ABSTRACT
Similar features and their successive change with increasing atomic number of the rare
earth metals are revealed in the dependence of solid solution decomposition on ageing
temperature. The main action of the ageing temperature is acceleration of solid solution
decomposition with increasing temperature of ageing and delay of the solid solution
decomposition when it lowers. This action is specific in different M g - R E systems. In
alloys with rare earth metals of the cerium subgroup, only the acceleration of solid solution
decomposition was revealed. Its magnitude may be seen in Figure 74, where dependence
of the hardness and electrical resistivity on ageing time are presented for the alloys M g - 2 . 3
mass % Pr [272], M g - 3 . 4 mass % N d [287] and M g - 5 . 6 5 mass % Sm [265]. A l l the alloys
were in the form of extruded rods. They were solution treated by quenching in cold water
from temperatures slightly below eutectic ones and then aged isothermally at different
temperatures. A s one can see in Figure 74a, where results for the alloy M g - 2 . 3 mass % Pr
are reproduced there is shift of the hardness curves in the direction to the less ageing time
with increasing ageing temperatures from 150 to 250°C. In the limits of the ageing
exposures used (0 .5-200 h) the hardness maximum is revealed for ageing at 175, 200 and
225°C. A t 150°C it is not reached, although it may be quite near. A t 250°C the hardness
maximum seems to be overpassed after the least used ageing time of 0.5 h, yet it exists.
