Shear transformation zone dependence of creep behaviors of amorphous phase in a CuZr-based bulk metallic glass composite

The creep behaviors of the amorphous phase in a CuZr-based bulk metallic glass composite(BMGC)are studied by nanoindentation.Samples fabricated via higher cooling rates are found to exhibit more prominent creep,but a smaller shear viscosity.The volume of the shear transformation zones(STZs)in the amorphous phase calculated based on a cooperative shear model increases with the cooling rate.The evolution of excess free volume created during creep deformation is clarified.A looser atomic arrangement leads to a larger STZ volume,thus facilitating creep deformation.This study gives a better understanding of the deformation behaviors of the amorphous phase in BMGCs.

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb（MC） has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown（H-B） standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion ＇c then σn tan φ＇ should replace ＇c plus σn tan φ＇ for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.

Influence of Oxygen Addition on Structure and Properties of Titanium Produced by Electroslag Remelting

The current work is devoted to the investigation of oxygen impact on the structure and properties of titanium. For this purpose, oxygen was introduced into titanium during chamber electro-slag remelting by three different methods: alloying by oxygen-rich residues from the Kroll process to final values between 0.053 wt.-% and 0.40 wt.-%, by reaction with the gas phase to 0.27 wt.-% and by introduction of TiO2 nanoparticles to 0.73 wt.-%. The influence of oxygen on microstructure of titanium during crystallization, heat treatment and deformation is determined as well as the effect of oxygen on the hardness and the mechanical properties of the material in different structural states. Furthermore, control methods of the structure formation process by thermal effects are proposed. Results show that the chamber electroslag remelting allows obtaining a homogeneous structure of the ingot in the investigated range of oxygen content in titanium. The hardness does not vary by more than 10 percent in longitudinal or radial direction in any of the remelted ingots.

Intelligent Fusion of Infrared and Visible Image Data Based on Convolutional Sparse Representation and Improved Pulse-Coupled Neural Network

Multi-source information can be obtained through the fusion of infrared images and visible light images,which have the characteristics of complementary information.However,the existing acquisition methods of fusion images have disadvantages such as blurred edges,low contrast,and loss of details.Based on convolution sparse representation and improved pulse-coupled neural network this paper proposes an image fusion algorithm that decompose the source images into high-frequency and low-frequency subbands by non-subsampled Shearlet Transform(NSST).Furthermore,the low-frequency subbands were fused by convolutional sparse representation(CSR),and the high-frequency subbands were fused by an improved pulse coupled neural network(IPCNN)algorithm,which can effectively solve the problem of difficulty in setting parameters of the traditional PCNN algorithm,improving the performance of sparse representation with details injection.The result reveals that the proposed method in this paper has more advantages than the existing mainstream fusion algorithms in terms of visual effects and objective indicators.

Analysis of mode Ⅲ crack perpendicular to the interface between two dissimilar strips

The present work is concerned with the problem of mode Ⅲ crack perpendicular to the interface of a bi-strip composite. One of these strips is made of a functionally graded material and the other of an isotropic material, which contains an edge crack perpendicular to and terminating at the interface. Fourier transforms and asymptotic analysis are employed to reduce the problem to a singular integral equation which is numerically solved using Gauss-Chebyshev quadrature formulae. Furthermore, a parametric study is carried out to investigate the effects of elastic and geometric characteristics of the composite on the values of stress intensity factor.

Some exact solutions of the oscillatory motion of a generalized second grade fluid in an annular region of two cylinders

The velocity field and the associated shear stress corresponding to the longitudinal oscillatory flow of a generalized second grade fluid, between two infinite coaxial circular cylinders, are determined by means of the Laplace and Hankel transforms. Initially, the fluid and cylinders are at rest and at t = 0＋ both cylinders suddenly begin to oscillate along their common axis with simple harmonic motions having angular frequencies Ω1 and Ω2. The solutions that have been obtained are presented under integral and series forms in terms of the generalized G and R functions and satisfy the governing differential equation and all imposed initial and boundary conditions. The respective solutions for the motion between the cylinders, when one of them is at rest, can be obtained from our general solutions. Furthermore, the corresponding solutions for the similar flow of ordinary second grade fluid and Newtonian fluid are also obtained as limiting cases of our general solutions. At the end, the effect of different parameters on the flow of ordinary second grade and generalized second grade fluid are investigated graphically by plotting velocity profiles.

Exact solutions for the flow of second grade fluid in annulus between torsionally oscillating cylinders

The velocity field and the associated shear stress corresponding to the torsional oscillatory flow of a second grade fluid, between two infinite coaxial circular cylinders, are determined by means of the Laplace and Hankel transforms. At time t = 0, the fluid and both the cylinders are at rest and at t = 0 ＋ , cylinders suddenly begin to oscillate around their common axis in a simple harmonic way having angular frequencies ω 1 and ω 2 . The obtained solutions satisfy the governing differential equation and all imposed initial and boundary conditions. The solutions for the motion between the cylinders, when one of them is at rest, can be obtained from our general solutions. Furthermore, the corresponding solutions for Newtonian fluid are also obtained as limiting cases of our general solutions.

Unraveling the threshold stress of structural rejuvenation of metallic glasses via thermo-mechanical creep

The competition between physical aging and structural rejuvenation determines the physical and mechanical properties of glassy materials.Thus,the rejuvenation-aging boundary must be identified quantitatively.In this work,we unravel a stress boundary to distinguish rejuvenation from aging via the thermo-mechanical creep of a typical Zr-based metallic glass.The crept glasses were rejuvenated into high-enthalpy disordered states when the applied stress exceeded a threshold that was numerically close to the steady-state flow stress;otherwise,the glasses were aged.A theoretical model for glass creep was adopted to demystify the observed stress threshold of rejuvenation.The model revealed that the thermo-mechanical creep beyond the threshold stress could activate sufficient shear transformations to create a net free volume,thus leading to structural rejuvenation.Furthermore,we derived the analytical expressions for the threshold and flow stresses.Both stresses can act as the rejuvenation-aging boundary,which is well supported by experimental creep data.The present work procures a deeper understanding of the rejuvenation mechanism of glasses and provides useful implications for abstaining from glass aging.

Effective Energy Density of Glass Rejuvenation

Glasses with rejuvenated structures usually exhibit improved room-temperature plasticity,which facilitates their applications.However,glass rejuvenation requires external energy injection to“shake up”the frozen-in disordered structure.In this work,we give the answer to how much the required energy is.According to the constitutive model of amorphous plasticity,we find that the applied stress higher than the steady-state flow value can effectively induce the structural disordering in terms of the generation of free volume.Therefore,the effective energy density(EED)of structural rejuvenation is defined as the integral of this effective stress on the corresponding strain.By tailoring the applied strain,strain rate,temperature and initial free volume,different degrees of structural rejuvenation are achieved,which show a generally linear correlation with the defined EED.This work deepens the understanding of glass rejuvenation from an energy perspective.

Dynamic mechanical relaxation and thermal creep of high-entropy La_(30)Ce_(30)Ni_(10)Al_(20)Co_(10)bulk metallic glass

Dynamic mechanical relaxation is a fundamental tool to understand the mechanical and physical properties of viscoelastic materials like glasses.Mechanical spectroscopy shows that the high-entropy bulk metallic glass(La_(30)Ce_(30)Ni_(10)Al_(20)Co_(10))exhibits a distinctβ-relaxation feature.In the present research,dynamic mechanical analysis and thermal creep were performed using this bulk metallic glass material at a temperature domain around theβrelaxation.The components of total strain,including ideal elastic strain,anelastic strain,and viscous-plastic strain,were analyzed based on the model of shear transformation zones(STZs).The stochastic activation of STZ contributes to the anelastic strain.When the temperature or external stress is high enough or the timescale is long enough,the interaction between STZs induces viscous-plastic strain.When all the spectrum of STZs is activated,the quasi-steady-state creep is achieved.

Effect of physical aging and cyclic loading on power-law creep of high-entropy metallic glass

The power-law relationship between creep rate decay and time is one of the intrinsic characteristics of metallic glasses.In the current work,a La_(30)Ce_(30)Ni_(10)Al_(20)Co_(10) high-entropy metallic glass was selected as the model alloy to test the influences of physical aging and cyclic loading on the power-law creep mechanism,which was probed by the dynamic mechanical analysis in terms of the stochastic activation,and contiguous interplay and permeation of shear transformation zones.It is demonstrated that a notable discrepancy appears between thermal treatment and mechanical treatment on the power-law creep mechanism of this high-entropy metallic glass.On the one hand,physical aging below the glass transition temperature introduces the annihilation of potential shear transformation zones which contribute to creep.On the other hand,cyclic loading can tailor the“forward”jump operations competing with the“backward”ones of shear transformation zones by controlling the interval time(recovery time).The current research offers a new pathway towards understanding the creep mechanism of high-entropy metallic glasses.

Microstructure and Mechanical Properties of NANOBAIN Steel

The microstructure and mechanical properties of NANOBAIN steel treated at different isothermal temperatures were investigated by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,uniaxial tensile tests and X-ray diffraction（XRD）.It was found that bainitic ferrite（BF）plate was made of basic shear transformation units arranged in the same direction of subunits.The existence of defects,such as nanoscale twinning and dislocation,suggested that the growth of transformation units was controlled by the surrounding defect plane with dislocation,which was consistent with the moving direction of BF/austenite interface parallel to the twinning plane.The behavior of work hardening indicated that mechanical stability of microstructures obtained at 250 ℃ and 300 ℃ was much more stable than that obtained at 210℃.The evolution of carbon partitioning in retained austenite and bainitic ferrite also indicated that austenite was enriched in carbon at the initial stage step by step;after the formation of BF,the austenite did not seem to be greatly enriched in carbon and the carbon content showed a little decrease instead;subsequently,aphenomenon of little decarburization of supersaturated bainitic ferrite has also been found.