The Ignition Behavior of Mg-Ca Binary Alloys: The Role of Heating Rate

The ignition temperature of Mg-(1-3) wt% Ca binary alloys has been tested at two different heating rates: a low heating rate (LHR) of 25 degrees C/min and a high heating rate (HHR) of 45 degrees C/min under a constant flow of extra dry air at a rate of 0.9 L/min. At both heating rates, the ignition temperature increased with Ca in the alloy. For LHR sample, the ignition temperature increased from 655 degrees C at 0 wt% Ca to 861 degrees C at 3 wt% Ca. For HHR samples, the increase in ignition temperature was lower, from 658 to 787 degrees C as Ca increased from 0 to 3 wt%. It was observed that Ca-rich interdendritic regions, when molten, resist oxidation more than when they are solid, an effect that can be attributed to the Delta G (Gibbs energy change) of CaO formation from the molten and solid phases. Using a LHR results in a smooth oxide scale and a higher ignition temperature, which is related to the formation of extensive molten pools, while a HHR yielded an irregular oxidation and lower ignition temperature due to the early oxidation of solid intermetallics.