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FCC-HCP Phase Stability and Grain Refinement Behavior in Cr20Mn20Fe20Co40-xNix High-Entropy Alloys
National Institute for Materials Science1
The concept of high-entropy alloys (HEAs), introduced independently by Cantor  and Yeh  in 2004, is to maximize a configurational entropy of the Gibbs free energy, which contributes to stabilize a solid-solution state with respect to ordered intermetallic compounds. Equimolar FCC alloy of CrMnFeCoNi, known as the Cantor alloy, exhibits a combination of remarkable strength and ductility at cryogenic temperatures as well as high fracture toughness . These properties were associated with the formation of nanotwins. Enhanced work-hardening and strength-elongation balance via the FCC-HCP transformation was also reported in Fe35Mn45Co10Cr10. Thus the FCC-HCP phase stability is one of the critical parameters to control the deformation behavior of the HEAs and is also closely related to stacking fault energy. Present study elucidates the effect of phase stability on deformation behavior and on grain refinement by severe plastic deformation in FCC high-entropy alloys.
Gibbs free energy difference between FCC and HCP phase was evaluated for Cr20Mn20Fe20Co40-xNix alloys by Calphad approach . The results indicated that decreasing Ni content stabilizes HCP phase. Based on the results, ingots with x = 0, 5, 10, 15, and 20 were prepared by high-frequency melting. The ingots were forged and caliber rolled at 1473 K into the form of 14 x14 mm2 square bars or 10mmf round bars. Final heat-treatment was given at 1173 ~ 1373 K for 1 hours followed by water quench.
Room temperature tensile tests revealed that the tensile strength increased with decreasing Ni content while the yield stress are similar for different alloys. The 10Ni and 15Ni alloys exhibited fairly good strength-ductility balance due to TRIP effect and TWIP effect, respectively.
Grain refinement behavior by severe plastic deformation was investigated using high-pressure torsion (HPT). 10 mmf disks of each alloy were deformed by HPT under an applied compressive load of 5 GPa up to 10 rotations of the anvils. Effect of HPT deformation in Vickers microhardness (Hv) was investigated. For the FCC stable alloys with 15~20Ni, HPT led to a drastic increase in Hv. The Hv values were doubled even after 0.25 rotation. BSD-SEM and TEM observations revealed that pronounced formation of lamellar structures with about 100 nm, separated by nanotwins. It was inferred that the nanoscale lamellar accelerates an increase in dislocation density and led to the formation of equiaxed nanograins (~50 nm) by continuous dynamic recrystallization. Thus the drastic increase in Hv can be attributed to rapid progress of grain refinement. Meanwhile, the hardness increase was less significant for the lower Ni content alloys (0Ni and 5Ni), where HCP phase was dominant and the HV values scattered. It was found the grain refinement mainly occurred in shear bands, which resulted in the heterogenous microstructures. Increase in the Hv was mainly due to the increase in the dislocation density in HCP phase and the contribution from the grain refinement was limited in the HCP stable alloys.
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