Research For High Temperature Oxidation Properties Of GH5188Superalloy

GH5188Co-based superalloy sheet1mm thickness was studied in the presentwork. The static oxidation behaviors of the sheet were investigated in air at800℃,900℃,1000℃and1100℃for100h. Cyclic oxidation behaviors of the sheet in airat1000℃for50N were also investigated. The high temperature oxidationmechanism and high temperature tensile of the alloy were studied by using XRD,SEM with EDS and TEM with EDS. The results can provide theoretical basis fordevelopment and application of honeycombed metal.The researches included:phase composition, morphology and microstructure of oxide film, microstructure,high temperature tensile and fracture analysis of base alloy.The results showed that oxidation kinetic of the static heat-treated followlinear law, parabolic law and gain-reduction cycle law in turn.Oxidation kinetic ofthe cyclic oxidation always follow gain-reduction cycle law. Cr, Mn and Si exitingin alloy were used to form oxide film in the course of static and cyclic oxidation.Cr was preferential oxidized to quasi-circular Cr₂O₃grain, which lead to aformation of dense oxide surface. A discontinuous amorphous layer of SiO₂wasformed in Cr depleted zone, which distributed in the Cr₂O₃oxide film/substrateinterface. Higher diffusion coefficient of Mn ion is the reason of Mn-Cr spinelforms at the outmost of the film. By the relevant thermodynamic and analysis, weknow that, Cr₂O₃, Mn-Cr spinel and SiO₂has good protection, belonging toexcellent oxidation resistance of the oxide composition.The oxide film of GH5188alloy crazed and spalled because of growth stressand thermal stress. The phase composition of oxide film affects its spalling.Because the thermal expansion coefficient of Cr₂O₃is bigger compared with alloysubstrate, and its PBR is relatively great. Crack initiated and extended easily in theCr₂O₃oxide layer, then resulting in fracture of the whole oxide film. Throughformation of grain clusters, Mn-Cr spinel made the increasing of partial oxide filmthickness, then the stress in the film increased resulting in the cracking spalling ofoxide film. Oxidation temperature, oxidation state and oxidation time alsoimpacted on the spalling of oxide film. With the increase of oxidation temperatureand oxidation time, the spalling was more seriously. Oxide film spalling of cyclicoxidation was more seriously than the same temperature static oxidation. In the1000℃static oxidation process, the morphology of Mn-Cr spinel transforms tolath-shaped with preferential growth. This long Mn-Cr spinel improved thespalling resistance. So the alloy oxidation resistance at1000℃was better thanother temperature. After high temperature tensile, GH5188alloy oxidized at high temperature,exhibited big elongation, good plasticity at the elevated temperatures. The tensilefracture surfaces of the alloy showed a typical characteristic of ductile fracture.However, its tensile strength was relatively low. With the increase of oxidationtemperature, the tensile strength decreased, and the elongation increased. Thecyclic oxidation state could reduce the tensile strength in the correspondingtemperature to a certain extent.