Cross-kink unpinning controls the medium- to high-temperature strength of body-centered cubic NbTiZr medium-entropy alloy

The deformation mechanisms of a NbTiZr body-centered cubic (BCC) medium-entropy alloy (MEA) are investigated by tensile testing at various temperatures. The yield strength (YS) shows a strong temperature dependence from 77 K to 300 K, while being insensitive to temperatures between 300 K and 873 K, followed by a significant drop at 1073 K. TEM investigations show that the alloy deformation is controlled by screw-dislocation slip. Screw-dislocations with cross-kinks/jogs are frequently observed at all temperatures except at 1073 K. The deformed microstructure at 473 K reveals dislocations loops/debris indicating the dominance of cross-kink strengthening at moderate to high temperatures, leading to a temperature insensitive YS. The behavior of NbTiZr is consistent with the cross-kink strengthening mechanism, as also confirmed by the comparison between observed and predicted values of the activation volume. The TEM investigations at 1073 K are consistent with the annihilation of cross-kinks/edge dipoles, which can explain the observed strength drop above this temperature.