H.H. Ruan

A Promising New Class of High-Temperature Alloys: Eutectic High-Entropy Alloys

High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous achievement of both still constitutes a tough challenge. The inferior castability and compositional segregation of HEAs are also obstacles for their technological applications. To tackle these problems, here we proposed a novel strategy to design HEAs using the eutectic alloy concept, i.e. to achieve a microstructure composed of alternating soft fcc and hard bcc phases. As a manifestation of this concept, an AlCoCrFeNi2.1 (atomic portion) eutectic high-entropy alloy (EHEA) was designed. The as-cast EHEA possessed a fine lamellar fcc/B2 microstructure, and showed an unprecedented combination of high tensile ductility and high fracture strength at room temperature. The excellent mechanical properties could be kept up to 700°C. This new alloy design strategy can be readily adapted to large-scale industrial production of HEAs with simultaneous high fracture strength and high ductility.

Local deformation models in analyzing beam-on-beam collisions

Abstract With the intention to reveal the local behavior of beam-on-beam collisions, several local deformation models in the collision between a free–free beam and a simply supported beam are proposed and examined. The conventional rigid, perfectly plastic material idealization is adopted in analyzing the global flexural deformation of the beams, while the coupling of the global flexural deformation with the local indentation is formulated. Numerical results demonstrate that various local models lead to similar estimate on the global energy partitioning, whatever they adopt the stick or non-stick assumption. It is also noted that the weaker local model results in higher local energy dissipation. In addition, the separation and multi-impact between the two beams are observed by using some local models with non-stick assumption.

Deformation mechanism and defect sensitivity of notched free-free beam and cantilever beam under impact

Abstract This paper studies the dynamic behavior of pre-notched free–free beam and pre-notched cantilever beam subjected to step loading at a free end, as typical structures with defects. Special attention is paid to the deformation mechanism and the defect sensitivity in the plastic energy dissipation. By employing the rigid, perfectly plastic material idealization, complete solutions are obtained for combinations of the notch size, notch position and the magnitude of load. Apart from a one-hinge mode, two- and three-hinge modes are observed and analyzed. It is revealed that the first hinge, which is the one closest to the loading point, dissipates most of the input energy. If the first hinge forms at the notched section, the energy dissipation is highly sensitive to the notch; otherwise, it is insensitive. It is also noted that when the load increases, the defect sensitive region shrinks to a narrower region close to the loading point, indicating that under a large load, the notch may affect the energy dissipation remarkably, only if it is very close to the loading point. To reveal the interaction between the traveling hinges and the notched section, a pre-notched cantilever beam subjected to a rigid striker on its free end is analyzed. It is found that for a heavy striker, the stationary hinge either on the root or on the notched section dissipates most of energy and only when the notched section is sufficiently close to the impact point, the traveling hinge can move across the notch, indicating that the energy dissipation is insensitive to the notch. However, for a very light striker, the defect sensitivity is influenced by the traveling distance of the hinge and the deformation mechanism in the modal phase.

On the plasticity event in metallic glass

Based on a systematic molecular dynamics analysis, this study reveals that plastic deformation of metallic glass is not through a uniform configuration change but via many localized plasticity events. These events are manifested by the atomic clusters of high kinetic energy and high strain rate, emerging even in the elastic deformation regime. The life of such a plasticity event is on the order of 10−12 s, during which the distribution of kinetic energy follows a power law. The study shows that yielding in metallic glass occurs at the sudden surge point of the number of plasticity events. In the steady plastic deformation regime, the continuous nucleation and annihilation of the plastic events lead to a steady flow stress and stabilized total potential energy.

Plastic modal approximations in analyzing beam-on-beam collisions

Dynamic behavior of a moving free–free beam striking the tip of a cantilever beam, as a typical example of collision between two deformable structures, is analysed by employing modal approximation techniques. The applicability of both rigid-plastic and elastic–plastic mode approximations is examined in predicting the energy partitioning between the two colliding beams. Three rigid-plastic modes (RP-Modes) are considered and the Lee’s functional is applied to select the appropriate mode. It is found that one of the beams would absorb all the initial kinetic energy, unless a higher-order RP-mode is adopted. To incorporate the effect of elastic deformation into the modal solution, an elastic, perfectly plastic mode (EP-Mode) approximation for the same problem is proposed. By replacing each of the plastic hinges in the RP-Mode with a nature hinge and an elastic–plastic rotational spring, the fundamental features of the dynamic elastic–plastic behavior of the two colliding beams are revealed. Both beams participate in energy dissipation, while the structural and geometrical parameters greatly influence the energy partitioning. It is shown from numerical examples that the EP-Mode solution provides a fairly good approximation compared with the RP complete solution and finite element simulation.

Viscosity of Amorphous Materials during Glass-Forming: More from the Adam-Gibbs Law

This study aims to investigate the microscopic origin of viscosity by simplifying an amorphous system to a mixture of many independent atomic subsystems. The response of the macroscopic system is then taken as an ensemble average of the relaxations of such subsystems. The result shows that with the reduction of temperature, the overall viscosity changes from the harmonic mean of the subsystems, which is dominated by the fast relaxations, to the arithmetic mean governed by the slowest relaxation. The successful application of our model to the amorphous Selenium indicates the model captures the fundamental mechanism of the viscosity variation.

Effect of Chain Morphology and Carbon-Nanotube Additives on the Glass Transition Temperature of Polyethylene

Glass transition temperature Tg is the most important parameter affecting the mechanical properties of amorphous and semi-crystalline polymers. However, the atomistic origin of glass transition is not yet well understood. Using Polyethylene (PE) as an example, this paper investigates the glass transition temperature Tg of PE with the aid of molecular dynamics (MD) simulation. The effects of PE chain branches, crystallinity and carbon-nanotube (CNT) additives on the glass transition temperature are analyzed. The MD simulations render a good agreement with the relevant experimental data of semi-crystalline PE and show the significant effects of crystallinity and addition of CNTs on Tg.

Implementation of Glass Transition Physics in Glass Molding Simulation

Glass transition is the most important factor in the thermo-forming of glass elements of precise geometries such as optical glass lenses. Among many attempts to model the physics of glass transition, the Master equations based on the potential energy landscape (PEL) appear to be apropos. In this study, we used Monte-Carlo approach to approximately solve the master equations and further implement the Monte-Carlo method in the finite element simulation. We used Selenium as an example since its PEL has been quantified. Through the FEM simulations, it is found that the geometrical replication quality is the best when the forming is performed at the viscosity around 105~106 Pa×s, that the residual stress developed in the cooling process can be minimized in the slow cooling process or through post-annealing process after moulding.

Probing Temperature-Dependent Viscoelastic Properties of Glassy Materials Using Impact Induced Vibration-Theory and Experiments

The viscoelastic properties of glass under different temperature are essential for the high-precision thermo-plastic-forming of glass. But it is exceptionally difficult to establish a quantitative relation between the thermal history and the viscoelasticity owing to the lack of constitutive model of glassy materials’ relaxation. The present work investigates the validity of Young’s modulus measurement in impulse excitation technology and then the viscosity predicted by Kelvin and Maxwell model. It is demonstrated that the classical Kelvin model, leads to the seemingly unphysical result that viscosity increases with temperature since the experimental loss rate of damped vibration increases with temperature. Although Maxwell model can be employed to explain the positive temperature dependence of loss rate, the magnitude is even smaller than the viscosity at glass transition temperature and is therefore also unreasonable. The further theoretical work suggests the intermediate zone of Kelvin and Maxwell model.

Plastic Deformation Clusters with High Kinetic Energy in Metallic Glass

Although localized atomic rearrangements have been considered to be the underlying mechanism of plastic deformation in metallic glass, the nucleation and evolution of such plasticity events are still elusive. With the aid of molecular dynamics analysis, this study revealed that a series of localized atomic rearrangement events would occur in metallic glass as demonstrated by the formation of high-kinetic-energy clusters. It was found that atomic clusters of average sizes of 1 to 2 nm nucleate during elastic deformation, and become prevailing after yielding. The cores of these clusters contain several high-velocity atoms, which drive the local structural change and accommodate plastic strain. The nucleation and evolution of the local plasticity events are shown clearly by the strong dynamic signature, attributed to the spontaneous structural reshuffling after crossing an energy barrier.