No abstract

High entropy alloys (HEAs) with a face-centered cubic (fcc) structure are considered as promising structural materials, in particular due to their impressive ductility and toughness at cryogenic temperature; at the same time strength of these HEAs is often quite low. An addition of interstitial elements like carbon substantially increases the strength of the fcc HEAs at room temperature, however the effect of C on cryogenic properties has not been properly studied. Therefore in this work we examined cryogenic tensile behavior of the fcc high entropy alloys with different carbon content (0e2 at.%). The alloys had non-equiatomic proportions of principal elements, i.e. Co 1 Cr 0.25 Fe 1 Mn 1 Ni 1 . The lower Cr concentration in comparison with the equiatomic alloy led to the higher solubility of carbon confirmed by both Ther-moCalc calculations and experimental results; only in the alloy with 2 at.% C a small (<1%) fraction of Crrich M 7 C 3 carbides was found in the as-cast condition. The microstructure of the alloys was not significantly affected by the carbon content and generally consisted of coarse (250e300 mm) fcc phase grains with dendritic segregations. In turn, the carbon content influenced on mechanical behavior substantially: the strength of the alloys progressively increased with the carbon content along with some reduction in ductility. Solid solution strengthening by carbon at 77 K was much stronger than that at room temperature: 67 MPa/at% and 178 MPa/at%, respectively. The increase in solid solution strengthening agreed well with an anticipated increase in lattice friction at lower temperatures. Plastic deformation was associated with dislocations slip both at 293 K and 77 K; a decrease in temperature and an increase in the carbon concentration increased the inclination to planar slip. The obtained results offer new approaches to increase the cryogenic properties of fcc HEAs.

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.

Effect of annealing of a cold-worked CoCrFeMnNi alloy at temperatures of 500-900°C for 1-50 h on the structure and mechanical properties was studied in the present work. Annealing for an hour resulted in: i) recrystallization of the face-centered cubic (fcc) matrix at 600-900°C; ii) precipitation of a Cr-rich body-centered cubic (bcc) phase at 500-700°C or a sigma phase particles at 600-800°C. Moreover, an increase in the annealing time to 50 h at 600°C resulted in a continuous growth of both the fcc grans and bcc/sigma particles and in an increase in the fraction of the sigma phase at the expense of the bcc phase particles. The fcc grains growth was found to be controlled by the pinning effect of the second phase particles. Soaking for an hour at 500-600°C resulted in a substantial increase in strength of the alloy due to the second phases precipitation. Meanwhile annealing at the higher temperatures as well as an increase in the annealing time at 600°C resulted in softening; however, even after 50 h annealing, the alloy demonstrated reasonably high strength. In the latter case fine fcc grains, preserved due to the pinning effect by the second phases particles, contributed to strength mainly.

Specimens of a CoCrFeMnNi high entropy alloy with different content of nitrogen (0.5e2.0 at.%) were prepared by induction melting. Microstructure and tensile mechanical behavior of the alloys in the ascast condition were analyzed. The alloys with a low amount of N (0.5e1.0 at.%) had a single fcc phase coarse-grained microstructure. An increase in the content of N to 2.0 at.% resulted in the precipitation of a small amount (~1%) of Cr-rich M 2 N nitride particles. The yield strength of the alloys increased in proportion to the percentage of nitrogen by 117 MPa/at% N at 293 K or by 316 MPa/at% N at 77 K. The observed increase in strength was attributed to solid solution hardening. Ductility of the alloy with 0.5 or 1.0 at.% of N increased with a decrease in the testing temperature while ductility of the alloy with 2 at.% dropped from 67% at 293 K to 8% at 77 K. The plastic deformation of the alloys at both 77 K or 293 K was mostly associated with planar dislocation slip. No signs of mechanical twinning were found even at 77 K. Thermo-Calc software was used to produce a quasi-binary CoCrFeMnNieN phase diagram for comparing the experimental and calculated results. The effect of N on strengthening and deformation mechanisms at different temperatures was discussed.