A temeparture dependence of the mechanical properties and fracture mechanism in cast multi-principle-elements FeMnCrNiCo(N) alloys

We study the temperature dependence of the mechanical properties and fracture mechanisms of cast multi-principle-elements alloys 20.0Fe-20.0Mn-20.0Cr-20.0Ni-20.0Co (high-entropy Cantor alloy), 19.7Fe-20.0Mn-20.0Cr-19.9Ni-19.0Co-1.4N and 20.4Fe-20.4Mn-20.3Cr-20.3Ni-17.0Co-1.6N (at.%) under uniaxial static tension in the temperature interval from 77 K to 473 K. The alloys possess an austenitic structure with dendritic segregations, and nitrogen-alloying is accompanied by the precipitation of the grain-boundary phase (at nitrogen concentration of 1.6 at.%). Alloying with nitrogen causes an increase in the values of a yield stress and an increase in the temperature dependence of σ_0.2(T) over the whole temperature range. For Cantor alloy, which does not contain interstitial atoms, a decrease in test temperature is accompanied by a simultaneous increase in strength and elongation. This alloy is characterized by a ductile fracture in the entire investigated temperature range. Both nitrogen-containing alloys have higher strength properties and plasticity than those for Cantor alloy. In the temperature range from 183 K to 473 K, both alloys fracture in transgranilar regime and contain numerous dimples on the fracture surfaces. Moreover, the secondary intergranular cracks are observed on the fracture surfaces of the alloy with 1.6 at.% nitrogen. With the decrease in test temperature down to 77 K, the elongation of both nitrogen-bearing alloys sharply decreases, and numerous brittle intergranular cracks are observed on the fracture surfaces. For alloy with 1.4 at.% nitrogen, some regions with ductile transcrystalline fracture have been also seen, while a portion of transgranular cleavage-like brittle facets is typical of the alloy with 1.6 at.% nitrogen. That is, cast nitrogen-doped multicomponent alloys exhibit a ductile-to-brittle transition in low-temperature deformation regime. As it has been revealed by the energy-dispersive SEM analysis, the low-temperature embrittlement of the alloy with 1.4 at.% nitrogen arises due to the formation of grain-boundary segregations. Both these factors contribute the intergranular fracture of the samples. In alloy with 1.6 at.% nitrogen, the presence of brittle quasi-cleavages inside austenite grains indicates the development of a “ductile-to-brittle” transition in the austenitic phase along with the brittle intergranular cracking due to the presence of grain-boundary nitrides.

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