Effect Of Interstitial Atoms On Microstructure And Mechanical Properties Of Fe₅₀Mn₃₀Co₁₀Cr₁₀ High Entropy Alloy

The high entropy alloy(HEA)made of multi-principal elements has broken the traditional alloy design concept.It is usually composed of five or more elements in equiatomic or nearly equiatomic,and has a high mixing entropy.The characteristics of multi-component and high mixing entropy make it have unique physical,chemical and mechanical properties,such as excellent low-temperature fracture toughness,radiation resistance,high temperature resistance,wide temperature range service and other excellent properties,which are expected to break the performance limit of existing materials,to meet the major needs of national defense,energy,environment,transportation and other fields.However,most metallurgical mechanisms for increasing strength lead to ductility loss,an effect referred to as the strength–ductility trade-off.Thus,the transformation-induced plasticity-assisted dual-phase HEA,by adjusting composition,provides the possibility to overcome the tradeoff between strength and ductility and has attracted increasing research interest.Meanwhile,it is shown that doping small size atoms in HEAs can control the deformation mechanism of materials and further improve the strength and ductility of materials.Therefore,effects of the interstitial atoms on the microstructure evolution and mechanical properties of typical dual-phase Fe₅₀Mn₃₀Co₁₀Cr₁₀HEA were studied in this dissertation:Firstly,effect of interstitial atomic(i.e.,carbon,nitrogen and boron)on the microstructure and mechanical properties at room temperature of Fe₅₀Mn₃₀Co₁₀Cr₁₀HEA were systematically studied.It suggests that the doped interstitial atoms decrease the grains size of homogenized and recrystallized samples.Also,the carbon-doping,nitrogen-doping and boron-doping can stabilize the?phase and retard the???transformation.The boron-doped samples present the best mechanical properties,followed by the carbon-doped,while the nitrogen-doped is the worst.The fracture morphology of matrix and carbon-and boron-doped samples is typical ductile fracture,and that of nitrogen-doped samples is mixture of cleavage fracture and ductile fracture.Secondly,based on the best mechanical properties of boron-doped samples,the microstructure and mechanical properties at room temperature of Fe_50-xMn₃₀Co₁₀Cr₁₀B_xHEA with different boron fraction were studied.It suggesting that the grain size of homogenized samples can be significantly refined by increasing boron fraction,while the grain size of recrystallized samples is not obvious.Also,the?grain size can affect the fraction of thermal induced?phase,i.e.,the fraction of?phase in homogenized state decreases with the increase of boron content,but there is no significant difference in recrystallized state.In addition,the size of?grain also affects the number of thermal induced?variants.Therefore,the number of?variants decreases with the addition of boron,and the number of?variants in recrystallized state is less than that in homogenized state.The tensile test shows that with the increase of boron fraction,the yield strength and tensile strength of the alloy increase gradually,but the elongation sacrificed.Lastly,the microstructure and deformation mechanism of matrix Fe₅₀Mn₃₀Co₁₀Cr₁₀HEA and the most typical boron-doped Fe₄₉Mn₃₀Co₁₀Cr₁₀B₁HEA are analyzed in detail.Firstly,two scales of grain sizes,i.e.,coarse grains(homogenized)and fine grains(recrystallized),have been designed for the matrix HEA.The?grain size can determine the activation orders of secondary?variants and{10(?)2}twin.Under the small strain,the secondary?variants and{10(?)2}twin can be activated in the coarse grains,rather than in the fine grains.Furthermore,under the higher strain,the saturated?-martensite are almost dominated by one persisting?variant for two?grain sizes.Secondly,the analysis of the deformation mechanism of the boron-doped samples have been carefully investigated.At the initial strain stage(10%),the dislocation slips in?phase and the activation of{10(?)2}twins in the?phase are dominant.As the strain increases(20%),a large number of stacking faults consider as the embryo of?martensite,which will promote the???transformation.At the ultimate strain state(40%),the???transformation is saturated and{10(?)1}and{10(?)3}twins in the?phase provides the plastic deformation.