Synthesis of Al₂Ca Dispersoids by Powder Metallurgy Using a Mg-Al Alloy and CaO Particles.

Junji Fujita,Junko Umeda,Katsuyoshi Kondoh

doi:10.3390/ma10070716

Abstract

The elemental mixture of Mg-6 wt %Al-1 wt %Zn-0.3 wt %Mn (AZ61B) alloy powder and CaO particles was consolidated by an equal-channel angular bulk mechanical alloying (ECABMA) process to form a composite precursor. Subsequently, the precursor was subjected to a heat treatment to synthesize fine Al2Ca particles via a solid-state reaction between the Mg–Al matrix and CaO additives. Scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS) and electron probe micro-analysis on the precursor indicated that 4.7-at % Al atoms formed a supersaturated solid solution in the α-Mg matrix. Transmission electron microscopy-EDS and X-ray diffraction analyses on the AZ61B composite precursor with 10-vol % CaO particles obtained by heat treatment confirmed that CaO additives were thermally decomposed in the Mg–Al alloy, and the solid-soluted Ca atoms diffused along the α-Mg grain boundaries. Al atoms also diffused to the grain boundaries because of attraction to the Ca atoms resulting from a strong reactivity between Al and Ca. As a result, needle-like (Mg,Al)2Ca intermetallics were formed as intermediate precipitates in the initial reaction stage during the heat treatment. Finally, the precipitates were transformed into spherical Al2Ca particles by the substitution of Al atoms for Mg atoms in (Mg,Al)2Ca after a long heat treatment.

Highlights

Magnesium alloys are remarkably lightweight due to the low density of Mg (~1.74 g/cm3 ).Their application to structural components in automobiles can improve fuel consumption [1,2,3,4].In particular, the weight reduction of engine blocks and transmission cases used at elevated temperatures (120–200 ◦ C) is very important [4,5,6]
This is because these network-structured compounds prevented both the deformation of α-Mg grains and grain boundary sliding at elevated temperatures
Scanning electron microscopy-energy-dispersive spectroscopy (SEM-energy-dispersive spectrometry (EDS)) and EPMA were applied to investigate the distribution of Al atoms in the AZ61B powder precursor

Summary

Introduction

Magnesium alloys are remarkably lightweight due to the low density of Mg (~1.74 g/cm3 ).Their application to structural components in automobiles can improve fuel consumption [1,2,3,4].In particular, the weight reduction of engine blocks and transmission cases used at elevated temperatures (120–200 ◦ C) is very important [4,5,6]. The tensile strength of Mg alloys drastically decreases at elevated temperatures This prevents their widespread use in the automotive industry [5,6,7]. Previous studies have reported that the addition of calcium to Mg–Al alloys improved the creep resistance at 200 ◦ C by the formation of Al2 Ca or (Mg,Al) Ca intermetallic compounds around the α-Mg grain boundaries [8,9,10]. This is because these network-structured compounds prevented both the deformation of α-Mg grains and grain boundary sliding at elevated temperatures. Fine dispersed particles of intermetallic compounds are more effective to improve the yield stress of Mg alloys via pinning effects

Methods

Results

Conclusion