Plastic Deformation Region Research Articles

AC loss characteristics of Bi2223/Ag sheathed tape wires subjected to mechanical strainsand stresses

The influence of uniaxial tensile stress–strain on the AC loss characteristics ofmultifilamentary Bi2223/Ag sheathed tape wires was investigated. The uniaxial tensilestress–strain was applied to the sample wire in liquid nitrogen at atmospheric pressure, andthe AC losses (transport, magnetization and total losses) were measured by an electricmethod. Two kinds of wire, oxide-dispersion strengthened Ag-alloy sheathed and Ag-alloysheathed wires, were tested. The stress–strain curves of the tested wires were divided inthree regions, i.e. elastic deformation, continuous plastic deformation and serrated-likeplastic deformation regions, though the ranges of those regions were different for differentkinds of wire. In the elastic and continuous plastic regions, the stress–strain curvewas smooth and continuous, and in the serrated-like plastic region, the curvewas rough. In the serrated-like plastic region, the wires kept elongating, whileincrease of the tensile stress was suspended. Dependences of the critical currents onthe stress–strain were generally as follows. While decreases of the wire criticalcurrents were in the range of less than 4% of the original values of the no-stresscondition, the critical currents of the wires were reversible, that is, the criticalcurrents recovered the original values at zero stress when the stress were released,regardless of whether the wires were in the elastic or continuous plastic region. In thecontinuous plastic region, the critical currents decreased up to 10%–15% of the originalvalues and the critical currents were irreversible when the degradations of thecritical currents exceeded about 4%. In the serrated-like plastic regions, the criticalcurrents were more severely degraded. The AC loss characteristics of the wiresare different in those regions. In the elastic and continuous plastic regions, theabsolute values of AC losses were dependent on the stress–strain. However, thedependences of those normalized losses on the stress–strain are negligibly small when thelosses are properly normalized by the stress–strain-dependent critical currents.In the serrated-like plastic region, the AC loss characteristics are different fromthose in the elastic and continuous plastic regions and can be characterized bydefects grown almost tape wide, causing serious degradation of the critical currents.There are various origins of defects causing the degradations of the wire criticalcurrents. However, as a result of this investigation, the influence of the tensilestress–strain on the AC losses can be basically estimated by knowing the degrees of thedegradations of the wire critical currents caused by the stress–strain and in whichregions the stress–strain characteristics are, regardless of the origins of the defects.

The effect of annealing temperature on the microstructure of nanoindented Au/Cr/Si thinfilms

The nano-mechanical properties of as-deposited thin Au/Cr films deposited on Si(100) substratesare investigated using a nanoindentation technique. Nanoindentation is performed to a maximumdepth of 1000 nm, and selected specimens are then annealed at temperatures of 250, 350 or450 °C for 2 min. The nanoindentation results show that the loading–unloading curve is continuousand smooth in both the loading and the unloading steps, which suggests that no debondingor cracking occurs. Furthermore, very little elastic displacement is observed in theunloading curve, which indicates that the deformation is primarily plastic in nature. Thehardness and Young’s modulus of the Au/Cr/Si thin films are found to vary with thenanoindentation depth, and have values of 1.7 GPa and 88 GPa, respectively, at themaximum indentation depth of 1000 nm. The microstructures of the as-deposited andannealed nanoindented specimens are examined using scanning electron microscopy (SEM)and transmission electron microscopy (TEM) techniques. The microstructural observationsreveal that nanoindentation induces an atomic reorganization, and results inthe formation of high-stress plastic deformation regions beneath the indenter. Inthe as-deposited specimens, the plastic deformation results in a pile-up of Auaround the entrance of the indentation. However, the diffusion of the Au atoms isenhanced at higher temperatures, and hence the annealing process prompts ahomogenization of the high-stress areas and leads to a full recovery of the pile-upeffect. The high temperature induced in the annealed thin film specimens alsoprompts a silicidation of the Cr layer, which results in a direct contact between theAu film and the Si substrate. As a result, annealing has a beneficial effect onthe interfacial bond strength. Following annealing at the highest temperature of450 °C, an Au–Si eutectic phase is formed, which further enhances the strength of the interfacialbond.

Mixed viscous flow and softening of bulk metallic glasses

Plastic deformation of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass is investigated in a wide range of the strain rates, ε˙, and temperatures, T. Based on these results, a map of the plastic deformation modes as a function of strain rate and temperature is depicted. The region of the mixed flow contiguous to the inhomogeneous plastic deformation region is identified. In this region, at large values of ε, the glass softens significantly. This effect is interpreted as a result of the cluster fragmentation.

Hydrogen thermal desorption behavior of Ni–Ti superelastic alloy subjected to tensile deformation after hydrogen charging

The hydrogen thermal desorption behavior of Ni–Ti superelastic alloy subjected to tensile deformation after hydrogen charging has been investigated. Cathodic hydrogen charging is performed with a current density of 10 A/m 2 in a 0.9% NaCl aqueous solution for 2 h at room temperature (25 °C). In this case, hydrogen desorption is observed from room temperature to 400 °C. For the specimen immediately after hydrogen charging, upon tensile loading covering the stress plateau region caused by stress-induced martensite transformation followed by unloading, hydrogen that desorbs at low temperatures (approximately 150 °C) is observed markedly. In contrast, for the specimen aged for 240 h at room temperature in air after hydrogen charging, most hydrogen that desorbs at low temperatures shifts to a higher-temperature region and diffuses toward the center of the specimen, although the charged hydrogen does not diffuse out. Variation in hydrogen desorption behavior is rarely observed, even upon tensile loading in the plastic deformation region of the martensite phase followed by unloading. The present results suggest that dynamic processes such as stress-induced martensite and reverse transformations affect hydrogen desorption behavior at low temperatures of hydrogen-charged Ni–Ti superelastic alloy.

Failure Analysis of a Diesel Engine Gear System Consisting of Camshaft and Crankshaft Gears

A failure investigation was conducted on a diesel engine gear system consisting of a driven camshaft and drive crankshaft gears that were used in a truck. The gears are made from a nitrided 42CrMo steel. Adjacent teeth fracture and plastic deformation regions appeared on the gears after a 400 h run test of the gear system. Fractography indicates that fatigue fracture is the dominant failure mechanism for the gear teeth. Although the appearance of needle-like nitrides in the nitrided layer and the narrow depth of the compound layer may decrease the fatigue strength of the camshaft gear, these do not suffice to lead to the premature fracture of the gear teeth. Geometrical analysis of the gears was performed and compared with an analysis of unfailed gears that had experienced a run test for 1800 h. The comparison reveals that the small fillet radius at the root area of the camshaft gear concentrated the stresses and is mainly responsible for fatigue fracture of the teeth. The camshaft gear is the component that initiated trouble in the gear system. The appearance of severe plastic deformation on the gear faces is caused by the fractured teeth crushing the teeth faces and being embedded in the grooves between teeth.

Indentation of lipid-based particle gels: An experimental, theoretical and numerical study

The rheological properties of lipid-based particle gels are critical in defining end-use properties. Although indentation is commonly utilized to investigate the end-use properties of these materials, a comprehensive methodology for acquiring and characterizing fundamental rheological properties via indentation is lacking. This study sought to develop the indentation method for obtaining the large-strain viscoplastic characteristics of three lipid systems that vary in composition and structure: shortening, margarine and butter. Uniaxial compression and conical indentation tests were performed at various rates. Constitutive behaviour was fitted with a viscoplastic model, with the rate-dependent plasticity defined by the overstress power law equation. The static stress–strain curve was defined by a linear elastic region, a strain hardening plastic deformation region, and a subsequent region of perfect plasticity. Forward predictions of the indentation load–displacement response were obtained via finite element analysis using compression test data. Good reverse predictions of the stress–strain properties from the indentation response were obtained through a novel method developed in this study.

Numerical simulation of residual stresses at the grain and sub-grain length scale using atomistic modeling∗

Abstract For modeling the deformation and the heat treatment related change of micro structural material properties, a crystal structure with several grains is analyzed using the molecular dynamics simulation. The generated atom arrangement has been equilibrated, sheared and tempered, and the resulting microstructures and stresses as well as their changes are presented. The shearing of the multiple grain model into the region of plastic deformation caused a significant change in its microstructure and introduced additional stress. On applying the heat treatment simulation, it was possible to show thermally induced relaxation processes in a microstructure using molecular dynamics.

Prediction of fracture characteristics of metals from high-frequency test data

A method is proposed for the prediction of cyclic crack resistance characteristics of metallic materials under low-frequency loading from high-frequency test data, which is based on a model of development of local plastic deformation regions during the accumulation of fatigue damages and fatigue crack growth with allowance for cyclic loading rate. We performed a comparative analysis of the results of prediction of fatigue fracture diagrams with test data for VT22, VT18U, VNS-25, and AMg6N alloys in a frequency range of 20 Hz–10 kHz.

Fracture of poly(vinylidene fluoride): a combined synchrotron and laboratory in-situ X-ray scattering study

Semi-crystalline polymers show a complex fracture mechanism, which is controlled by the micro-mechanisms associated with formation and breakdown of a plastic deformation region. Such regions develop at notches, cracks or other stress-raising defects. In the present paper, we use time-resolved synchrotron X-ray scattering techniques during the deformation process in poly(vinylidene fluoride) to study the plastic zone formation and fracture processes at different strain rates. This gives new insight into the micro-mechanisms of cavitation, lamellae separation and fibril formation in this particular material.

Fracture toughness and adhesion of thermally grown titanium oxide on medical grade pure titanium

The mechanical properties and adhesion characteristics of a thin thermally grown titanium oxide film on commercially pure titanium were examined. Tensile tests were used to introduce controlled strains in the thermally grown oxide (TGO), through the titanium substrate, to study the damage evolution and to quantitatively evaluate the intrinsic strength and fracture toughness of the TGO layer. Details of the TGO adhesion behaviour were explored. Nanoindentation was used to determine the Young's modulus and hardness of the TGO. Tensile loading resulted in multiple cracking to occur in the TGO layer along with distinctive inclined cracking driven by shear band deformation in the titanium substrate. The fracture toughness of the TGO was determined to be 1.8 ± 0.6 MPa m 1/2. Localised delamination of the TGO was observed but only when the titanium substrate was strained well within the plastic deformation region by more than about 2.5%.

21509 原子間力顕微鏡を用いたナノ加工に関する研究(微細加工,OS.13 微細加工)

In order to obtain the basic data to clarify the material removal mechanism in the nano-grinding, fundamental experiments to generate the nanoscale grooves of Single-crystal Si has been tried using an Atomic Force Microscope combined with a two-axis capacitive force/displacement transducer. When increasing the normal force, three different regions in the normal force in which the different mechanism of groove formation take places are found. They are an elastic deformation region without remaining deformation, a plastic deformation region in which the groove is dominantly formed by plastic flow and a removal process region in which the groove is dominantly formed by removal process The minimum normal force f_<nR> at which the process begins is about 34.9μN and in this case the groove with a depth of about 0.42nm is formed. The critical normal force f_<nR> is identified from a characteristic change in a ratio of normal and tangential forces with increasing normal force.

Application of Diamond-Like-Carbon Coating to a Coronary Artery Drug-Eluting Stent

The surfaces of medical materials are coated with polymer molecules or drugs in order to functionalize the surfaces in such ways as imparting biocompatibility. But in the case of inorganic materials there is the problem that polymers will have poor adhesiveness and be liable to peel. A coronary artery stent holds the vessel lumen patent by being expanded to the plastic deformation region, and if the material surfaces are coated with DLC, the coating must follow the plastic deformation of the base material. Accordingly we created a thickness-wise concentration gradient in the Si content that is added to the DLC, and discovered that thereby a DLC nanocoating is produced that does not crack under the plastic deformation required in a stent, and that has superior adhesion. In this way we succeeded in improving adhesion of the drug coating when used in combination with thin film material obtained via application of plasma surface treatment techniques, and in producing a practical stent that exhibits high biocompatibility even after slow release of the antithrombotic agent into the body is complete.

The characterization of plastic deformation in Ce-based bulk metallic glasses

The plastic deformation behavior of Ce 68Al 10Cu 20Nb 2 and Ce 70Al 10Cu 20 bulk metallic glasses (BMGs) at room temperature was studied by depth-sensing nanoindentation and microindentation. It is shown that the two BMGs exhibit a continuous plastic deformation without distinct serration at the all of the studied loading rates during nanoindentation. An obvious creep displacement was observed during the holding-load segment at the maximum load for the two alloys, and the magnitude of creep during holding-load increases with loading rate. The subsurface plastic deformation zone of the two BMGs after indentation at various loading rates was investigated through bonded interface technique using depth-sensing microindentation. A highly developed shear banding pattern can be observed in the plastic deformation region, though the global load–depth curves illuminate a “homogeneous flow”. The plastic deformation behavior of the Ce-based BMGs during indentation measurements is discussed in terms of localized viscous flow.

Study of serrated flow and plastic deformation in metallic glasses through instrumented indentation

The plastic deformation behavior and serrated flow in seven bulk metallic glass (BMG) systems were investigated through instrumented indentation. These materials include Ce 65Al 10Ni 10Cu 10Nb 5, Mg 65Cu 25Gd 10, Pd 43Ni 10Cu 27P 20, Cu 60Zr 20Hf 10Ti 10, Pt 57.5Cu 14.7Ni 5.3P 22.5, Ni 60Nb 37Sn 3 and Fe 43Cr 16Mo 16C 15B 10 BMGs, which show a glass transition temperature ( T g) ranging from 360 to 908 K at a heating rate of 0.33 K/s. Remarkable difference in deformation behavior was found among these BMGs in the load–depth curves during nanoindentation. Prominent serrations are observed in Mg-, Pt- and Pd-based BMGs with medium T g during the loading process, whereas no distinct serrated flow was found in Ce-, Ni- and Fe-based BMGs with quite low or high T g. The subsurface plastic deformation regions after indentation were investigated using depth-sensing microindentation to characterize the shear band feature developed in various BMG systems. The size of the shear band upset is found to be larger in the alloys with lower T g. The effect of T g on the operation of shear bands and the serrated flow behavior in various BMG systems were discussed.

THERMAL STABILITY OF PLATINUM BOTTOM ELECTRODE FOR BISMUTH TITANATE THIN FILMS

ABSTRACT Platinum (Pt) films were widely used as an electrode in device applications of ferroelectric thin films. However, the surface morphology and microstructure were deformed during ferroelectric layer preparation. In this study, consequently, thermal stability of Pt bottom electrode for ferroelectric bismuth titanate (Bi4Ti3O12) thin film fabrication was investigated. The deposited Pt film with thick Ti adhesion layer exhibits the different aspect areas, which contain Ti and/or Si element diffused into Pt grain boundaries. Therefore, the adhesion layer thickness was thinned as much as possible. Compressive stress was relaxed by hillock formation in plastic deformation region. Therefore, It is important to maintain the tensile stress from the substrate during the ferroelectric thin film formation. Bi4Ti3O12 thin films deposited on stabilized Pt bottom electrode, exhibit superior dielectric properties. The deposited film exhibited ferroelectric properties. The polarization value at zero field of the strongly c-axis oriented one was 1.75 μC/cm2. We think that the choke of silicon and/or titanium elements, diffused into Pt electrode, were effective against suppress hillock formation.

Positron Annihilation in Metals Defected by Action of the Tensile Force

Results of experimental investigations of uniaxially elongated mono- and polycrystalline samples of several metals (Fe, Ta, Pd, Ag, and Au), performed using the positron annihilation methods, are reviewed. The dependences of the S-parameters and positron lifetimes on the relative elongation of the samples were presented. The data obtained for polycrystalline samples indicate that in the proportionality and limited proportionality regions the changes in the physical properties are governed mainly by generation of vacancies and by kinetics of formation and transformations of vacancy clusters occurring flrst of all on the grain boundaries of monocrystallites. In the region of plastic deformations the dominant defects are dislocations and vacancies and their aggregates generated due to the formation and movement of the dislocations of the primary and secondary slip. In the case of monocrystalline samples it was found that the dynamics of the dislocations and vacancy generation during the sliding of some crystallographic planes depends on the crystallographic direction.

Analytical modeling of stress–strain behavior at 1873 K of alumina/YAG composite compressed parallel and perpendicular to the solidification direction

The strain rate-dependence of the stress–strain curve and flow stress at 1873 K of the directionally solidified eutectic alumina/YAG composite under applied compressivse stress parallel (0°) and perpendicular (90°) to the solidification direction were analyzed by a finite element model and by the isostrain and isostress models based the rule of mixtures. The experimentally measured stress–strain curve and flow stress in the steady state plastic deformation region of the composite was described by a finite element model for both 0° and 90° compression directions. The influence of the microstructural deviation of alumina and YAG from the theoretical isostrain and isostress models on the overall flow stress was discussed for both 0° and 90° compression directions, based on the calculated spatial distribution of the equivalent strain rate.

Numerical Simulation of Residual Stresses at the Grain and Sub-Grain Length Scale Using Atomistic Modeling

For modeling the deformation and the heat treatment related change of micro structural material properties, a crystal structure with several grains is analyzed using the molecular dynamics simulation. The generated atom arrangement has been equilibrated, sheared and tempered, and the resulting microstructures and stresses as well as their changes are presented. The shearing of the multiple grain model into the region of plastic deformation caused a significant change in its microstructure and introduced additional stress. On applying the heat treatment simulation, it was possible to show thermally induced relaxation processes in a microstructure using MD.

Ultrafine-grain-sized zirconium by dynamic deformation

A polycrystalline zirconium alloy (Zircadine 702, containing 0.7% Hf) was subjected to high plastic strains (shear strains of 25–100) at a high strain rate (∼10 4 s −1) in an experimental setup comprising of a hat-shaped specimen deformed in a split Hopkinson bar. A narrow region of intense plastic deformation (10–25 μm thick) is produced which was analyzed by scanning (electron backscattered diffraction) and transmission electron microscopy. The microstructure within the shear band is characterized by equiaxed grains with an average size of 200 nm. The temperature excursion undergone by the deforming region is calculated on the basis of a Zerilli–Armstrong constitutive equation. The calculated temperature for a shear strain of 100 is equal to 930 K, corresponding to 0.43 T m. Electron backscattered diffraction reveals that plastic deformation by shear leads to a strong 〈 1 1 2 ¯ 0 〉 fiber texture prior to the breakup of the existing grain structure. The ultrafine-grain structure observed is similar to that obtained in conventional severe plastic deformation processes such as equal channel angular extrusion, suggesting that the mechanism of grain refinement is the same in both processes, in spite of the differences in strain rate and thermal excursion. A mechanism is proposed for the breakup of the existing equiaxed microstructure (with grain size ∼14 μm) into an ultrafine structure. It has three stages: (1) formation of elongated cells and subgrains; (2) increased misorientation between neighboring grains and breakup of elongated grains into smaller units; and (3) rotation of boundaries by grain boundary rotation and formation of equiaxed structure.

Static and dynamical cavitation for incompressible hyperelastic-plastic material

Bifurcation problems both for static and dynamical cavitation in a solid sphere composed of the incompressible hyperelastic-plastic material, under uniformly distributed tensile boundary dead load were studied. For each problem, cavity forms at the center of the sphere when the tensile load is larger than its critical value. Bifurcation curves and the growth curves for the plastic deformation region were given. For static cavitation, the deformation displays three stages, namely, fully elastic, elasto-plastic and fully plastic stages. For dynamical cavitation, the cavity grows without bound and the sphere displays plastic flow.