266 MRS BULLETIN • VOLUME 44 • APRIL 2019 • www.mrs.org/bulletin © 2019 Materials Research Society Introduction Most materials are in a thermodynamically metastable state in some stages during synthesis, processing, and service. Microstructure is actually defined as the collective ensemble of all features in a material that are not in thermodynamic equilibrium (i.e., interfaces, dislocations, stacking faults, com- position gradients, and dispersed precipitates). These defects, though not in thermodynamic equilibrium, are often retained in materials due to their local mechanical stability and slow relaxation and annihilation kinetics. As these microstructure ingredients endow most materials (e.g., metallic alloys) with their characteristic good load-bearing properties, thermody- namic metastability is a desired material state and the main tar- get behind practically all processing steps that follow primary synthesis. In contrast, alloys in thermodynamic equilibrium are a rare exception with little relevance for applications. While the arrangement and density of specific defect class- es such as dislocations and grain boundaries are frequently addressed topics in microstructure research, less attention has been placed on the role of bulk and local chemical compo- sition and partitioning effects on phase metastability, and its relation to the activation of specific deformation mechanisms. This applies particularly to scenarios where site-specific and self-organized equilibrium segregation to lattice defects changes their local chemical composition, leading to regions of spatially confined metastability. Such a trick, also referred to as segregation engineering, 1–5 can be utilized for enabling activation of athermal transformation effects that are spatially confined only to the metastable defect region. Athermal or dif- fusionless transformation mechanisms of particular interest in this context include martensitic and twinning-induced plasticity ( Figure 1). The upper rows in the figure show representative microstructures with the decorating atoms in red. The bottom rows show the local chemical composition. Some of these lattice defect regions become metastable when decorated. While bulk composition tuning for achieving well-defined phase metastability is a typical design target (e.g., for transfor- mation induced plasticity [TRIP], 6–11 twinning induced plastic- ity [TWIP], 12–19 duplex, 20 and quench-partitioning steels, 21–24 as well as for some high-entropy alloys 25–47 ) it is less com- monly used for designing specific chemical decoration states at lattice defects. 1–5,48–50 Spatially confined or compositionally graded metastable states are sometimes accidentally inher- ited from solidification and processing, but they are usually not engineered. The aim of the metastability alloy design (MAD) concept lies in the compositional, thermal, and microstructure tuning of metastable phase states for triggering diffusive (e.g., spinodal Metastability alloy design Dierk Raabe, Zhiming Li, and Dirk Ponge This article reviews the concept of metastability in alloy design. While most materials are thermodynamically metastable at some stage during synthesis and service, we discuss here cases where metastable phases are not coincidentally inherited from processing, but rather are engineered. Specifically, we aim at compositional (partitioning), thermal (kinetics), and microstructure (size effects and confinement) tuning of metastable phases so that they can trigger athermal transformation effects when mechanically, thermally, or electromagnetically loaded. Such a concept works both at the bulk scale and also at a spatially confined microstructure scale, such as at lattice defects. In the latter case, local stability tuning works primarily through elemental partitioning to dislocation cores, stacking faults, interfaces, and precipitates. Depending on stability, spatial confinement, misfit, and dispersion, both bulk and local load-driven athermal transformations can equip alloys with substantial gain in strength, ductility, and damage tolerance. Examples include self-organized metastable nanolaminates, austenite reversion steels, metastable medium- and high-entropy alloys, as well as steels and titanium alloys with martensitic phase transformation and twinning- induced plasticity effects. Dierk Raabe, Max-Planck-Institut für Eisenforschung, Germany; raabe@mpie.de Zhiming Li, Department of Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung, Germany; zhiming.li@mpie.de Dirk Ponge, Max-Planck-Institut für Eisenforschung, Germany; d.ponge@mpie.de doi:10.1557/mrs.2019.72 https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2019.72 Downloaded from https://www.cambridge.org/core. Max-Planck-Institut fuer Eisenforschung, on 10 Apr 2019 at 06:16:23, subject to the Cambridge Core terms of use, available at