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Additively manufactured (AM) metals exhibit highly complex microstructures,particularly with respect to grain morphology which typically featuresheterogeneous grain size distribution, anomalous and anisotropic grain shapes,and the so-called columnar grains. In general, the conventional morphologicaldescriptors are not suitable to represent complex and anisotropic grainmorphology of AM microstructures. The principal aspect of microstructural grainmorphology is the state of grain boundary spacing or grain size whose effect onthe mechanical response is known to be crucial. In this paper, we formallyintroduce the notions of axial grain size and grain size anisotropy as robustmorphological descriptors which can concisely represent highly complex grainmorphologies. We instantiated a discrete sample of polycrystalline aggregate asa representative volume element (RVE) which has random crystallographicorientation and misorientation distributions. However, the instantiated RVEincorporates the typical morphological features of AM microstructures includingdistinctive grain size heterogeneity and anisotropic grain size owing to itspronounced columnar grain morphology. We ensured that any anisotropy arising inthe macroscopic mechanical response of the instantiated sample is mainlyassociated with its underlying anisotropic grain size. The RVE was then usedfor meso-scale full-field crystal plasticity simulations corresponding touniaxial tensile deformation along different axes via a spectral solver and aphysics-based crystal plasticity constitutive model. Through the numericalanalyses, we were able to isolate the contribution of anisotropic grain size tothe anisotropy in the mechanical response of polycrystalline aggregates,particularly those with the characteristic complex grain morphology of AMmetals. Such a contribution can be described by an inverse square relation.

journal of materials science and technology/journal of materials science and technology

The anisotropic grain size effect on the mechanical response of  polycrystals: The role of columnar grain morphology in additively  manufactured metals