Two quite different primary Mn-rich phases in Al-Si-Cu-Mn heat-resistant alloy and its effect to mechanical properties

Two quite different primary Mn-rich phases were observed by optical microscope and scanning electron microscope in the surface and central parts of Al-12 wt%Si-4 wt%Cu-2 wt%Mn heat-resistant alloy casting. Deep-etching observation reveals that one is as highly developed 3D-dendrites and the other is as simple 2D-laths. The results of SEM-EDS show the constituents of these two Mn-rich phases are quite different too. The results of TEM-EDS verify it further. By selected area electron diffraction pattern, the lath-shaped Mn-rich phase is identified as: Al13Mn4Si8, simple tetragonal, a = b = 1.240 nm, c = 0.489 nm. The tension test indicates the contribution to strength from dendritic Al15Mn3Si2 phase is larger than Al13Mn4Si8 phase whether at room temperature or at high temperature. The reason for it is thought to be tied with the much larger difference in elastic modulus of Al13Mn4Si8 phase with aluminum matrix than that of Al15Mn3Si2.