Stability maps to predict anomalous ductility in B2 materials

Abstract

While most B2 materials are brittle, a new class of B2 (rare-earth) intermetallic compounds are observed to have large ductility. We analytically derive a necessary condition for ductility (dislocation motion) involving 111 versus 001 slip and the relative stability of various planar defects that must form. We present a sufficient condition for antiphase boundary bistability on {110} and {112} planes that allows multiple slip systems. From these energy-based criteria, we construct two stability maps for B2 ductility that use only dimensionless ratios of elastic constants and defect energies -calculated via density-functional theory. These two conditions fully explain and predict enhanced ductility (or lack thereof) for B2 systems. In the 23 systems studied, the ductility of YAg, ScAg, ScAu, and ScPd, ductile-to-brittle crossover for other rare-earth B2, and brittleness of all classic B2 alloys and ionic compounds, are correctly predicted.

Materials Prediction via Classification Learning

“…In our case, either an RM intermetallic is ductile or brittle, and therefore, we cannot write a phenomenological free energy expansion in terms of the order parameter. Alternatively, we can employ the recently developed mesoscale dislocation mechanics 32 that uses energy-based stability criterion to validate our misclassifications. According to this approach, the necessary and sufficient crystallographic conditions that must be satisfied by a B2 material for enhanced ductility are that should be the dominant slip direction, yet slip should also be possible with the formation of anti-phase boundaries (APBs) and APBs should have bistable existence on both and planes, respectively.…”

“…On the other hand, ductile B2 alloys are nearly isotropic. Sun and Johnson suggested the use of the Zener anisotropy ratio \ [A , where c 11 , c 12 , and c 44 are the elastic constants of the B2 cubic structure], as a qualitative figure of merit to classify the mechanical properties 32 . In ductile B2 systems (similar to those explored in this paper), the value of A should be close to one 32 .…”

“…Sun and Johnson suggested the use of the Zener anisotropy ratio \ [A , where c 11 , c 12 , and c 44 are the elastic constants of the B2 cubic structure], as a qualitative figure of merit to classify the mechanical properties 32 . In ductile B2 systems (similar to those explored in this paper), the value of A should be close to one 32 . Wang et al 33 have calculated the A-ratio for ScIr as 1.584 from DFT calculations (using GGA approximation), which is closer to that of the ductile ScCu (1.5) as opposed to that of the brittle YZn (1.985).…”

First-principles study on Ni3Al (111) antiphase boundary with Ti and Hf impurities

“…Finally, we note that theoretical studies of mechanical behaviors such as yield-stress anomaly and ductility often involve ratios of APB energies corresponding to various slip systems, which are used in stability maps derived from mesoscale dislocation theory [31,32]. The method presented in this paper can be extended to investigate the temperature dependence or the concentration dependence of these phenomena from first principles.…”

Predicting ductility in quaternary B2 -like alloys

“…The two slip systems and the ductility associated with 001 slip result in two criteria for ductility in B2 materials: 001 slip must be the dominant slip direction and multiple slip in the 111 direction should be possible. Sun and Johnson [13] implement these two criteria to create a quantitative metric for B2 ductility. Their metric uses antiphase boundaries (APBs) and stacking faults (SFs) in the two slip systems, specifically the 111 {110} APB, 001 {110} SF, and 111 {112} APB (denoted here as APB{110}, SF{110}, and APB{112}, respectively).…”