Abstract
Analytical electron microscopy, supported by microchemical and microdiffraction techniques, was applied to assess the microstructural changes accompanying semisolid molding of a creep-resistant magnesium alloy with nominal chemistry of Mg–5%Al–2%Sr. In an ingot precursor, the Sr containing precipitates exhibited a morphology of Al 4Sr platelets within eutectic colonies and Al–Mg–Sr films, both residing at grain/cell boundaries, while sub-boundaries were occupied by fine Mg 17Sr 2 plates. Trace quantities of Mg 17Al 12 were present as plate-shaped continuous precipitates within selected grain/cell interiors. Semisolid processing of mechanically comminuted ingots generated thixotropic morphologies with globular solids of α-Mg, having well-developed sub-structures. The quickly-solidified liquid fraction, enriched in Al and Sr, led to the formation of increased volumes of a lamellae type Al 4Sr phase, surrounding the secondary α-Mg. Traces of Mg 17Al 12 still existed, predominantly as irregular shape compounds located mainly in grain interiors. Both the primary solid and high melting point precipitates of Mn–Al modified with Sr, acted as nucleation substrates for secondary α-Mg grains. The possible implications of semisolid processing on the creep behavior of magnesium alloys are discussed.
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