Comparative study of dynamic impact properties and microstructures of weldable and unweldable aluminium–scandium (Al–Sc) alloys

The dynamic deformation behaviours of two Al–Sc alloys, one weldable and one unweldable, are investigated at strain rates ranging from 1·2 × 10³ to 5·9 × 10³ s^–1 and temperatures of –100, 25 and 300°C respectively, using a compressive split Hopkinson pressure bar. The fracture features and microstructures of the impacted specimens are examined using scanning electron microscopy and transmission electron microscopy respectively. The stress–strain relationships indicate that for both alloys, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Moreover, the flow stress, work hardening rate and strain rate sensitivity are higher in the unweldable Al–Sc alloy than in the weldable alloy. In both alloys, the activation volume is dominated by a thermally activated mechanism and increases as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. In describing the plastic deformation behaviour of the two Al–Sc alloys using the Zerilli–Armstrong fcc constitutive model, the error between the predicted flow stress and the measured stress is found to be <5%. The scanning electron microscopy observations reveal that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of a ductile fracture mode. The dimple like structures on the fracture surfaces of the unweldable Al–Sc alloy are shallower than those on the fracture surfaces of the weldable Al–Sc alloy, which indicates that the weldable Al–Sc alloy has a better ductility than the unweldable Al–Sc alloy. The transmission electron microscopy images show that the microstructures of the two alloys contain different types of precipitates. Specifically, the unweldable Al–Sc alloy contains both Al₃Sc and Al₂Cu particles, whereas the weldable Al–Sc alloy contains Al₃Sc precipitates only. These particles prevent dislocation motion, and therefore prompt a significant strengthening effect. The transmission electron microscopy observations also reveal that in both alloys, the dislocation density increases with increasing strain rate, but decreases with increasing temperature. Furthermore, it is found that the dislocation density of the unweldable Al–Sc alloy is higher than that of the weldable Al–Sc alloy. In other words, the dislocation cells in the unweldable Al–Sc alloy are smaller than those in the weldable Al–Sc alloy. Thus, it is inferred that the unweldable Al–Sc alloy has a higher flow stress than the weldable Al–Sc alloy.