PROPERTIES OF SQEEZE CAST MgZnAl AND MGZNALCA ALLOYS WITH ICOSAHEDRAL PHASE DURING ANNEALING WITH CONSTANT HEATING RATES
Abstrakt
The Mg-11.5wt.%Zn-3.0wt.%Al and Mg-5.1wt.%Zn-3.3wt.%Al-0.1wt.%Ca alloys were squeeze cast under a protective gas atmosphere (Ar + 1% SF6). Precipitation reactions were studied by differential scanning calorimetry at heating rates of 0.5 - 30 K/min.
Electrical resistometry at 78 K and microhardness (HV0.5) at room temperature were performed additionally. The specimens were subjected to isochronal annealing with steps of 20 K/20 min up to 300 oC.
The thermal measurements revealed two exothermic effects during linear heat treatment in the temperature range of 100 - 250 oC at heating rates of 2 - 30 K/min. In agreement to the thermal response, two stages of electrical resistivity decrease were observed in the same temperature range.
The lower thermal and absolute resistivity changes were observed in the alloy with Ca-addition. Activation energies obtained from the thermal measurements using the Kissinger method for the two mentioned processes were calculated as Q1 = (124 +/- 17) kJ/mol and Q2 = (133 +/- 4) kJ/mol in both alloys.