XPS and Electron Microscopy Study of Oxide-Scale Evolution on Ignition Resistant Mg-3Ca Alloy at Low and High Heating Rates

Abstract

Earlier work by the authors suggested that the formation of molten eutectic regions in Mg-Ca binary alloys caused a discrepancy in ignition temperature when different heating rates are used. This effect was observed for alloys where Ca content is greater than 1 wt%. In this work, the effect of two heating rates (25 °C/min and 45 °C/min) on the ignition resistance of Mg-3Ca is evaluated in terms of oxide growth using X-ray Photoelectron Spectroscopy. It is found that the molten eutectic regions develop a thin oxide scale of ~100 nm rich in Ca at either heating rate. The results prove that under the high heating rate, solid intermetallics are oxidized forming CaO nodules at the metal/oxide interface that eventually contribute to the formation of a thick and non-protective oxide scale in the liquid state.

1. Introduction

The development of ignition-resistant magnesium (Mg) alloys for cabin components of commercial planes has gained a renewed interest since their ban was lifted in an update of the SAE aerospace standard AS8049C (Gwynne, 2015) in 2014. The use of Ca to increase ignition resistance has been particularly preferred for both commercial and experimental alloys (Sakamoto et al., 1997; Shih et al., 2004; Tekumalla & Gupta, 2017; Villegas-Armenta & Pekguleryuz, 2020). However, some underlying mechanisms such as the sensitivity of ignition temperature (T_i) to heating rate and the effect of microstructure evolution have been missing. The main factor driving ignition resistance, is the slowing down of Mg oxidation, which increases the surface temperature drastically and leads to Mg vaporization and ignition. An earlier study by the authors (Villegas-Armenta et al., 2020) has found that the T_i of Mg-2wt%Ca and Mg-3wt%Ca alloys were affected by heating rate, with little to no effect on pure Mg and Mg-1wt%Ca. The results showed that, unlike observations on biomass materials (Cai et al., 2018; Huang et al., 2018; Mishra & Mohanty, 2018), where a low heating rate leads to longer oxidation times and low ignition temperature, the low heating rate in binary Mg-2wt%Ca and Mg-3wt%Ca alloys leads to a high ignition temperature. The longer time elapsed allowed the formation of molten eutectic pools covering the alloy surface, which are resistant to oxidation. A high heating rate quickly oxidizes Ca, which forms a protective scale at first, but becomes crack-prone upon thickening and leads to extensive Mg oxidation, vaporization and early ignition. In this paper, we present a complementary analysis of the oxide scale growth on a Mg-3Ca alloy during continuous heating. Two heating rates, which exhibited very drastic differences in ignition behavior, were studied.

2. Methods

A Mg-3Ca binary alloy was synthesized by permanent mold cast as per the method presented in (Villegas-Armenta et al., 2020). Likewise, details of sample preparation and the interrupted continuous heating tests were presented in the same work (Villegas-Armenta et al., 2020). Two different continuous heating rates were used for the interrupted tests: 25 °C/min (Low Heating Rate – LHR) and 45 °C/min (High Heating Rate – HHR). The tests were interrupted at 350 (solid), 560 (semi-solid) and 700 (liquid) °C. The chemical composition of the Mg-3Ca alloy was determined by the NADCAP certified laboratory Genitest Inc via Inductively Coupled Plasma – Atomic Emission Spectrometry (ICP-AES). The resulting composition was (in wt%): 2.98 Ca, 0.005 Mn, 0.006 Al, 0.003 (Cu, Ni, Si) and balance Mg. Characterization was carried out using a SU3500 Hitachi Scanning Electron Microscope (SEM), a Thermo Scientific K-Alpha X-ray Photoelectron Spectroscopy (XPS) and a FEI Tecnai G² F20 200 kV Cryo-Scanning Transmission Electron Microscope (TEM). TEM samples were extracted using a FIB-SEM Helios Nanolab 660 DualBeam. Additionally, the software CrysTBox (Klinger, 2017) was used as an auxiliary tool to define the diffraction patterns.

3. Results

Fig. 1 presents the heating curves Mg-3Ca samples obtained at HHR and LHR. The T_i for Mg-3Ca alloy measured previously by the authors (Villegas-Armenta et al., 2020) is also indicated in Fig. 1 for both heating rates. Fig. 2 shows the oxide scale evolution analyzed though XPS at each interrupted test. The vertical red dotted line describes the etch level at which the Mg metallic signal in the Mg KLL region was detected. This is considered to be the metal/oxide interface. Fig. 3 shows the surface of both HHR and LHR samples at 560 °C. At this stage (semi-solid state) samples start displaying significant microstructural differences. The LHR sample show a widespread covering of molten and re-solidified eutectic phases while the HHR shows a more restricted covering along with internally oxidized CaO-rich nodules coming from remaining solid intermetallics. The small micrographs on the upper right corner of each image show the cross sections extracted through FIB that were analyzed using TEM. In Fig. 4, the SAED patterns confirmed that the re-solidified intermetallics are in fact Mg₂Ca using either heating rate, while the oxide scale over them is thin (80 to 110 nm) and rich in CaO with apparently no MgO being formed.

Fig. 1. Ignition curves of the Mg-3Ca alloy heated using a HHR and a LHR. The liquidus and solidus temperatures are indicated as thin dotted lines. The interrupted ignition test lines are indicated as thick dotted lines.

Fig. 2. XPS surface analysis of the HHR and LHR. Samples interrupted at 350, 560 and 700 °C. The red dotted line represents the approximate position of the metal/oxide interface.

Fig. 3. SEM micrographs of the sample surface in the semi-solid state (560 °C) (a) LHR; (b) HHR. (c) EDS line scan of one of the oxide nodules observed in the HHR sample.

Fig. 4. TEM bright field images of FIB-extracted cross-sections for (a) LHR and (b) HHR. SAED pattern (left) of the re-solidified phase and EDS map (right) of the oxidized surface.

XPS results indicate that Ca tends to be higher at the metal/oxide interface for both heating rates with a similar approximate thickness. For the HHR however, the Ca content increases significantly in the semi-solid and liquid stages, confirming the previously exposed theory of profuse Ca oxidation being the driving force for early ignition (Villegas-Armenta et al., 2020). The re-solidified regions in the semi-solid state have the same characteristics regardless of the heating rate used. CaO nodules are formed from the solid intermetallics that oxidize in the HHR sample. In the LHR sample, the slower heating allows the partial melting of the interdendritic regions instead of their localized oxidation.

4. Conclusions

The present work confirmed the observations made previously by the authors; using XPS surface analysis to study the oxide scale growth of Mg-3Ca alloy during ignition testing indicate that Ca concentrates at the metal/oxide interface. Despite both samples having similar characteristics regarding oxide characteristics and re-solidified phases in the semi-solid state, the internally oxidized CaO nodules in the HHR sample promote the formation of a thick oxide scale prone to cracking once the liquid state is reached.