| Single-phase high-entropy alloys(Single-phase HEAs)possess many advantages,such as simple phase composition,uniform microstructure and good processing properties.Especially,some single-phase HEAs with face centered cubic structure(FCC)(e.g.CoCrFeNi,CoCrFeMnNi,etc.)also have excellent plasticity which exhibit great application potential in engineering field.However,the hardness and strength of the single-phase HEAs are relatively low.Although cold rolled could improve the hardness and strength of the alloy,it decreases significantly after recrystallization annealing.Therefore,how to effectively improve the hardness and strength of single-phase HEAs with FCC structure is an urgent problem to be solved.In-situ carbide reinforced CoCrFeNi composites were fabricated by mechanical alloying and subsequently hot pressed sintering in this study.X-ray diffraction(XRD),field emission scanning electron microscope(FSEM),energy disperse spectroscopy(EDS),transmission electron microscope(TEM),Vicker’s microhardness tester,universal electron tester,electrochemical workstation and vibrating sample magnetometer(VSM)were used to characterize the composites.The effects of preparation process(i.e.,ball milling and sintering process)and the Ti,C content on the microstructure,mechanical properties,corrosion resistance and magnetic properties of the composites were systematically studied.The main conclusions of the study are as follows:1.The Co,Cr,Fe and Ni powders experienced the deformation-cold welding-crushing-cold welding-crushing process during ball-milling.A single-phase CoCrFeNi solid solution with FCC structure,fine particles(2~5μm)and homogeneous chemical composition was obtained after milled at a speed of 350rpm and 20:1 ball to powder ratio for 10h.2.The density of sintered TiC-CoCrFeNi bulk(6.9-7.1g/cm~3)reached over96%of the theoretical density(7.18g/cm~3)after hot pressing sintered at 1200℃for 1h with a pressure of 50MPa,indicating the milled powders achieved essentially complete densification.3.TiC and Cr7C3 structured carbides were formed and dispersed in the CoCrFeNi solid solution after hot pressing sintering.Milling time did not change the phase composition of the sintered bulk,however,it significantly affacted the size and content of TiC and Cr7C3 structured carbides.The carbides increased and exhibited different coarsen degrees with an increase in milling time.When the milling time reached 10h,large amount of fine TiC phase(300~500nm)uniformly dispersed in the matrix and the hardness and yield strength of the composite reached the maximum values of 671HV and 1440MPa,respectively.4.The results of potentiodynamic polarization corrosion curves of sintered materials with various milling times in 3.5wt%NaCl solution showed that the corrosion potential of the composites was between-0.3775V~-0.23239V,and the corrosion current density was between 1.7308E-7~5.1431E-6A/cm~2,which was similar to that of the 316 stainless steel(-0.19245V、3.2628E-7A/cm~2).The results of immersion experiment indicated that the corrosion resistance of the composites in10wt%HNO3 solution decreased with an increase in milling time.5.The hardness of TixCxCoCrFeNi composites increased with increasing Ti and C contents,while the strength first increased and then declined.When x reached 0.22and 0.34,the composites exhibited excellent comprehensive mechanical properties.The hardness,yield strength and compressive elongation of the sintered bulks reached500HV,645MPa,47.89%and 594HV,950MPa,21.03%,respectively.The results of immersion experiment in 10wt%HNO3 solution indicated that the corrosion resistance of the composite exhibited a decrease trend with increasing Ti and C content. |
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