Effect of cold working on the rate sensitivity in high entropy lamellar CoCrFeNi(Nbx/Tay) eutectic composites

Abstract

The strain rate sensitivity (m) and the activation volume (V*) of arc melted ingots (AMIs), suction cast rods (SCRs) of lamellar CoCrFeNiNbx (0.45 ≤ x ≤ 0.65) and CoCrFeNiTay (0.2 ≤ y ≤ 0.5) eutectic high entropy alloys (EHEAs) comprising of FCC γ-Ni and Fe2Nb/Co2Ta-type Laves phases have been studied at strain rates of 8 × 10-5- 8 × 10-3/s. Furthermore, specimens were cold worked up to 55% strain to investigate the effect of prior deformation on the rate sensitivity in bulk specimen using jump test under compression as well as the localized m in the nanoeutectic matrix and micrometer-size pro-eutectic phases using nanoindentation. The m value of the bulk EHEAs with varying lamellae thickness (λw) of 70–202 nm lies in between 0.0070 and 0.0082, and the V* value lies in between 24.8b3 and 43.7b3. Whereas, the localized m value of the eutectic matrix increases from 0.0066 to 0.0075 upon cold working of x = 0.5 SCR. Cold working increases the dislocation density up to 1016/m2, forms cells, sub-boundaries and distorts the lamellae interface, thus reducing the dislocation mean free path (Ʌ) and V*. The local lattice distortion, lamellae thickness, and the dislocation pile-up at the lamellae interface are correlated with the rate sensitivity and related mechanisms in the solute concentrated EHEAs.