Research On Microstructure And Properties Of ODS High-entropy Alloy And Nickel-base Alloy Repair Materials

Oxide dispersion strengthened high entropy alloy（ODS-HEA）as a kind of composite with ODS and HEA characteristics,through the oxide composition design,the ODS-HEA exhibits excellent mechanical properties and wear resistance.In this dissertation,the mechanical alloying behavior of the different oxide content ODS-HEA,the effects sintering pressure and oxide content on the microstructure and properties of ODS-HEA were studied.The powder of ODS-HEA with different Y₂O₃ content,OY₂O₃-CoCrFeMnNi,0.25Y₂O₃-0.4Ti-CoCrFeMnNi and 0.5Y₂O₃-0.4Ti-CoCrFeMnNi,were prepared by mechanical alloying（MA）and ODS-HEA bulks were prepared by spark plasma sintering（SPS）.During MA process of 0Y₂O₃-CoCrFeMnNi,with the increase of milling time,alloying degree increased,grain size decreased,the lattice strain increased,and the average particle size of powder decreased first and then increased.When the milling time reached 50 h,three kinds of ODS-HEA powders were all completely alloyed,had uniform composition,and formed a solid solution with FCC and BCC crystal structure,with the increase of oxide content,ODS-HEA powders grain size decreased,the lattice strain increased.After 50 MPa and 40 MPa SPS,ODS-HEA bulk by the FCC and BCC coexistence phase,into a single FCC solid solution,during pressure sintering,the occurrence of BCC phase to FCC phase transition happened.A small number of elements segregation existed in ODS-HEA bulk local microstructure.Under 50 MPa pressure,the average grain size of alloy is larger than the average grain size under 40 MPa pressure.According to Hall-Petch law,alloy hardness is lower under 50 MPa pressure.High pressure sintering is beneficial to densification of sinter and increase of density of sintered sample.Under 50 MPa pressure SPS,the strength and elongation of ODS-HEA increased first and then decreased with the increase of oxide content.The tensile fracture morphology of ODS-HEA with different Y₂O₃ content all had ductile fracture and brittle fracture mechanisms,in which the ductile fracture is the main fracture mechanism.Friction and wear experiments of ODS-HEA which were prepared under 40 MPa pressure SPS were carried out.The result showed that the average friction coefficient under the load of 5 N and 10N were very close,and the oxide content has little effect on the friction coefficient of the composites.The wear mechanism of ODS-HEA with different Y₂O₃ content is the result of abrasive wear,adhesive wear and oxidation wear.The addition of oxide enhanced the wear resistance of ODS-HEA,but the excessive oxide addition decreased the wear resistance of the alloy.In addition,the friction and wear properties of three kinds of spray forming Ni based repair alloys were investigated in this thesis.Results reveal that the addition of high titanium and cobalt is beneficial to the optimization of the friction property and wear resistance of the alloys.The increase in titanium and cobalt content will result in the increase of γ/γ’ misfit,grain size,and coherent γ’ size,and enhance the wear surface oxidation and the formation of good oxides lubricants,which leads to a decrease of the coefficient of friction.Moreover,with increasing titanium and cobalt content,the predominant wear mechanism changes from delamination wear to abrasive wear and adhesive wear.The high titanium and cobalt content give a large improvement in wear resistance by strongly hindering dislocations motion and reducing crack growth rate,which resists delamination wear and improves alloy wear resistance.