Effects of HCP elements Ti and Zr on the plastic deformation behavior of Ti-Zr-Nb medium-entropy alloys with the BCC structure

The deformation behavior of Ti-Zr-Nb medium-entropy alloys (MEAs) with the body-centered cubic (BCC) structure has been investigated as a function of the HCP (hexagonal close-packed)-element (Ti and Zr) concentration in compression at temperatures between 50 and 1173 K and is discussed in terms of the extent of lattice distortion quantified as mean-square atomic displacement (MSAD). The temperature dependence of yield stress observed for all the Ti-Zr-Nb MEAs investigated is essentially the same as that observed for conventional BCC metals and alloys, featuring a rapid decrease at low temperatures (T<T1), a plateau at intermediate temperatures (T1≤T<T2) and a rapid decrease again at higher temperatures (T≥T2). The deformation mechanism is converted from the Peierls mechanism at low temperatures (T<T1) to an athermal process involving local pinning of screw dislocations at superjogs presumably by solute cluster arising from non-random distributions of the constituent elements at plateau temperatures (T1≤T<T2), as evidenced by the arc-shaped configurations of screw dislocations bowing-out between pinning points. The extent of lattice distortion (MSAD) increases with higher HCP-element concentration and enhances the resistance to dislocation motion, thereby raising both the thermal stress at 0 K and the athermal stress (i.e., the plateau stress). The increased lattice distortion also leads to the steeper temperature dependence of yield stress at low temperatures (T<T1) resulting in the lower onset temperature (T1) for the plateau.