A family of ductile intermetallic compounds

Abstract

Stoichiometric intermetallic compounds have always been touted for their attractive chemical, physical, electrical, magnetic and mechanical properties, but few practical uses have materialized because they are brittle at room temperature. Here we report on a large family of fully ordered, stoichiometric binary rare-earth intermetallic compounds with high ductility at room temperature. Although conventional wisdom calls for special conditions, such as non-stoichiometry, metastable disorder or doping to achieve some ductility in intermetallic compounds at room temperature, none of these is required in these unique B2 rare-earth compounds. Ab initio calculations of YAg, YCu and NiAl crystal defect energies support the observed deformation modes of these intermetallics.

Stability maps to predict anomalous ductility in B2 materials

“…So, it is possible that the splittings could be different than the usual APBs considered here. As a starting point, all B2 systems treated equally, we assume that there are well defined APBs on {110} and {112} planes with the displacement vector 1 2 111 and also a stacking fault on {110} planes with the vector 1 2 001 . However, the existence of such metastable faults is by no means guaranteed -indeed they are system specific.…”

“…The symmetry does not provide the guarantee of the stability of these faults. On {110} planes there may be metastable 1 2 111 faults in some materials, e.g., CuZn or FeTi. 64 However, the vectors corresponding to metastable faults may differ from 1 2 111 ; for example, simulations using empirical potentials found in NiAl that the APB with 1 2 111 is not actually stable but other faults on {110} planes existed.…”

“…Indeed, it has been reported 59 that 111 dislocations are metastable in NiAl and that they have been observed in the Y-based compounds. 1 The first central finding: The B2 stability map in Fig. 3 identifies candidates for multiple slip, and only a subset of Y-and Sc-based systems and some others qualify, namely, YAg, YCu, YIn, YRh, YZn, ScAg, ScAu, ScCu, ScPd, (ScPt), ScRh, NiAl, CsI and (CsCl), where the parenthesis reflect a borderline case that should be checked.…”

“…3 In contrast to B2 alloys, this new class of RM compounds have an exact stoichiometry, and are nearly elastically isotropic. 1 Following the discovery of the RM compounds, there have been several experimental 4,8,9 and theoretical 2,10-12 studies. Morris et al have hypothesized that the enhanced ductility in the Y-based compounds is due to the competing structural stability of B33 and B27 phases, obtained by introducing a periodic array of a 2 001 {110} superintrinsic stacking faults (SISF) to the B2 lattice.…”

Structural stabilities, elastic constants, generalized stacking fault energetics, and the martensitic transformation mechanisms for the Ni50−xTiPtx(x= 0 − 30) ternary system: ab initio investigation

“…This is met for each of the structures and stoichiometries studied. A second criterion for ductility in metals is that the anisotropy factor is greater than 0.8 and Poisson's ratio is less than 0.35 [18]. For this criterion Ni 50−x Ti 50 Pt x does not satisfy either component.…”