Grain boundary complexions in multicomponent alloys: Challenges and opportunities

Abstract

Grain boundaries (GBs) can undergo first-order or continuous phase-like transitions, which are called complexion transitions. Such GB transitions can cause abrupt changes in transport and physical properties, thereby critically influencing sintering, grain growth, creep, embrittlement, electrical/thermal/ionic conductivity, and a broad range of other materials properties. Specifically, the presence of multiple dopants and impurities can significantly alter the GB complexion formation and transition. This article reviews and discusses several GB adsorption (segregation) and prewetting/premelting type complexion models in multicomponent alloys, in which the interactions among multiple adsorbates not only provide a route to control GB properties but also produce novel phenomena. Specifically, various ternary GB diagrams, including both GB adsorption complexion diagrams with well-defined transition lines calculated from a lattice model (without considering interfacial disordering) and GB  diagrams that predict useful trends for average general GBs to disorder at high temperatures and related sintering phenomena, are constructed to quantitatively describe the GB behaviors as functions of bulk compositions. Finally, we propose a new opportunity of utilizing "high-entropy GB complexions" to stabilize nanocrystalline alloys.

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

“…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.…”

“…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]. GB complexion diagrams with first-order transition lines and critical points have been constructed using diffuse-interface (phase-field) [1,23,24] and latticetype [16,[25][26][27][28] models, which have not yet been validated with experiments systematically, particularly by direct HRTEM or STEM characterization.In this study, we computed a GB complexion diagram for average general GBs in Bi-doped Ni, for which a recent experimental study found ubiquitous formation of a bilayer complexion at virtually all general GBs at 700°C and 1100°C [29]. Furthermore, we calculated the GB transition lines for two special GBs based on the 0 K density functional theory (DFT) calculations reported in two prior studies [30,31] for Scripta Materialia 130 (2017) [165][166][167][168][169] …”

“…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]. GB complexion diagrams with first-order transition lines and critical points have been constructed using diffuse-interface (phase-field) [1,23,24] and latticetype [16,[25][26][27][28] models, which have not yet been validated with experiments systematically, particularly by direct HRTEM or STEM characterization.…”

Stabilization of nanocrystalline alloys at high temperatures via utilizing high-entropy grain boundary complexions

“…We should note that generally (∂γ GB /∂ T) P , X i N 0 for a specimen of a constant bulk composition (X Bulk i ) due to thermally-induced desorption. In a recent article [26], we proposed that a high-entropy GB effect can be achieved for a saturated specimen (in equilibrium with precipitates), when the bulk composition moves long the solvus line, where the solutes' bulk chemical potentials are pinned by the precipitates so that (∂ γ GB / ∂ T) P , X Bulk i on solvus ≈ (∂ γ GB /∂ T) P , μ i = − S XS ; thus, γ GB can be reduced with increasing temperature for a high-entropy GB with positive and …”