Onset of plastic deformation in non-equiatomic fcc CoCrFeMnNi high-entropy alloys under high-speed loading

A.V. Korchuganov


The main mechanisms of plastic deformation under high-speed compression and tension of CoCrFeMnNi single crystals are the formation of stacking faults and bands with a hcp lattice, the subsequent twinning occurs only upon stretching. Deviation from the equiatomic composition significantly improves the physical and mechanical properties of this alloy.On the basis of molecular dynamics computer simulation, the features of plastic deformation nucleation and evolution under mechanical loading of CoCrFeMnNi high-entropy alloy single-crystals with different stoichiometric compositions were studied. To obtain thermodynamically equilibrium distribution of chemical elements the relaxation of samples was carried out using the Monte Carlo method. Calculations showed that increase in Co and Ni fraction or decrease in Cr, Fe, and Mn fraction leads to increase of Young's modulus of the alloy. Based on these calculations, two samples with different stoichiometric compositions were selected and investigated: Co30Cr30Fe10Mn10Ni20 (Co30Cr30-sample) and Co10Cr10Fe30Mn30Ni20 (Fe30Mn30-sample), with high and low Young's modulus, respectively. A comparison was also made with the equiatomic alloy. Regardless of the composition, the onset of plasticity in CoCrFeMnNi single-crystals is realized through the formation and growth of intrinsic stacking faults and bands with hcp lattice. The mechanism for their formation is the rearrangement of the lattice from fcc to bcc and then to hcp structure. Structural and mechanical response of the samples differ substantially for different alloy compositions, types and rates of mechanical loading. Nucleation and growth of stacking faults and hcp bands are suppressed in Co30Cr30-sample, therefore, its yield stress is more than twice the values for Fe30Mn30-sample. After the formation of these defects twinning takes place at tension in all samples and it is not observed at compression. Plasticity mechanisms of high-entropy alloy and fcc nickel are in many respects similar. The obtained results allowed establishing a relationship between the stoichiometric composition and the atomic mechanisms of plastic deformation of high-entropy alloys under various types of mechanical loading.

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