Effect of carbon on cryogenic tensile behavior of CoCrFeMnNi-type high entropy alloys

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.

Carbide Formation in Refractory Mo15Nb20Re15Ta30W20 Alloy under a Combined High-Pressure and High-Temperature Condition

“…Because of the low concentration and small atom size of C, the effect from doped C to lattice constant of HEA is negligible. Similar observations on HEA such as Mo 0.5 NbHf 0.5 ZrTi [ 20 ] and CoCrFeMnNi [ 15 ] were reported.…”

“…Beside the contained elements in HEA, doping with interstitial elements such as carbon to HEA has been announced to be an effective way to improve the mechanical properties [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. The tensile yield strength of CoCrFeMnNi could increase from 371 MPa to 792 MPa just by adding 3.0 at.% C [ 22 ].…”

Interstitials in f.c.c. High Entropy Alloys

“…% C at 77 K and 64 MPa/at. % C at 293 K. It is worth noting that σ y for CoCr 0.25 FeMnNi [38] is similar to the values reported at both 77 K and 293 K for equiatomic CoCrFeMnNi [27][28][29]. εf decreased slightly with increasing carbon content at both temperatures, e.g., at 293 K from 64% with no carbon to 53% for 2.11 at.…”

“…The increase of WHR with increasing strain in carbon-doped HEAs delays the onset of necking, which further enhances εf. The carbon addition produced a higher WHR (calculated from data in Table 2 [38]) at both temperatures, i.e., from 1376 to 1394 MPa for 0 and 0.53 at. % C to 1521 MPa for 0.95 at.…”

“…Klimova et al [38] examined the effect of three different (measured) carbon levels (0.53, 0.95 and 2.11 at. %) on the strength of as-cast, large-grained (150-200 µm) non-equiatomic CoCrFeMnNi, i.e., CoCr 0.25 FeMnNi, at both 77 K and 293 K: only the highest carbon level was reported to contain particles (<1 vol.…”