320 MRS BULLETIN • VOLUME 41 • APRIL 2016 • www.mrs.org/bulletin © 2016 Materials Research Society Introduction Twins impede dislocation glide. They act as barriers to disloca- tions, promoting their aggregation and decreasing their mean free path. 1–11 Twinning depends on the stacking-fault energy (SFE). 12–17 In face-centered-cubic (fcc) alloys, the SFE can be tuned by alloying. 18–21 This opens compositional access to “optimal strain-hardening design.” 2,11,21–23 This term refers to a state where twins and the associated substructures do not form instantly upon initial yielding, but rather, they form over a wide loading range, so as to enable permanent dynamic reduc- tion of the dislocation free path and hence continuous strain hardening. 8 We can apply this design concept to twinning- induced plasticity (TWIP) steels, as described in this article. To understand how strain hardening responds to a change in SFE via alloy tuning, the first step lies in identifying its dependence on composition. With this knowledge, the descrip- tion of the strain-hardening modes can be rendered chemistry sensitive. The next step is to understand the underlying strain- hardening mechanisms. These are twin nucleation and growth, dislocation cross-slip, and twin–slip, slip–slip, and twin–twin interactions. 1,4–8,17 While the first step is accessible to thermodynamic calcu- lations on the basis of ab initio-derived interface energies that can be obtained by density functional theory (DFT) calcula- tions, the second step can be formally rendered by mapping the individual strain-hardening effects into mean field rate equations that are based on internal variables and by coupling the twin nucleation rate to the dislocation substructure and to the SFE. This combination of ab initio-derived thermodynamics and constitutive internal variable models renders this alloy design approach into a multiscale plasticity model where ab initio-derived quantities are linked with constitutive microstructure evolution equations ( Figure 1). The constitutive parameters are validated by the measurement of dislocation densities and twin volume fraction fractions using electron channeling contrast imaging (ECCI) and transmission elec- tron microscopy (TEM). 2,8,24 Texture and grain size effects are mapped using electron backscatter diffraction. 18 Ab initio-guided design of twinning-induced plasticity steels Dierk Raabe, Franz Roters, Jörg Neugebauer, Ivan Gutierrez-Urrutia, Tilmann Hickel, Wolfgang Bleck, Jochen M. Schneider, James E. Wittig, and Joachim Mayer The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain- hardening mechanisms. Dierk Raabe, Max-Planck-Institut für Eisenforschung, and RWTCH Aachen University, Germany; d.raabe@mpie.de Franz Roters, Max-Planck-Institut für Eisenforschung, Germany; f.roters@mpie.de Jörg Neugebauer, Max-Planck-Institut für Eisenforschung, Germany; email neugebauer@mpie.de Ivan Gutierrez-Urrutia, National Institute for Materials Science, Japan; gutierrezurrutia.ivan@nims.go.jp Tilmann Hickel, Department of Computational Materials Design, Max-Planck-Institut für Eisenforschung, Germany; t.hickel@mpie.de Wolfgang Bleck, Steel Institute, RWTH Aachen University, Germany; wolfgang.bleck@iehk.rwth-aachen.de Jochen M. Schneider, RWTH Aachen University, Germany; schneider@mch.rwth-aachen.de James E. Wittig, Vanderbilt University, USA; j.wittig@vanderbilt.edu Joachim Mayer, Central Facility for Electron Microscopy, RWTH Aachen University, and Ernst Ruska-Centre, Forschungszentrum Jülich, Germany; mayer@gfe.rwth-aachen.de DOI: 10.1557/mrs.2016.63