First-Principles Design Maps for Predicting Anomalous Ductility in B2 Materials

Recently, a new class of B2 intermetallic compounds, e.g., YAg and YCu, were experimentally shown to have significant ductility comparable to face-centered cubic (fcc) Al, in contrast to other B2 materials like CuZn or NiAl. As of yet, there has been no explanation for the enhanced ductility in this class of materials. In order to provide understanding and a means to predict for such behavior, we derive, using mesoscale dislocation mechanics, criteria of h111i slip versus h001i slip, as well as the relative stability of antiphase boundaries (APB) and stacking faults (SF), together giving the necessary condition for ductility. Combined with the sufficient condition, which requires APB bistability on f1¹10g and f11¹2g planes, stability maps are constructed using dimensionless ratios of the calculated lattice constants, elastic constants, a 2 h111i f1¹10g and a 2 h111i f11¹2 g APB energies, and a2 h001i f1¹10g SF energies. To obtain required input to the stability maps, we have performed ¯rst-principles density-functional theory (DFT) calculations on three types of B2 materials: the Y-based B2 compounds (YAg, YCu, YIn, YRh, and YMg), the classic B2 alloys (NiAl, FeAl, AuCd, AuZn, CuZn, and AgMg), and the CsCl-type ionic compounds (CsCl, CsI, TlBr, and TlCl). In all the B2 materials, only YAg and YRh satisfy both necessary and su±cient conditions for enhanced ductility, while, like classic B2 alloys and ionic compounds, the YIn and YMg systems are predicted to be brittle, where the latter has been experimentally con¯rmed. This general combined dislocation mechanics and DFT approach provides predictive maps for use in alloy design and understanding of anomalous ductility in B2 systems