| AZ91D is one of the most popular Mg alloys due to its good cast ability and mechanical properties. However, it is easy to form porosities during traditional die casting because of its wide solidification temperature interval. Compared with the traditional die casting, thixoforming can significantly reduce or eliminate porosities, and the mechanical properties of formed components can be further improved through heat treatment. Unfortunately, But Mg is a kind of very active metal and Mg alloys are easy to be corroded in most of environments, especially as structural materials, magnesium alloys suffer from the combination actions of mechanical stress and corrosion medium, and stress corrosion cracking (SCC) is easy to take place. SCC is a common local corrosion and its destructive effect is very great. The SCC behaviors the traditional casting Mg alloys have been studied, but there are lots of problems to waited for clarifying. Investigations about SCC behaviors of thixoformed Mg alloy shave not been involved. Therefore, in this thesis, the static stress corrosion behavior and the SCC mechanism of thixoformed AZ91D alloy were studied through analyzing the effects of NaCl concentrations and PH values of the corrosive solutions and stress on the SCC. The results are as followings:(1) Compared with the permanent mould casting AZ91D alloy, the thixoformed AZ91D alloy has relatively higher SCC resistance. This is attributed to the differences in the microstructures of these two alloys. One difference is that the shape and distribution a and β phases are different. The β phase in the thixoformed alloy continuously distributed around the spherical primary a particle while the β phase in the permanent mould casting alloy discontinuously distributed between the primary a dendrites. Succession distributed |3 phase as a kind of border is to hinder corrosion media and improve stress corrosion ability. The other difference is that the amount and distribution of inclusions and porosities which the SCC is sensitive to are different. Thixoforming obviously decreased the inclusions and porosities.(2) The stress corrosion process of the thixoformed AZ91D alloy includes fourstages: destroy of the passive oxide film, pitting corrosion, crack formation and propagation, and fracture. The surface oxide film and the hydrogen formed from the corrosion reaction played an important role. The corrosion rate of the permanent mould casting alloy is higher than that of the thixoformed alloy during pitting corrosion stage. As soon as the pitting occurred, the speeds of crack formation and propagation were also rapider for the permanent mould casting alloy.(3) After solution and age heat treatment, the stress corrosion resistance of thixoformedAZ91D alloy was obviously improved. This is mainly expressed in three aspects. First, thei time to fracture for the heat treated alloy is obviously longer than that of the not heat treatedalloy. Second, there are laminar structure characteristics on the fracture surface of the heat treated alloy. Mainly because after heat treat, appear phase hindered crakle expand, make the crakle expand speed decreased, thus lengthened fracture time. Third, both the intercrystalline fracture and transcrystalline fracture were observed during testing for the heat treated alloy.The size of strengthening precipitates of p-MgpAl^ became finer and their distribution was more uniform because the precipitates experienced a process of dissolving and subsequent precipitating, which improved the SCC resistance. |
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