Effect of heat input on nanomechanical properties of wire-arc additive manufactured Al 4047 alloys

Heat input is one of the most important process parameters during additive manufacturing (AM). It is of great significance to understand the effect of heat input on the microstructure and nanomechanical properties, as well as the underlying mechanisms. Wire-arc additive manufactured (WAAM-ed) Al 4047 alloys under different heat inputs were produced and studied in this work. The as-manufactured Al alloys showed hypoeutectic microstructure that consisted of primary Al (α-Al) dendrite and ultrafine Al–Si eutectic. The effect of heat input on hardness and strain rate sensitivity (SRS) were investigated through nanoindentation. The nanohardness decreased with the increasing heat input, in accordance with the trend of yield strength and microhardness in the previous studies, in which the mechanism was usually explained by the grain growth model and Hall-Petch relationship. This work suggests a distinct mechanism regarding the effect of heat input on nanohardness, which is the enhanced solid solution strengthening produced by lower heat input. In addition, the heat input had little effect on the SRS and activation volume. It is hoped that this study leads to new insights into the understanding of the relation between heat input and nanomechanical properties, and further benefits to improve the targeted mechanical properties and engineering applications of the AM-ed materials.publishedVersio

A Surfactant for Enhanced Heavy Oil Recovery in Carbonate Reservoirs in High-Salinity and High-Temperature Conditions

In view of the difficulty of producing heavy oil from carbonate reservoirs, the surfactant SDY-1 was synthesized by homogeneous solution polymerization with a homogeneous solution polymerization technique using aliphatic amine polyoxyethylene ether (PAEn) H(OCH2CH2)nNR(CH2CH2O)nH as the raw material, epichlorohydrin as the reaction intermediate, tetrabutylammonium bromide and pentamethyldivinyltriamine as the promoters, and alkylphenol as the catalyst. Based on the analysis of reservoir fluid and rock properties, the performance of the surfactant SDY-1 was evaluated in terms of its heat resistance, its salinity tolerance, its ability to change the heavy oil–water interfacial tension and rock wettability and its oil washing efficiency. The results show that when the salinity of the formation water is 2.23 × 105 mg/L, the addition of surfactant SDY-1 can lower the super-heavy oil–water interfacial tension with an asphaltene concentration of 30.19 wt.%, which is aged at a temperature of 140 °C for 3 days, from 22.41 to 0.366 mN/m. In addition, the surfactant SDY-1 can change the contact angle of super-heavy oil–water–rock from 129.7 to 67.4° and reduce the adhesion of crude oil to the rock surface by 99.26%. The oil displacement experiment indicates that the oil washing efficiency of the surfactant SDY-1 can reach 78.7% after ageing at a temperature of 140 °C for 3 days. Compared with petroleum sulfonate flooding, the addition of SDY-1 can improve the displacement efficiency by 33.6%, and the adsorption loss is only 0.651 mg/g oil sand. It has broad application prospects for heavy oil reservoirs with high temperatures, high pressures and high asphaltene contents

Assembly graphene platelets for bioinspired, stimuli-responsive, low ice adhesion surfaces

Design and fabrication of functional materials for anti-icing and de-icing attract great attention from both academic research and industry. Among them, the study of fish-scale-like material has been proved that enabling sequential rupture is an effective approach for weakening the intrinsic interface adhesion. Here, graphene platelets were utilized to construct fish-scale-like surfaces for easy ice detachment. Using a biomimicking arrangement of the graphene platelets, the surfaces were able to alter their structural morphology for sequential rupture in response to external forces. With different packing densities of graphene platelets, all the surfaces showed universally at least 50% of reduction in atomistic tensile ice adhesion strength. Because of the effect of sequential rupture, stronger ice-surface interactions did not lead to an obvious increase in ice adhesion. Interestingly, the high packing density of graphene platelets resulted in stable and reversible surface morphology in cyclic tensile and shearing tests, and subsequently high reproducibility of sequential rupture mode. The fish-scale-like surfaces built and tested, together with the nanoscale de-icing results, provided a close view of ice adhesion mechanics, which can promote future bio-inspired stress-responsive anti-icing surface designs

Effects of Morphology and Temperature on the Tensile Characteristics of Carbon Nitride Nanothreads

Very recently synthesized carbon nitride nanothreads (CNNTs) by compressing crystalline pyridine show outperformance in chemical and physical properties over diamond nanothreads. Here, using first-principle based ReaxFF molecular dynamics (MD) simulations, a comprehensive investigation on mechanical characteristics of seven experimentally synthesized CNNTs is performed. All the CNNTs exhibit unique tensile properties that change with molecular morphology, atomic arrangement and the distribution of nitrogen in the skeleton. CNNTs with more effective loading covalent bonds at cross-sections are more mechanically robust. Surprisingly, tiny CNNT with periodic unit structures of 5462-cage shows extreme ductility because of formation of linear polymer via 4-step dissociation-and-reformation of bonds at extremely low temperature of 1 to 15 K, however, it yields by brittle failure at one cross-section with low ductility at higher temperature, similar to other CNNTs at different temperatures, which offers a feasible way to design a kind of lightweight material that can be used in ultra-low temperature conditions, for example, harsh deep space environment. Results also show that temperature significantly affects the fracture stress and rupture strain but not the effective stiffness. Analysis of atomic bond orders and bond lengthening reveals that the unique nonlinear elasticity of CNNTs is attributed to the occurrence of local bond transformations. This study provides physical insights into the tensile characteristics of CNNTs for design and application of the CNNT-based nanostructures as multifunctional materials

Tensile Mechanical Characteristics of Thinnest Carbon Sulfur Nanothreads in Orientational Order

Carbon sulfur nanothreads (CSNTs) mainly composed of two chiral long alkane chains have been recently fabricated from thiophene by a pressure-induced phase transition in low-temperature, but their mechanical properties remain unexplored. Here, the critical roles of morphology and temperature on the tensile characteristics of CSNTs are for the first time examined using molecular dynamic simulations with first-principles-based ReaxFF forcefield. It is revealed that CSNTs exhibit high tensile Young’s modulus, high tensile strength and excellent ductility, and their tensile properties are morphology and temperature dependent. Morphologically, atomic arrangement with various configurations makes every CSNTs possess unique mechanical properties. Thermally, as temperature varies from 1-1500 K, CSNTs become mechanically weakened. In comparison with conventional diamond nanothreads (DNTs) and carbon nitride nanothreads (CNNTs), CSNTs show distinct axial elongation mechanisms, with relatively insignificant changes in chemical bond orders and bond length in the skeleton prior to the final rupture. Instead, the stretching of bond angle and dihedral angle mainly contribute to the global axial elongation, while the torsional deformation is limited due to their perfect global symmetry in the configuration. This study provides fundamental insights into the mechanics of ultra-thin CSNT structures

Nanomechanical Characteristics of Trapped Oil Droplets with Nanoparticles: A Molecular Dynamics Simulation

Nanoparticles (NPs) possess great potentials in applications to enhanced oil recovery (EOR), the underlying mechanisms of which however remain to be explored. In this study, the motion of NPs and the local pressure distribution in a trapped oil droplet/nanofluid system in confined nanochannels are scrutinized by molecular dynamic simulations. Depending on the particle wettability, three different motion patterns have been observed: hydrophilic NPs are more likely to be adsorbed on the solid surface of the channel and stay close to the three-phase contact areas, hydrophobic NPs tend to move inside the oil droplet as clusters, and NPs with mixed hydrophobicity are prone to be trapped at the oil-water interface. It is shown that the existence of NPs introduces high local pressure in the nanochannels, especially at locations where NPs aggregate. Significantly, in the three-phase contact area for hydrophilic NPs, the local pressure distribution features the postulated structural disjoining pressure reported in the literature. For the first time, our molecular dynamics simulation results elucidate nanoparticle-induced structural disjoining pressure at the atomistic scale. The results thus provide a better understanding on the fundamentals of nanofluids in confined channels and serve as guidelines for the design of NPs for EOR applications

Microstructure and nanomechanical behavior of an additively manufactured (CrCoNiFe)94Ti2Al4 high-entropy alloy

Recent substantial studies indicate that high-entropy alloys (HEAs) possess superior mechanical performance, including exceptional strength, high creep resistance, etc. However, additive manufacturing (AM), a burgeoning manufacturing method, may induce extraordinary impacts on the resulting mechanical properties. For the additively manufactured (AM-ed) HEAs, the nanoscale mechanical performance and deformation mechanisms in accordance with the microstructural properties remain unclear. In this work, the microstructure and nanomechanical properties of an AM-ed (CrCoNiFe)94Ti2Al4 HEA were investigated. The local mechanical properties including hardness, elastic modulus, and nanoscale creep deformation, were explored by nanoindentation-based measurement. Simultaneously, the crystallographic orientation dependence on the mechanical behavior of AM-ed HEA was carried out by combining with electron backscattered diffraction (EBSD). It is found that the {101}-grain has the highest hardness and elastic modulus, whereas the creep resistance of {111}-grain is the greatest, with the indicators of the creep mechanism showing lattice diffusion is the dominant mechanism. Two different states of HEA, as-printed and heat-treated, were utilized to explore the effect of heat treatment. Heat treatment in the current study can increase the hardness and elastic modulus but decrease the creep resistance slightly. This work elucidates the underlying mechanisms of grain orientation dependence on nanomechanical properties and the effects of heat treatment. Moreover, it also sheds light on the particular creep behavior at the nanoscale and creep mechanism of the AM-ed (CrCoNiFe)94Ti2Al4 HEA

A framework for predicting the local stress-strain behaviors of additively manufactured multiphase alloys in the sequential layers

The additive manufacturing (AM) process often results in non-uniform microstructure and different mechanical properties in sequential layers, impacting the overall performance of the AM-ed component. However, it is extremely challenging to evaluate the local stress-strain behavior of each individual layer, owing to the limited size of the AM-ed layered structure. To this end, a framework for characterizing and predicting the mechanical evolution of AM-ed multiphase alloys by combing nanoindentation and microstructure-based finite element method (FEM) was proposed. The sample used in this study was superduplex stainless steel (SDSS) manufactured by wire arc additive manufacturing (WAAM), and the microstructure varied from layer to layer. Firstly, the mechanical properties of the two constituent phases in each layer, including elastic modulus and hardness, were obtained by nanoindentation, and the indentation size effect (ISE) was also evaluated. The yield strength and hardening exponent of each phase were subsequently estimated by reverse analysis method, and therefore the constitutive behaviors of the individual phase, which served as input parameters for FEM, were acquired. By aid of real microstructure-based FEM under uniaxial tension, the overall stress-strain behaviors of each layer and the distributions of the stress and strain during the deformation process were investigated. This work provides a new avenue for the characterization of the multiphase alloys in AM industry, beneficial to the understanding of the mechanical evolution in AM-ed materials

Microstructure and nanomechanical behavior of an additively manufactured (CrCoNiFe)94Ti2Al4 high-entropy alloy

Recent substantial studies indicate that high-entropy alloys (HEAs) possess superior mechanical performance, including exceptional strength, high creep resistance, etc. However, additive manufacturing (AM), a burgeoning manufacturing method, may induce extraordinary impacts on the resulting mechanical properties. For the additively manufactured (AM-ed) HEAs, the nanoscale mechanical performance and deformation mechanisms in accordance with the microstructural properties remain unclear. In this work, the microstructure and nanomechanical properties of an AM-ed (CrCoNiFe)94Ti2Al4 HEA were investigated. The local mechanical properties including hardness, elastic modulus, and nanoscale creep deformation, were explored by nanoindentation-based measurement. Simultaneously, the crystallographic orientation dependence on the mechanical behavior of AM-ed HEA was carried out by combining with electron backscattered diffraction (EBSD). It is found that the {101}-grain has the highest hardness and elastic modulus, whereas the creep resistance of {111}-grain is the greatest, with the indicators of the creep mechanism showing lattice diffusion is the dominant mechanism. Two different states of HEA, as-printed and heat-treated, were utilized to explore the effect of heat treatment. Heat treatment in the current study can increase the hardness and elastic modulus but decrease the creep resistance slightly. This work elucidates the underlying mechanisms of grain orientation dependence on nanomechanical properties and the effects of heat treatment. Moreover, it also sheds light on the particular creep behavior at the nanoscale and creep mechanism of the AM-ed (CrCoNiFe)94Ti2Al4 HEA