a b s t r a c tMicrostructural evolution during cold sheet rolling to 80% thickness strain and annealing at 600e1100 C for 30 min of the CoCrFeNiMn high entropy alloy doped with 1 at.% of C and resulting mechanical properties of the alloy are reported. It is shown that in the initial homogenized (24 h at 1000 C) condition the alloy has single fcc phase structure. Cold rolling is accompanied by dislocation slip, deformation twinning and formation of shear bands. Annealing at 600 C after 80% cold rolling results only in partial recrystallization of cold-deformed structure, while an increase of the annealing temperature produces fully recrystallized microstructure. Comparison with the data on undoped CoCrFeNiMn alloy demonstrates that the addition of carbon pronouncedly increases dislocation activity simultaneously retarding deformation twinning during rolling and decreases the fraction of twin boundaries in the annealed condition. The effect of carbon can be attributed to an increase of stacking fault energy of the carbon-containing alloy. Cold rolling results in a substantial strengthening of the alloy; its ultimate tensile strength approaches 1500 MPa, but at the expense of low ductility. Good combination of strength and ductility can be obtained after annealing. For example, after annealing at 800 C, the alloy has yield strength of 720 MPa, ultimate tensile strength of 980 MPa, uniform elongation of 21% and elongation to fracture of 37%. It is shown that the high strength of the annealed alloy can be attributed to (i) strong grain boundary strengthening; (ii) solid solution strengthening by carbon.

The effect of cold rolling on the microstructure and mechanical properties of an Al- and C-containing CoCrFeNiMn-type high-entropy alloy was reported. The alloy with a chemical composition (at %) of (20–23) Co, Cr, Fe, and Ni; 8.82 Mn; 3.37 Al; and 0.69 C was produced by self-propagating high-temperature synthesis with subsequent induction. In the initial as-cast condition the alloy had an face centered cubic single-phase coarse-grained structure. Microstructure evolution was mostly associated with either planar dislocation glide at relatively low deformation during rolling (up to 20%) or deformation twinning and shear banding at higher strain. After 80% reduction, a heavily deformed twinned/subgrained structure was observed. A comparison with the equiatomic CoCrFeNiMn alloy revealed higher dislocation density at all stages of cold rolling and later onset of deformation twinning that was attributed to a stacking fault energy increase in the program alloy; this assumption was confirmed by calculations. In the initial as-cast condition the alloy had low yield strength of 210 MPa with yet very high uniform elongation of 74%. After 80% rolling, yield strength approached 1310 MPa while uniform elongation decreased to 1.3%. Substructure strengthening was found to be dominated at low rolling reductions (<40%), while grain (twin) boundary strengthening prevailed at higher strains.

Microstructure and mechanical properties of the Fe-Mn-Cr-Ni-Al system non-equiatomic high entropy alloys with a different Al content (x ¼ 0e14 at.%) were studied in the present work. The Fe 40 Mn 25 Cr 20 Ni 15 alloy was composed of the face-centered cubic (fcc) matrix phase with a small amount of coarse bodycentered cubic (bcc) particles. Addition of a small amount of Al (x ¼ 2e6) resulted in an increase in the fraction of the bcc phase to 26% and the formation of fine B2 precipitates within the bcc phase. At higher amounts of Al (x ¼ 10 and x ¼ 14) the microstructure consisted of coarse bcc matrix grains with the B2 precipitates inside. The alloys tend to become stronger with an increase in the Al content from 0 to 10 at.%; further increase in Al concentration did not influence strength considerably. The alloys exhibited pronounced softening with an increase in testing temperature from 25 to 400 Ce600 C. Ductility of the alloys was high enough (>50%) at all temperatures. A quasi-binary Fe 40 Mn 25 Cr 20 Ni 15-Al phase diagram was constructed using a ThermoCalc software and a TCHEA2 database; reasonable agreement between the experimental and predicted phase compositions of the alloys was obtained. It was suggested that an addition of the strong bcc-stabilizing and compound-forming Al to a bcc-prone Fe 40 Mn 25 Cr 20 Ni 15 alloy is beneficial for the development of the alloys with the disordered bcc matrix and the embedded B2 precipitates having attractive mechanical properties.

A new approach to increase the tensile performance of high entropy alloys (HEAs) by producing a duplex ultrafine-grained (UFG) structure was reported in this work. A novel HEA based on the CoCrFeNiMn system with substantial amounts of Al and C was used for the illustration of this approach. In the as-cast condition the alloy had almost entirely a single face-centered cubic (fcc) phase structure with an insignificant amount of M 23 C 6 carbides. After cold rolling and annealing at 800-1000°C an increased amount of fine second phases, namely M 23 C 6 carbides and B2 phase, effectively pinned boundaries of recrystallized fcc grains. As a result, a duplex UFG structure composed of the recrystallized fcc grains and M 23 C 6 and B2 particles was produced. The alloy with the UFG structure demonstrated attractive mechanical properties. For example, after annealing at 900°C the alloy had the yield strength of 785 MPa, the ultimate tensile strength of 985 MPa, and elongation to fracture of 32%. The phase composition of the alloy in different conditions was compared with the equilibrium phase diagram obtained using a Thermo-Calc software. Strengthening mechanisms were qualitatively analyzed, and some possibilities for further improvement of strength of the alloy were discussed.

The effect of cold rolling to 80% thickness reduction and annealing at 973-1373 K for 1 h on the microstructure and mechanical properties of a C-containing CoCrFeNiMn high-entropy alloy was studied. Cold rolling significantly strengthened the alloy to the yield strength of 1310 MPa. Annealing at 973 K or 1073 K resulted in incomplete recrystallization of an fcc matrix and M 23 C 6-type carbide precipitations aligned with highly elongated grains/subgrains. Complete recrystallization occurred during annealing at 1173 − 1373 K. The ordered arrangement of the carbides was not observed after annealing at 1273 K or 1373 K. The volume fraction of carbides decreased with increasing the annealing temperature that can be reasonably described by a Thermo-Calc prediction. The coarsening behavior of the microstructure constituents was studied during isothermal annealing at 1173 K for 1-50 h. It was found that the grain growth and the particle coarsening can be expressed by power law functions of annealing time with grain/particle size exponents of about 2 and 3, respectively. An increase in the annealing temperature from 973 K to 1373 K led to a gradual softening of the alloy; the yield strength decreased from 870 MPa to 320 MPa, whereas total elongation increased from 24% to 47%, respectively. Contributions of various strengthening mechanisms into the overall strength of the alloy were discussed.