Boundary Complexions Research Articles

Grain boundary complexions and pseudopartial wetting

The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.

Expanding time–temperature-transformation (TTT) diagrams to interfaces: A new approach for grain boundary engineering

Bulk phase transformation kinetics were not well understood before Davenport and Bain developed time–temperature-transformation (TTT) diagrams for steel alloys in the 1930s. These powerful diagrams revolutionized the heat treatment of steel and other alloys. Grain boundaries and internal interfaces are now known to behave in a phase-like manner, referred to as ‘complexions’, and their transitions can be represented on TTT diagrams. We present experimental grain boundary complexion TTT diagrams for polycrystalline Al2O3 and Y2O3. Grain boundary mobility discontinuities in Y2O3 occur at different temperatures for different annealing times, an unusual pattern that becomes understandable when viewed on a complexion TTT diagram. Similarly, the anisotropy of Al2O3 complexion kinetics can be visualized with these diagrams. Complexion TTT diagrams are a graphical tool to control interface-related phenomena such as diffusion, creep, oxidation, and microstructure evolution. They could explain why two-step sintering produces dense nanocrystalline ceramics and offer insight into other processes as well.

High-Temperature Stability and Grain Boundary Complexion Formation in a Nanocrystalline Cu-Zr Alloy

Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were created with mechanical alloying and their thermal stability was studied from 550–950°C. Annealing drove Zr segregation to the grain boundaries, which led to the formation of amorphous intergranular complexions at higher temperatures. Grain growth was retarded significantly, with 1 week of annealing at 950°C, or 98% of the solidus temperature, only leading to coarsening of the average grain size to 54 nm. The enhanced thermal stability can be connected to both a reduction in grain boundary energy with doping as well as the precipitation of ZrC particles. High mechanical strength is retained even after these aggressive heat treatments, showing that complexion engineering may be a viable path toward the fabrication of bulk nanostructured materials with excellent properties.

Abnormal grain growth in the Potts model incorporating grain boundary complexion transitions that increase the mobility of individual boundaries

The Potts model of grain growth was adapted for the purpose of simulating abnormal grain growth (AGG) resulting from grain boundary complexion transitions. The transition in grain boundary structure between specific complexion types results in changes in properties. Where the transitions decrease energy and increase the mobility of boundaries, AGG occurs provided that such transitions predominantly occur via propagation to adjacent boundaries. Thus the model increases the mobility of selected boundaries on the basis of their adjacency to zero, one, or a multiplicity of boundaries that have already transitioned. The effect of transitions to a high mobility complexion was investigated separately from the effect of changes in energy. The influence of the frequency of complexion transitions with different adjacencies on the occurrence of AGG was explored. The simulations show how propagating complexion transitions can explain the AGG observed in certain ceramic systems.

Developing grain boundary diagrams for multicomponent alloys

Impurity-based, premelting-like, intergranular films (IGFs, a common type of grain boundary complexion) can form in various materials and influence sintering, creep, and microstructure development. A thermodynamic framework is presented to forecast the formation and stability of these premelting-like grain boundary complexions (a.k.a. interfacial “phases” that are thermodynamically two dimensional) in multicomponent alloys to consider the interactions of multiple alloying elements. Key thermodynamic parameters that control the interfacial segregation and disordering behaviors have been identified and systematically examined. Subsequently, ternary and quaternary grain boundary diagrams have been computed and used to forecast the sintering behaviors of W–Ni–M (M=Fe, Co, Cr, Zr, Nb and Ti) and Mo–Si–B–M (M=Ni, Co and Fe) systems. This work supports a long-range scientific goal of extending bulk computational thermodynamics and CALPHAD methods to interfaces and developing grain boundary complexion (interfacial “phase”) diagrams as extensions to bulk phase diagrams, which can be a generally useful materials science tool.

A grain boundary mobility discontinuity in reactive element Zr-doped Al2O3

A grain growth study on polycrystalline 180 wt. ppm Zr-doped Al2O3 has revealed a discontinuous increase in grain boundary mobility. The mobility increases by over an order of magnitude within a narrow temperature window between 1550 and 1650 °C. The discontinuity is attributed to a grain boundary complexion transition that occurs at about 1600 °C. This grain boundary complexion transition has important implications for creep and oxidation protection, and technological issues such as scale growth in high-temperature alloys.

Influence of grain boundary energy on the nucleation of complexion transitions

To test the effect of grain boundary energy on the nucleation of a transition from a low mobility to a high mobility grain boundary complexion, polycrystalline yttria-doped alumina was sandwiched between two single crystals of sapphire. Thermal groove measurements showed that the distribution of grain boundary energies at the single crystal–polycrystal interface is biased by the single crystal orientation. When annealed above the critical temperature for the complexion transition, the high mobility complexion preferentially nucleated at the higher energy interface.

Grain Growth Anomaly and Dielectric Response in Ti-rich Strontium Titanate Ceramics

A grain growth anomaly in Ti-rich strontium titanate ceramics is reported. Here we show that three discontinuities on the temperature dependence of grain growth take place with drops in the grain size at temperatures around 1500, 1550, and 1605 °C. We also show that similar discontinuities can be observed in the dependence of the grain boundary activation energy for conductivity and in the grain boundary thickness, assessed by impedance spectroscopy (IS). These notable coincidences are reported for the first time and strongly support the formation of different grain boundary complexions in polycrystalline oxides with transitions in between the observed grain growth regimens, which may be correlated to different grain boundary mobility and dielectric properties. These results call into question the discussion on the role of nonstoichiometry of SrTiO3 and complexions on the microstructure development and open opportunities to design properties of functional materials.

Grain boundary complexions

Grain boundaries exhibit phase-like behavior in which their structure, chemistry and properties may change discontinuously at critical values of thermodynamic parameters such as temperature, pressure and chemical potential. Therefore, grain boundaries (and other interfaces such as surfaces and heterophase boundaries) can be treated as thermodynamically stable interfacial states. To differentiate these interfacial states from bulk phases, the term “complexion” has been introduced. A variety of terminology has been used to describe complexions and complexion transitions. In many cases, several terms exist that describe essentially the same phenomenon. We give an overview of complexion-related terminology, suggest a preferred nomenclature and discuss a classification framework that can be used to categorize complexions and complexion transitions. The field of grain boundary complexions has evolved rapidly in the past decade due to advances in experimental equipment – in particular, aberration-corrected transmission electron microscopy – and progress in computational simulation methods. Grain boundary complexion transitions are the root cause of a wide variety of materials phenomena – such as abnormal grain growth, grain boundary embrittlement and activated sintering – that have defied mechanistic explanation for years. In this overview, we review the history and theory of grain boundary complexion transitions, their role in materials processing and their effect on materials properties.

Kinetics and thermodynamics associated with Bi adsorption transitions at Cu and Ni grain boundaries

The grain boundary diffusivity of Au in Cu and Cu-Bi, and Cu in Ni and Ni-Bi are characterized by secondary ion mass spectroscopy depth profiling. Samples are equilibrated in a Bi containing atmosphere at temperatures above and below the onset of grain boundary adsorption transitions, sometimes called complexion transitions. A simple thermo-kinetic model is used to estimate the relative entropic contributions to the grain boundary energies. The results indicate that the entropy term plays a major role in promoting thermally and chemically induced grain boundary complexion transition.

The Relationship between Grain Boundary Energy, Grain Boundary Complexion Transitions, and Grain Size in Ca-Doped Yttria

The thermal groove technique has been used to measure relative grain boundary energies in two 100 ppm Ca-doped yttria samples. The first has a normal grain size distribution and the boundaries have a bilayer of segregated Ca. In the second sample, there is a combination of large grains and small grains. The boundaries around the large grains are known to have an intergranular film. The results show that the relative energies of boundaries in the sample with normal grain growth and the boundaries around small grains far from larger grains in the second sample are similar. Also, boundaries surrounding the largest grains and small grains immediately adjacent to them have the same and significantly lower energies. The results indicate that grain boundaries with an intergranular film have a lower energy than those with bilayer segregation and that the intergranular film extends beyond the periphery of the largest grains, but not throughout the entire sample.

Grain boundary complexion transitions in WO3- and CuO-doped TiO2 bicrystals

Several grain boundary complexions (grain boundary interfacial phases) have been identified in TiO2 bicrystals by high-resolution transmission electron microscopy (TEM) and aberration-corrected scanning TEM (STEM). An intrinsic grain boundary with no apparent impurity segregation was observed in an undoped TiO2 bicrystal. In a WO3-doped TiO2 bicrystal, WO3 second-phase particles formed along the boundary, with a nominally clean, intrinsic-type grain boundary in between the particles. In a CuO-doped bicrystal, a remarkable series of three distinct grain boundary complexions with abrupt structural transitions was discovered coexisting at the grain boundary, and the existence of a fourth equilibrium complexion at the annealing temperature was implied. Thus, the WO3- and CuO-doped TiO2 bicrystals exhibit dramatically different solute partitioning behavior which can be understood in terms of the relative interphase boundary energies of these two systems. STEM–electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy analysis of the nanoscale lens-shaped films of amorphous material in the CuO-doped TiO2 bicrystal demonstrated an excess of CuO, as expected, yet also revealed the unintentional presence of SiOx. The multiple grain boundary complexions in CuO-doped TiO2 offer an explanation for the CuO-enhanced grain growth and sintering of TiO2 that has been reported in the literature. Conversely, the intrinsic grain boundary complexion observed in WO3-doped TiO2 is consistent with previous work showing that WO3 has no effect on grain boundary mobility in TiO2. A phenomenological thermodynamic model is proposed to explain the physical origin of these observed grain boundary complexions and the abrupt, first-order complexion transitions that are believed to occur upon cooling of the CuO-doped TiO2 bicrystal.

Identification of a bilayer grain boundary complexion in Bi-doped Cu

Using aberration-corrected scanning transmission electron microscopy, we have directly observed a bilayer grain boundary complexion in Bi-doped Cu, akin to that observed in Ni–Bi [Science, 333: 1730 (2011)]. In comparison with the Ni–Bi bilayer, the Cu–Bi bilayer appears to exist in a much narrower chemical potential window attributable to the fact that Cu–Bi and Ni–Bi have different pair-interaction potentials. Furthermore, these bilayers often form in conjunction with nanoscale faceting. This study demonstrates that direct imaging of the atom columns provides a more accurate understanding of the structure, chemistry and distribution of the adsorbates in a grain boundary and their role in embrittlement.

Effects of sintering aids on the densification of Mo–Si–B alloys

The effects of seven sintering aids (0.5 at.% Ni, Co, Fe, Cr, Zr, Nb, and Pd) on the densification of Mo–Si–B alloys of six different compositions (Mo, Mo–0.2Si, Mo–0.2Si–0.02B, Mo–2.5Si–2.5B, Mo–7Si–5B, and Mo–8.9Si–7.7B at.%) are systematically investigated. It was found that Ni, Co, and Fe are effective in enhancing densification of Mo–Si–B alloys, and Ni is the most effective sintering aid. This study supports a previously proposed hypothesis that activated sintering results from enhanced mass transport in the sintering-aid-induced quasi-liquid intergranular films (a type of grain boundary complexion). The relative effectiveness of these sintering aids can be rationalized by analyzing several key thermodynamic parameters that control the stability of premelting-like grain boundary complexions. Future studies are needed to develop interfacial thermodynamic models and methods for computing “grain boundary complexion (phase) diagrams” for multicomponent systems, which can be a useful component for the “Materials Genome” project that will enable better predictions of the activated sintering and other materials phenomena.

Changes in the Grain Boundary Character and Energy Distributions Resulting from a Complexion Transition in Ca-Doped Yttria

The grain boundary character distribution and the relative grain boundary energy of 100 ppm Ca-doped yttria were measured before and after a previously identified grain boundary complexion transition. The grain boundary character distribution of samples exhibiting normal grain growth (before the complexion transition) favored {111} planes, whereas those exhibiting abnormal grain growth (after the complexion transition) favored {001} planes. Additionally, the relative grain boundary-to-surface energy ratios in the sample exhibiting abnormal grain growth were 33 pct lower than in the sample exhibiting normal grain growth. The results also indicate that the complexion transition increased the anisotropy of the grain boundary energy, and this may be responsible for the increase in the anisotropy of the grain boundary character distribution.

Grain Boundary Complexions in TiO2 Bicrystals Doped with CuO

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Influence of interface energies on solute partitioning mechanisms in doped aluminas

The experiments described in this paper have been designed to understand how particular dopants in alumina (Ca, Mg, Si, and Y) affect microstructural development through the energetics of their associated precipitates. Specifically, the role of the interphase boundary energy and precipitation activation energy are considered to be in competition with grain boundary complexion (disorder) transitions for partitioning excess solute. The results reveal a relationship between the relative precipitation activation energy and the temperature at which grain boundary complexion transitions occur. The large differences in activation energy primarily derive from the interphase boundary energy. Precipitates that form lower interphase boundary energies tend to suppress complexion transitions, while systems that contain precipitates with high interphase boundary energies are more susceptible. Based on the findings, a new criterion for additive selection to control complexion transitions and abnormal grain growth is proposed that is based on interfacial energies between the host and precipitate.

Grain boundary plane distributions in aluminas evolving by normal and abnormal grain growth and displaying different complexions

Abstract The grain boundary character distributions of selected doped aluminas were measured from normal and abnormal populations. The relative energies of the A-, C-, and R-planes of undoped alumina were also measured. There is an inverse relationship between the population of grain boundaries and the relative energies of grain boundary planes in undoped alumina. This relationship is found to be qualitative for abnormal grains, whose interfacial anisotropy may be affected by kinetic factors. It is found that the relative grain boundary anisotropy correlates with the temperature dependence of grain boundary complexion transitions in a particular system that is prone to abnormal grain growth. However, there is no direct correlation between the total anisotropy of the grain boundary character distribution and that system's propensity to undergo a particular complexion transition. Therefore, anisotropy will significantly affect the microstructural evolution in systems that are prone to abnormal grain growth, but the magnitude of anisotropy is not a sufficient selection criterion for determining which systems will undergo abnormal grain growth.

Grain boundary complexions in ceramics and metals: An overview

The concepts of grain boundary phase transitions and complexions offer clues to resolve old problems such as abnormal grain growth and solid-state activated sintering as well as new opportunities for mechanism-informed materials design.

Grain boundary complexions: The interplay of premelting, prewetting, and multilayer adsorption

A thermodynamic model for coupled adsorption and disordering transitions at grain boundaries is developed by combining diffuse-interface and lattice-gas models and incorporating colloidal type interfacial forces. This model produces a systematical spectrum of interfacial phenomena for grain boundaries, including first-order and continuous coupled prewetting and premelting transitions, critical points, multilayer adsorption, layering and roughening, and complete wetting and drying, and it produces a series of grain boundary “phases” (complexions) with character similar to those observed by [Dillon et al., Acta Mater. 55, 6208 (2007)]. The presence of dispersion and electrostatic forces in ceramic materials can appreciably change grain boundary transitions.