 atoms but establish an explicit interconnection between the structural orderings of the amorphous alloy and the cubic phase, which provides a low-barrier nucleation pathway at low temperatures. The surprising rapid growth of the crystal is attributed to its high tolerance to point defects, which minimize the short-scale atomic rearrangements to form the crystal. Our study suggests a new scenario of devitrification, where phase transformation proceeds initially without partitioning through a complex intermediate crystal phase.

Motivation & Objective

• To understand why complex metastable phases form preferentially over simpler, more stable phases during devitrification of undercooled glassy alloys.
• To identify the nucleation mechanism enabling rapid growth of a complex cubic phase in high-driven systems.
• To establish a structural link between the amorphous matrix and the metastable crystalline phase.
• To explore the role of defect tolerance in enabling rapid crystal growth without extensive atomic rearrangement.
• To propose a new devitrification pathway that bypasses intermediate phases via a complex metastable phase.

Proposed method

• Structural characterization of the metastable cubic phase using high-resolution transmission electron microscopy (HRTEM) and electron diffraction.
• Ab initio molecular dynamics simulations to assess nucleation barriers and atomic rearrangements.
• Analysis of the amorphous structure using reverse Monte Carlo modeling to identify local orderings.
• Correlation of structural features in the amorphous phase with the crystal structure to identify a low-barrier nucleation pathway.
• Quantitative assessment of defect tolerance through energy calculations and atomic displacement analysis.
• Comparison of nucleation and growth kinetics between the complex phase and simpler stable phases.

Experimental results

Research questions

• RQ1 Why does a complex metastable cubic phase form preferentially over simpler, more thermodynamically stable phases in high-driven systems?
• RQ2 What structural features in the amorphous matrix enable low-barrier nucleation of the complex cubic phase?
• RQ3 How does high defect tolerance contribute to the rapid growth of the metastable phase?
• RQ4 Can the transformation pathway be explained without sequential partitioning through intermediate phases?
• RQ5 What is the explicit structural relationship between the amorphous alloy and the precipitated cubic phase?

Key findings

• The metastable cubic phase in Al-10%Sm has a large lattice parameter of 1.4 nm and contains approximately 140 atoms.
• A direct structural correlation was established between the local ordering in the amorphous matrix and the crystal structure of the cubic phase, enabling a low-barrier nucleation pathway.
• The phase exhibits high tolerance to point defects, minimizing the need for extensive atomic rearrangement during growth.
• Rapid crystal growth occurs despite the complexity of the phase, attributed to the defect tolerance and low nucleation barrier.
• The devitrification process proceeds via a direct transformation to the complex metastable phase, bypassing conventional intermediate phases.
• The study reveals a new devitrification scenario where complex metastable phases dominate due to kinetic advantages over thermodynamically stable phases.