The role of lattice defects, element partitioning and intrinsic heat effects on the microstructure in selective laser melted Ti-6Al-4V

The microstructure and phase composition in selective laser melted (SLM) Ti-6Al-4V plays a key role for its mechanical performance. The microstructure evolution in SLM Ti-6Al-4V was studied in the as-built condition and after sub-transus heat treatments between 400 degrees C and 800 degrees C focusing on elemental partitioning and the role of lattice defects on precipitation of the beta phase. With SLM parameters corresponding to low volume energy density (E-V = 77 J/mm(3)) the as-built microstructure consisted of acicular martensite and showed a higher density of lattice defects than that synthesized under high E-V = 145 J/mm(3) condition. High energy X-ray synchrotron diffraction indicated the presence of similar to 2 wt.% beta-phase at this high E-V. Moreover, atom-probe tomography revealed enrichments in beta-stabilizers at one-and two-dimensional lattice defects. These fine enriched one-dimensional columnar and two-dimensional features are identified as precursors of beta-phase, revealing the role of lattice defects for beta-precipitation. Upon annealing at 400 degrees C and 530 degrees C, beta-films began to fragment into beta-platelets and nanoparticles, whereas annealing at 800 degrees C led to a coarse-lamellar alpha/beta-microstructure. Moreover, alpha(2)-Ti3Al was found in the 400 degrees C annealed condition. In line with the microstructure changes, Vickers hardness increased upon annealing at temperatures up to 530 degrees C and dropped when coarsening occurred at higher temperatures. Substantial element partitioning occurred during thermally driven martensite decomposition, which was significantly stronger for Fe than for V. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.