Open Access

Zn coatings are typically applied to steels in the liquid state. During spot welding, this coating is also presumed to remelt, and the subsequent embrittlement of the welded steel is therefore commonly associated with the presence of liquid Zn. However, whether the liquid phase itself, and hence temperatures above the Zn melting point, are necessary for the embrittlement phenomenon has so far remained unclear. In a series of thermodynamic investigations [1-4], we show that a Zn-segregation transition in BCC-Fe grain boundaries occurs over a broad temperature range, associated with a magnetic miscibility gap in the Fe-Zn system, and can play a central role in the embrittlement. Although this range includes the melting temperature of Zn, the segregation transition can occur fully in the solid state. The resulting nanoscale, high-concentration Zn patches at grain boundaries are shown to constitute viable nucleation sites for a Zn-rich liquid phase and/or a Γ precipitate. More importantly, the work of separation is found to decrease drastically already before any phase change takes place. The present results indicate that Zn-induced embrittlement in steels cannot be viewed solely as a liquid-metal embrittlement phenomenon. Instead, it should be understood more generally as a segregation-mediated grain-boundary instability, in which interfacial thermodynamics, local chemical transitions, and decohesion are the primary events, while liquid or intermetallic phase formation may follow as secondary consequences. This perspective opens a route toward a more unified thermodynamic description of Zn-induced liquid and solid metal embrittlement in steels.

Radiation-induced segregation (RIS) and phase change phenomena have traditionally been framed as a transport-centric problem, and for good reason: it is fundamentally non-equilibrium diffusion governed by irradiation-generated point defects and their coupling to solute fluxes, which drive the redistribution of alloying elements toward or away from microstructural defects. Yet, this emphasis on transport aspects has created an asymmetry: The thermodynamic counterpart of the problem, namely the local free-energy landscape of the receiving defect, remains under-formalized: In most RIS frameworks, the governing description is closed through coupled transport equations for solutes and irradiation-generated point defects, augmented by source, recombination, and sink terms. Accordingly, the microstructural defects typically enters through sink strength, capture-efficiency, geometry, or boundary conditions, whereas their thermodynamic identity is represented in simplified form. Even where segregation energetics are included, they are often projected onto bulk thermodynamic factors or effective interfacial properties, rather than being based on a sink-specific free-energy landscape. In this Opinion Paper, I argue that a more complete understanding of RIS can emerge when sink-specific thermodynamic landscapes and their interaction with non-equilibrium fluxes are taken explicitly into account.

Liquid-metal embrittlement (LME) in galvanized advanced high-strength steels (AHSS) represents a critical challenge during resistance spot welding, yet the early stages of crack initiation remain poorly understood. In this work, the time-dependent evolution of grain-boundary microstructures during interrupted welding is investigated using high-resolution electron microscopy and thermodynamic modeling. The results reveal rapid Zn diffusion along grain boundaries, followed by the formation of nanoscale Γ-phase (Fe3Zn10) intermetallics within milliseconds of welding. Crack initiation is found to occur preferentially at evolving α/Γ interfaces, while Zn segregation and grain-boundary density reduction promote interfacial instability and phase separation. Density-based grain-boundary phase diagram calculations further indicate the presence of a Fe–Zn miscibility gap that drives segregation transitions and intermetallic formation below the classical ductility trough temperature. These findings provide new insight into the nanoscale mechanisms governing the onset of LME in steels and establish a framework for alloy- and grain-boundary-engineering strategies to mitigate embrittlement.

Chemically complex materials (CCMats) 􀀀 including high-entropy alloys, oxides, and related multi-principal element systems 􀀀 offer a paradigm shift in materials design by leveraging chemical diversity to simultaneously optimize functional, structural, and sustainability criteria. The vastness of the compositional and structural space in CCMats propels the field into an expanding exploratory state. To reconcile functional and structural performance across this immense parameter space remains an open challenge. This Perspective evaluates the opportunities and challenges associated with harnessing chemical complexity across a broad spectrum of applications, such as hydrogen storage, ionic conductors, catalysis, magnetics, dielectrics, semiconductors, optical materials, and multifunctional structural systems. It is delineated how three central design strategies: targeted substitution (SUB), defect engineering (DEF), and diversity management (DIV) enable the reconciliation of high functional performance with long-term structural stability and environmental responsibility. Advances in computational thermodynamics, microstructure simulations, machine learning, and multimodal characterization are accelerating the exploration and optimization of CCMats, while robust data infrastructures and automated synthesis workflows are emerging as essential tools for navigating their complex compositional space. By fostering cross-disciplinary knowledge transfer and embracing data-driven design, CCMats are poised to deliver next-generation materials solutions that address urgent technological, energy, and environmental demands.

The density-based phase-field model for grain boundary (GB) thermodynamics and kinetics has offered a broad range of applications in alloy and microstructure design. Originally, this model is based on a potential energy terms that is connected to the cohesive energy of a given substance. A more rigorous approach, however, is a full consideration of an interatomic potential over the possible range of distance and therefore density. In Manuscript I of this series, we developed and thoroughly analyzed the coarse-graining of atomistic GB structures. In this work (Manuscript II), we complete the coupling between atomic and mesoscale modeling of GBs by incorporating the full interatomic potentials into the density-based free energy functional. Using GB energies calculated from atomistic simulations, the coarse-graining approach and the atomistic-integrated density-based Gibbs free energy, we effectively evaluate the density gradient energy coefficient. We found that coupling the density-based model with atomistic potentials reveal physically-sound trends in the GB equilibrium properties. A universal equation was derived to describe the potential energy contribution to the GB energy and the gradient energy coefficient for BCC-Fe and -Mo GBs, similar to the universal equation for GB excess free volume presented in Manuscript I. The proposed approach provides a mesoscale density-based model rooted in atomic-scale characteristics for reliable predictions of GB properties.