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

Abstract

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 °C/min and a high heating rate (HHR) of 45 °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 °C at 0 wt% Ca to 861 °C at 3 wt% Ca. For HHR samples, the increase in ignition temperature was lower, from 658 to 787 °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 ΔG (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.