Effects of manganese on microstructure and mechanical properties of A206 alloys containing iron
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The effects of Mn and Fe contents on the mechanical properties of aluminum-based A206 alloys were investigated quantitatively. Results showed that the addition of Fe caused a loss in both ductility and yield strength. Further addition of Mn could recover the ductility, but it caused a further loss in yield strength. In low-Mn alloys (0.29 wt% Mn) the primary constituent was the needle shape of Cu2FeAl7. Upon further addition of Mn, the Chinese script configuration of Mn-bearing particles formed instead. The Cu2Mn3Al20 particles formed in high-Mn alloys during solution treatment and resulted in grain-growth inhibition. The needle, Mn-bearing, and Cu2Mn3Al20 particles caused the solid solution level of copper in the matrix to decrease; meanwhile, increasing the Mn solution level retarded the precipitation of the strengthening phase. Differential scanning calorimetry analyses showed the kinetics and amount of decrease in ⬘ phase precipitation when the contents of Fe and/or Mn were increased. The smaller grain size induced by the Cu2Mn3Al20 particles and the ⬘ phase were the factors that determined the hardness of A206 alloys under as-quenched and T7-treated conditions, respectively. I. INTRODUCTION
The precipitation-hardened A206 Al–Cu alloy is widely used in the aerospace and military industries.1,2 Its solidification structure mainly consists of a network Al–CuAl2 eutectic phase at the ␣–Al interdendrites; sometimes CuMgAl2 and other complex eutectics can be found depending on the cooling rate.3 It is well known that the optimal strength and ductility can be obtained by precipitating large amounts of the ⬘ phase through T6 heat treatment, but to increase the stress corrosion resistance of A206 alloy, a T4 or T7 heat treatment is used instead of T6.4 After precipitation, the precipitated phases may include the major strengthening phase of CuAl2 intermediate phase (⬘ phase) and a little of the CuMgAl2 intermediate phase (S⬘ phase). Differential scanning calorimetry (DSC) analysis has been used to investigate the precipitation kinetics of these precipitates.5–10 The sequence of precipitation and the amount of precipitates forming at each stage during nonisothermal aging can also be obtained from this analysis. Iron is an important impurity generally found in commercial aluminum alloys and will form intermetallic compounds easily. The various iron-containing compounds affect the mechanical behavior, corrosion protection, and surface character of the castings, depending on their compositions, morphologies, and distributions. In general, they are detrimental to the strength and ductility
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e-mail: [email protected] J. Mater. Res., Vol. 17, No. 9, Sep 2002
http://journals.cambridge.org
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of materials, especially when present as a needle shape with a sharp tip.11–14 It is well known that the richer the content of iron in aluminum, the more feasible it is to form the Fe-rich intermetallic compounds. Jacob et al.13 pointed out that the needle shape compound of Cu2FeAl7 formed in Al–5% Cu al
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