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Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

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HEAs’ compositional space allows the design of hierarchical microstructures that contribute to the exceptional strength values.

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Progress in materials science through thermodynamic modelling may rest crucially on access to a database, such as that developed by Scientific Group Thermodata Europe (SGTE) around 1990. It gives the Gibbs energy Gfalse(Tfalse) of the elements in the form of series as a function of temperature, i.e. essentially a curve fitting to experimental data. In the light of progress in theoretical understanding and first‐principles calculation methods, the possibility for an improved database description of the thermodynamics of the elements has become evident. It is the purpose of this paper to provide a framework for such work. Lattice vibrations, which usually give the major contribution to Gfalse(Tfalse), are treated in some detail with a discussion of neutron scattering studies of anharmonicity in aluminium, first‐principles calculations including ab initio molecular dynamics (AIMD), and the strength and weakness of analytic model representations of data. Similarly, electronic contributions to Gfalse(Tfalse) are treated on the basis of the density of states Nfalse(Efalse) for metals, with emphasis on effects at high T. Further, we consider Gfalse(Tfalse) below 300 K, which is not covered by SGTE. Other parts in the paper discuss metastable and dynamically unstable lattices, Gfalse(Tfalse) in the region of superheated solids and the requirement on a database in the calculation of phase diagrams.

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