Heterogeneous microstructure-mediated ductile fracture of twin-roll cast Al-Mn strip

The microstructure and mechanical properties of the twin-roll cast (TRC) and traditional direct-chill cast (DC) Al-Mn strip have been compared using electron backscattered diffraction, scanning electron microscope, transmission electron microscope, and atom-probe tomography. The TRC Al-Mn strip showed a heterogeneous microstructure and texture along the thickness direction of the strip, forming a 'hard shell-soft core' structure. This heterogeneous microstructure resulted in a mutual constraint between the surface region and the center region, enabling the TRC strip to be plastically deformed to a total elongation >30% prior to fracture. In comparison, the DC strip, with a relatively homogeneous microstructure and texture, exhibited a uniform elongation close to zero. The distinct tensile properties presented by the two strips have been rationalized in terms of the differences in their microstructural features, including grain size/orientation, dislocation density, coarse constituent particles and solute atoms. A micromechanics model has been employed to understand the superior ductility of the TRC strip.