Developing grain boundary diagrams as a materials science tool: A case study of nickel-doped molybdenum

Abstract

Impurity-based, premelting-like, grain boundary (GB) "phases" (complexions) can form in alloys and influence sintering, creep, and microstructural development. Calculation of Phase Diagrams (CalPhaD) methods and Miedema-type statistical interfacial thermodynamic models are combined to forecast the formation and stability of subsolidus quasiliquid GB phases in binary alloys. This work supports a long-range scientific goal of developing "GB (phase) diagrams" as a new materials science tool to help controlling the materials fabrication processing and resultant materials properties. Using nickel-doped molybdenum as a model system, a type of GB diagram (called "λ diagram") is computed to represent the temperature-and composition-dependent thermodynamic tendency for general GBs to disorder. Subsequently, controlled sintering experiments are conducted to estimate the GB diffusivity as a function of temperature and overall composition, and the experimental results correlate well with the computed GB diagram. Although they are not yet rigorous GB-phase diagrams with well-defined transition lines, the predictability and usefulness of such λ diagrams are demonstrated. Related interfacial thermodynamic models and computational approaches are discussed.

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.…”

The development of phase-based property data using the CALPHAD method and infrastructure needs

“…Efforts to estimate the composition dependent interfacial energies for a given set of solid phases were reviewed by Costae Silva et al [103], who noted that while thermodynamic approaches can provide reasonable estimates, fundamental firstprinciples approaches [104] should be used as the reference values. More recently, Shi and Luo [105] developed a method to estimate the interfacial energy associated with a grain boundary between two phases. Combining these and other methods with data available from coarsening experiments [106,107] and single-sensor DTA nucleation cooling experiments [101] a CALPHAD based databases could be established, where the interfacial energies are defined by two phases.…”