Abstract
Nonplanar penta-graphene and planar phagraphene, which are connected by carbon pentagons and penta–hexa–hepta carbon rings, respectively, are two allotropes of graphene. Graphene as a star ...material in two-dimensional materials has been widely studied. However, the studies around penta-graphene and phagraphene are still insufficient. We are interested in both materials’ response to temperature, hydrostatic pressure, and stress. In this work, the thermal expansion, linear compressibility, and Poisson’s ratio of penta-graphene and phagraphene have been investigated systematically. It is found that both materials can exhibit abnormal negative thermal expansion behavior, while their linear compressibility behavior is normal. The negative Poisson’s ratio behavior only occurs in penta-graphene, which is consistent with other work. Through an analysis of the lattice vibrations and associated mode Grüneisen parameters, it is found that there are anomalies in the phonon spectra of both penta-graphene and phagraphene. It is noted that acoustic phonons contribute most to their respective anomalies, especially the transverse acoustic mode. The librational motion of the lowest-frequency optical phonon of both materials is identified and also associated with their novel properties. In general, the unique topological arrangement of carbon atoms can play a decisive role in determining the performances of penta-graphene and phagraphene.
Vacancy-ordered superstructural phases of zirconium carbide have been intermittently observed at low temperatures for over 50 years. However, little is known about these ordered phases as they have ...proven to be challenging to fabricate experimentally, although theoretical predictions suggest that they should be significantly more stable than the more-observed vacancy-disordered solid solution ZrCx (x ≤ 1) phase at low temperatures. The stability and structural properties of the vacancy-ordered and vacancy-disordered phases are investigated using first-principles calculations. The stability of the ordered superstructural phases is related to the driving force from the relative instability of certain vacancy configurations, which are preferred or avoided in ordered structures. The trend of the vacancy ordering and the underlying mechanisms of the relative instability are explained in terms of the geometry of the crystal structures and the electronic charge distribution and atomic bonding features.
•A crystal plasticity-based approach is implemented to predict creep-fatigue life.•Creep and fatigue indicator parameters are introduced to describe damage evolution.•A potential methodology for ...conservative creep-fatigue life evaluation was provided.
A numerical process based on crystal plasticity finite element (CPFE) was implemented to predict creep-fatigue crack initiation life. CPFE-based model can describe the macroscopic cyclic deformation and reveal grain-level damage mechanism. A new life prediction approach was then constructed by introducing fatigue and creep indicator parameters. Furthermore, a series of strain-controlled creep-fatigue tests in GH4169 superalloy at 650℃ were used to validate predicted accuracy of this model, where most of the data points lied within ±1.5 error band. Finally, a potential methodology for conservative creep-fatigue life evaluation was provided by flexible creep-fatigue damage summation rule in engineering applications.
In this study, the change of the strength of B-rich Fe-based amorphous alloy was investigated as a function of its microstructure controlled by the annealing at temperatures from 713 to 873 K. The ...strength was measured by using a micro-tensile test in a focused ion beam apparatus, and its machinability was tested by a blanking test. It was found that the static strength and machinability of the alloy varied drasticaly depending on its microtexture. In the micro-tensile test, the as-received sample exhibited ductile fracture with a tensile strength of about 1200 MPa, concurring with the bulk material. During the annealing up to 773 K, fine Fe3B crystals stared to precipitate sparsely in the matrix, and α-Fe crystals grew around Fe3B the crystals. Due to this microstructural change, the tensile strength dropped down to about 300 MPa, and the fracture mode changed from ductile to brittle, and the toughness of the alloy decreased significantly. However, when the annealing temperature exceeded 773 K, the microtexture changed to clear polycrystalline and thus, large plastic deformation reappeared, and the tensile strength returned to about 900 MPa. The blanking test revealed that the annealing reduced the blanking resistance significantly, in particular, in the sample annealed at 763 K. High-quality machining traces without burrs and shear droops were obtained from this sample. From the viewpoint of machinability, it was concluded that the microstructure with the fine precipitates dispersed in the amorphous matrix was most appropriate for this amorphous alloy.
Display omitted
•We propose a new blanking method based on the transformation of amorphous alloys.•Material strength is evaluated in micrometer-order regions where blanking occurs.•Static strength and machinability are markedly affected by microstructural changes.•Tensile strength decreases from 1200 to 300 MPa owing to microstructural changes.•Structure of fine crystals dispersed in the amorphous phase is ideal for blanking.
This paper proposes that it will be an effective way to discover and explore organic negative thermal expansion (NTE) materials based on the specific topologies in inorganic NTE materials. Various ...NTE behaviors from the uniaxial, area, and volume-NTE can be achieved by adjusting the topology, for instance, quartz-like and diamond-like. Zn(ISN) 2 and InH(BDC) metal–organic frameworks (MOFs) with quartz-like topology have been studied by first principles calculations. The calculated area-NTE of Zn(ISN) 2 and uniaxial-NTE of InH(BDC) within quasi-harmonic approximation (QHA) agree well with the experimental data. Through the calculation of Grüneisen parameters, it is shown that low-frequency optical phonons appear dominant resulting in their NTE, but the coupling to high-frequency phonons is of greater ultimate importance. The lattice vibrational modes of great contribution to area-NTE of Zn(ISN) 2 and uniaxial-NTE of InH(BDC) are analyzed in detail. Also, four MOFs with diamond-like topology are predicted to exhibit volume-NTE behavior. Moreover, it is found that there is a bulk modulus anomaly in some studied MOFs with the quartz-like and diamond-like framework, where the temperature dependence of bulk modulus does not follow the inverse dependence on that of volume. These specific topologies provide key geometric frameworks for various NTE behaviors of MOFs, and meanwhile, the local structure and bond environment in MOFs can lead to abnormal interatomic force, i.e. , bulk modulus anomaly. This abnormal elastic property also deserves more attention.
Display omitted
•Hole microspheres MoS2@reduced graphene oxide composites were successfully synthesized.•MoS2@reduced graphene oxide on Ti plate exhibited high catalytic activity.•Enhanced activity ...was attributed to edge sites, high specific surface area and synergic effect.
The pure MoS2 and MoS2@reduced graphene oxide were synthesized on commercially available Ti plate by one-step hydrothermal method. The hole microspheres were successfully synthesized. The resulting MoS2@reduced graphene oxide electrode on Ti plate exhibited higher catalytic activity than pure MoS2 electrode on Ti plate for hydrogen evolution reaction. In addition, the excellent cycling stability was also obtained in MoS2@reduced graphene oxide electrode on Ti plate. Such enhanced catalytic activity and excellent cycling stability occurred due to the abundance of catalytic edge sites, high specific surface area and the unique synergic effects between the MoS2 and reduced graphene oxide.
In this study, intermittent creep‐fatigue tests were applied on Ni‐based Alloy 617 and Alloy 625. Scanning electron microscope (SEM) observation and electron back‐scatter diffraction (EBSD) analysis ...were employed to the evaluation of the degradation of the crystallinity under the creep‐fatigue loads. It was found that the initial damage under creep‐fatigue load basically appeared as intergranular cracks. In addition, the lifetimes of Alloy 625 samples fluctuated significantly due to the growth of NbC precipitates. The initial damage of these two alloys was dominated by the growth and accumulation of dislocations and vacancies around the interface that consisted of large lattice mismatch. Local atomic diffusion was activated when the summation of the nominal stress and local stress caused by the large lattice mismatch exceeded a critical value. The stress‐induced acceleration of the degradation of the crystallinity of the alloys was analyzed by applying the modified Arrhenius equation. Instead of the most popular inversion methods according to the final failure mode, the quantitative description in life prediction was developed based on the dynamic progress for acceleration mechanism of the degradation. It is of importance to perfecting the frontiers of damage mechanics approach.
Highlights
The degradation mechanism was obtained by analyzing the damage behavior of two alloys.
IQ value was used to characterize the degree of degradation under creep‐fatigue load.
The degree of degradation and life assessment was expressed quantitatively.
•Filling the gaps of creep-fatigue tests under non-proportional loading.•Enriching dataset with stress triaxiality in the range of 0 to 0.33.•Revealing interactions between addition hardening and ...creep.•Improving a viscoplasticity model for multiaxial constitutive behavior.•Providing reasonable life prediction results with an energy-based method.
In this paper, a series of strain-controlled fatigue and creep–fatigue tests under proportional/non-proportional loadings were performed for type 304 stainless steel at 873 K. Then, post-test metallographic observations were performed through the electron back scattered diffraction (EBSD) and transmission electron microscope (TEM) combinative characterizations. In this aspect, the wavy slip dominated deformation mechanism under non-proportional loadings was considered as the essence for additional hardening, while the introduction of creep resulted in further microstructure evolutions by facilitating recrystallization. Afterward, a unified viscoplasticity constitutive model was proposed to simulate the cyclic stress–strain responses, in which an additional hardening parameter combined with a loading-path parameter was used to describe the cyclic hardening curves. Concurrently, stress triaxiality was introduced to provide accurate descriptions for the stress relaxation behavior. Semi-physical continuum damage models involving multiaxial damage factor and non-proportional strain energy parameter was proposed to predict the multiaxial creep–fatigue damage evaluations. Good agreements between experimental data and simulated results were achieved with the help of the proposed numerical procedures.
Recently, substantial attention has been paid to the strain sensitivity of the carbon nanotubes＇ （CNTs＇） electronic properties. In this study, the relationships between the geometric structures and ...electronic states of zigzag CNTs under uniaxial compressive strain were investigated. We found that different factors dominate the electronic states of zigzag CNTs depending on the strain regions： the initial stage of the strain loading, which lasts until column-buckling deformation begins, and the strain regions corresponding to column- and shell-buckling deformations. Because shell-buckling deformation significantly increases the re-orbital angle, the angle between the π orbital axis vectors of adjacent atoms, strong localization of the density of states （LDOS） occurs in the buckled area. We also analyzed the current able to pass through deformed CNTs using a tight-binding-based Green＇s function approach and determined that the current can be significantly suppressed by applying uniaxial compressive strain. Our method of predicting the electronic state of a deformed CNT based on the π-orbital angle is expected to be useful for predicting the electronic properties of CNT-based electronic devices and sensors.
•Explore degradation of material mechanical properties in creep-fatigue process.•A damage variable Dm is defined based on tensile plastic strain energy density.•Critical values of damage summation ...rules degrade as a power function of Dm.•3D damage diagram is constructed for evaluating creep-fatigue damage level.
Progressive degradation of material mechanical properties in the low cycle fatigue (LCF) and creep-fatigue (CF) interaction at high temperature affects the safe operation of in-service materials. By considering material degradation, the present work aims to establish a method for evaluating LCF and CF damage levels with wide applicability. Material-level data accumulations as well as theoretical foundations of LCF and CF are presented, including interrupted LCF and CF tests, subsequent tensile tests and energy-based damage models. A damage variable representing the degradation of material mechanical properties is then defined based on the tensile plastic strain energy density (TPSED), the physical mechanism of which is reflected in the microstructure evolution and fracture appearance. By taking into consideration the material degradation threshold in the traditional damage summation rule, a new three-dimensional (3D) damage interaction diagram is established, where the additional third axis indicates the material degradation level. Finally, taking GH4169 alloy and P92 steel as examples, this work demonstrates the implemented procedures of damage level evaluation, which has been validated via the experimental data.
PDF
