Insight into the ageing effect on incipient plasticity of non-equiatomic TiNbZr body-centered cubic multicomponent alloy via using nanoindentation experiments

The incipient plasticity of as-homogenized and aged body-centered cubic (BCC) non-equiatomic TiNbZr multicomponent alloys (MCAs) is investigated using an instrumented nanoindentation method. The average critical shear stress tau max required for the occurrence of the first pop-in event triggering the incipient plasticity is decreased from 1.67 GPa to 1.48 GPa after the ageing treatment. The activation volumes Omega v of the first pop-in events are measured as 1.2 Omega and 1.6 Omega (Omega is the unit atomic volume for BCC-structured crystal) respectively for the as-homogenized and aged alloys, suggesting a dominant heterogeneous mode of dislocation nucleation via the atom-vacancy exchange method to motivate the onset of incipient plasticity. The results of microstructural characterization suggest that nano-sized second phase, namely long-ranged chemical orders with a decussate profile are generated via a typical spinodal decomposition in the aged specimen. Such chemical orders rich in Zr but poor in Nb, are found to promote the dislocation nucleation since the dissociation energy of the Ti-Zr bond is much less than that of the Ti-Nb one. Moreover, mobile dislocation can be readily hindered by the chemical orders, reducing the plastic zone size beneath the indent and leading to the lower factor f (the ratio of plastic zone radius to the indenter tip radius) of the aged alloys ( 4.01, while 4.29 for the as-homogenized counterpart). The above results further contribute to the theoretical correlation between the microstructural features and mechanical properties of metastable MCAs.