Phase Transformation and Strengthening Effect Induced by Al Doping and Annealing in Nanostructural Medium Entropy Alloy Films

In this work, we have investigated the effects of Al content and annealing on the phase transformation and mechanical properties of nanocrystalline (CoCrFe)100-xAlx medium entropy alloy films prepared by DC magnetron co-sputtering. It is found that the as-deposited films have the face-centered cubic (fcc) structure even if Al content is as high as 20 at.%. However, after the annealing treatment, the fcc phase structure transforms into body-centered cubic structure (bcc) plus nanoscale ordered b2 phase. The nanoindentation test results show that the contribution of Al doping has little contribution on the hardness of the as-deposited films. After annealing treatment, the maximum hardness of the films increased to 11.7 GPa, which is 1.7 times higher than that of the as-deposited ones. Both bcc phase and nanoscale ordered b2 phase precipitation cause the superb strengthening effect of (CoCrFe)100-xAlx films.