The effect of manganese additions on the structure changes proceeding in rapidly solidified Al‐Cu alloys

The effect of Mn additions (0… 2 wt%) on the decomposition of rapidly solidified Al‐4.0 wt% Cu alloys (cooling rate 10 3 to 10 4 K/s; LQ treatment) were studied during ageing between RT and 450 °C by hardness, X‐ray methods and electron microscopy. The results were compared with alloys homogenized in the region of the solid solution (SQ treatment). (i) The LQ treatment results in a quite better homogeneous distribution of the alloyed elements than the SQ one, that is less particles of intermetallic phases are present in the ascast state. (ii) At T < 250 °C Mn additions affect the decomposition kinetics by trapping of vacancies (retardation) and the diminution of the solubility of Cu atoms (acceleration). The first effect dominates in the stage of G.P. zone formation, the second one during precipitation of intermediate phases. (iii) At T ≧ 300 °C the intermetallic compound Cu 2 Mn 3 Al 20 forms associated with a significant increase of the hardness.