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Here we propose to employ electron beam surface melting (EBSM) to refine microstructural length scale and obtain suited microstructure morphology of Nb-Si based alloys. As characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT), nano-lamellar eutectics are created in Nb-20Si (at.%) alloys upon EBSM. Theoretical analysis based on the competitive growth models demonstrates the preferred formation of Nb solid solution (Nbss)/Nb 3 Si eutectics under selected processing parameters. The large cooling rate during EBSM, higher than 8 × 104 K/s, promotes the formation of the nano-scale eutectics. Compared to the initial coarse eutectics by arc melting, simultaneous enhancement of hardness and fracture toughness is enabled owing to the controlled microstructural length scale and morphology. Display Omitted • Nano-lamellar eutectics are created by electron beam surface melting. • Theoretical analysis based on the competitive growth models is performed. • Simultaneous enhancement of hardness and fracture toughness is achieved. [ABSTRACT FROM AUTHOR]

Abstract The performance of tungsten heavy alloys (WHAs) is related to the characteristic of matrix/W interfaces, which depends directly on the wettability of the matrix materials on W. This work designs a novel WHA that selects an AlCrFeNiV-system high entropy alloy (HEA) as matrix based on its wetting behavior on polycrystalline W. The wetting experiments show that this W/HEA system exhibits low contact angles at the temperatures for liquid phase sintering (LPS), and dissolutive wetting with a moderate W solubility in the HEA. This favorable wetting behavior benefits the densification process during LPS, leading to clean matrix/W interfaces with high bonding strength and hence a high ultimate tensile strength of 1132 MPa in the resulting W/HEA composite. These results indicate that the AlCrFeNiV-system HEA is a promising matrix material for WHAs. Graphical abstract Image 1 Highlights • A novel tungsten heavy alloy using high entropy alloy as matrix (W/HEA composite) is prepared by liquid phase sintering. • Dissolutive wetting of W by HEA melt with low contact angles is investigated. • W/HEA composite has fine W particle size of 13.15 μm and strong interfacial bonding. • W/HEA composite has a high yield strength of 705 MPa and high ultimate tensile strength of 1132 MPa. [ABSTRACT FROM AUTHOR]

The control of solidification microstructure is critical to successful laser processing of single-crystal (SX) nickel-base superalloys and a practical tool for the microstructural control is processing–microstructure maps. However, the maps presented in literature do not consider the effects of powder feeding during laser additive manufacturing (LAM) of SX superalloys. This paper therefore presents a simple and feasible strategy to deal with the effects of powder feeding and to extend the combined numerical model used to calculate processing–microstructure maps. A characteristic ratio of epitaxial SX growth was defined to quantitatively compare the final solidification microstructure. Resulting processing–microstructure maps can estimate the influence of most processing variables, especially powder feeding rate, on the extent of epitaxial SX growth and the position of columnar-to-equiaxed transition. Using the processing parameters selected according to these processing–microstructure maps, a multi-layer SX deposit with fine dendrites was successfully fabricated by LAM. This successful SX LAM indicates that these new processing–microstructure maps involving powder feeding are reliable and useful because they can determine proper processing windows for LAM of SX superalloys and further advance the understanding of the processing–microstructure relationship in powder-feeding LAM process. [ABSTRACT FROM AUTHOR]

Experimental process optimization is essential to obtain reliable processing conditions prior to performing actual laser additive manufacturing (LAM) of single-crystal (SX) nickel-base superalloys. The influence of processing parameters on deposited productivity and epitaxial SX growth in powder-feeding LAM process was systematically investigated by the orthogonal experiment (OE) method, a statistical experiment design method. This method can rapidly and economically estimate the effect of each processing parameter by a small number of experiments. Resulting relationship between the processing variables and each of deposited productivity and microstructure formation contributes to the selection of detailed processing conditions to balance the factors crucial to successful SX LAM, which means that appropriate adjustment of the processing parameters during actual SX LAM is easy to be performed. On the basis of the analyses of the OE results, a combination of relatively high power, low scanning velocity and moderate powder feeding rate is beneficial to both deposited productivity and epitaxial SX growth during powder-feeding LAM, and allows the preparation of good multilayer SX deposits with fine dendrites. [ABSTRACT FROM AUTHOR]

The solidification path of single-crystal nickel-base superalloys containing minor carbon was investigated under various laser rapid directional solidification (LRDS) conditions. By controlling the solidification rate, LRDS processing can provide the evidence whether some diffusion-controlled phase transformations occur because such transformations will be suppressed under high cooling rates. Results show that the solidification path and final solidification microstructure depend upon the cooling rate; the microstructure without γ-γ′ eutectic can be obtained as long as the cooling rate is high enough. A peritectic transformation in carbon-containing single-crystal superalloys was first experimentally verified by controlling the cooling rate during LRDS processing. [ABSTRACT FROM AUTHOR]

Primary dendritic arm spacing (PDAS) is an important microstructure feature in the nickel-base single-crystal (SX) superalloys by laser rapid directional solidification (LRDS). A combined numerical model was developed in this paper to investigate the influence of laser processing parameters on the PDAS. This model consists of (1) the theoretical PDAS models which relate PDAS to the solidification conditions and (2) the heat-flux calculations of laser processing which provide the solidification conditions as a function of the processing parameters. It is therefore able to immediately relate the PDAS to the processing parameters and obtain corresponding processing–microstructure maps. To verify the predicted accuracy, the PDAS values calculated by different theoretical models were compared with those produced under different processing conditions. Results show that the PDAS firstly decreases and then increases at a lower laser power whereas it decreases with increasing scanning velocity at a higher laser power, leading to nose-shape contour lines. The predicted accuracy depends on appropriate selections of material thermo-physical properties. These processing–microstructure maps can accurately capture the trends of the PDAS variation with the processing parameters and contribute to the control and optimization of dendritic microstructure while determining relevant processing windows for controlled SX laser processing. [ABSTRACT FROM AUTHOR]

The microsegregation in single-crystal nickel-base superalloys, which may result in many undesired precipitated phases, can be effectively reduced by rapid solidification. A new microsegregation model for rapid solidification multicomponent alloys was developed in this paper to predict the microsegregation in the single-crystal superalloys of laser rapid directional solidification and study the influence of processing parameters on the microsegregation behavior. To verify the predicted accuracy of the present model, the concentration profiles and microsegregation ranges under different processing conditions were examined and compared with those calculated by different models. Compared with previous models, this multicomponent model not only has definite physical meaning but also takes nonequilibrium solidification and dendritic tip undercooling into account. Therefore, it is suitable for a wide range of solidification conditions and can obtain better predicted results for complex multicomponent alloys. Moreover, although the microsegregation behavior depends primarily on the solidification velocity, a high temperature gradient also contributes to reducing the microsegregation. Consequently, the microsegregation can be effectively controlled by using a set of processing parameters that can obtain high temperature gradient and solidification velocity. Such a model contributes to the control of the microsegregation while determining relevant processing windows for the preparation of superalloy components using a controlled single-crystal laser processing. [ABSTRACT FROM AUTHOR]

The stray-grain formation and epitaxy loss at the substrate during laser surface remelting of single-crystal nickel-base superalloys, which immediately lead to the break of the single-crystal growth, were investigated by means of modeling and experiments. Results indicate that the stray-grain formation and epitaxy loss at the substrate are attributed to the increased trend of the formation of equiaxed grains resulting from the composition segregation at the substrate interdendritic region. Accordingly, a solution treatment for the substrates prior to laser processing can effectively avoid the SG formation and epitaxy loss, which reveals a basic precondition, homogenized solution treatment of substrates, for successful single-crystal laser processing. [ABSTRACT FROM AUTHOR]

The influence of prior-β-grain (PBG) size on the tensile behavior of a laser-deposited α/β Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy was investigated to reveal the strengthening mechanism of PBGs. Results show that the strength increases with increasing PBG size. This special coarse-PBG strengthening predominantly depends on the strong constraints resulting from the strain incompatibility among PBGs rather than PBG boundary resistance to dislocation motion. With increasing PBG size, increasing constraints are developed in the adjacent PBGs, and thus the applied stress needed to maintain the compatible deformation among these PBGs increases. In particular, the strengthening effect is not significantly affected by increasing temperature, which reveals a potential strengthening method that can strengthen a laser-deposited α/β Ti alloy at room and elevated temperatures simultaneously by controlling its PBG size. [ABSTRACT FROM AUTHOR]

Abstract: The compositional variation and the microstructural evolution in laser additive manufactured Ti/Ti–6Al–2Zr–1Mo–1V graded structural material were investigated. An anomalous compositional variation and its good correlation with hardness were found. Although, during the forming process, the ratio of Ti–6Al–2Zr–1Mo–1V in feeded powders abruptly changes from 0% to 100%, all the elements exhibit a gradual variation, which is preferred to a graded material because such gradient change can decrease the discontinuity of its microstructures and properties and can remarkably reduce the costs due to the simplified manufacturing process and the rational distribution of materials. [Copyright &y& Elsevier]

The mechanical behavior of commercial purity Ti (CP-Ti)/Ti–6Al–2Zr–1Mo–1V structurally graded materials (SGMs) fabricated by laser additive manufacturing was investigated via uniaxial tensile experiments. Although the strength of all SGM specimens is higher than that of the monolithic CP-Ti specimens, only the SGM specimen of Ø10mm gauge diameter with bulky α-grains exhibits excellent plasticity comparable to the CP-Ti specimen under the same conditions. The SGM presents a potential for use as load-bearing components due to its good strength–ductility balance. [ABSTRACT FROM AUTHOR]

Every strengthening/hardening method has its own limit. However, it is difficult to couple multiple extreme hardening in a bulk material, especially for highly strengthened ones. Here we demonstrate a strategy to further harden an ultrastrong precipitation-hardened high entropy alloy (HEA) with ultrafine-grain (UFG)/nanograin (NG) bands. These UFG/NG bands with nanoprecipitates were obtained by combining local severe plastic deformation (SPD) from cryogenic rolling and the pinning effect of precipitates during recrystallization. We found that the bands, with combined hardening of NGs and nanoprecipitates, provide an amazing yield strength of ~2.8 GPa and an ultrahigh hardness of ~9.7 GPa. Such nanoprecipitation-hardened UFG/NG bands in the bulk HEA contribute to an extra strengthening close to 300 MPa and an extremely high tensile strength of more than 2.2 GPa. This research presents a possibility for obtaining NG structure in bulk metals which will open new avenues for developing stronger alloys. [ABSTRACT FROM AUTHOR]

The susceptibility to adiabatic shear banding (ASB) of tungsten heavy alloys (WHAs) is a key factor to determine their penetration performance for kinetic energy penetrators (KEPs). This work reports a novel WHA with strong susceptibility to ASB that uses a precipitation-hardening high-entropy alloy (HEA) as the binding phase of W particles. The results find that nanoscale L1 2 precipitates and high dislocation density in the matrix contribute a high dynamic yield strength of ∼2300 MPa. Also, a narrow ASB with a width of ∼12 μm forms in the novel WHA, which is only 6% of the width of ASB in traditional WHAs like W-NiFe, exhibiting a great susceptibility to ASB. The redissolution of precipitates into the matrix because of temperature rise during shearing could sharply soften the shearing location, which is suggested to be a very important factor of destabilizing mechanism to promote the development of ASB. • The novel W-HEA alloy exhibits a peak compressive dynamic strength of ∼2300 MPa. • High susceptibility to adiabatic shear band (ASB) is observed in the W-HEA alloy. • ASB in W-HEA alloy has a narrow width of ∼12 μm and melting traces. [ABSTRACT FROM AUTHOR]

Deformed alloys commonly soften after annealing because of the decrease in defects. This work reports an annealing hardening that occurs in a cryo-rolled CoCrFeNi high-entropy alloy. This abnormal behavior is related to continuous twinning in nanoscale, which would have occurred in cryo-rolling but was belated until annealing was performed. This is because the stress accumulated in cryo-rolling was not enough to activate the reaction between dislocations and stacking faults, so as to require additional thermal activation (by annealing) for supplying the energy requirement for twinning. Due to the extra strengthening from stable twin boundaries, the annealed samples exhibit an ultrahigh yield strength close to 1.4 GPa (~6 times higher than the solid-solution counterpart), even after annealing for 24 h. These results confirm a possibility for introducing numerous stable nanotwins in bulk FCC metals to improve their mechanical properties. [ABSTRACT FROM AUTHOR]

Preparing bulk WNiFe alloys with high strengths and low W content (<80 wt%) is challenging. Here, we present a strategy, powder-fed laser melting deposition (LMD), to solve this problem. The LMD-prepared WNiFe (LMD WNiFe) samples show high mechanical strengths and low W contents of 50 and 75 wt%. The W particles in the LMD WNiFe samples uniformly distribute in the matrix (γ), exhibiting fine particle sizes of approximately one-fifth of those of conventional 93WNiFe alloys prepared by liquid phase sintering (LPS). The ultimate tensile strengths of the 50 and 75 wt% samples are 1120 and 1316 MPa, respectively, which are 24.44 and 59.33% higher than those of the LPS-prepared 93WNiFe alloy (LPS 93WNiFe). These results suggest a useful strategy for preparing low-W-content, high-strength WNiFe alloys with fine, uniformly distributed W particles. This finding offers new potential applications of WNiFe alloys in additive manufacturing. Unlabelled Image • WNiFe with W particle size of 5.5 μm is prepared by laser melting deposition (LMD). • WHAs with low W content (no W skeleton to keep shape) can be achieved by LMD. • W particles in LMD WHAs distribute uniformly in γ matrix. • The yield strength of LMD WHAs is 59.3% higher than that of LPS WHAs. [ABSTRACT FROM AUTHOR]

Tungsten heavy alloys with a L1 2 precipitation-hardening high-entropy alloy matrix exhibit high susceptibility to adiabatic shear, but the relationship between the size and morphology of L1 2 precipitates and the adiabatic shear behavior is not well understood. This work systematically studies the effect of different size and morphology L1 2 precipitates on the mechanical properties and susceptibility to adiabatic shear of the tungsten heavy alloy with an AlCrFeNiV-system high-entropy alloy matrix (W-HEA). The results show that the W-HEA with square L1 2 precipitates has tensile strength of 1300 MPa and dynamic compression strength of 2100 MPa at a stain rate of 6000 s−1. The higher interfacial energy of square L1 2 precipitates leads to the faster re-dissolution of square L1 2 precipitates into the matrix than ultrafine and lath-like L1 2 precipitates under temperature rise during shearing, which can sharply soften the shearing location and contribute to the narrowest adiabatic shear band (ASB) with width of 15 μm. [Display omitted] • The matrix of 3D printed fine-grained tungsten heavy alloy with high-entropy alloy matrix (W-HEA) contains different morphology L1 2 precipitates. • Effects of the morphology and size of the L1 2 precipitates on the adiabatic shear sensitivity of the W-HEA was revealed. • Large interfacial energy of the square L1 2 precipitates is the main reason for the formation of adiabatic shear band. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposing a solid-state transformation control solution to additive manufacturing of fine-grain alloys. • Developing a high-scanning-speed laser deposition technique to prepare fine-grain fully lamellar TiAl. • The fine-grain fully lamellar TiAl overcome the conflict between room-temperature properties and creep resistance. • The fine-grain fully lamellar TiAl can be applied to a high temperature up to 900 °C. Additive manufacturing (AM), or 3D printing, has attracted increased attention in producing metallic parts with complex geometries, but it has proved difficult to prepare equiaxed fine-grain parts because the high thermal gradient in solidification commonly conduces the formation of coarse columnar grains. This work shows a solution to fine-grain titanium aluminide (TiAl) alloys by designing a high-frequency thermal cycling to control the solid-state phase transformations in layer-by-layer AM. After solidification, the specially-designed high-frequency thermal cycling can significantly refine the microstructure by repeatedly inducing the nucleation of new grains and suppressing the growth of these newborn fine grains. Therefore, even if solidification leads to coarse columnar grains, equiaxed fine-grain microstructure can still be obtained by solid-state phase transformations. The resulting TiAl alloys have fine heteromorphic grains (∼50 μm) and a fully lamellar microstructure. These fine grains contribute to good strength-ductility balance at room temperature, and their irregular shape and fully lamellar microstructure restrict the flow and distortion of grains at high temperatures, which stands a chance to significantly increase the operating temperature of TiAl parts. [ABSTRACT FROM AUTHOR]

Coherent ordered nanoprecipitation (CON) hardening is an efficient strategy for breaking the strength-ductility trade-off. Revealing the precipitation mechanism and its influence on mechanical property is significant for further optimization of CON-strengthened alloys. In this work, we investigated the precipitation and mechanical behavior of the coherent FCC/L1 2 spinodal nanostructure and nano-lamellar BCC phase in a newly developed CON-strengthened Al 0.5 Cr 0.9 FeNi 2.5 V 0.2 high entropy alloy (HEA). We found that high-density defects induced by pre-deformation promoted the segregation of Cr, resulting in composition redistribution. The thermodynamics state of the matrix shifted into a spinodal regime and the ordering energy of L1 2 phase increased, facilitating the formation of coherent FCC/L1 2 spinodal nanostructure. Meanwhile, the Cr segregation promoted the precipitation of Cr-enriched nano-lamellar BCC phase. During tensile deformation, the FCC phase yielded first, followed by L1 2 and BCC phases in sequence. The L1 2 and BCC strengthening phases contributed to an ultrahigh macroscopic yield strength. Due to the low-misfit interconnected spinodal nanostructure, the differences in phase stress between FCC and L1 2 phases was insignificant, which could avoid the stress concentration and retain ductility. The nano-lamellar feature of the stiff BCC phase delayed the crack initiation and propagation. Our research revealed not only the significant effect of pre-deformation on CONs formation and refinement, but also the deformation mechanism of CONs, which shed new light on the microstructural optimization and mechanical property improvement of CON-strengthened alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Including ductile solid-solution phases into NiAl intermetallic alloys can toughen these materials, but based on conventional alloying strategy, this approach either degenerates the strength or has no visible improvement in the room-temperature (RT) ductility. Here we show a feasible strategy to improve the RT ductility of NiAl alloys by replacing conventional dilute solid solutions with complex concentrated (i.e. , high-entropy) solid solutions as the ductilizing phases. The resulting alloy possesses a composite nanostructure, which combines B2-structured Ni(Al, Fe) phase with body-centered-cubic (BCC) and face-centered-cubic (FCC) structured high-entropy phases (HEPs) in different scales. We find that the applied stress is mainly partitioned to the B2 phase, contributing the alloy to high strength, while the ductile BCC and FCC HEPs, although bearing low loading, cooperate with the stress-induced martensitic transformation to accommodate the deformation, enhancing the sample ductility. Accordingly, our alloy presents superior tensile properties in a wide temperature range, including high strength (>1500 MPa at RT and >360 MPa at 700 °C) and good ductility (>9% at RT and ~8% at −50 °C), which achieves a simultaneous improvement in the ductility and strength compared with the conventional multiphase NiAl alloys. • Proposing a strategy to improve the ductility of multiphase NiAl alloys. • Revealing the distinctive ductilizing effect of high-entropy solid solutions. • Presenting the contribution of B2→L1 0 martensitic transformation to the ductility • Achieving a simultaneous improvement in the ductility and operating temperature. [ABSTRACT FROM AUTHOR]

• Fine-grained tungsten heavy alloy (FG-WHA) is prepared by liquid phase sintering. • Novel matrix that reduces W grain growth rate brings about FG-WHA. • Fine W grain in WHA contributes to a high ultimate tensile strength of 1267 MPa. • The FG-WHA exhibit a nearly full density and a good elongation of 10%. It is difficult to prepare fine-grain tungsten heavy alloys (WHAs), e.g. W-NiFe, by liquid phase sintering (LPS) mainly because of high solubility of W in the NiFe matrix. To obtain fine W grains, this work proposes to reduce W grain growth rate by selecting a high entropy alloy (HEA) with low diffusivity and solubility of W as the matrix of WHA. The results show that the LPS W-HEA has a fine W grain size of ~9.7 μm, which is about a third of the value in the conventional LPS W-NiFe. The fine W grains in the as-sintering W-HEA contribute to high yield strength and ultimate tensile strength of 840.8 MPa and 1267.3 MPa, respectively. These results suggest a new pathway for obtaining a fine-grain WHA by tailoring the matrix composition. [ABSTRACT FROM AUTHOR]