Development of creep resistant Mg-Gd-Sc alloys with low Sc content

High thermal stability and good mechanical properties are crucial for the wider future application of magnesium alloys. One of the most promising directions is the alloying of Mg with rare earth elements as Gd. The fine dispersion of metastable beta' phase (c-base centred orthorhombic, a = 0.641 nm, b = 2.223 nm, c = 0.521 nm), already known from commercially successful WE alloys (Mg-Y-Nd-Zr), precipitated in all three possible orientation modes during T6 treatment causes very pronounced age hardening in binary Mg-Gd system and inhibits very effectively the dislocation motion during the creep. The stable beta phase Mg5Gd, f.c.c. structure, a = 2.234 nm ensures the creep resistance comparable to WE alloys. A high content of Gd (above 10 wt.%) is necessary to attain the required microstructure. The addition of Se (below 1 wt.%) and Mn (about 1.5 wt.%) suppresses the solubility of Gd in Mg considerably. The complex precipitation process involving the precipitation of very stable Mn2Sc, Mn and Gd containing phase and metastable beta' phase is responsible for superior creep properties of MgGd5Sc0.3Mn1 alloy at elevated temperatures. Even at 300 degreesC the creep resistance is markedly better than for WE43 alloy. The increased Gd and Sc contents in MgGd10Sc0.8Mn1 alloy do not further improve the creep resistance.