Crystallographic orientation relationships between the aggregated intermetallic phases in a casting Mg-Ag-Al alloy

https://doi.org/10.1016/j.matdes.2020.108561Get rights and content
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Highlights

  • Both binary and ternary intermetallic phases were observed in Mg-Ag-Al alloy.

  • Three aggregated intermetallic phases are semi-coherent with each other.

  • Two orientation relationships between ε-Mg3Ag and γ-Mg17Al12 were revealed.

  • Mg-6Ag-1Al alloy has high-strength/ductility due to aggregated intermetallic phases.

Abstract

Aggregated eutectic intermetallic phases in a casting Mg-Ag-Al alloy were characterized using transmission electron microscopy. Three kinds of intermetallic phases, namely ε-Mg3Ag (or ε-Mg26xAg7+x), β′-Mg2(Al,Ag)3, and γ-Mg17Al12, were observed to aggregate together following certain crystallographic orientation relationships (ORs) as: (001)ε//(11¯2)γ and [100]ε//[110]γ (OR1), (001)ε//(110)γ and [001]ε//[110]γ (OR2), (011¯)β//(1¯10)γ and [100]β//[110]γ, and (001)β//(010)ε and [100]β//[100]ε. TEM observations combined with the corresponding phase diagrams reveal that the ε phase firstly formed during solidification, then the γ phase nucleated on the ε phase, and finally, the β′ phase nucleated on the γ phase. This special intermetallic phase composition which owns numerous fine particles and many semi-coherent interphase boundaries results in high-strength and high-ductility for the casting QA61 alloy.

Keywords

Magnesium alloys
Transmission electron microscopy (TEM)
Intermetallic phase
Orientation relationship
Formation mechanism

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