| 其他摘要 | DA718 alloy, which has excellent mechanical and thermal processing properties, is one of the most important materials in producing key parts for engine turbines. With extensive applications of those engine turbines in the hot corrosive environment, it is of theoretical and practical significance to investigate the hot corrosion behavior and mechanism of DA718 alloy. In addition, the beneficial effect of phosphorus in DA718 alloy has been a hot topic in the wrought superalloy area. Phosphorus tends to be segregated highly along the grain boundaries. It provides new insight into the grain boundary effect of phosphorus by investigating its effect on the hot corrosion behavior of DA718 alloy. So the hot corrosion behavior of DA718 alloy around working temperature and the effect of phosphorus have been studied, and the effect of oxidation and hot corrosion pre-treatment on the mechanical properties has also been investigated.
The hot corrosion test is performed at 650℃ and 680℃ in the mixed salt of 75 wt.%Na2SO4 and 25 wt.%NaCl. Cr, Fe, Al, Ti and Nb are oxidized by the oxygen dissolved in the melting salt, and only Ni is sulfidized during the hot corrosion process. The main corrosion products are Cr2O3, FeCr2O4, NiO, TiO2, Al2O3 and Ni3S2 at 650℃, and at 680℃ Fe2O3 can also be detected. A layered scale structure can be obtained at both temperatures. The extenal layer is comprised of oxides and the internal layer is comprised of nickel sulfide. Only when the temperature is around 650℃, the periodic formation of the sulfide and oxide layers can be expected. The hot corrosion process of DA718 alloy in the mixed salt can be described as an intercrossed and repeated process of oxidation, sulfidation and basic dissolution. In the beginning Cr, Al and Ti are selectively oxidized and later Fe is then oxidized. Because of the outward diffusion of metal atoms, the surface is covered completely by a layer of oxides. Na2SO4 diffuses through the oxide layer to the scale/substrate interface and reacts with nickel to form nickel sulfide and oxide as well as oxygen ions. NiO is then dissolved by O2- to form which diffuses outwardly into the oxide layer and then dissociates into NiO. The existence of NaCl inhibits the formation of a compact and protective oxide layer. The oxidation and sulfidation processes take place at the same time. The interaction between these two processes can accelerate each other. Only at 650℃ the nickel-nickel sulfide could exist as a solid and does not flow through the scale. This is the basis of the periodic formation of the sulfide and oxide layers. However at 680℃, nickel-nickel sulfide eutectic exists as liquid, and flows into the oxide layer, so no periodic formation can be achieved. Phosphorus does not affect the final hot corrosion products, but it can inhibit the diffusion of sulfur along grain boundaries.
The effect of oxidation on mechanical properties of DA718 alloy is evaluated by comparing the properties of the alloys after thermal exposing and aging at 650℃ for 300h. The room temperature and 650℃ tensile properties of DA718 hardly change, but the 650℃/700MPa rupture and 650℃/725MPa creep properties decrease slightly by thermal exposing. The diffusion of oxygen atoms into grain-boundaries does not affect the first and second creep stage, but promotes crack propagation. Phosphorus could protect DA718 alloy from the attack of oxygen, but this is not the main reason for its beneficial effect.
The immersion of the alloy in the salt mixture at 650℃ for 100h scarcely reduces the tensile properties at room temperature and at 650℃, but decreases the tensile plasticity slightly, and the 650℃/700MPa stress rupture properties noticeably due to the decohesion of grain boundaries by sulfur. The constitution and the microstructure of the surface are greatly changed by the hot corrosion pre-treatment which causes the formation of cracks along the surface. The interaction between the crack tip and sulfur atom will promote the diffusion of sulfur into the grain boundaries and the dynamic embrittlement, which accelerates the crack propagation. The hot corrosion is accelerated when a stress is added. |
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