Developing Interfacial Phase Diagrams for Applications in Activated Sintering and Beyond: Current Status and Future Directions

Abstract

This article critically assesses the current status and future directions for the development of interfacial phase diagrams for applications in activated sintering and other fields. The origin of solid‐state activated sintering is attributed to the enhanced mass transport in sintering‐aid‐based, nanoscale, quasi‐liquid, interfacial films that are stabilized below the bulk solidus line. Interfacial thermodynamic models have been developed via extending a phenomenological premelting theory and incorporating the computational thermodynamic (CalPhaD) methods. A primitive type of interfacial phase diagrams, λ‐diagrams, have been computed, and these diagrams have been validated by experiments and proven useful. More rigorous interfacial phase diagrams with well‐defined transition lines and critical points may also be constructed. A long‐range scientific goal is proposed to develop interfacial phase diagrams as a new materials science tool. Future studies should be conducted in several areas to achieve this goal, and special efforts should be made to predict the complex interfacial phase behaviors in multicomponent ceramic materials. Potential broad applications are envisaged.

Calculation and validation of a grain boundary complexion diagram for Bi-doped Ni

“…The computed GB complexion (phase) diagram has been systematically validated with experiments, including direct AC STEM characterization (both previously reported data for specimens quenched from 700°C and 1100°C [29] and a new experiment on a specimen quenched from 1400°C) and an early AES study of GB segregation [32]. In addition to the earlier development GB λ-diagrams that can forecast the trends in GB disordering and related sintering behaviors at high temperatures [9,14,15,[17][18][19], this study constructed a more rigorous (yet simple) GB complexion diagram with well-defined transition lines that can be verified by experiments. Future research should be conducted to compute and validate other GB complexion diagrams to realize a potentially-transformative scientific goal of systematically constructing GB complexion diagrams as a new materials science tool.…”

“…Similar to the free-energy functions of bulk phases, different functions of γ GB can be written for different GB complexions and their intersections define GB transitions, e.g., the occurrence of a premelting transition from one complexion with low S xs to another complexion with high S xs with increasing temperature or an adsorption transition from one complexion with low Γ to another complexion with high Γ with increasing chemical potential difference. In this study, we focus on modeling GB adsorption (vs. prior studies [9,10,[13][14][15] that forecasted interfacial disordering).…”

“…the formation of nanoscale, impurity-based, quasi-liquid complexions) and subsequently constructed a class of "GB λ diagrams" to represent the thermodynamic tendency for general GBs to disorder at high temperatures. Although the predicted trends have been validated with direct high-resolution transmission electron microscopy (HRTEM) [9,[14][15][16][17][18][19] and proven useful for forecasting sintering behaviors [9,10,[13][14][15]20], these GB λ diagrams are not yet rigorous GB complexion diagrams with well-defined transition lines. An early report in 1999 [21] also constructed a GB complexion diagram for Cu-Bi via a rather simple model that considered GBs as "quasi-liquid layers" to explain the GB segregation behaviors measured by Auger electron spectroscopy (AES), but more recent aberration-corrected scanning transmission electron microscopy (AC STEM) observed an ordered bilayer complexion in Cu-Bi instead [22].…”

“…complexion) diagrams as an extension to bulk phase diagrams and a generally-useful materials science tool. To support this goal, recent studies [9,10,[13][14][15][16] have extended bulk CALPHAD (CALculation of PHAse Diagrams) methods to model coupled GB premelting and prewetting (a.k.a. the formation of nanoscale, impurity-based, quasi-liquid complexions) and subsequently constructed a class of "GB λ diagrams" to represent the thermodynamic tendency for general GBs to disorder at high temperatures.…”

“…Materials scientists have long recognized that phase-like behaviors at GBs can often control the fabrication processing, microstructural evolution, and materials properties [3,[6][7][8][9][10][11][12]. Thus, a potentially-transformative research is represented by the development of GB "phase" (a.k.a.…”