Nanocrystalline Materials at Equilibrium: A Thermodynamic Review
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DOI: 10.1007/s1183701516369
 Cite this article as:
 Kalidindi, A.R., Chookajorn, T. & Schuh, C.A. JOM (2015) 67: 2834. doi:10.1007/s1183701516369
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Abstract
The instability of nanocrystalline materials against both grain growth and bulk phase separation is a principal challenge in their production and usage. This article reviews the thermodynamic stabilization of nanocrystalline structures by alloying, where a nanocrystalline state is considered to be stable if the nanostructure has the lowest free energy available to the alloy system, such that it is stable both against grain growth and the formation of bulk second phases. The thermodynamic accessibility of nanocrystalline structures in the alloy phase space introduces configurational degrees of freedom both at the atomic scale of the grain boundary structure and at the mesoscale level of the grains and grain boundary topology, which should be considered when identifying the equilibrium state. This article presents a survey of the kinds of thermodynamic models and simulations that have been developed to search for equilibrium nanocrystalline states. The review emphasizes the utility of Monte Carlo simulations to assess the thermodynamic stability of nanocrystalline states, including methods that have been proposed to account for degrees of freedom at both the atomic and grain scales. Although atomic scale simulations provide detailed segregation energetic information, the topological degrees of freedom in nanoscale polycrystals seem to be more critical considerations in the free energy description for identifying whether a nanocrystalline state is stable, and these are better addressed with mesoscale latticebased simulation methods. A variety of interesting new nanostructural alloy states awaits further exploration by computational methods.
Introduction
The high volume fraction of grain boundaries in nanocrystalline materials pits structural instability against improved performance. The nanoscale grain structure provides these materials with an interesting set of properties such as high strength and wear resistance, but introduces a large driving force for grain growth out of the desired nanocrystalline regime. Alloying has been used to oppose grain growth through at least two different mechanisms: by treating the symptom—grain boundary motion—through solute drag or second phase pinning1, 2, 3, 4, 5 or by treating the underlying cause—the excess free energy of the grain boundary, which can be relieved by solute segregation.6, 7, 8, 9, 10, 11 Whereas slowing grain boundary motion can be sufficient for some applications, the kinetic nature of the drag mechanism often yields a merely transient stability to nanostructures; they eventually prefer to coarsen. Furthermore, solute drag is typically only effective at relatively low homologous temperatures due to the strong temperature dependence of mobility. On the other hand, if alloying can eliminate the driving force for grain growth, then nanocrystalline alloys can be developed that reliably retain their structure for a longer timeattemperature or at higher temperatures, providing better control over nanostructure, and possibly a wider range of processing routes and applications.
The implications of this result are limited by the dilute solution assumption and the imposition of solutesaturated grain boundaries. To alleviate these assumptions, Trelewicz and Schuh12 used a regular solution approach to develop the regular nanocrystalline solution (RNS) model. In addition to a crystalline grain interior region, the RNS model includes a grain boundary region, defined by two variables: the grain boundary volume fraction, which in their model mapped monotonically to grain size, and the grain boundary solute concentration, which is a measure of the degree of grain boundary segregation. As in the classical regular solution model, the free energy of the alloy state is determined from the internal energy, calculated by summing the energies of the bonds (assuming a random distribution of solute in the crystalline and grain boundary regions and considering nearestneighbor pairwise interactions), and from the configurational entropy of the full system. In this way, the free energy calculation was not restricted to dilute solutions or saturated grain boundaries, thus allowing a more detailed representation of free energy as a function of a variety of structural and chemical parameters. This is shown for a specific pair of variables (grain size and grain boundary solute content) in Fig. 1b and compared with a free energy plot from Weissmüller in Fig. 1a, which is presented only against a single variable (grain size). The stable state is determined by finding the minimum in free energy, which for an alloy with a nanocrystalline state stable against grain growth occurs at a finite grain size and an equilibrium solute concentration at the grain boundary (Fig. 1b).13 The Trelewicz–Schuh RNS model has been refined and extended by other authors to different more specific situations, with similar general outputs in each case.14,15
The stability of a nanocrystalline state as described in these early analytical models, and models based on these same concepts that are not detailed here,16, 17, 18, 19, 20 is largely determined by the enthalpy of segregation of the alloy and the energy penalty of the unalloyed grain boundary. Experimental studies of such thermodynamic stability have been conducted by alloying with a solute species that has a large difference in atomic radius compared with the solvent element to promote segregation driven by elastic mismatch.11,21, 22, 23, 24, 25 Typically, it is observed that these alloys can retain nanocrystalline states at low homologous temperatures but undergo phase separation at higher temperatures, which triggers loss in solute enrichment at grain boundary sites and results in rapid grain growth. Thus, the local minimum in free energy used to identify stable nanocrystalline alloys based on these models alone is only sufficient to claim a metastability of the segregated grain boundary state: These alloys may be stable against grain growth but not necessarily against secondphase precipitation.
Recently, some models have been used to identify hypothetical alloys in which a grainboundary segregated solid solution is quantitatively of lower free energy than the known competing bulk states, including second phases as well as coarser grains.9,13,26, 27, 28, 29, 30, 31, 32, 33 Designing for this type of stabilization requires considerations based primarily on the chemistry both in the bulk material and specifically at grain boundaries. As a result, models of thermodynamic stabilization are decidedly different from grain growth models as they are concerned with the energies of different nanocrystalline configurations as opposed to the dynamics of grain growth. In this article, we review this recent work on the thermodynamic stabilization of nanocrystalline alloys against both grain growth and phase separation. In these alloys, nanocrystallinity is expected to be preserved to higher temperatures and possess a more reliable and predictable stability due to its thermodynamic nature. Additionally, the proposed existence of nanocrystalline ground states marks a shift in how grain boundaries are considered thermodynamically.
Nanocrystallinity at Equilibrium
The thermodynamic accessibility of nanocrystalline states also suggests a need for a reevaluation of typical assumptions built into alloy phase diagrams. Zhou and Luo26 extended the approach of Murdoch et al.9,13 by using a CALPHAD evaluation of free energies to produce a corresponding phase diagram for FeZr alloys, shown in Fig. 2b. They included nanocrystalline states computed using a regular solution model for grain boundary segregation developed by Wynblatt and Chatain.35 In this case, the segregated nanocrystalline states were less energitcally favorable than the Fe_{23}Zr_{6} compound, and thus only metastable grain size information could be provided. In an alloy exhibiting true nanocrystalline stability, phase diagrams are expected to include phase transitions between bulk phases and nanostructured states, as well as twophase regions possessing unique nanostructural features. As the thermodynamic understanding of nanocrystalline alloys matures, it should lead to phase diagrams that explore a more extensive collection of possible nanocrystalline states.
A multiscale description of the grain boundary state is difficult to capture analytically, and as such the analytical models discussed above rely on the definition of a single average grain boundary site, and the fraction of those sites in the system then becomes the core descriptor of a nanocrystalline structure. Atomistic simulations, on the other hand, have demonstrated great utility in describing the atomistic environment of the grain boundaries of nanocrystalline states36 and the grain topology in studies of grain growth. Taking advantage of this, Millett et al.37, 38, 39 performed molecular statics and molecular dynamics (MD) studies of stability against grain growth. Using a LennardJones potential, their simulations showed that placing sufficiently larger solute atoms at the grain boundaries successfully arrested grain growth by reducing the excess grain boundary energy to zero. However, from a thermodynamic perspective, two critical features are missing from such an approach for studying nanocrystalline ground states. First, grain boundary segregation should occur thermodynamically, such that the chemical potential of the alloy is constant throughout the system, which is not obeyed by artificially placing solute at the grain boundary. Second, the simulation must be sufficiently long such that bulk phases can nucleate, which is not easy to capture in an MD simulation because of the longer time scales associated with diffusion. For the time and length scales required to study the thermodynamics of nanostructured states, a statistical mechanicsbased approach offers many advantages, and we turn our attention to these models in the next sections.
Thermodynamic Monte Carlo Simulations
In statistical mechanics, the equilibrium behavior of an alloy is determined by taking thermal averages at the atomic level. For closed systems at a fixed temperature (i.e., in the canonical ensemble), the probability that a particular alloy configuration m is the equilibrium state depends on the energy of the configuration E_{m} as \( P_{\text{m}} = \left( {{\text{e}}^{{  \frac{{E_{\text{m}} }}{kT}}} } \right)/Q \), where k is the Boltzmann constant and T is the absolute temperature. The partition function \( Q = \mathop \sum \limits_{i} e^{{  \frac{{E_{i} }}{kT}}} \) represents the size of the configuration space and can be related to thermodynamic quantities, such as entropy and free energy. Thus, if the energies of all configurations are known, then the canonical ensemble partition function and the probabilities of each configuration can be determined and used to identify the preferred state of the alloy and calculate relevant thermodynamic information.
According to this method, if the new configuration has a lower energy, then the system always transitions into it, but if it has a higher energy, then the transition only occurs with a probability determined by the energy increase and the temperature (with transitions to higherenergy states more probable at elevated temperatures). Typically, an initial configuration is chosen, and transitions are attempted and performed according to the Metropolis algorithm until the system reaches equilibrium, at which point any dependence on the choice of initial configuration has been eliminated.
When defining transitions, the goal is to explore the configuration space efficiently with respect to its degrees of freedom. For example, for bulk crystalline alloys, the alloy configuration space can be simplified to consider the distribution of solute on a lattice, as is done in the Ising model.41 In closed systems, atom swaps are used to transition through the configuration space, where a random solute atom and solvent atom from within the lattice have their lattice positions exchanged, leading to a new configuration. The nature of this transition has two important features. First, this mechanism is not meant to represent a physical process but rather to sample the configuration space without getting trapped in metastable states corresponding to local minima in the free energy, which is assisted by the longrange nature of these swaps. At the same time, a single atom swap does not produce an independent sample configuration from the phase space; it is highly dependent on the previous configuration. Therefore, to collect uncorrelated samples of the phase space and to satisfy the ergodic hypothesis, many swaps must be performed in between samples before including a new state in calculations of macroscopic thermodynamic quantities. At moderate temperatures, it has been demonstrated in a number of studies that this sampling approach successfully finds the global minimum in free energy and captures the expected phase equilibria as well as enthalpic and entropic behavior of binary alloys.42, 43, 44, 45
Monte Carlo Simulations Considering Nanocrystalline States
This general Monte Carlo formulism can be adapted to include grain boundaries in the description of the configurational space of the alloy. As discussed, the presence of grain boundaries and their interaction with segregants introduces additional degrees of freedom at both the atomistic scale, related to the structure of the grain boundary, and at the mesoscale, related to the topology and crystallography of the grain boundary network. Two types of Monte Carlo simulations have been developed, each designed to focus at a particular scale, and to provide a different level of information regarding the stability of nanocrystalline states.
Monte Carlo Simulations at the Atomic Level of Grain Boundaries
A particular grain boundary can take on different equilibrium atomic configurations, known as complexions,46 with varying grain boundary thicknesses, disorder, and chemistry. As such, Monte Carlo studies of grain boundary segregation at the atomic level should consider the added positional degrees of freedom available for atoms at the grain boundary by including appropriate transition operations to reach the equilibrium grain boundary structures for a particular nanocrystalline alloy. Simulations to study grain boundary segregation in this way were developed to measure the extent of segregation at different grain boundaries and to analyze structural transitions that occur within the grain boundary.47, 48, 49, 50, 51 Such methods have been more recently adapted by Detor and Schuh28 and Purohit et al.29,30 to study grain boundary stabilization.
The transition event used to sample the phase space of the alloy must allow atoms to relax locally. A common approach is to accompany each solute–solvent atom swap with atomic relaxation of the system to maintain zero hydrostatic stress, for example by straining the system incrementally and using conjugate gradient relaxations to allow the atoms to utilize the offlattice degrees of freedom at the grain boundary. The new, depressurized state is then considered for transition according to the Metropolis algorithm, where the energies of each state are typically calculated using manybody potentials. Such a simulation produces the lowest free energy state for the alloy, but it is constrained to the initial grain topology provided because the transition event does not create or remove grain boundary area, and the relaxations permitted do not allow for large atomic reconfiguration. Thus, to assess nanocrystalline stability, simulations at fixed grain sizes must be compared with singlecrystal simulations to determine the more favorable state, as these states are not considered simultaneously in the Monte Carlo framework.
The shortcoming of atomistic Monte Carlo simulations for considering the stability of nanocrystalline states is that the portion of the phase space considered in a given simulation does not simultaneously include both nanocrystalline and singlecrystalline states. Moreover, to identify the stable nanocrystalline state, all possible nanocrystalline configurations at the topological level should be considered in the free energy minimization to identify the nanostructure of the equilibrated state. As a result, whereas the atomic position degrees of freedom at the grain boundary are important at the grain boundary structure level, the topological degrees of freedom of the grain boundary network must be explored as part of the Monte Carlo sample space to determine whether a nanocrystalline state exists at equilibrium and to assess the mesoscale structure of the state.
Monte Carlo Simulations at the Topological Level of Grain Boundaries
To sample configurations with different grain boundary topologies, in addition to atom swaps, two types of grain swaps were proposed by Chookajorn and Schuh: A random grain boundary site can change its grain number to that of an adjacent grain (grain boundary motion) or to an entirely new grain number (nucleation of a new grain). This evolution of the grain topology is based on the classical Potts model52 and is likewise vulnerable to grain faceting and pinning,53 which increases the likelihood of being trapped in a local energy minimum and falsely identifying a stable nanocrystalline state. To address this, the temperature can be decreased slowly at the beginning of each simulation to facilitate grain boundary motion during the early stage.
In the cases where stable nanostructures are not thermodynamically the most favorable state, this model replicates bulk alloy thermodynamics that are consistent with the Ising model. For alloys that favor grain boundary segregation, however, the Monte Carlo method provides results that are consistent with the analytical models introduced earlier: Nanocrystalline states emerge as preferred structures. The model also uncovers unique behaviors that arise from the synchronicity between the equilibration of both grain and chemical structures, which are not predicted by regular solution models, and to this point, remain unique predictions of the Monte Carlo model.
In Fig. 6, the bulk structure (red region, Fig. 6b), and segregated nanocrystalline structure (green region, Fig. 6e) regions are the classic states predicted by the RNS model and are similar to the RNSbased map regions plotted on different axes in Fig. 2a. However, the simulations are not limited to studying fully segregated grain boundary states and bulk phases. Duplex nanostructures (blue region, Fig. 6d) exhibit simultaneous solute segregation and precipitation, and phaseseparated polycrystals (yellow region, Fig. 6c) do not exhibit segregation but are characterized by precipitates as well as grain boundaries. The latter two nanostructures, which have solute precipitates as well as grain boundaries at equilibrium, are not the minimum internal energy configurations possible; in both cases, the lowest internal energy state would be a single crystal, precipitated state. However, in the duplex and phaseseparated polycrystal regions of the stability map, grain boundary states exist with internal energies that are in between the precipitated and disordered singlecrystal states, and as such at intermediate temperatures the higher entropy available in duplex nanostructures and phaseseparated polycrystals leads to a minimum free energy polycrystalline state. The ability of the latticebased simulation to predict the stability of a wide range of structures makes it a valuable tool for alloy design, and it has been shown to compare well with some limited experimental studies of WTi (segregated nanocrystal)32 and WCr (duplex nanostructure).33
Stability maps of the kind described by Chookajorn et al.9,13,31 have only been published for positive enthalpy of mixing alloys. When using the RNS model, this is convenient as the miscibility gap can be also calculated as a function of the enthalpy of mixing, which is not necessarily the case in negative enthalpy of mixing alloys. In principle, Monte Carlo simulations can capture ordering effects more relevant to negative enthalpy of mixing systems, albeit only at compositions near the stoichiometry of the ordered phase when pairwise bond descriptions are used. Monte Carlo methods can thus provide a natural pathway to the extension of nanostructure design maps to a broader range of alloys. Figure 6 extends previously published maps to include the other quadrants outside of the positive enthalpy of mixing range that has been explored in prior work. The lines separating the regimes of different structures in quadrants II, III, and IV of this plot are derived in the same manner as proposed by Chookajorn and Schuh,31 by merely examining the enthalpic preference on the basis of the lowest bond energies. Figure 6f and g shows a series of sample structures from negative enthalpy of mixing regions of Fig. 6a, calculated using the Chookajorn–Schuh Monte Carlo algorithm. The negative enthalpy of mixing notwithstanding, the simulated structures resemble those produced in the same region using analogous positive enthalpy of mixing alloy parameters.
The analysis used to construct the stability map assumes that the formation enthalpy of the ordered compound depends only on the pairwise bond energies provided in the model description, which is too restrictive to identify stable nanostructured phases that involve ordered compounds. Because many alloys are based on negative enthalpy of mixing pairs, clearly more nuanced treatments should be developed to add detail to these regions of the stability maps. With the addition of methods that better capture ordered compounds in the Monte Carlo simulations, these regions might indeed prove to contain additional nanostructured phases in which both ordering and grain boundary segregation are at play.
Challenges and Opportunities
As demonstrated by lattice Monte Carlo results such as those described previously, considering the topological degrees of freedom of the grain boundary network in the thermodynamic assessment of alloys allows the simulation to determine whether grain boundaries should exist in equilibrium or not, as part of its free energy minimization. The implementation used in the published studies so far has proven suitable to identifying stable nanostructures of some particular alloy systems at finite temperatures, but it does rely on local grain swap events; local events such as these are more likely to be susceptible to trapping in metastable states, which would become more acute as the temperature of interest is lowered. Local trapping would also prohibit the ergodic sampling of the ensemble at a fixed temperature, to assess quantitatively macroscopic thermodynamic quantities such as free energy and entropy. A much larger correlation time is associated with the sampling of the topology than with the atom swaps in sampling the chemical distribution, if the topology is only probed via processes of local grain boundary motion. Improving this method for sampling nanostructures such that the grain topology is not strongly correlated between samples would present an important step for conducting Monte Carlo calculations of the free energy of nanocrystalline states and for assessing their stability at lower temperatures.
The effective use of any coarsegrained model of nanocrystalline stability, including, e.g., analytical RNStype models or mesoscale Monte Carlo simulations, for the purpose of screening of candidate nanostructured alloy systems also requires accurate estimation of “average” or “effective” energetic parameters describing grain boundary segregation and ordering interactions. The enthalpy of grain boundary segregation is not well known for most alloy systems, and therefore it is the limiting input parameter for studying a wide range of materials, with Miedematype models and elaborate experimental techniques that measure the concentration at grain boundaries as the predominant methods. Instead, atomiclevel Monte Carlo simulations at a variety of “typical” grain boundaries could be used to estimate these enthalpies.
Summary
Preferential segregation of solute to grain boundaries can reduce their excess free energy, and in certain alloy systems it can, in principle, entirely relieve the energetic penalty associated with the boundaries. Such an alloy possesses a minimum in its free energy of mixing at a finite grain size, and if this minimum energy is lower than that of the competing phases, then the nanostructured state is the equilibrium state of the alloy and is stable against both grain growth and second phase formation.
The possibility of thermodynamically stable nanocrystalline states expands the portion of the phase space that should be considered in the calculation of macroscopic thermodynamic properties, both at the atomic level, where atoms are freer to rearrange locally in grain boundaries, and at the topological level, where different grain size distributions can exist. Whereas analytical models such as the regular nanocrystalline solution model and its variants provide a firstorder approach to capturing grain boundary states, they rely on assumptions at each of these two scales. On the other hand, Monte Carlo simulations can be targeted at each of these different scales to address some of the open interesting questions in nanostructure stabilization. Simulations at the atomistic level can capture precise grain boundary structures, for example revealing the functional form by which the enthalpy of segregation decreases with increasing solute, or the influence of grain boundary crystallography on segregation behavior. However, such local atomistic models are not currently able to discern whether a single crystal or a nanocrystalline structure is the true equilibrium state due to the difficulty of calculating the free energy of each equilibrated structure. On the other hand, at the mesoscale it is possible to incorporate topological degrees of freedom in a lattice Monte Carlo simulation, so that both the nanocrystalline state and the ground state are simultaneously considered in minimizing the free energy. While such methods so far are not amenable to precise calculation of the free energy, the lowest free energy structure is determined intrinsically within the simulation.
One important direction for future work that calls for more effort on computer simulation is the interesting set of structures in which multiple bond configurations are energetically similar, and as a result, twophase regions and phase transitions containing equilibrium nanostructures are expected. For example, “duplex nanostructures” are those in which the ground state is proposed to be precipitation, but the first excited state of the system as temperature is raised is the decoration of grain boundaries by solute. When the alloy space is broadened to accommodate further alternative configurations, e.g., ordered compound formation in negative enthalpy of mixing systems, a variety of nontrivial nanostructured states might be possible.
Acknowledgement
This work was supported by the U.S. Army Research Office under grant W911NF1410539 and through the Institute for Soldier Nanotechnologies at MIT. A.K. acknowledges a National Science Foundation Graduate Research Fellowship.
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