Effect of secondary phase particles on thermal stability of ultra-fine grained Mg-4Y-3RE alloy prepared by equal channel angular pressing

As-cast magnesium alloy WE43, containing yttrium and rare earth elements, was processed by equal channel angular pressing (ECAP). The processing led to a significant grain refinement together with a massive precipitation of the secondary phase particles. Thermal stability of the ultra-fine grain (UFG) structure together with microstructural changes due to exposure to elevated temperatures were studied by several complementary techniques in the temperature range of 160-500 degrees C. It was found that UFG structure consisting of grains with size of similar to 340 nm and high density of Mg5RE particles is stable up to 280 degrees C for 1 h of annealing. Moreover, only negligible change of the microstructure occurred after annealing for 16 hat 250 degrees C. Excellent thermal stability of UFG structure was caused by fine Mg5RE particles, which suppressed the grain growth. Exceeding the limit of thermal stability of these particles above 280 degrees C resulted in material softening. Moreover, statistically significant hardening of the UFG material occurred in the temperature range of 200-280 degrees C. Segregation of yttrium and rare earth elements and eventually precipitation at grain boundaries was proved to be responsible for observed hardening by in-situ and ex-situ transmission electron microscope and positron annihilation spectroscopy analysis. Finally, individual effect of particles dissolution and grain growth on the material softening was investigated and discussed.