In this paper, a new way for synthesising nanoscale AlN and Si3N4 co-reinforced aluminium matrix composites (AMCs) by means of laser powder bed fusion (LPBF) is presented. The method entails irradiating AlSi10Mg powder with a laser beam, thereby forming an Al matrix melt pool in an atmosphere of dilute nitrogen (N2) concentration; simultaneously, the nearby N2 undergoes pyrolysis, generating reactive nitrogen ions that react with dissolved Al and Si atoms within the melt pool to form AlN and Si3N4 precipitates. Benefiting from rapid solidification characteristic of LPBF, these nitrides nucleate and grow in an extremely short temporal window before precipitating in the melt pool. Finally, the (AlN + Si3N4)/AMC is fabricated through successive layer-by-layer application of this synthesis strategy. Herein, a comparative investigation was conducted between AMCs processed under varying N2 concentrations (2 vol%, 5 vol%, and 10 vol%) and a reference AlSi10Mg alloy produced in a pure argon (Ar) atmosphere (0 vol% N2). Multiple analytical techniques (XRD, SEM, EDS, HR-TEM, and XPS analyses) verified nitrides incorporation into the AMCs matrix, meanwhile, EBSD characterised the grain morphology, dimension, and orientation. The strengthening-toughening mechanisms of the composites were systematically elucidated through comprehensive microstructural and mechanical property evaluations.

As additive manufacturing (AM) advances, the use of gas as a reaction medium to optimize material properties becomes more and more important. In this study, we manufactured titanium matrix composites (TMCs) with uniformly dispersed reinforcement TiN by gas–liquid in-situ reaction using an integrated strategy combining AM and laser gas nitriding (LGN) in low nitrogen (N2) concentrations (2 vol% and 8 vol%). Meanwhile, both good mechanical and electrochemical properties were achieved. As the N2 concentration was raised from 2 vol% to 16 vol%, the content of N in the samples gradually increased, accompanied by enhanced TiN generation, and the compositional variation triggered significant microstructural evolution, which affected the overall properties of the material. Notably, the strength and plasticity of the samples prepared in 2 vol% N2 concentration reached the maximum value simultaneously. In addition, the samples prepared in low N2 concentrations (2 vol%, 8 vol%) possessed superior wear resistance and corrosion resistance. The mechanisms behind the strengthening and toughening as well as the tribological and corrosion mechanisms were studied. These findings provide a new possibility for fabricating tailorable high-performance TMCs.

Controlling boride structure and morphology is critical for enhancing the performance of B-containing TiAl alloys. This work proposes and demonstrates for the first time the effectiveness of segregation control in regulating the morphology/structure of borides, achieving exclusive formation of monolithic B27-TiB needles in Ti-45Al-2Mn-2Nb-1B via segregation-controlled directional solidification, suppressing detrimental multiphase borides. By reverse solute rejection, we maintain a sustained low-Al environment that stabilizes the L -> beta+ B27-TiB eutectic reaction. B27-TiB exhibits dual refining functions: serving as preferential nucleation sites for L -> beta -> alpha -> alpha 2+ gamma phase transformations while simultaneously impeding phase growth. In contrast, TiB2-containing borides formed in Al/B-enriched interdendritic regions fail to refine primary solidification due to spatial/temporal constraints, although both functions are equivalent in solid-state transformations. This segregation control strategy enables unprecedented control over boride phase, establishing a new pathway for regulating boride morphology and investigating refinement mechanisms of B-modified TiAl alloys.

Laser powder bed fusion (LPBF) has garnered considerable attention for its capacity to fabricate near-net-shaped complex components, particularly in lightweight high-strength Al alloys. The rapid solidification conditions inherent to LPBF can theoretically engineer unique microstructure to surpass mechanical property limits. However, the practical performance of many high-strength age-hardenable Al alloys (such as 2XXX, 6XXX and 7XXX series) processed by LPBF remains non-competitive owing to their high susceptibility to cracking. Herein, we demonstrate a design strategy to synergistically regulate solidification and precipitation behaviors through a minor addition of LaB6 particles (1.0 wt%), enabling the fabrication of crack-free high-strength Al-Cu alloys. We show that LaB6 addition promotes significant grain refinement during solidification and facilitates the formation of dense multiscale precipitates during subsequent aging. This microstructural modification simultaneously improves LPBF processability and enhances alloy strength. We further reveal that the introduced LaB6 particles dissolve within the melt pool and reprecipitate during cooling. The reprecipitated LaB6 particles act as effective heterogeneous nucleation sites for alpha-Al, while residual La solutes suppress the growth of theta-Al2Cu phase under non-equilibrium solidification. Additionally, LaB6 promotes the precipitation of a high density of submicron T-Al20Cu2Mn3 dispersoids and nanoscale theta '-Al2Cu precipitates during solid-state phase transformation. These synergistic effects of crack elimination, grain refinement and nanoscale precipitation result in mechanical properties with a yield strength of 271 +/- 1.8 MPa, an ultimate tensile strength of 341 +/- 6.1 MPa and an elongation of 6.5 +/- 0.8%. The dual roles of LaB6 provide a new strategy for designing multifunctional regulators fabricated by LPBF.

This study employs an innovative gas–liquid in-situ reaction approach combined with laser powder bed fusion (LPBF) technology to fabricate pure titanium matrix functionally graded composite materials (FGMs) under a progressively decreasing nitrogen (N2) atmosphere. Consequently, the TiN reinforcement was successfully synthesized within a pure Ti matrix, leading to the fabrication of TiN-reinforced titanium matrix FGMs with well-defined gradient features and minimal defects. Furthermore, the continuous decrease in N2 concentration along the LPBF construction direction leads to a corresponding reduction in both the volume fraction and particle width of in-situ formed TiN reinforcement. As a result, a continuously graded microstructure evolves, characterized by the progressive refinement of the α′-Ti phase and prior β grain from the top to the bottom of the fabricated FGMs. This graded microstructure, in turn, gives rise to smooth gradients in microhardness and wear resistance, which stem from the synergistic effects of nitrogen solid solution strengthening, the variation in content and size of TiN reinforcement, and the refinement strengthening of the matrix grain.

All metallic materials begin their life through solidification. A detailed understanding of solidification phenomena is critical for controlling microstructural evolution and enhancing material performance. This state-of-the-art review presents the recent developments in grain refinement during the solidification processing of aluminum alloys, focusing on the two primary interrelated factors governing grain size: nucleant particles and solute elements. The review commences with an overview of established grain refinement theories, including heterogeneous nucleation, constitutional undercooling and the interdependence theory (Section 2). The subsequent review emphasizes the effect of key factors influencing grain refinement performance: nucleant particles potency, size distribution, kinetic behaviors and solute elements (Section 3). A comprehensive discussion then covers the recent progress in novel grain refiners, their efficiency and grain refinement mechanisms under the near-equilibrium solidification (conventional casting, Al-Ti-B-X refines) and rapid solidification (additive manufacturing, in-situ Al3X phases and ex-situ nanoparticles). The article concludes by outlining major findings and presenting an outlook on future development directions including quality control and evaluation, intelligent grain refiner design, standardized grain refinement theories and multifunctional agent development. This review aims to provide holistic insights into grain refinement for conventionally cast and additive manufactured Al alloys, thereby controlling the microstructure and advancing the development of high-performance of Al alloys.

With the in-depth study of additive manufacturing (AM) technologies, the use of gas as a reaction medium to optimize material properties becomes more and more important. In this study, we manufactured titanium matrix composites (TMCs) with uniformly dispersed reinforcement TiN by gas-liquid in-situ reaction using a new strategy combining AM and laser gas nitriding (LNG) in different nitrogen (N2) concentrations (2 vol.%, 8 vol.% and 16 vol.%), respectively. Meanwhile, both good mechanical and electrochemical properties were achieved. Subsequently, the microstructure, mechanical and electrochemical properties were characterized. The results showed that the strength and plasticity of the samples prepared in 2 vol.% N2 concentration reached the maximum value at the same time. In addition, the samples prepared in low N2 concentrations (2 vol.%, 8 vol.%) possessed more excellent wear resistance and corrosion resistance. We have analyzed the mechanisms behind the strengthening and toughening as well as the tribological and corrosion mechanisms. These findings provide a new possibility for the preparation of high-performance TMCs.

This paper proposes a new powder recoating mechanism named forward rotating flexible roller (FRFR) which aims to solve the poor powder flowability problem brought by the aggregation effect of the fine particles in the mixed 30 wt% 5 μm SiC and 55 μm AlSi10Mg powder. An initial extrusion force instead of a gap between the flexible roller and powders is used to improve the recoated powder density, while the recoated powder layer thickness is proved to be controllable. An industrial camera and a laser displacement sensor were used to estimate the recoated surface filling rate and recoated surface profile. The mixed powder was recoated with an FRFR on a baseplate at first, the achieved surface filling rate was the same as that of 55 μm AlSi10Mg recoated with a rubber blade. Then 120 layers of mixed powder were recoated continuously with FRFR, the recoated surface profile within a single layer and between multiple layers were compared with the rubber blade recoated AlSi10Mg, their difference is insignificant. Furthermore, composite parts with different shapes were successfully made with the FRFR. This technique is proved to be promisihfcang for laser powder bed fusion of fine particle ceramic and metal mixed powder.

Herein, we report a novel method for in-situ synthesis of nanometer-scaled TiC-reinforced Ti6Al4V-matrix composites (TMCs) via laser powder bed fusion (LPBF). The formation mechanism can be summarized as insitu additive manufacturing (AM) via a gas-liquid reaction. Here, gas means the laser-induced pyrolysis methane gas (CH4) generates gaseous carbon (C) atoms/ions and liquid means that the laser beam irradiates Ti6Al4V powder to create a Ti matrix melt pool. TiC, which is the reaction product of gaseous C atom/ion with liquid Ti atom, initially undergoes nucleation and growth, and subsequently precipitates from the Ti matrix melt pool during fast LPBF cooling process. Finally, the TiC-reinforced TMCs are fabricated via a layer-wise gas-liquid reaction. With this method, three nanocomposites (Sample 2, 3 and 7) fabricated in low CH4 concentration (9 vol % and 19 vol%) exhibited good dispersion, clean interface and strong interfacial bonding between the TiC reinforcement and Ti matrix. Thus, it achieved a good combination of simultaneously high strength and high plasticity. The effects of CH4 concentration, laser power and scanning speed on the microstructures and mechanical properties including microhardness, compression behaviors and wear resistance were systematically studied. And the strengthening and toughening mechanisms of the TMCs were elucidated. The proposed in-situ AM method via the gas-liquid reaction would carve a new path for manufacturing uniformly dispersed nanophase reinforced composites with excellent mechanical properties and complex geometries.

Titanium alloys are extensively used in areas such as aerospace and biomedicine. However, inadequate mechanical qualities (e.g., low hardness and poor wear resistance) and poor machinability limit the scope of their application expansion. To directly manufacture near-net-shape titanium alloy components with complicated architectures and improved performances, titanium matrix composites (TMCs) were fabricated based on the Ti–N reaction by introducing nitrogen gas (N2) in the process of selective laser melting (SLM) of Ti6Al4V. The formation principle of this novel method is as follows: Laser-induced N2 decomposition near the melt pool of Ti6Al4V generates N atoms or ions, which react with Ti atoms in the melt pool to in-situ synthesize TiN-reinforcement particles. In turn, TiN-reinforced Ti6Al4V matrix composites are manufactured layer-by-layer. This approach has some important advantages, which are as follows: Above all, in-situ gas-liquid synthesized reinforcements are equally dispersed due to N2, good diffusivity and dispersibility. Furthermore, extremely small gas molecules have the potential to produce nanoscaled reinforcement. Moreover, the in-situ reaction mode produces a clean interface and strong interfacial bonding between the matrix and reinforcement. In this study, TMCs were prepared by SLM in three different N2 volume fractions of 3%, 10% and 30%, which were compared to the Ti6Al4V alloy fabricated in an argon atmosphere. The microstructures were observed by SEM. Interstitial solid solutions of N in the Ti lattices were confirmed by XRD patterns. The presence of TiN was verified by EDS. The high-resolution transmission electron microscope (HR-TEM) picture indicated that the matrix and reinforcement were TiN and Ti, respectively. Such in situ synthesized nitride reinforcements were uniformly distributed; in particular, numerous nanoscale reinforcements were uniformly dispersed in the composites manufactured in low volume fraction N2 atmospheres (3% and 10%). Additionally, the improved strength and plasticity were simultaneously achieved in a diluted N2 atmosphere (10%). The effect of varying N2 concentrations on the microstructure and mechanical characteristics of the TMCs was investigated. The content of the TiN particles increased with increasing N2 concentration due to the increased availability of N atoms and ions for nucleation and growth of the reinforcement. Nevertheless, the TMC produced in a high N2 atmosphere (30%) demonstrated degradation of the mechanical properties (particularly plasticity and ultimate strength) because of the presence of excessive N solid solutions and brittle TiN particles. The strengthening mechanisms were primarily grain refinement strengthening of the Ti matrix due to the "pinning" effect of TiN particles, the precipitation hardening and dispersion strengthening effects of uniformly distributed reinforcement particles, interstitial solid solution strengthening caused by the results from the portion of N in the Ti lattices, Orowan strengthening caused by the in-situ synthesis of nanoscaled reinforcements, and the load transfer effect from Ti matrix to TiN reinforcements because of the clean interface.

The Ti6Al4V alloys were fabricated by selective laser melting (SLM) technology. The effect of laser energy density (LED) on the relative density, microhardness, compression strength and plasticity of the Ti6Al4V alloys was studied. Effects of N-2 concentration on microstructure and mechanical properties of the Ti6Al4V matrix composites were studied. The results show that the optimum LED processing window for SLM of Ti6Al4V alloys is in the range of 84.8 similar to 163.6 J/mm(3). In the optimum LED window, the TiN reinforced Ti6Al4V matrix composites were fabricated by SLM in different N-2 concentrations ( 3vol%, 10vol% and 30vol%) atmospheres based on gas-liquid reaction. The composite fabricated by SLM in 3vol% N-2 atmosphere exhibits a good combination of high strength and high plasticity. The strengthening and toughening mechanisms were studied.

In this study, we proposed a novel method for in-situ additive manufacturing of TiC reinforced Ti6Al4Vmatrix functionally graded composite materials (FGMs) based on gas-liquid reaction. Laser-induced pyrolysis of methane gas (CH4) near the surface of the Ti6Al4V melt pool provides carbon atoms/ions which react with titanium atoms to in-situ synthesize TiC reinforcements. Clean interface and excellent dispersion between the TiC reinforcement and Ti matrix were achieved owing to in-situ gas-liquid reaction, good diffusivity and dispersibility of the gaseous carbon source. As anticipated, spatially varying volume fraction and grain size of the TiC reinforcement, continuously varying microhardness and wear resistance were attained in the TiC/Ti6Al4V FGM by controlling continuous variation of CH4 concentration. (c) 2021 Elsevier B.V. All rights reserved.

In situ synthesised TiN reinforced Ti6Al4V matrix composites were fabricated via laser powder bed fusion (L-PBF) in an atmosphere of 10vol.-% N2 and 90vol.-% Ar based on novel gas–liquid reaction between N2 and Ti6Al4V melt pool. With lower laser energy density (LED), the microstructures of the composites mainly consisted of acicular α′ martensites, while the martensites were shortened and coarsened with increasing LED. The X-ray diffraction analysis demonstrated that the interstitial solid solution of N was formed in the Ti lattice. High-resolution transmission electron microscopy analysis confirmed the generation of TiN. The microhardness, yield strength, ultimate strength and plasticity of the composites initially increased and then decreased with increasing LED. The strengthening mechanisms of the composites were elucidated.

Processing titanium (Ti)-based materials in high-concentration nitrogen gas (N2) atmosphere is credited with reducing ductility/plasticity arising from the excessive formation of brittle agglomerated TiN. However processing these materials in dilute N2 may reduce the risk. In this study, a novel method is developed for laser additive manufacturing of in-situ synthesized TiN and AlN co-reinforced Ti6Al4V matrix composites (NTMCs) by the gas–liquid reaction in low-concentration N2 atmospheres. The manufacturing process of the NTMCs involves melting Ti6Al4V powders followed by laser-induced pyrolysis of N2 near the melt pool. The process is facilitated by the reaction between the decomposed nitrogen and molten Ti6Al4V, dissolution and precipitation of nitrides, and formation of the composites layer-by-layer. The formation of the nitride precipitates was verified by XRD, SEM, EDS, and HR-TEM. Such in-situ synthesized nanoscale reinforcements exhibited good dispersion and strong interfacial bonding with the matrix alloy in the composites. The microhardness, 0.2% compressive yield strength, and ultimate compressive strength of the NTMCs significantly increased with increasing N2 concentration in additive manufacturing; their maxima were 511 HV, 1721 MPa, and 2010 MPa, respectively, increased by 36.3%, 67.9%, and 16.8% from those of the Ti6Al4V alloy. The formation and strengthening mechanisms of the NTMCs were elucidated.

A novel laser additive manufacturing method is proposed to manufacture the agglomeration-free nanoscale TiC-reinforced titanium-matrix composites by in-situ compounding the reinforcement via gas–liquid reaction. The clean interface and strong interfacial bonding between the in-situ nanoscale TiC reinforcement and the matrix alloy can significantly improve grain boundary behavior and grain boundary strength, besides, the mechanisms of achieving simultaneously high strength and high plasticity in the TiC/Ti6Al4V composites were studied. The proposed in-situ additive manufacturing method using pyrolysis-reaction would open a new route for manufacturing of the homogeneous and agglomeration-free nanocomposites parts with high-complexed structures and excellent mechanical properties.

Four groups of Ge-Sb-Se chalcogenide glasses with compositions of GexSb10Se90-x (10 <= x <= 32.5), GexSb15Se85-x (5 <= x <= 27.5), GexSb20Se80-x (5 <= x <= 25), and Ge12.5SbxSe87.5-x (5 <= x <= 25) were prepared to systematically study their structural, thermal, and optical properties over a very wide compositional range. The structures, densities, glass transition temperatures, refractive indices, Vis-NIR and IR transmission spectra of the Ge-Sb-Se glasses show strikingly similar transitional features in the stoichiometric compositions. The transition behaviors of the structural, thermal, and optical properties of the Ge-Sb-Se glasses can be ascribed to the 'demixing' of the glassy network above the stoichiometric compositions.

Carbon nanotubes were blended into a Ti–6Al–4V matrix to synthesize titanium carbide (TiC) in situ, via spark plasma sintering. The microstructure and mechanical properties of both the monolithic Ti–6Al–4V alloys and the TiC/Ti–6Al–4V composites were studied to evaluate the strengthening effects of TiC on the Ti–6Al–4V matrix. The morphologies obtained by scanning electronic microscopy and optical microscopy indicated that the grain size of both the Ti–6Al–4V alloy and the TiC/Ti–6Al–4V composite decreased with increasing planetary ball-milling (PBM) speed, leading to an increase in the hardness of the investigated materials. The compressive yield strength of the monolithic Ti–6Al–4V alloys and the TiC/Ti–6Al–4V composites initially increased and then decreased with increasing PBM speed. The strengthening and fracture mechanisms were studied.

Ti-6Al-4V samples were fabricated by selective laser melting (SLM) with a series of different laser energy inputs. The microstructure presented a feature of fine acicular martensite due to the high cooling rate of the SLM process. All the relative density, micro-hardness and tensile properties initially increased and then decreased with increasing laser energy density. The fracture morphology exhibited a mixed mode of complex ductile and brittle fracture. All the results indicated that the laser energy density has a crucial effect on the microstructures and mechanical properties of the SLMed Ti-6Al-4V alloy. The correlations between the laser energy density and the microstructures and mechanical properties of the SLMed Ti-6Al-4V alloy were studied.

Three-dimensional parts of the 30CrMnSiA steel were successfully fabricated using selective laser melting (SLM). The microstructures and mechanical properties of the SLM-processed 30CrMnSiA samples were investigated by scanning electron microscope and transmission electron microscopy. The results indicate that the microstructures of the 30CrMnSiA samples consist mainly of lath martensite and acicular martensite. The value of the surface roughness decreases with increasing laser energy density (LED) before it reaches a minimum and then increases with further increasing LED. The relative density, microhardness and ultimate tensile strength of the SLM-processed samples initially increase and then decrease with increasing LED. By taking the relative density, surface roughness, microhardness and ultimate tensile strength into account, the optimized LED should be in the range of 46.15–51.28 J mm −3 for the SLM-processed 30CrMnSiA alloys. In addition, the differences in the microstructures and mechanical properties between the conventionally wrought 30CrMnSiA sample and SLM-processed 30CrMnSiA samples were also studied.

Monolithic ZK61 magnesium alloy and carbon nanotube (CNT)-reinforced ZK61 matrix composites were successfully prepared via spark plasma sintering. The effects of the sintering temperature on the microstructure and mechanical properties of monolithic ZK61 were studied, and the microstructural and mechanical properties of CNT/ZK61 composites were investigated as functions of the CNT content. The grain sizes of the CNT/ZK61 composites are smaller and more homogeneous than those of the monolithic ZK61 powder, and the width of the grain boundary is greater than those present in ZK61. The Vickers hardness and compressive yield strength of the CNT/ZK61 composites are observed to initially increase with increasing CNT content, reaching a maximum at 1.5 wt.% CNT, however, these values begin to decrease as the content increases further. We believe that this is owing to the grain refinement effect and load transfer mechanism of the CNT.