Effect of processing strain rate and temperature on interfacial segregation of zinc in a magnesium alloy

We show that, while interfaces formed in a Mg-3Zn-0.5Y (at%) magnesium alloy by severe plastic deformation (SPD) by high pressure torsion (HPT) at room temperature (RT) are segregated with Zn (without aid of any thermal treatment), only a small fraction of grain boundaries are segregated when extruded at 573 K (300 °C). We have examined the effect of strain rate and temperature on the diffusion behavior and segregation of zinc in magnesium alloys. At first, we have established the driving force for segregation by evaluating segregation energy and the interface energy gain by density functional theory calculations. The mechanism of segregation is established through calculation of excess vacancy concentration and critical dislocation velocity as a function of strain rate. It is estimated that Zn atoms are transported by SPD induced vacancy flux in case of HPT at RT, whereas the Zn atoms are dragged by equilibrium vacancies and dislocations on extrusion at 573 K (300 °C). The amount of segregation to the different twins and grain boundaries with different interfacial energies have been calculated by thermodynamic parameters and found to be in the range of 1-8 at% of Zn for the case of HPT process and 0.7-1.6 at% of Zn for extrusion processed specimens. These estimates correspond to the solute concentrations determined experimentally.