Unique phase transformation in high entropy alloy films driven by size and stress

In this work, the phase stability of Fe 50 Mn 30 Co 10 Cr 10 and Fe 50 Mn 30 Co 10 Ni 10 high entropy alloy (HEA) films and their mechanical properties was studied. During non-equiatomic magnetron sputtering deposition, the size-driven FCC-to-HCP phase transformation is observed in both single HCP-phase Fe 50 Mn 30 Co 10 Cr 10 and HCP/FCC dual-phase Fe 50 Mn 30 Co 10 Ni 10 films compared to their bulk sibling. The phase structure of Fe 50 Mn 30 Co 10 Cr 10 films remain unchanged after deformation, due to the bidirectional transformation. While the stress-driven HCP-to-FCC phase transformation is observed in Fe 50 Mn 30 Co 10 Ni 10 films, which is explained in terms of the nucleation of stacking faults, serving as the nuclei for phase transformation. Furthermore, the Fe 50 Mn 30 Co 10 Ni 10 films possess higher hardness than Fe 50 Mn 30 Co 10 Cr 10 , which is attributed to the transformation strengthening and abundant stacking faults/nanotwins. The current work provides a new pathway to tailor the size-induced and stress-induced phase transformation and thereby synthesize high strength metallic materials.