Development of Al-Mn-Mg 3xxx Alloys for Applications at Elevated Temperature

The general objective of the present study is to develop a new aluminum wrought alloy which can be fabricated by conventional ingot metallurgy route for elevated-temperature applications (250°C-350°C). Al-Mn-Mg 3xxx alloys were chosen to be the base alloy. In order to improve the elevated-temperature mechanical properties, the compositions of materials need to be optimized. The influence of Mg, Si, Sc, Zr and Cu elements on the microstructure and mechanical properties at both ambient and elevated temperatures were investigated. Moreover, the nucleation mechanism of alpha-Al(MnFe)Si dispersoids was studied.;In this study, transmission electron microscope, scanning electron microscope and optical microscope equipped with an image analysis system were used to observe and quantitatively analyze the material microstructure. The mechanical properties at ambient temperature were evaluated by Vickers micro-hardness measurements and compression yield strength tests. The elevated-temperature mechanical properties as well as the creep properties were measured by compression yield strength tests and creep tests at elevated temperature. The results obtained were divided into following four parts.;In the first part, the effects of magnesium and silicon addition on microstructure, elevated-temperature yield strength and creep resistance of Al-Mn-Mg 3xxx alloys were investigated. Results revealed that both magnesium and silicon had an important influence on the distribution and volume fraction of precipitated dispersoids in 3xxx alloys. Without Mg or Si addition, dispersoids could hardly form during the precipitation heat treatment; hence, the alloys free of Mg or Si possessed low yield strength and creep resistance at elevated temperature. A significant improvement in elevated-temperature yield strength and creep resistance was obtained over a wide range of Mg (0.5-1.5 wt%) and Si (0.25-1 wt%) content studied due to the precipitation of a large number of dispersoids. The best combination of yield strength and creep resistance at 300 °C was obtained by the alloy containing 1.0 wt% Mg and 0.25 wt% Si with the maximum volume fraction of dispersoids and the minimum volume fraction of dispersoid free zone.;In the second part, the effect of metastable Mg2Si and dislocations on the formation of alpha-Al(MnFe) Sidispersoids were studied by a close examination of the dispersoid precipitation process using the quench technique and TEM observation. Special attentions were paid on the nucleation mechanisms. Mg plays an important role in promoting the formation of alpha-Al (Mn,Fe)Si dispersoids. The number density and volume fraction of dispersoids in the Mg containing alloy are much higher than that in the Mg-free control, resulting in a strong dispersoid strengthening effect. During heating process in the Mg containing alloy, metastable Mg2Si precipitated and dissolved, leaving local Si-rich areas on pervious metastable Mg2Si, which provide favorable nucleation sites for alpha-Al (Mn,Fe)Si dispersoids. It is found that beta'-Mg2Si precipitates were more effective on the promotion of the dispersoid nucleation than beta''-Mg 2Si. In the deformed sample, the dislocations become the preferable sites for the alpha-Al (Mn,Fe) Si dispersoid nucleation. By reducing dispersoid free zones, the dispersoid distribution became more uniform compared to the non-deformed sample. The dispersoid nucleation mechanisms based on both metastable Mg2Si and dislocations are proposed and discussed.;In the third part, Sc and Zr were added in Al-Mn-Mg 3004 alloy to form two populations of strengthening particles (50-70 nm alpha-Al (Mn,Fe)Si dispersoids and 6-8 nm Al3(Sc,Zr)precipitates) and their strengthening effects on mechanical properties and creep resistance at ambient and elevated temperatures were studied. Results showed that the microhardness and yield strength at ambient temperature greatly increased due to the Sc and Zr addition. The creep resistance at 300 °C significantly improved due to the precipitation of fine Al3 (Sc,Zr). However, the yield strength at 300 °C did not change with increasing Sc and Zr contents. The combined effects of alpha-Al (Mn,Fe)Si dispersoids and Al3(Sc,Zr)precipitates on the yield strengths at 25 °C and 300 °C were quantitatively analyzed based on the Orowan bypass mechanism and the dislocation climb mechanism.;In the fourth part, the effect of Cu addition on the dispersoid precipitation, mechanical properties and creep resistance were investigated. Cu addition promotes the dispersoid precipitation by increasing the number density and decreasing the size of dispersoids. Metastable Q-AlCuMgSi and beta'-Mg2Si precipitates were observed during heating process and both can provide favorable nucleation sites for dispersoids. The addition of Cu improves the thermal stability of dispersoids during a long-term thermal holding at 350 ºC for 500 h. Results of mechanical testing show that the addition of Cu significantly improves the hardness at ambient temperature as well as yield strength and creep resistance at 300 ºC, which is mainly attributed to the dispersoids strengthening and Cu solid solution strengthening. (Abstract shortened by ProQuest.).