Nowadays, it is challenging to completely eliminate low angle or high angle grain boundaries (LAGBs or HAGBs) from Nickel-based single crystal (SX) superalloys manufactured using the conventional ...directional solidification technique. The additions of C, B and Hf have been found to be an effective measure in improving the damage resistance of grain boundary (GB) defects, and thus increasing the creep resistance. However, the strengthening mechanism through their additions is still unclear. In this study, a double-seed solidification technique with two misorientation levels, i.e., 5° and 20°, was used to produce a series of bicrystal superalloys with different contents of Hf and B. It is the first report of an alloy with joint Hf and B addition that demonstrates tolerance to GBs with a misorientation as high as ∼20° under all of the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. Interestingly, the effect of individual additions of Hf or B was not as pronounced as that of the joint Hf and B addition. To understand the influence of these additions on the creep mechanism in nickel-based superalloys with GB defects, a detailed characterization of the microstructures in the vicinity of the LAGBs or HAGBs was carried out, and the elemental distribution at the HAGBs was analyzed with various techniques. This study will be beneficial for understanding the role of Hf and B additions on improving the GB tolerance, and optimizing the Hf and B additions in nickel-based single crystal superalloys.
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In this study, the heterogeneous anisotropic microstructure and mechanical properties of additively manufactured (CoCrFeMnNi)99C1 high-entropy alloy (HEA) are comprehensively investigated using ...experimental and theoretical analyses. For the present alloys, the selective laser melting (SLM) process produced orthogonally anisotropic microstructure with not only strong macroscopic morphological but also sharp microscopic crystallographic textures. Moreover, due to the complex thermal gradient and history in the melt pools, the columnar grains were heterogeneously evolved along the building direction with alternatively arranged layers of fine and coarse grains parallel to the laser scanning direction. This unique morphological texture played a dominant factor for the big difference in tensile properties between different loading directions in the early stage of deformation. In particular, the alternatively arrangement of fine and coarse grains could generate high hetero-deformation induced (HDI) hardening along the scanning direction in the as-built samples by profuse evolution of geometrically necessary dislocation at the boundaries of each layer. On the other hand, upon the last stage of plastic deformation, the crystallographic texture played a crucial role in directional flow behavior by modulating twinning activity. The combined contribution of the various anisotropic microstructural factors to the tensile properties of the SLM-processed HEAs was clarified both qualitatively and quantitatively. This work will shed light on effective utilization of both heterogeneity and anisotropy of the structural parts for customized performance via expanding multi-scale freedom of design in additive manufacturing.
•There are synergistic effects between FA and MEA on the expansion behavior and mechanical properties of cement paste.•The combination of FA and MEA results in only the gel pores and transition pores ...in the cement matrix.•The MEA-enhancing alkaline environment promotes the formation of more fibrous C-S-H on the surface of FA.
To expand the use proportion of fly ash (FA) and MgO expansive additive (MEA) in engineering, this paper explores the feasibility of a new cementitious material, which conforms to the dual-carbon background. The synergistic effects of FA and MEA on the expansion behavior and mechanical properties of cement paste are confirmed by designing experiments with the dosage and curing temperature as variables. Moreover, the formation mechanism of synergism is analyzed through isothermal calorimetry, XRD, TG, 29Si NMR, MIP, SEM and EDS in combination. The experimental results show under 40 °C water curing, the addition of 40 % FA and 10 % MEA not only amplifies the early expansion, but also controls well the later expansion in a stable state. Besides, the 90-day compressive and flexural strengths increase by up to 20.8 % and 15.6 %, respectively. FA reduces the effect of MEA on the hydration heat of cement significantly, making the hydration rate curves of the two systems about Krstulovic-Dabic model highly consistent. The MEA-enhanced alkaline environment promotes the secondary hydration of cement by improving the hydration degree of FA. The reduction of cumulative pores volume and refinement of pore size are promoted obviously, resulting in only the gel pores and transition pores in the matrix. The existence of FA avoids the centralized distribution of Mg-containing hydration products, while there is more fibrous C-S-H generated on the surface of FA. In addition, the hydration model of the new cementitious material has been established.
A series of Co-free CrFeNiAlxTiy medium-entropy alloys (MEAs) were designed and prepared. The effect of Al and Ti addition on phase evolution, microstructure, nano- and marco-mechanical behaviors ...were systematically investigated and analyzed. The density of the CrFeNiAlxTiy MEAs was measured by the Archimedes method. With the content of Al and Ti increasing, the density of the CrFeNiAlxTiy system decreased. The density of the CrFeNiAl0.3Ti0.3 MEA is only 6.9294 g/cm3. The microstructural analysis indicated that the addition of Al element increased the volume fraction of BCC phase, while the Al and Ti addition increased the volume fraction of BCC and B2 phases. The mechanical tests suggested that the hardness, compressive yield strength, fracture strength, specific yield strength, and specific fracture strength all increased with the Al and Ti addition. The CrFeNiAl0.3Ti0.3 MEA possessed high Vickers hardness, compressive yield strength, fracture strength, specific yield strength, and specific fracture strength, whose values are 551 HV, 1712 MPa, 3700 MPa, 247.06 MP cm3/g, and 533.96 MP cm3/g, respectively. Furthermore, the compressive plasticity of the CrFeNiAl0.3Ti0.3 MEA is also very large (~39.8%). The CrFeNiAl0.4Ti0.2 MEA also displayed outstanding mechanical behaviors just as the CrFeNiAl0.3Ti0.3 MEA, whose Vickers hardness, yield strength, and compressive plasticity are 580 HV, 1600 MPa, and 45%, respectively. The good combination of strength and compressive plasticity of CrFeNiAl0.3Ti0.3 and CrFeNiAl0.4Ti0.2 MEAs could attribute to the peculiar microstructure. The phase formation criteria and strengthening mechanism were discussed. This study provides insights not only into the BCC/B2-containing MEAs but also into the future development of MEAs with high-performance, low-cost, and low-density for industrial applications.
To realize the systematic comparison of the hot workability and guide the further hot-processing of powder metallurgy (PM) and ingot metallurgy (IM) Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloys, the hot ...deformation behaviour and microstructural evolution of the two alloys were investigated at a wide temperature range of 700 °C–1100 °C and strain rate of 0.001 s−1-10 s−1. The activation energy maps and processing maps for both PM and IM alloys were constructed, as well as the specific deformation mechanisms were identified for each processing region. The results showed that PM alloy has lower deformation resistance, smaller activation energy and larger optimal processing windows than those of IM alloy. The dynamic α precipitation mechanisms in PM alloy were diffusional globularization and coarsening, rather than diffusionless shearing and fracturing in IM alloy. The extensive dynamic recrystallization (DRX) happened at 900 °C–1050 °C for PM alloy and at 1000 °C–1100 °C for IM alloy. The DRX process was dominated by discontinuous dynamic recrystallization (DDRX) for PM alloy while continuous dynamic recrystallization (CDRX) for IM alloy. Furthermore, PM alloy had smaller flow instability region than IM counterpart in the hot processing map. The schematic deformation mechanism maps were eventually developed for both PM and IM Ti-5553 alloys.
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•Ti-5553 alloy with better workability than ingot-casting (IM) counterpart was prepared by fast powder-consolidation (PM).•Activation energy maps and processing maps of PM and IM alloys were constructed to evaluate their hot workability.•Comprehensive comparison of deformation mechanisms of the alloys is achieved by the deformation mechanism maps.•Different dynamic recrystallization mechanisms were identified in PM and IM alloys.•Different dynamic α precipitation mechanisms were verified and characterized in PM and IM alloys.
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•Deformation parameters have significant influences on the grain microstructure.•The dislocation substructure is also very sensitive to the deformation parameters.•The bulging of ...initial grain boundaries is the main nucleation mechanism of DRX.•The complete DRX domains are determined by DRX volume fractions contour map.
Hot compressive tests of a nickel-based superalloy are performed under the strain rate range of 0.001–1s−1 and deformation temperature range of 920–1040°C. Optical microscopy (OM) and transmission electron microscopy (TEM) are employed to investigate the evolution of dynamic recrystallized (DRX) grain and dislocation substructure. It is found that the effects of deformation degree, strain rate and deformation temperature on DRX grain are significant. When the deformation degree or temperature is increased, the number of DRX grains rapidly increases. But, the increase of strain rate reduces the number of DRX grains. The dislocation substructure is also very sensitive to the deformation degree, strain rate and deformation temperature. With the increase of deformation degree, the evolution of dislocation substructure can be characterized as: high dislocation density→dislocation network→subgrain→DRX grain. Under high deformation temperatures or low strain rates, the dislocation substructure can be easily annihilated and rearranged because of the occurrence of DRX. Based on the evaluated DRX volume fractions, the contour map is constructed to optimize the hot deformation parameters.
•The ductility of Inconel 718 ultrathin sheet increases notably with assistance of ultrasonic vibration.•The softening effect is unapparent due to the distinct properties of superalloy.•The grain ...misorientations are significantly changed by the ultrasonic vibration.•A hybrid constitutive model is developed considering stress superposition and acoustic softening separately.
Ultrasonic vibration (UV) has been widely applied in metal forming process due to its ability to improve the material properties. However, the mechanical response of difficult-to-deform materials with the assistance of UV is unclear, especially for the ultrathin sheet metals. Meanwhile, the current constitutive models are deficient for the description of UV assisted deformation behavior of the ultrathin material. In this research, UV assisted tensile tests with different amplitudes were performed on the ultrathin superalloy sheet, and the influences on the mechanical characteristic and microstructural evolution were explored. The experimental results indicated that the UV induced softening effect is not evident for high strength material, which is attributed to the tensile deformation characteristic of ultrathin sheet and the distinct mechanical property of superalloy. Nevertheless, the ductility of the superalloy sheet is notably enhanced under the ultrasonic amplitude of 3.18 μm, featured with the increase of elongation from 20.7% to 25.5%. Moreover, the texture is enhanced with increasing ultrasonic amplitude, and the grain misorientations are significantly changed under different UV conditions. To accurately describe the constitutive behavior of superalloy under the assistance of UV, a hybrid constitutive model separately considering stress superposition and acoustic softening effect was developed based on the dislocation evolution theory. The predicted results are in good agreement with the experimental data. These findings provide a more in-depth understanding on the mechanism of UV assisted forming and promote the application of UV assisted manufacturing processes for difficult-to-deform alloys.
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The semi-solid 6061 aluminum alloy slurry was prepared by a serpentine channel pouring process. The effects of pouring temperature, bend number and bend diameter on the microstructures were ...investigated. Microstructural evolution mechanism of the semi-solid slurry during the pouring process was also analyzed. The results show that the grain is refined and the grain roundness is improved by controlling the pouring temperature close to the liquidus temperature, and the nucleation rate of primary α(Al) grains is effectively increased via increasing the bend number and decreasing the bend diameter. The primary grains are not only formed directly from the alloy melt via chilling nucleation and heterogeneous nucleation, but also evolved from the fractured dendrite fragments. Meanwhile, the heat exchange between the melt and the serpentine channel is increased by the “self-stirring” effect in the melt, which also promotes the refinement and spheroidization of primary α(Al) grains.
We investigated hardness and tribological properties of ultra-fine grain (UFG) aluminum samples prepared by accumulative rolling bonding (ARB) and by physical vapor deposition (PVD). We have found ...wear-induced grain refinement in this material for the first time and we have identified a transition from grain growth to grain refinement as a function of the mean contact stress. This trend is the same for the PVD and ARB samples, which means it does not depend on the synthesis method. The hardness was found to increase with a decreasing grain size, but the results deviated from the Hall-Petch relation. Among ARB samples, UFG Al with the highest initial hardness was found to be the most wear-resistant. However, PVD-grown UFG Al was found to be most wear-resistant among all samples, even though it was not the hardest. The reason lies in the different dislocation contents of the samples prepared by different methods and in a subsequent wear-induced evolution of dislocation networks. The mechanisms of microstructural evolution in UFG Al were analyzed using transmission electron microscopy and the main mechanism identified is the dynamic recrystallization (DRX), including both continuous DRX and discontinuous DRX.
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