Effect of minor elements Al and Ti on dynamic deformation and fracture of CoCrNi-based medium-entropy alloys

Abstract

Three texture-free CoCrNi-based medium-entropy alloys (MEAs) with similar grain sizes, CoCrNi, (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, are investigated under plate impact loading. Free surface velocity histories are obtained along with microstructure characterizations of postmortem samples. The influences of the addition of minor elements Al and Ti on the microstructures, dynamic mechanical properties and underlying deformation and damage mechanisms of MEAs are elucidated. Compared to the single-phase CoCrNi MEA, spherical nanosized L12 precipitates and micron sized body-centered-cubic phase are found in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively. The addition of minor elements Al and Ti leads to an increase in the dynamic yield stress approximately by 70% (to 1.0 GPa) and 130% (to 1.4 GPa) for (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively, due to the solid solution strengthening and precipitate strengthening. Multiple deformation mechanisms are observed. Planar dislocation glide and stacking faults are the dominant deformation mechanisms for all the MEAs. However, deformation twinning is deactivated in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5 with small additions of Al and Ti. For spallation, damage is ductile in nature for CoCrNi and (CoCrNi)94Al3Ti3, but brittle for (CoCrNi)90Al5Ti5. Under similar shock stress, the spall strength of (CoCrNi)94Al3Ti3 is comparable to that of CoCrNi, ∼ 150% higher than that of (CoCrNi)90Al5Ti5 because intergranular brittle fracture activation does not require substantial plastic deformation. The minor addition of Al and Ti in medium-entropy alloys significantly affects their dynamic mechanical properties, deformation and damage mechanisms.