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A precipitation hardening high-entropy alloy (HEA) (FeCoNi)86Al7Ti7 was fabricated by selective laser melting (SLM) and ageing treated under different temperatures and time conditions. Yield strength of the aged HEA increases substantially from 710 to 934 and then to 1203 MPa. Theoretical analyses reveal that the coherent L12 precipitate contributes most of the improved strength for the aged HEAs, whereas recovery during ageing causes the decrease of dislocation density thus exerts a softening effect. In addition, it is found that ductility decreases with increasing volume fraction of the incoherent L21 precipitates. Based on a void growth model, the trend is qualitatively explained. Moreover, a new fracture mode, intercellular fracture, is proposed to account for the strong dependence of fracture dimple size on the dislocation cells, also directly validated by delicate microstructural examination. The findings provide an effective strengthening method and propose a unique fracture mechanism for the additively manufactured HEA.

Strength and ductility were simultaneously enhanced in the additively manufactured CoCrFeMnNi high-entropy alloy by laser powder bed fusion (LBPF) under reactive N2 atmosphere. It was found that nitrogen atoms picked up during additive manufacturing line-up to form ordered nitrogen complexes (ONCs) in the octahedral interstitial position of the HEA matrix. Dislocation multiplication is then facilitated by the formation of ONCs during LPBF, leading to a higher dislocation density with smaller dislocation cells. Dislocation strengthening, combined with interstitial strengthening, endows the additively manufactured HEA with the yielding strength of 690 MPa, 15% higher than that of the counterparts fabricated under inert atmosphere. More interestingly, the ONCs stimulate dislocation nucleation and engender more heterogeneous microstructure, giving rise to an outstanding ductility of 15.3%, with an increment of 34%. As a result, the strength–ductility trade-off was successfully reversed by the nitrogen doping during LPBF under reactive atmosphere.

Lightweight and high-strength polymer-derived SiOC ceramics with varied lattice structures have been successfully produced using different polysiloxanes as preceramic polymers (PCPs) via photopolymerisation-based digital-light-processing 3D printing and pyrolysis. Photocurable precursor resins were prepared by simple mixing of polysiloxanes with photosensitive acrylate monomers, achieving good flowability and preserving desirable stability under different heating and oscillation conditions. Complex micron-sized structures were manufactured with high precision via the optimisation of polymer formula and printing parameters. The printed PCPs pyrolysed at 600–1000°C preserved fine features with uniform shrinkage. The skeletons were almost fully dense, with smooth and flawless surfaces at macro/micro scale. Porosities and mechanical properties, including apparent compressive strength, elastic modulus, and indentation hardness, were characterised. XRD, FT-IR, Raman spectroscopy, and XPS were used to explore the chemical variations in elements and atomic bonds. High specific compressive strength to density ratios was obtained for the SiOC lattices compared with other porous ceramics.

This study investigated the effects of hot isostatic pressing (HIP) on the microstructures and mechanical properties of Ti6Al4V fabricated by electron beam melting (EBM). The differences of surface morphologies, internal defects, relative density, microstructures, textures, mechanical properties and tensile fracture between the as-built and HIPed samples were observed using various characterization methods including optical metallography microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) and tensile tests. It was found that the main effects of HIP on microstructures include—the increase of average grain size from 7.96 ± 1.21 μm to 11.34 ± 1.89 μm, the increase of α lamellar thickness from 0.71 ± 0.15 μm to 2.49 ± 1.29 μm and the increase of β phase ratio from 4.7% to 10.5% in terms of area fraction on the transversal section. The combinatorial effects including densification, increase of grain size, α lamellar thickness, β phase ratio, reduction of dislocation density and transformation of dislocation patterns contributed to the improvement of elongation and ductility of EBM-fabricated Ti6Al4V. Meanwhile, these effects also resulted in a slight reduction of the yield strength and UTS mainly due to the coarsening effect of HIP.