Effect of silicon additions on structure and stability of Al-7.5wt.%Mn alloy melt-spun ribbons

Al-Mn-Si alloys offer an alternative to Al-Fe-Si as a base for development of rapidly solidified Al-base alloys for service at 0.6–0.8Tm. This study investigated the effect of Si level on the characteristics of the α-AlMnSi dispersoid which forms as a stable phase in these alloys. Al-7.5%Mn-2.7 to 9.8%Si (wt.%) alloys were studied in the form of melt-spun ribbons. X-ray diffraction (XRD) indicated the presence of α-Al, α-AlMnSi and icosahedral phases as-spun, the latter transforming to α-AlMnSi during heat treatment at 340–540 °C. Transmission electon microscopy (TEM) of as-spun material showed a decrease in average α-Al cell size from 570 to 175 nm and of average icosahedral dispersoid size from 750 to 320 nm with increase in Si content, with the intercellular α-AlMnSi decreasing in thickness from 80 to 30 nm. XRD indicated that the αa-Al lattice parameter was a maximum between 4 and 7 wt.% Si and increased to its equilibrium value within 1 h at 540 °C. Microhardness as-spun increased from 120 ± 10 to 190 ± 20 kg mm−2 with increase in Si content over the range studied, this increment of approximately 50% in hardness being maintained after 1 h heat treatments at temperatures up to 450 °C. The significance of these results on the hardening mechanisms operating in this system is discussed.