Annual Review of Materials Research - Volume 49, 2019

During the past two decades, density-functional (DF) theory has evolved from niche applications for simple solid-state materials to become a workhorse method for studying a wide range of phenomena in a variety of system classes throughout physics, chemistry, biology, and materials science. Here, we review the recent advances in DF calculations for materials modeling, giving a classification of modern DF-based methods when viewed from the materials modeling perspective. While progress has been very substantial, many challenges remain on the way to achieving consensus on a set of universally applicable DF-based methods for materials modeling. Hence, we focus on recent successes and remaining challenges in DF calculations for modeling hard solids, molecular and biological matter, low-dimensional materials, and hybrid organic-inorganic materials.

First-principles methods can provide insight into materials that is otherwise impossible to acquire. Density functional theory (DFT) has been the first-principles method of choice for numerous applications, but it falls short of predicting the properties of correlated materials. First-principles DFT + dynamical mean field theory (DMFT) is a powerful tool that can address these shortcomings of DFT when applied to correlated metals. In this brief review, which is aimed at nonexperts, we review the basics and some applications of DFT + DMFT.

Understanding and predicting materials corrosion under electrochemical environments are of increasing importance to both established and developing industries and technologies, including construction, marine materials, geology, and biomedicine, as well as to energy generation, storage, and conversion. Owing to recent progress in the accuracy and capability of density functional theory (DFT) calculations to describe the thermodynamic stability of materials, this powerful computational tool can be used both to describe materials corrosion and to design materials with the desired corrosion resistance by using first-principles electrochemical phase diagrams. We review the progress in simulating electrochemical phase diagrams of bulk solids, surface systems, and point defects in materials using DFT methods as well as the application of these ab initio phase diagrams in realistic environments. We conclude by summarizing the remaining challenges in the thermodynamic modeling of materials corrosion and promising future research directions.

Mesoscale modeling and simulation approaches provide a bridge from atomic-scale methods to the macroscale. The phase field (PF) method has emerged as a powerful and popular tool for mesoscale simulation of microstructure evolution, and its use is growing at an ever-increasing rate. While initial research using the PF method focused on model development, as it has matured it has been used more and more for material discovery. In this review we focus on applying the PF method for material discovery. We start with a brief summary of the method, including numerical approaches for solving the PF equations. We then give seven examples of the application of the PF method for material discovery. We also discuss four barriers to its use for material discovery and provide approaches for how these barriers can be overcome. Finally, we detail four lessons that can be learned from the examples on how best to apply the PF method for material discovery.

There is increasing awareness of the imperative to accelerate materials discovery, design, development, and deployment. Materials design is essentially a goal-oriented activity that views the material as a complex system of interacting subsystems with models and experiments at multiple scales of materials structure hierarchy. The goal of materials design is effectively to invert quantitative relationships between process path, structure, and materials properties or responses to identify feasible materials. We first briefly discuss challenges in framing process-structure-property relationships for materials and the critical role of quantifying uncertainty and tracking its propagation through analysis and design. A case study exploiting inductive design of ultrahigh-performance concrete is briefly presented. We focus on important recent directions and key scientific challenges regarding the highly collaborative intersections of materials design with systems engineering, uncertainty quantification and management, optimization, and materials data science and informatics, which are essential to fueling continued progress in systems-based materials design.

Understanding mesoscale ferroelectric domain structures and their switching behavior under external fields is critical to applications of ferroelectrics. The phase-field method has been established as a powerful tool for probing, predicting, and designing the formation of domain structures under different electromechanical boundary conditions and their switching behavior under electric and/or mechanical stimuli. Here we review the basic framework of the phase-field model of ferroelectrics and its applications to simulating domain formation in bulk crystals, thin films, superlattices, and nanostructured ferroelectrics and to understanding macroscopic and local domain switching under electrical and/or mechanical fields. We discuss the possibility of utilizing the structure-property relationship learned from phase-field simulations to design high-performance relaxor piezoelectrics and electrically tunable thermal conductivity. The review ends with a summary of and an outlook on the potential new applications of the phase-field method of ferroelectrics.

We review recent theoretical progress in the understanding and prediction of novel topological semimetals. Topological semimetals define a class of gapless electronic phases exhibiting topologically stable crossings of energy bands. Different types of topological semimetals can be distinguished on the basis of the degeneracy of the band crossings, their codimension (e.g., point or line nodes), and the crystal space group symmetries on which the protection of stable band crossings relies. The dispersion near the band crossing is a further discriminating characteristic. These properties give rise to a wide range of distinct semimetal phases such as Dirac or Weyl semimetals, point or line node semimetals, and type I or type II semimetals. In this review we give a general description of various families of topological semimetals, with an emphasis on proposed material realizations from first-principles calculations. The conceptual framework for studying topological gapless electronic phases is reviewed, with a particular focus on the symmetry requirements of energy band crossings, and the relation between the different families of topological semimetals is elucidated. In addition to the paradigmatic Dirac and Weyl semimetals, we pay particular attention to more recent examples of topological semimetals, which include nodal line semimetals, multifold fermion semimetals, and triple-point semimetals. Less emphasis is placed on their surface state properties, their responses to external probes, and recent experimental developments.

Many materials crystallize in structure types that feature a square net of atoms. While these compounds can exhibit many different properties, some members of this family are topological materials. Within the square-net-based topological materials, the observed properties are rich, ranging, for example, from nodal-line semimetals to a bulk half-integer quantum Hall effect. Hence, the potential for guided design of topological properties is enormous. Here we provide an overview of the crystallographic and electronic properties of these phases and show how they are linked, with the goal of understanding which square-net materials can be topological, and predict additional examples. We close the review by discussing the experimentally observed electronic properties in this family.

Three-dimensional (3D) topological semimetals represent a new class of topological matters. The study of this family of materials has been at the frontiers of condensed matter physics, and many breakthroughs have been made. Several topological semimetal phases, including Dirac semimetals (DSMs), Weyl semimetals (WSMs), nodal-line semimetals (NLSMs), and triple-point semimetals, have been theoretically predicted and experimentally demonstrated. The low-energy excitation around the Dirac/Weyl nodal points, nodal line, or triply degenerated nodal point can be viewed as emergent relativistic fermions. Experimental studies have shown that relativistic fermions can result in a rich variety of exotic transport properties, e.g., extremely large magnetoresistance, the chiral anomaly, and the intrinsic anomalous Hall effect. In this review, we first briefly introduce band structural characteristics of each topological semimetal phase, then review the current studies on quantum oscillations and exotic transport properties of various topological semimetals, and finally provide a perspective of this area.

Circular economy's (CE) noble aims maximize resource efficiency (RE) by, for example, extending product life cycles and using wastes as resources. Modern society's vast and increasing amounts of waste and consumer goods, their complexity, and functional material combinations are challenging the viability of the CE despite various alternative business models promising otherwise. The metallurgical processing of CE-enabling technologies requires a sophisticated and agile metallurgical infrastructure. The challenges of reaching a CE are highlighted in terms of, e.g., thermodynamics, transfer processes, technology platforms, digitalization of the processes of the CE stakeholders, and design for recycling (DfR) based on a product (mineral)-centric approach, highlighting the limitations of material-centric considerations. Integrating product-centric considerations into the water, energy, transport, heavy industry, and other smart grid systems will maximize the RE of future smart sustainable cities, providing the fundamental detail for realizing and innovating the United Nation's Sustainability Development Goals.

Cold sintering is an unusually low-temperature process that uses a transient transport phase, which is most often liquid, and an applied uniaxial force to assist in densification of a powder compact. By using this approach, many ceramic powders can be transformed to high-density monoliths at temperatures far below the melting point. In this article, we present a summary of cold sintering accomplishments and the current working models that describe the operative mechanisms in the context of other strategies for low-temperature ceramic densification. Current observations in several systems suggest a multiple-stage densification process that bears similarity to models that describe liquid phase sintering. We find that grain growth trends are consistent with classical behavior, but with activation energy values that are lower than observed for thermally driven processes. Densification behavior in these low-temperature systems is rich, and there is much to be investigated regarding mass transport within and across the liquid-solid interfaces that populate these ceramics during densification. Irrespective of mechanisms, these low temperatures create a new opportunity spectrum to design grain boundaries and create new types of nanocomposites among material combinations that previously had incompatible processing windows. Future directions are discussed in terms of both the fundamental science and engineering of cold sintering.

The iron aluminides discussed here are Fe–Al-based alloys, in which the matrix consists of the disordered bcc (Fe,Al) solid solution (A2) or the ordered intermetallic phases FeAl (B2) and Fe3Al (D03). These alloys possess outstanding corrosion resistance and high wear resistance and are lightweight materials relative to steels and nickel-based superalloys. These materials are evoking new interest for industrial applications because they are an economic alternative to other materials, and substantial progress in strengthening these alloys at high temperatures has recently been achieved by applying new alloy concepts. Research on iron aluminides started more than a century ago and has led to many fundamental findings. This article summarizes the current knowledge of this field in continuation of previous reviews.

Reducing the weight of automobiles is a major contributor to increased fuel economy. The baseline materials for vehicle construction, low-carbon steel and cast iron, are being replaced by materials with higher specific strength and stiffness: advanced high-strength steels, aluminum, magnesium, and polymer composites. The key challenge is to reduce the cost of manufacturing structures with these new materials. Maximizing the weight reduction requires optimized designs utilizing multimaterials in various forms. This use of mixed materials presents additional challenges in joining and preventing galvanic corrosion.

Topological magnetic structures such as domain walls, vortices, and skyrmions have recently received considerable attention because of their potential application in advanced functional devices. Tuning the magnetic order of the topological structures can result in emergent functionalities and thus lead to novel application concepts. Strain engineering is one promising approach with which to control magnetic order via magneto-elastic coupling in ferromagnets. By introducing lattice deformation, mechanical strain not only can trigger the magnetic phase transition but also can be used to manipulate topological magnetic orders in ferromagnets. The present review is based on magneto-elastic coupling as the coherent basis of the mechanical control of different topological magnetic orders. Following a description of magneto-elastic coupling, we review recent progress in the mechanical control of the magnetic phase transition and topological structures, including magnetic domain walls, vortices, and skyrmions. The review concludes by briefly addressing the future research directions in the field.

X-ray microscopy has been an indispensable tool to image nanoscale properties for materials research. One of its recent advances is extending microscopic studies to the time domain to visualize the dynamics of nanoscale phenomena. Large-scale X-ray facilities have been the powerhouse of time-resolved X-ray microscopy. Their upgrades, including a significant reduction of the X-ray emittance at storage rings (SRs) and fully coherent ultrashort X-ray pulses at free-electron lasers (FELs), will lead to new developments in instrumentation and will open new scientific opportunities for X-ray imaging of nanoscale dynamics with the simultaneous attainment of unprecedentedly high spatial and temporal resolutions. This review presents recent progress in and the outlook for time-resolved X-ray microscopy in the context of ultrafast nanoscale imaging and its applications to condensed matter physics and materials science.