Microstructure and Phase Changes Due to Heat Treatment of Squeeze Cast Mg-Sc-(Ce)-Mn Alloys

Abstract

The time dependence of the electrical resistivity of MgScMn and MgScCeMn alloys due to isothermal annealing at 500 and 600 °C confirms the results of thermodynamic calculations that a homogenisation heat treatment (T4) and consequently a full age hardening (T6) is impossible. The peak age hardening of the as-cast material (T5) is due to the precipitation of fine dense Mn2Sc particles in both materials accompanied by the precipitation of Mg12Ce particles in Ce containing alloys. This microstructure accounts for the excellent creep resistance of these materials which is superior to the high temperature creep resistant alloy WE43. The decrease in electrical resistivity during isochronal annealing in the temperature range 300–400 °C (MgScMn) and 300–480 °C (MgScCeMn) has been shown by TEM observations to be due to the same precipitation processes. TTT diagrams drawn from peak hardness values obtained by the isothermal heat treatment also confirm the complex precipitation process in MgScCeMn alloys. Short pseudo-homogenisation heating at 600 °C preceding the isochronal annealing causes another earlier resistivity decrease at 200–300 °C in quaternary alloys corresponding to a different precipitation sequence.