Kinetics of Amorphous Defect Phases Measured Through Ultrafast Nanocalorimetry

TL;DR

This study investigates the kinetics and stability of amorphous defect phases at grain boundaries in Al-based alloys using ultrafast nanocalorimetry, revealing critical cooling rates and remarkable microstructural stability at high temperatures.

Contribution

It introduces a novel application of ultrafast differential scanning calorimetry to measure the kinetics of amorphous defect phases in nanocrystalline alloys, linking phase evolution to material stability.

Findings

Critical cooling rate of 2,400 °C/s for disordered-to-ordered transition in Al-Ni

Amorphous defect phases exhibit stability after repeated annealing above 90% of melting temperature

Correlation between phase evolution and grain stability established through TEM and nanocalorimetry

Abstract

Recognition of the role of extended defects on local phase transitions has led to the conceptualization of the defect phase, localized thermodynamically stable interfacial states that have since been applied in a myriad of material systems to realize significant enhancements in material properties. Here, we explore the kinetics of grain boundary confined amorphous defect phases, utilizing the high temperature and scanning rates afforded by ultrafast differential scanning calorimetry to apply targeted annealing/quenching treatments at high rates capable of capturing the kinetic behavior. Four Al-based nanocrystalline alloys, including two binary systems, Al-Ni and Al-Y, and two ternary systems, Al-Mg-Y and Al-Ni-Y, are selected to probe the materials design space (enthalpy of mixing, enthalpy of segregation, chemical complexity) for amorphous defect phase formation and stability, with correlative transmission electron microscopy applied to link phase evolution and grain stability to nanocalorimetry signatures. A series of targeted isothermal annealing heat treatments is utilized to construct a Time-Temperature-Transformation curve for the Al-Ni system, from which a critical cooling rate of 2,400 {\deg}C/s was determined for the grain boundary confined disordered-to-ordered transition. Finally, a thermal profile consisting of 1,000 repeated annealing sequences was created to explore the recovery of the amorphous defect phase following sequential annealing treatments, with results indicating remarkable microstructural stability after annealing at temperatures above 90% of the melting temperature. This work contributes to a deeper understanding of grain boundary localized thermodynamics and kinetics, with potential implications for the design and optimization of advanced materials with enhanced stability and performance.