FCC metal-like deformation behaviour of Ir₃Nb with the L1₂ structure

The deformation behaviour of Ir₃Nb with stoichiometric and off-stoichiometric compositions has been investigated as functions of crystal orientation and deformation temperature. The critical resolved shear stress (CRSS) for (111)[1-01] slip in stoichiometric Ir₃Nb exhibits a marginal temperature dependence at all temperatures with neither strong negative temperature dependence at low temperatures nor positive (anomalous) temperature dependence at high temperatures. This behaviour is similar to that usually observed in many pure FCC metals such as Al and Ni and cannot be classified into any of the categories of L1₂ compounds previously proposed based on the temperature dependence of CRSS in relation to the dissociation scheme of the [1-01] dislocation. The CRSS for (111)[1-01] slip exhibits orientation dependence, neither. The [1-01] dislocation dissociates into the anti-phase boundary (APB)-type scheme and is observed to be smoothly curved on the (111) slip plane at all temperatures, indicating the planar core structure. This is exactly what is known for the perfect dislocation in many pure FCC metals and is the reason for the observed FCC metal-like deformation behaviour of Ir₃Nb. The absence of yield stress anomaly in Ir₃Nb is discussed in terms of anisotropy in planar fault energies and elastic constants calculated by first principles calculations and experimentally determined in the present study.