Metastability alloy design
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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, composition 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, thermodynamic metastability is a desired material state and the main target 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 classes 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 composition 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 diffusionless 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 transformation induced plasticity [TRIP],6–11 twinning induced plasticity [TWIP],12–19 duplex,20 and quench-partitioning steels,21–24 as well as for some high-entropy alloys25–47) it is less commonly used for designing specific chemical decoration states at lattice defects.1–5,48–50 Spatially confined or compositionally graded metastable states are sometimes accidentally inherited 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
Dierk Raabe, Max-Planck-Institut für Eisenforschung, Germany; [email protected] Zhiming Li, Department of Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung, Germany; [email protected]
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