Formation of endogenous MgO and MgAl2O4particles and their possibility of acting as substrate for heterogeneous nucleation of aluminum grains

Keehyun Kim

doi:10.1002/sia.5726

Abstract

Aluminum containing 4 wt.% magnesium was oxidized at a temperature for different oxidation times and analyzed by high-resolution electron microscopy. A thin oxidized layer of about 5 µm, which is composed of MgO, forms at short oxidation time and gradually increases. High-resolution microstructures reveal that the oxidized layers are porous regardless of oxidation time. After extended oxidation time, discrete MgAl2O4 particles formed as a result of the reaction of initially formed MgO, liquid aluminum, and oxygen introduced from air through the porous MgO. Furthermore, it is clear by high-resolution lattice images that MgAl2O4 particles are covered with thin Al2O3, whereas MgO is bonded intimately to aluminum. Therefore, MgAl2O4 particles that form naturally during oxidation are difficult to act as a direct substrate for nucleation of aluminum grains because of the coverage of Al2O3. In contrast, MgO shows the possibility of acting as a substrate for the aluminum nucleation. The formation mechanism of MgO and MgAl2O4 and their possibility of acting as substrates for nucleation of aluminum grains suggest that atomic level bonding and mismatches of nucleant/nucleus metal should be considered for correct evaluation of the possibility of heterogeneous nucleation of metallic matrix on a potent nucleant. © 2015 The Authors. Surface and Interface Analysis Published by John Wiley & Sons, Ltd.

Highlights

Wetting between reinforcements and metallic matrix is the most important requirement to obtain uniform dispersion of reinforcements
In order to know the morphology of as-oxidized sample for different oxidation times, the surfaces were observed at the same magnification of 1000×
magnesium oxide (MgO) formed initially near the surface of aluminum–magnesium melt, and they were covered with aluminum

Summary

Introduction

Wetting between reinforcements and metallic matrix is the most important requirement to obtain uniform dispersion of reinforcements. MgAl2O4 with the density of 3.65 g/cm exhibits a congruent melting point at 2315 °C in the MgO-Al2O3 phase diagram and shows low coefficient of thermal expansion, good thermal shock resistance, and high electrical resistivity.[21,22,23] The lattice parameter for the close-packed oxygen anion sublattice of the MgAl2O4 unit cell is nearly equal to that of aluminum unit cell with the same face-centered cubic structure. This similar crystal structure can form low energy interfaces for any orientation. The lattice mismatch of the MgAl2O4 (400) and aluminum (200) is about 0.2%, which is one of the lowest among all reinforcements in aluminum matrix.[21,24,25] several studies suggested that a stable oxide that can act as the substrate for heterogeneous nucleation of aluminum grains is probably MgAl2O4.[17,20] it was shown that MgO (not MgAl2O4) can act as the substrate for School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, West Midlands, B15 2TT, UK

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Discussion

Conclusion