C15 NbCr2 Laves phase with mechanical properties beyond Pugh’s criterion

• The elastic properties of C15 NbCr2 Laves phase are calculated.
• NbCr2 Laves phase with mechanical properties beyond Pugh’s criterion.
• Other Laves phases and other compounds also with mechanical properties beyond Pugh’s criterion.
• New insights into Pugh’s B/G criterion are discussed.

Pugh’s criterion in terms of the bulk/shear modulus ratio (B/G) is widely used to distinguish the ductility and brittleness of metals, with the recommended B/G ratio of 1.75 for the boundary ratio between ductility and brittleness. The present first-principles calculations yield a B/G = 3.69 for the C15 Laves phase NbCr2, suggesting the ductility of NbCr2 according to Pugh’s criterion. However, experiments indicate that NbCr2 is a brittle phase at room temperature with the fracture toughness of 1.0–1.4 MPa m1/2. NbCr2 is hence a compound with its ductile and brittle behavior beyond Pugh’s criterion, and this finding also validates for other Laves phases and compounds. New insight into Pugh’s B/G criterion is also discussed.

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