Unravelling the deformation mechanisms in Ni-rich high entropy alloy with tailored Ti content: An experimental and atomistic approach

Ti-containing face centred cubic (FCC) high entropy alloys (HEAs) have garnered significant attention due to their exceptional mechanical properties. Nevertheless, the role of Ti on contributory strengthening mechanisms and the corresponding deformation behavior remains less explored till date. The present study sheds light on evolution of microscale plastic deformation mechanism and the associated strengthening effects induced by Ti addition in a novel spark plasma sintered Ni_46-xCo_18-xAl₁₂Cr₈Fe₁₂Mo_4-yTi_2z (x = 0, y = 0, z = 0; x = 0, 1 and 2, y = 2, z = 1, 2 and 3 at. %) HEA through a combination of experimental analyses and molecular dynamics (MD) simulations. The sintered compacts were composed of FCC solid solution with presence of minor amounts of brittle Cr-rich and Mo-rich sigma (σ) phases, along with essential L1₂ phase in the FCC matrix. Yield strength and compressive strength increased continuously with increasing Ti content, from 1130 MPa and 1809 MPa in Ti-free HEA to 1452 MPa and 2011 MPa in 6 at. % Ti containing HEA, respectively, while maintaining an appreciable fracture strain > 26 % in all the consolidated HEAs. Such remarkable mechanical properties are primarily attributed to inherent solid solution strengthening from Ti-induced lattice distortion, along with synergistic effect of narrow twin boundaries, finer grain size and precipitation strengthening from L1₂ phase. Furthermore, MD simulation revealed that increasing Ti content lowered stacking fault energy of the HEAs and promoted formation of deformation twins (DTs) and stacking faults (SFs). Characterization of deformed microstructures at sequential strain levels showed that plastic deformation in Ti-free HEA was primarily mediated by ordinary dislocation slip, whereas with increase in Ti content, plastic deformation predominantly proceeded through formation of SF networks and DTs, alongside dislocation gliding. Additionally, increased dynamic recrystallization fraction in higher Ti-containing HEAs during loading, attributed to increased pre-existing strain within grains, contributed in retaining impressive ductility. This study provides comprehensive insights into the deformation mechanisms in Ti-added Ni-rich FCC HEAs and offers guidance for designing high-performance HEAs.