Dear Colleagues,

Additive manufacturing (AM) enables the fabrication of complex, near-net shape components with high geometric freedom because of the layer-by-layer build-up. Metal AM technologies have found their way into industry, and are still attracting growing research interest. Key challenges remain with controlling the metal AM processes characterized by extremely high cooling rates (>10⁴ K/s) and directional heat extraction via underlying material which then experiences a repetitive heat treatment. Thus, the evolution of metastable microstructures is kinetically favored. Crystalline phases form during metal AM processing although they are not thermodynamically stable, while the formation of stable crystalline phases can be suppressed. In the most extreme case, the supercooled liquid does not crystallize and instead vitrifies, resulting in the synthesis of metallic glass. Crystal growth is strongly affected by the diverse processing conditions and complex thermal cycles, resulting in peculiar microstructural features and defects that influence the mechanical properties of the resulting component. To be more precise, additively manufactured metals can show elongated grains, cellular solidification structures eventually demarcated by a network with high dislocation density and the segregation of elements. The evolution of anisotropic microstructures can be effectively hindered by the incorporation of high-melting ceramic nucleants, as is done by processing of powder blends with a reinforcing phase.

Although there has been a vast number of studies published in this field, in recent years, new metastable materials fabricated by different metal AM technologies have continued to emerge. Potential topics of the present Special Issue include, but are not limited to, the AM of Al, Co, and Fe-based alloys, high-entropy alloys, as well as bulk metallic glasses and composites fabricated by processing respective powder blends. In addition to the characterization of the metastable microstructure, we seek a deeper understanding of the relationship between processing, microstructure, and properties.

Dr. Konrad Kosiba
Guest Editor

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• additive manufacturing
• laser powder bed fusion
• laser metal deposition
• wire arc additive manufacturing
• metastable phase formation
• metal matrix composites
• bulk metallic glass