Abstract
The MgYNdScMn alloy was squeeze-cast under a protective atmosphere. Precipitation reactions were studied by differential scanning calorimetry.
Additionally the electrical resistometry and hardness measurements were performed. Transmission electron microscopy of specimens quenched from temperatures of significant thermal changes were used to identify microstructural processes responsible for these changes.
The measurements revealed three exothermic effects connected to complex precipitation processes. No direct evidence was observed for the first effect.
As this early stage is associated with small resistivity and hardness changes, we suppose a submicroscopic formation of a transient beta'' phase there. Precipitation of the transient beta' phase in the form of fine plates was observed at 270 oC.
A weak thermal reaction corresponding to this process was detected only for heating rates of 1 and 5 K/min. The I' phase subsequently transforms into the stable I phase.
Discs of the Mn2Sc phase parallel to basal planes and thin basal Mn-rare earth plates of a transient hexagonal phase were observed simultaneously after annealing up to 390 oC. The main resistivity decrease in the range 270 - 390 oC is caused by precipitation of these three phases.
The apparent activation energy for the early stage, Q = (119 +- 10) kJ.mol-1, and for I phase, Q = (147 +- 10) kJ/mol, corresponds excellently to the results obtained in our previous study in ternary MgYNd and WE43 alloys, respectively.