On the possibility of cleavage in Nb

The bcc element, Nb, has the interesting and atypical property that it is predicted to fail by ⟨111⟩ {112} shear, rather than by {100} cleavage, when strained to its ideal strength in uniaxial tension. This suggests that pure Nb will not exhibit the classic bcc ductile–brittle transition when fractured at low temperature. The experimental data, while limited and ambiguous, seems to support this view. However, for inherent immunity to cleavage fracture at low temperature the shear failure mode must be preserved in the triaxial stress field at the crack tip. First-principles calculations of the ideal strength of Nb under combined uniaxial and hydrostatic tension show that the inherent shear failure mode is preserved over the range of stresses that pertain at the crack tip, while the ideal strength increases in the presence of hydrostatic tension. It is also shown that the ideal strength of Nb decreases under hydrostatic compression, which helps to explain recent observations of the behavior of Nb-rich Ti alloys in nanoindentation.