| Monel is a type of nickel copper base alloy of vital importance, which has been widely used in national defense applications and national product applications due to its excellent mechanical properties and corrosion resistance. But the application is limited to the inner land due to the blockade on manufacturing and application technology from abroad, for fluorochemical plant in atomic energy engineering. This work, as part of a national key project,focused on erosion corrosion problem, have highlighted the research on mechanical properties and corrosion resistance to fluorides, aiming at offering strengthening methods and corrosion control theories to the next development of this alloy system. Based on the designation of strengthening from γ’ precipitates promoted by adding mainly aluminum plus defined amount of silicon and titannium, this thesis, giving a view from alloying point, investigate the effect of Al, Al+xSi, Al+xTi content on promoting precipitates, strength and corrosion resistance to fluorides. Finally, the aging treatment was determined, and the mechanism of strengthening and resistance to fluorides was observed, drawing main conclusions as follows:The influences of Al, Al+xSi, Al+xTi addition on microstructure were investigated, and results indicate that there is no obvious effect of these three elements on crystal grain size and twin boundary. The y’phase is strongly promoted by addition of aluminum, silicon, titannium. The critical amount of aluminum to precipitate y’is about3.6%by weight in alloy with no addition of silicon or titannium, and the critical amount of silicon is below0.32%by weight in alloy with3.5%aluminum. The coarsening tendency of y’precipitates is increased when silicon content is below0.5%.Various aging treatments were performed, results show:the greatest hardness value is obtained at about595℃, while the temperature for lower silicon content is about550℃. There is a trend that the peak aging temperature is to be promoted by increasing aluminum, silicon, titanium content. Prolonging the aging treatment for6hours will give a better strengthening effect, while31hours will have the best aging strength. The best aging treatment is595℃x(6-31)h. Results caculated by Thermo-Calc indicate the uper temperature at which y’to precipitate is promted by increasing silicon and titanium contents, and titanium have better effect than silicon.The effects of aluminum, silicon, titanium on mechanical properties were observed, results show that the strength is greatly promoted by increasing amounts of aluminum, silicon, titanium. While addition of0.84%silicon will have a best strengthening effect, room temperature tensile strength902MPa, yield strength472.5Mpa, and380℃tensile strength735MPa, yield strength440Mpa. Alloy with aluminum have a better plasticity, while addition of titanium could promote plasticity of silicon-bearing alloys. The mechanism of aging strengthening was observed, conclusion was drawn that the strength of alloy is promoted by interaction between secondary particle y’and dislocation, the strengthening effect is related to amount of y’ precipitates.Effects of aluminum, silicon, titanium on anodic polarization character and resistance to fluorides were also examined, results show that anodic passivity could be promoted by improving titanium content while silicon will have an opposite effect. Pitting corrosion maybe poorer with addition of silicon. Corrosion resistance to HF solution is lowered by inceasing aluminum, silicon, titanium content. |
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