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Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory high-entropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ∼100 μm-thick gradient nanostructured layer is dramatically refined from the original ∼200 μm to only ∼8 nm in the top surface layer. The microhardness is therefore gradually increased from ∼240 HV in matrix to ∼650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase body-centered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich body-centered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZrHf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.

Isotropy in microstructure and mechanical properties remains a challenge for laser powder bed fusion (LPBF) processed materials due to the epitaxial growth and rapid cooling in LPBF. In this study, a high-strength TiB2/Al-Cu composite with random texture was successfully fabricated by laser powder bed fusion (LPBF) using pre-doped TiB2/Al-Cu composite powder. A series of advanced characterisation techniques, including synchrotron X-ray tomography, correlative focussed ion beam–scanning electron microscopy (FIB-SEM), scanning transmission electron microscopy (STEM), and synchrotron in situ X-ray diffraction, were applied to investigate the defects and microstructure of the as-fabricated TiB2/Al-Cu composite across multiple length scales. The study showed ultra-fine grains with an average grain size of about 0.86 μm, and a random texture was formed in the as-fabricated condition due to rapid solidification and the TiB2 particles promoting heterogeneous nucleation. The yield strength and total elongation of the as-fabricated composite were 317 MPa and 10%, respectively. The contributions of fine grains, solid solutions, dislocations, particles, and Guinier–Preston (GP) zones were calculated. Failure was found to be initiated from the largest lack-of-fusion pore, as revealed by in situ synchrotron tomography during tensile loading. In situ synchrotron diffraction was used to characterise the lattice strain evolution during tensile loading, providing important data for the development of crystal-plasticity models.

Additive manufacturing (AM) enables the processing of heterogeneous materials with customised architectures to improve strength-ductility synergy. Herein, layerwise-heterostructured high-strength steels were processed by AM of two steel powders following the designed architectures. The fabricated high-strength steels with hierarchical heterogeneous characteristics at the layer, melt-pool and grain scales, exhibit good strength-ductility combination, reaching a strength of 1.32 GPa together with an elongation of 7.5%. The increased strength attributes to hetero-deformation induced strengthening. In-situ deformation monitoring reveals many unique deformation bands in heterostructure materials, which delays necking and improves ductility. The findings demonstrate a novel potential approach to circumvent material property trade-offs. Additively manufactured heterogeneous materials with controllable architectures achieved a good strength-ductility combination; the underlying strengthening mechanism and unique deformation behaviour are revealed.

β-titanium (Ti) alloys combined with high strength and good ductility have attracted extensive research interest for use in many advanced industrial applications. In this study, laser powder bed fusion (LPBF) parameters were firstly optimised to fabricate highly dense metastable β Ti–12wt% Mo alloy with largely homogeneous structure from low-cost elemental powders. When the laser area energy density (AED) increased to 4 J/mm2 using the simple scan mode, the refractory Mo powder melted and dissolved into the Ti matrix due to the increased melting pool temperature and increased cycles of remelting. However, when the same high AED was used in the chess scanning mode, keyhole-induced defects emerged along the island boundaries. The laser beam delay (LBD) and island spacing (IS) settings were then optimised to eliminate keyhole defects. Additionally, it is found that using low and high AED (e.g. 1.6 and 4 J/mm2 respectively), the builds show significantly different microstructure and mechanical properties. In view of this, a Ti–Mo functional gradient composite (FGC) was fabricated via alternating AEDs of 1.6 and 4 J/mm2 layer-wise. A gradient distribution of α” phase with varied size and quantities across the designed layer boundaries was produced. The Ti–Mo FGC possessed a high compressive yield strength of 1173 (±15) MPa and improved strain hardening capacity. The developed approach demonstrated the potential for the fabrication of FGCs using an in situ alloying based LPBF.

Tumor-induced bone loss is the main reason causing bone tumor therapy failure. Rational design of implants with both anti-tumor and bone tissue regeneration functions is urgently needed. This study presents a 3D-printed implant that simultaneously releases anti-cancer drugs and growth factors for anti-tumor therapy and osteogenesis. Such an implant was realized by alternatively assembling polydopamine (PDA)-hybridized nanosized zeolitic imidazolate framework-8 (pZIF-8 nanoMOFs) and PDA-decorated-hydroxyapatite nanoparticles (pHA NPs) on the surfaces of the 3D-printed gelatin-based scaffolds through PDA-assisted layer-by-layer (LbL) assembly strategy. The synthesis of the pZIF-8 nanoMOFs was based on mussel-inspired catechol chemistry, which endowed the nanoMOFs with versatile adhesiveness, high drug loading efficiency, good physiological stability, and tumor environment-sensitive degradability. By using the pZIF-8 nanoMOFs as drug nanocarriers, it was possible to define the distinct spatial distribution and environmental-adaptive release patterns for BMP-2 and cisplatin from the scaffold. In vitro and in vivo studies confirmed that the scaffold possessed good osteoinductivity to induce osteogenic differentiation and to promote new bone formation. By responding to stimuli in the tumor microenvironment, the scaffolds efficiently released cisplatin and inhibited tumor growth. In short, this PDA-hybridized nanoMOF offers a new avenue to functionalize biomaterials with smart and responsive therapeutic ability for diverse biomedical applications.

Biocompatible β Ti-alloys with high strength and low modulus are of interest for additive manufacturing of biomedical implants. In this study, a metastable β Ti-12Mo-6Zr-2Fe (TMZF) alloy with highly dense structure was successfully fabricated by laser powder bed fusion (LPBF) from low-cost elemental powders. The applied different scanning strategies (simple and chess scan), and post heat treatment can regulate both the texture and secondary phases. The formation of strong {100} texture leads to the low elastic modulus of TMZF alloys, while nano-sized α" phases induce significantly strengthening effect. The as-fabricated TMZF alloy via simple scanning strategy shows considerably high strength due to the high density of α", ω phases and sessile dislocations, but it is brittle owing to the presence of ω phase. The TMZF alloy, manufactured using chess scanning strategy, possessed high yield strength of 1,026 MPa, low modulus of 85.7 GPa and good ductility of 12.7%. This results from its unique hierarchical microstructure containing α" phases, heterogeneous grains and the formation of {100} texture. After solution heat treatment, the specimens exhibit stronger {100} texture, hence lower modulus of 70.9 GPa. High yield strength of 943 MPa was maintained due to the formation of plate-like α" precipitates. The TMZF alloy fabricated by in-situ alloying based LPBF demonstrates comparable strength to that of Ti-6Al-4V alloy, but much lower elastic modulus, suggesting that it could be a potential candidate for some implant applications.