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

Abstract

a b s t r a c t a r t i c l e i n f oA grain boundary (GB) "phase" (complexion) diagram is computed via a lattice type statistical thermodynamic model for the average general GBs in Bi-doped Ni. The predictions are calibrated with previously-reported density functional theory calculations and further validated by experiments, including both new and old aberrationcorrected scanning transmission electron microscopy characterization results as well as prior Auger electron spectroscopy measurements. This work supports a major scientific goal of developing GB complexion diagrams as an extension to bulk phase diagrams and a useful materials science tool.© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Grain boundaries (GBs) can undergo phase-like transformations, for example, premelting [1] or adsorption [2] transitions, which can influence a broad range of materials properties [3][4][5]. Thus, GBs can be treated as "interfacial phases" that are thermodynamically two-dimensional (2-D) despite that they have thermodynamically-determined interfacial widths as well as through-thickness compositional and structural gradients. A new term "complexion" was introduced to differentiate such 2-D interfacial phases from the conventional bulk phases defined by Gibbs [1,3,4,6,7].Phase diagrams are one of the most useful tools for materials engineering. 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. complexion) diagrams as an extension to bulk phase diagrams and a generally-useful materials science tool. To support this goal, recent studies [9,10,13-16] 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. 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]. 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]...

Probing the densification mechanisms during flash sintering of ZnO

“…Following the earlier successes of constructing GB l diagrams to predict useful trends in high-temperature GB disordering and to subsequently forecast activated sintering behaviors (and potentially other materials properties such as creep resistance) in several W-and Mo based metallic alloys [25e31] (as well as more rigorous computed GB complexion diagrams with well-defined transition lines, such as that for Bi-doped Ni in a most recent report [50]), this study made a first successful attempt to extend the model and method to compute the first GB l diagram for a ceramic system (CuO-doped TiO 2 ). In future studies, similar or more rigorous "GB diagrams" should be developed for other ceramic systems, which can have broad applications.…”

Review of grain boundary complexion engineering: Know your boundaries

“…Complexion diagrams that include both complexion type and their mechanical response have been developed in metal systems, including Ni‐Bi and Cu‐Zr . Recently, fracture toughness measurements were correlated with complexion types in Al 2 O 3 and MgAl 2 O 4 .…”

Grain Boundary Complexions and Phase Transformations in Al- and Cu-Based Alloys

“…When the first evidence of the presence of GB phase transformations appeared, these experimental findings are discussed in terms of transitions "pure (or dry) GB → few nanometer thin GBF → thick GB layer of bulk phase". Later, it has been shown that the variety of possible GB states (called complexions) is much richer (see for example References [17,[19][20][21][22][23][24][25][26][27]87,169,170]). One can imagine the transitions from clean GB to single-layer GBF, then to bi-layer, tri-layer etc.…”

“…One can imagine the transitions from clean GB to single-layer GBF, then to bi-layer, tri-layer etc. [17,[19][20][21][22][23][24][25][26][27] Some of these transition lines can finish not only at the lines of bulk transitions but also in critical points in the middle of one-phase area of a bulk phase diagram [17,19,20,23,27,87].…”

“…Different reviews discuss the experimental works done after the predictions of References [1,2], and various aspects of GB phase transitions have been studied [3][4][5][6][7][8][9][10][11][12][13]. As one of the important results of these works, the new additional lines of GB phase transformations appeared in conventional 3D-phase diagrams [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. It was quite simple to study the GB phase transformations in the two-phase (or multiphase) regions of the bulk phase diagrams.…”