Showing papers on "Creep published in 2002"

Creep-resistant magnesium alloys

Abstract: The development of creep-resistant alloys over the past 20 years is discussed, ranging from the WE series developed early in the 1980s and which represents the present state of the art to MgSc and MgGd ternary alloys that show an improvement in creep resistance of two orders of magnitude.

Effects of Creep and Thermal Drift on Modulus Measurement Using Depth-sensing Indentation

Abstract: In modulus measurement by depth-sensing indentation, previous considerations assume elastic recovery to be the sole process during unloading, but in reality creep and thermal drift may also occur, causing serious errors in the measured modulus. In this work, the problem of indentation on a linear viscoelastic half-space is solved using the correspondence principle between elasticity and linear viscoelasticity. The correction term due to creep in the apparent contact compliance is found to be equal to the ratio of the indenter displacement rate at the end of the load hold to the unloading rate. A condition for nullifying the effect of thermal drift on modulus measurement is also proposed. With this condition satisfied, the effect of thermal drift on the calculated modulus is negligible irrespective of the magnitude of the drift rate.

Structural aspects of high performance Mg alloys design

Abstract: Magnesium–gadolinium binary alloys exhibit good mechanical properties and high creep resistance comparable to or better than commercial WE type (Mg–Y–Nd–Zr) alloys. Combining scandium and manganese with a particular rare earth element (R.E.–Gd, Y, Ce) has a beneficial effect on the creep behaviour of complex Mg–R.E. alloys, at lower R.E. contents than in WE type alloys. They stabilise high creep resistance up to high temperatures (above 300°C) by precipitation of the stable phase Mn2Sc and by precipitation of basal plates of a Mn and R.E.-containing hexagonal phase.

Mechanical Properties of Engineered Materials

Abstract: Featuring in-depth discussions on tensile and compressive properties, shear properties, strength, hardness, environmental effects, and creep crack growth, "Mechanical Properties of Engineered Materials" considers computation of principal stresses and strains, mechanical testing, plasticity in ceramics, metals, intermetallics, and polymers, materials selection for thermal shock resistance, the analysis of failure mechanisms such as fatigue, fracture, and creep, and fatigue life prediction. It is a top-shelf reference for professionals and students in materials, chemical, mechanical, corrosion, industrial, civil, and maintenance engineering; and surface chemistry.

Creep and microstructure of magnesium-aluminum-calcium based alloys

Abstract: This article describes the creep and microstructure of Mg-Al-Ca-based magnesium alloys (designated as ACX alloys, where A stands for aluminum; C for calcium; and X for strontium or silicon) developed for automotive powertrain applications. Important creep parameters, i.e., secondary creep rate and creep strength, for the new alloys are reported. Creep properties of the new alloys are significantly better than those of the AE42 (Mg-4 pct* Al-2 pct RE**) alloy, which is the benchmark creep-resistant magnesium die-casting alloy. Creep mechanisms for different temperature/stress regimes are proposed. A ternary intermetallic phase, (Mg,Al)2Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance of the alloys.

An examination of the constitutive equation for elevated temperature plasticity

Abstract: It was 25 years ago that the symposium on rate processes in plasticity was organized. Since then, advances in transmission electron microscopy, large-scale computation as well as molecular dynamics simulation, etc. have contributed much to our understanding of elevated temperature plasticity. The constitutive relation that links the stress–strain rate–grain size–temperature relation (Mukherjee–Bird–Dorn, MBD correlation) was presented in 1968/1969 to describe the elevated-temperature crystalline plasticity. This equation has held up well during the intervening quarter of a century. It has been applied to metals, alloys, intermetallics, ceramics, and tectonic systems, and it has worked equally well. It made the depiction of deformation mechanism maps in normalized coordinates a reality and provided a rationale for estimating life prediction by giving a quantitative estimate of the steady-state creep rate in creep damage accumulation relationship. In the case of particle-dispersed systems as well as metal matrix composites, the introduction of the concept of a threshold stress was a substantial improvement in creep studies. One of the significant applications of the MBD relation has been in superplasticity. The concept of scaling with either temperature or with strain rate, inherent in this relationship, seems to be obeyed as long as the rate-controlling mechanism is unchanged. The application of this relation to high strain-rate superplasticity and also to low-temperature superplasticity has been illustrated. Experimental data demonstrate that superplasticity of nanocrystalline metals and alloys follows the general trend of the constitutive relation but with important differences in the level of stress and strain hardening rates. It is shown that in the nanocrystalline range, molecular dynamics simulation has the potential to yield data on stress–grain size–temperature dependencies at very low grain size ranges where experimentalists cannot conduct their studies yet.

Impression creep and other localized tests

Abstract: Impression creep and impression fatigue, both using cylindrical indenters, are reviewed in this paper. For impression creep, analytical solutions and computer simulations for different situations are presented. Materials tested include metals and alloys, superplastic materials, weldments, glasses, ceramics and polymers. Viscosity measurements using indentation techniques and impression creep of thin films are discussed also. For impression fatigue, a steady state per cycle is shown and the power law dependence of maximum stress is presented. Underloading and overloading effects, as well as delayed retardation, are described. Other localized tests, such as nanoindentation, stress relaxation, impression recovery and the adhesion energy determined by impression testing, are briefly discussed also.

Development of 9Cr-ODS Martensitic Steel Claddings for Fuel Pins by means of Ferrite to Austenite Phase Transformation

Abstract: For use as fuel cladding of liquid metal fast reactors, Fe-0.12C-9Cr-2W ODS martensitic steel claddings were developed by cold-rolling under the softened ferrite phase induced by slow cooling from austenite phase, subsequently by ferrite to austenite phase transformation to break up substantially elongated grains produced by cold-rolling at the final heat-treatment. The produced claddings showed noticeable improvement in tensile and creep rupture strength that are considerably superior to PNC-FMS and even austenitic PNC316 at higher temperature and extended time to rupture. The strength improvement is mainly attributed to titanium addition in ODS martensitic steels through its reduction of Y2O3 particle size and shortening inter-particles spacing. The behavior of oxide particle size reduction is associated with stoichiometry between Y2O3 and TiO2.

Si and O diffusion in olivine and implications for characterizing plastic flow in the mantle

Abstract: [1] The relationship between diffusion of individual species and creep of silicates is unclear and has long been debated, factually anchored by the central observation that the activation energies for creep are higher than the activation energy for diffusion of any species. There have been numerous attempts to explain this difference. New advances in experimental technology enable us to demonstrate that this difference does not exist and was an artefact of the limited experimental resolution of earlier studies – the activation energies of creep and diffusion of Si in olivine are identical, 530 kJ/mol. This allows the creep mechanism in olivine to be understood as one of simple climb (consistent with microstructural observations) and opens possibilities of estimating, understanding and predicting the (much simplified) creep behavior of diverse silicates under a wide range of P-T-X conditions through the use of diffusion data–measured or computed.

Characterization of high temperature creep properties in recrystallized 12Cr-ODS ferritic steel claddings

Abstract: The high temperature strengthening mechanism of previously manufactured 12Cr-ODS ferritic steel claddings was clarified. In the recrystallized 12Cr-2W-0.3Ti-0.24Y2O3-ODS ferritic steel cladding, αY2TiO5 type complex oxide formation was responsible for the drastic reduction of oxide particle size and the resulting shortened distance between particles, which led to superior internal creep rupture strength at 973 K because of the high resistance to gliding dislocation. Internal creep deformation was considered to be controlled by the grain boundary sliding associated with grain morphology: the near Σ11, Σ and Σ19 coincidence boundaries with a (110) common axis.

Indentation creep of Ge–Se chalcogenide glasses below Tg: elastic recovery and non-Newtonian flow

Abstract: Chalcogenide glasses from the Ge–Se system behave viscoelastically at room temperature. It follows that indentation measurements are time- or rate-dependent. The study of the dependence of hardness ( H ) on the loading duration for Ge x Se 1− x glasses with x between 0 and 0.4 shows that the penetration displacement is the sum of an elastic component which reaches values as high as 60% of the total displacement, and a creep one, which is strongly non-Newtonian (shear thinning), and leads to a significant decrease of H with an increase of the loading time. The apparent viscosity and activation energy for flow were derived from the H ( t ) data on the basis of a theoretical analysis of the indentation process, and the results are in good agreement with those obtained from conventional viscosity measurements.

Characterisation and high temperature mechanical properties of zirconium boride-based materials

Abstract: Two different ZrB 2 -based materials were produced by hot pressing: pure ZrB 2 and ZrB 2 +4wt.% Ni. The relative densities of the two materials were 86.5 and 98.0%, respectively. Several physical and mechanical properties were measured in ambient air. From these data it appears that the porosity of 13.5% of pure ZrB 2 strongly affects the properties. However at high temperature the presence of Ni-rich phases dominates the fracture behaviour and is responsible for the dramatic strength degradation (especially at 1200 °C). The high temperature creep was evaluated by uniaxial compression tests. Samples of both materials were deformed in argon atmosphere at temperatures between 1400 and 1600 °C and at stresses ranging between 47.0 and 472.3 MPa (pure ZrB 2 ) and 10–63.5 MPa (Ni-doped ZrB 2 ). Pure ZrB 2 showed a ductile behaviour under these conditions. On the other hand, Ni-doped ZrB 2 failed catastrophically for stresses higher than 25 MPa, approximately, at relatively low strains, showing a ductile behaviour only at lower stresses. This behaviour may be related to the presence of Ni-rich grain boundary phases at triple points of the grain structure.

Influence of long-chain branches in polyethylenes on linear viscoelastic flow properties in shear

Abstract: This contribution presents a survey on the influence of long-chain branching on the linear viscoelastic properties zero shear-rate viscosity and steady-state recoverable compliance of polyethylene melts. The materials chosen are linear and slightly long-chain branched metallocene-catalyzed polyethylenes of narrow molecular mass distribution as well as linear and highly long-chain branched polyethylenes of broad molecular mass distribution. The linear viscoelastic flow properties are determined in shear creep and recovery experiments by means of a magnetic bearing torsional creep apparatus. The analysis of the molecular structure of the polyethylenes is performed by a coupled size exclusion chromatography and multi-angle laser light scattering device. Polyethylenes with a slight degree of long-chain branching exhibit a surprisingly high zero shear-rate viscosity in comparison to linear polyethylenes whereas the highly branched polyethylenes have a much lower viscosity compared to linear samples. Slightly branched polyethylenes have got a higher steady-state compliance in comparison to linear products of similar polydispersity, whereas the highly branched polyethylenes of broad molecular mass distribution exhibit a surprisingly low elasticity in comparison to linear polyethylenes of broad molecular mass distribution. In addition sparse levels of long-chain branching cause a different time dependence in comparison to linear polyethylenes. The experimental findings are interpreted by comparison with rheological results from literature on model branched polymers of different molecular topography and chemical composition.

Microstructure and creep behavior in AE42 magnesium die-casting alloy

Abstract: The micro structural analysis of die-cast AE42 reveals a correlation between micro structure and creep strength. A lamellar-phase Al11RE3, which dominates the interdendritic microstructure of the alloy, partly decomposes above 150‡C into Al2RE and Al (forming Mg17Al12). The increased solubility of aluminum in magnesium at higher temperatures may also promote the decomposition of Al11RE3. The creep strength decreases sharply with these phase changes. A mechanism for the decrease in creep strength of AE42 is proposed whereby the reduced presence of lamellar Al111RE3 and/or the presence of Mg17Al12 contribute to the observed poor creep strength at higher temperatures.

Micromechanical analysis of viscoelastic properties of asphalt concretes

Abstract: A methodology for relating the microstructure of asphalt concretes to their viscoelastic behavior is described. Imaging techniques are used to capture the asphalt concrete microstructure, and the finite element method (FEM) is used to model its stress-strain behavior in the time domain. Aggregates are modeled as linear elastic, and the binder is modeled through mechanistic models as either linear viscoelastic or nonlinear viscoelastic. The binder viscoelastic properties are input into the FEM algorithm by two methods: a built-in viscoelastic function and a user-specified material characterization subroutine. The latter handles non-linearity in an iterative piecewise linear fashion, whereby the mechanistic binder model parameters are updated as a function of the strain level. For each strain level, mechanistic models are fitted to describe binder viscoelastic behavior based on dynamic shear rheometer data. The two approaches used for specifying binder viscoelastic properties into the FEM algorithm were ver...