Microstructure and mechanical properties of Co36Ni(37-x)Cr20Al3Ti2Nbx medium-entropy alloys
Co36Ni(37-x)Cr20Al3Ti2Nb,(x=2,3 and 4,at％)medium-entropy alloys(MEAs)were prepared by a vacuum arc-melting furnace,and the microstructure,mechanical properties,strengthening mechanism and deformation mechanism of the alloys were studied using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)analysis,transmission electron microscopy(TEM)and tensile testing machine.The results show that with the increase of Nb content,the average size of the alloys grains increases,and the composition and shape of precipitates at grain boundaries also change.When the Nb content is 2％and 3％,the precipitates are sheet-like precipitates rich in Al and Nb.When the Nb content increases to 4％,block like precipitates rich in Ti and Nb appear.With the increase of Nb content,the strength of the alloy increases and the elongation decreases.The Nb content increases from 2％to 4％,the yield strength of the alloys increases from 940 MPa to 1177 MPa,the ultimate tensile strength increases from 1246 MPa to 1271 MPa,and the elongation decreases from 13.67％to 3.75％.The reason for the increase in the alloy strength is that the addition of Nb element enhances solid solution strengthening and precipitation strengthening.In the early stage of alloy deformation,it is mainly achieved by the stacking faults-induced deformation,while in the later stage of alloy deformation,it is mainly achieved by dislocation movement.
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