The possibility of partial melting and its relations to the superplasticity at high strain rates were studied with transmission electron microscopy and differential scanning calorimetry in AlCuMg (2124), AlMg (5052), and AlMgSi (6061) alloys reinforced with Si3N4 particles. Calorimetry measurements of all three composites showed a sharp endothermic peak at an optimum superplastic temperature. At the same temperature, transmission electron microscopy showed the melting of grain boundaries and interfaces, suggesting direct correlations between partial melting and the superplasticity. Solute segregation was also observed at boundaries and interfaces, and was discussed as causes for partial melting.
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