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The effect of the addition of yttrium (Y) and Al–5Ti–1B modifiers mixing melt (3M) on the microstructure and mechanical properties of hypereutectic Al–20Si alloy was studied in the present work. The as-cast specimens were examined by scanning electron microscope equipped with energy spectrometer, electron probe microanalysis and X-ray diffraction. The results demonstrated that the morphology of primary Si could be refined from coarse irregular star-like/plate-like structures into fine blocks with the addition of 0.6 wt% Y and 1.0 wt% Al–5Ti–1B at 650 °C. The average grain size of primary Si was reduced from 82 to 29 µm, and the aspect ratio was decreased from 1.81 to 1.47. Similarly, the eutectic Si structure was modified from coarse needlelike/flake-like structures into fine coral fibrous structures and partial granular structures, with the mean roundness of eutectic Si decreased from 7.8 to 2.32. In addition, the coarse α-Al dendrites were significantly refined into the uniform equiaxed dendrites. With the refinement and homogenization of Si phases, the optimal modified alloy obtained the optimal ultimate tensile strength (UTS) and elongation (EL). The UTS was enhanced from 94 to 154 MPa, and the EL was increased from initial 1.12 to 1.79%. Furthermore, the refinement and modification mechanism of the addition of yttrium and Al–5Ti–1B modifiers mixing melt on Al–20Si alloy were also discussed. [ABSTRACT FROM AUTHOR]

The effects of Y additions (0.2, 0.4, 0.6, 0.8 and 1.0 wt%) on the microstructure and mechanical properties of hypereutectic Al-20Si alloy have been studied in the present work. The as-cast specimens were examined by scanning electron microscope (SEM) equipped with energy spectrometer (EDS), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD). The results demonstrated that the morphology of primary Si could be refined from coarse irregular star-like and plate-like shape to fine block-like when the addition contents of Y increased to 0.8%. The average size of primary Si reduced by 62.9% from 89 µm to 33 µm and the aspect ratio decreased by 38.6% from 2.07 to 1.27. In addition, the eutectic Si structure was modified from coarse needle / flake to fine coral fibrous structure and partial particles. With the refinement and homogenization of Si phases, the optimal elongation (El) increased by 22.5% from 1.13% to 1.36% when the Y content was 0.6%. However, the ultimate tensile strength (UTS) enhanced by 47.9% from 94 MPa to 139 MPa when the addition content of Y was up to 0.8%. Moreover, the refinement and modification mechanism of Y on Al-20Si alloy was also discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interlayer remelting strategies can regulate the microstructure evolution, martensitic transformation, and superelasticity of EBF3-fabricated NiTi SMAs. • A one-step reversible transformation (B2 ↔ B19′) occurred upon heating/cooling. • The critical stress (σMs) has a strong dependence on the martensitic transformation behavior. • The broadening and stabilization of martensite is the key factor for the deterioration of the superelastic response. In this work, different interlayer remelting strategies are applied to regulate the microstructure evolution, martensitic transformation, and superelastic features of NiTi shape memory alloys prepared using the EBF3 additive manufacturing technique. The NiTi deposits prepared under different remelting beam currents are all composed of the B2 austenite, residual B19′ martensite, and submicron-scale Ti 4 Ni 2 O x precipitates, and exhibit a one-step phase transformation (B2 ↔ B19′). Meanwhile, the crystallographic orientation, grain boundaries, and residual strain of these alloys present a distinct variation with the application of different remelting beam currents. During mechanical testing, the critical stress (σMs) of the EBF3-fabricated NiTi alloys was seen to possess a significant dependence on the martensitic transformation behavior, namely with the amount of B19′ martensite and the corresponding M s. However, the broadening and stabilization of lamellar martensite created by dislocation pile-ups and plastic deformation in the cyclic loading–unloading procedure is the key reason for the deterioration of the superelastic response of the NiTi deposits. This work confirms that the mechanical and functional performances of NiTi alloys produced via EBF3-technique can be modified upon the application of proper interlayer remelting strategies, which can be extrapolated to the directed energy deposition of shape memory alloys or other metal components. [ABSTRACT FROM AUTHOR]

Because of their superior mechanical performance at ultra-high temperatures, refractory niobium–silicon-based alloys are attractive high-temperature structural alloys, particularly as structural components in gas turbine engines. However, the development of niobium–silicon-based alloys for applications is limited because of the trade-off between room temperature fracture toughness and high-temperature strength. Here, we report on the fabrication of a Nb-18Si alloy with dispersion of hafnium carbide (HfC) particles through selective laser melting (SLM). XRD and SEM-BSE were used to examine the effects of scanning speed on the microstructure and the phase structure of the deposited Nb-18Si-5HfC alloy. The results show that when the scanning speed rises, the solid solubility of the solid solution improves, the interlamellar spacing of eutectics slowly decrease into nano-scale magnitude, and the corresponding hafnium carbide distribution becomes more uniform. We also discover the hafnium carbide particles dispersion in the inter-lamella structure, which contributes to its high fracture toughness property of 20.7 MPa∙m1/2 at room temperature. Hardness and fracture toughness are simultaneously improved because of the control of microstructure morphology and carbide distribution. [ABSTRACT FROM AUTHOR]

A novel Al–20Si–2.5Fe–2Mn master alloy containing α-Al15(Fe, Mn)3Si2 particles was developed to modify Si phases of hypereutectic Al–20Si alloy. The microstructure of as-cast specimens was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy-dispersive spectrometers (EDS), electron probe micro-analysis (EPMA), and X-ray diffraction (XRD). The morphology of primary Si phases changed from coarse irregular pentagonal and polygonal shapes to fine blocky shapes as the adding content of the master alloy increased up to 6.0%. The mean size of the primary Si phases decreased by 70.3% from 118 to 35 μm. Moreover, the shape of eutectic Si structure changed from coarse flakes or acicular shape to fine fibrous structure and granular particles. Furthermore, the Si phases were refined and homogenized when 6.0% master alloy was added into Al–20Si alloy. The optimum elongation (El) increased by 45.8% from 0.85 to 1.24%, and the optimum ultimate tensile strength (UTS) increased by 34.6% from 130 to 175 MPa comparing with unmodified Al–20Si alloy. In addition, the refinement and modification mechanisms of Si phases are also discussed on the hypereutectic Al–20Si alloys with the addition of master alloy. [ABSTRACT FROM AUTHOR]

• A novel modifier of AlCoCrFeNiTi high-entropy alloy (HEA) is prepared. • The HEA can effectively modify the eutectic silicon structure of Al-7Si alloy. • The HEA can significantly refine the α-Al grains of Al-7Si alloy. • The mechanical properties of alloy with 0.2 wt%HEA are significantly improved. In this work, the AlCoCrFeNiTi high-entropy alloy (HEA) was prepared as an inoculant to refine the α-Al and silicon phases in Al-7Si alloy. The modification mechanism and the existing form of HEA in Al-7Si alloy were studied. The results show that Fe-rich intermetallics including Al, Si, Co, Fe and Ni, precipitated when the 0.2% AlCoCrFeNiTi HEA was added into Al-7Si alloy. In addition, the elements of Ti and Cr uniformly distributed within Al-7Si alloy. The morphology of the eutectic silicon changed from long needle structure to short rod and granular structure. Moreover, the size of α-Al grains significantly decreased in Al-7Si-0.2HEA alloy. The mechanical properties of Al-7Si alloy significantly improved. [ABSTRACT FROM AUTHOR]

Electron beam freeform fabrication (EBF3) additive manufacturing technique has been successfully employed to print the non-weldable Al-5Cu thin-walled parts through an optimized set of process parameters. Post-heat treatment is conducted to engineer the desirable microstructure containing multiple nanostructures, i.e., the nanoscale θ" and θ' precipitates, stacking faulted zones and Al-Cu-Cd clusters, leading to a high-strength Al-5Cu alloy with good ductility. The grain formation mechanisms are revealed via a combinatorial simulation and experimentation. It is shown that EBF3 process can produce three different types of grain microstructures: fully equiaxed, fully columnar, and the mixture. In the layerwise melt pool solidification process, wide partially melted zone is observed to favor the formation of equiaxed grains due to its inheritance characteristic. After solution and aging heat treatment, Al-Cu-Cd nanoclusters (∼ 2 nm in Dia.) are generated, which facilitates the formation of nanoscale stacking faulted zones (∼11 nm). Meanwhile, as heterogeneous nucleation sites, Al-Cu-Cd nanoclusters effectively promote θ'-Al 2 Cu phase (∼ 19 - 114 nm) precipitation to enable the coexistence with fine θ"-Al 3 Cu precipitates (∼ 7 - 37 nm). The multiscale microstructure, especially incorporating the newly identified nanoscale stacking faulted zones, activates multiple strengthening mechanisms. Consequently, its tensile properties at room temperature reach an ultimate tensile stress of ∼ 496.5 MPa, yield strength of ∼ 435.7 MPa, and elongation of ∼ 9.6%, which are superior to other as-cast and additively manufactured Al-Cu alloys. This work offers a promising processing route to directly fabricate high-strength near-net-shape Al-Cu alloy parts for the aerospace and automotive industries. [Display omitted] [ABSTRACT FROM AUTHOR]

Herein, the Ti-6.5Al–2Zr–Mo–V (TA15) alloy with gradient characteristics is fabricated via electron beam freeform fabrication (EBF3) technology. The gradient microstructure and mechanical properties are investigated in detail. Specifically, the microstructural evolution containing the morphology and size along the building direction and its impact on microhardness and tensile properties are systematically illustrated. Besides, the numerical simulation is developed for the establishment of the EBF3 temperature field thermal cycling profile to further elaborate the effect of temperature and thermal history on the microstructural evolution. The results reveal that the columnar to equiaxed transition and coarsening of the prior β grains, as well as successive increase and decrease of thickness of α lamellae along the building direction. The formation of fine lamellar α colony within the layer band is associated with the survived α phase when the peak temperature is just below T β. The gradient microstructure endows the TA15 deposit with different mechanical properties. Specifically, the microstructure with fine α lamellae manifests marginally higher microhardness in the layer-band-free region. The elemental dilution, loss and partitioning result in relatively low microhardness in the region with fairly fine α lamellae immediately adjacent to the substrate. The top region of the TA15 deposit, characterized by fine basket-weave microstructure, is imparted with better comprehensive properties relative to the middle and bottom regions, i.e., 1007.4 ± 30.8 MPa, 915.31 ± 28.7 MPa, and 12.08 ± 1.96% for tensile strength, yield strength and elongation, respectively. The integrated effect of coarsened prior β grains and refined α lamellae allow the middle and bottom regions to exhibit similar tensile properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel iron-based shape memory alloy was successfully deposited by wire and arc additive manufacturing. • As-deposited material was predominantly composed by FCC γ phase with a columnar dendritic morphology. • High mechanical performance was observed for yield strength, fracture stress, elongation, cyclic repetibility and hardness. • Uniaxial loading induced the FCC γ → HCP ε phase transformation and resulted in a ductile fracture. Shape memory alloys (SMA) are a class of smart materials with inherent shape memory and superelastic characteristics. Unlike other SMAs, iron-based SMAs (Fe-SMA) offer cost-effectiveness, weldability, and robust mechanical strength for the construction industry. Thus, applying these promising materials to advanced manufacturing processes is of considerable industrial and academic relevance. This study aims to present a pioneer application of a Fe–Mn–Si–Cr–Ni–V-C SMA to arc-based directed energy deposition additive manufacturing, namely wire and arc additive manufacturing (WAAM), examining the microstructure evolution and mechanical/functional response. The WAAM-fabricated Fe-SMAs presented negligible porosity and high deposition efficiency. Microstructure characterization encompassing electron microscopy and high energy synchrotron X-ray diffraction revealed that the as-deposited material is primarily composed by γ FCC phase with modest amounts of VC, ε and σ phases. Tensile and cyclic testing highlighted the Fe-SMA's excellent mechanical and functional response. Tensile testing revealed a yield strength and fracture stress of 472 and 821 MPa, respectively, with a fracture strain of 26%. After uniaxial tensile loading to fracture, the γ → ε phase transformation was clearly evidenced with post-mortem synchrotron X-ray diffraction analysis. The cyclic stability during 100 load/unloading cycles was also evaluated, showcasing the potential applicability of the fabricated material for structural applications. [ABSTRACT FROM AUTHOR]

Herein, we employ electron beam surface melting (EBSM) technology to process Ti-6Al-3Nb-2Zr-1Mo alloys and compare their microstructure and corrosion performance with wrought counterparts. The mechanism of microstructural evolution has been illustrated. The EBSM sample exhibits a better corrosion resistance than the wrought one, which can be ascribed to the formation of a relatively more stable passive film due to a higher fraction of phase boundaries induced by the fine α′-phase, and the alleviated heterogeneous distribution of compositions between α (α′)- and β-phases stemming from rapid cooling during EBSM process, further leading to the absence of strong galvanic-corrosion between different phases. [Display omitted] • Effects of EBSM on microstructure and corrosion property of Ti6321 alloy are studied. • The formation mechanism of primary and secondary α′ martensite is discussed. • The EBSM Ti6321 alloy exhibits much better corrosion resistance than the wrought one. • Explore the origins of distinct corrosion property for wrought and EBSM Ti6321 alloys. • Corrosion mechanisms of Ti6321 alloy in artificial seawater and 5 M HCl are proposed. [ABSTRACT FROM AUTHOR]

• A novel TiAl/Ti 2 AlNb laminated composite has been successfully fabricated. • The microstructure evolution in the fabrication process is detailly studied. • A high toughness is achieved and its toughening mechanisms are investigated. TiAl/Ti 2 AlNb laminated composite has been successfully fabricated by hot pressing combined with the foil-foil metallurgy. The corresponding details about the microstructure evolution are discussed, and the metallurgical interface bonding can be achieved in the composite. Fracture toughness tests indicate the fracture toughness of this composite is increased by 77% over the TiAl matrix; meanwhile, the former behaves an enhanced crack damage tolerance ability. At last, the toughening mechanisms of the laminated composite are illuminated in detail. [ABSTRACT FROM AUTHOR]