Rate-controlling Deformation Mechanism Research Articles

Low temperature deformation of neutron irradiated iodide zirconium, titanium and zircaloy-2

Abstract Several investigations relating to low temperature deformation of hard hcp metals (with c/a < 1.633) have been reported in literature in recent years with a view to establish the rate controlling deformation mechanisms. In the case of zirconium and titanium metals, thermally activated deformation studies have been conducted on polycrystalline specimens having varying amounts of oxygen content, employing strain rate change tensile tests. It has been observed that whereas in high purity metals, Pierels-Nabarro stress is the rate controlling mechanism at low temperatures, oxygen-dislocation interaction becomes rate controlling in metals having relatively higher interstitial content.1–5 The results of thermally activated deformation studies including those on Ti and Zr have been recently reviewed by Lukac.6 Neutron irradiation of metal gives rise to defects and defect clusters which by offering resistance to dislocation motion cause radiation hardening.7,8 This effect is also reflected in values of t...

Production of an Aluminium-Rich Magnesium Silicide Alloy from Atomized Powder

0_ Mg EXPERIMENTAL Material The air-atomized AI-Mg-Si (HE9) alloy powder, supplied by Borax Consolidated Limited, nominally contained 0·5 wt.-%Mg, 0·5 wt.-%Si, balance AI. Initially the powder particle size distribution was obtained using standard 200 mm BS sieves stacked on an Endrock mechanical shaker to sieve a standard weight of powder (175 g) for a specified time (1 h). The results are shown in Table I. Samples of the powder were analysed by the Imperial College Analytical Services Laboratory. The chemical composition is given in Table II. precipitates may together be expected to alter the heattreatment reaction. There appears to be no reported literature on this subject. The AIMg-Si system, when aluminium-rich, is among the most widely used aluminium alloys, having architectural and structural applications. The object of the present work was to examine the rate-controlling deformation mechanism, the nature of product densification, the alloy structure, and the effect of oxide dispersions on the ageing process.

Neutron-irradiation hardening in irons and ferritic steels

Radiation hardening in α-iron and mild and low-alloy ferritic steels is comprehensively reviewed and rationalized in te rms of the neutron damage production mechanisms and the migration, impurity trapping, and clustering behaviour of the resulting vacancies and interstitials. The effects of the principal metallurgical (grain size, initial strength, purity) and irradiation (neutron dose, dose rate, spectrum, irradiation temperature) variables on the magnitude of the hardening are described and are discussed in terms of these basic processes. The deformation characteristics of the irradiated materials and the post-irradiation recovery of the hardening are also considered. Finally, an account is presented of the analyses of the rate-controlling deformation mechanisms after irradiation.

Dominant Creep Mechanism and the Descending Lithosphere

Summary The descending of the lithospheric slab into the asthenosphere is governed by gravitational body forces generated by thermal volume contraction and by phase boundary elevation in the colder slab. The dynamics of sinking depends on the rate controlling deformation mechanism in the lithospheric slab and in the adjacent mantle; it is a function of the thermal conditions, the state of shear stress, the grain size, and the elastic moduli. A finite element initial strain, incremental procedure has been used to model the sinking of the lithospheric slab into the asthenosphere. Both diffusional flow and power law creep have been incorporated as dominant deformation mechanisms separately. Computed principal stresses and maximum shear stresses are in good agreement with observations of seismicity and focal mechanisms in those models which involved power law creep mechanisms with a power of n= 3. Similarly, computed creep displacements and velocities support the view that the power law creep mechanism is the dominant mechanism in the mantle region of slab subduction. The resistance of the mantle to a slab sinking with a velocity of 8 cm/yr requires a stress-strain rate equation equivalent to the one based on experimental and theoretical results, involving constants, however, which render strain rates five orders of magnitude smaller than those in experiments. A corresponding strain rate map of the upper mantle based on the computed shear stress suggests a maximum rate of 3 × 10−14s−1 in the asthenosphere and an average rate of 10−19s−1 in the mesosphere, both taken at 100 bars shear stress. Translation into effective viscosities leads to a reasonable agreement with viscosities derived from glacio-isostatic rebound data.

The warm and hot working of dynamically strain ageable aluminum alloys

The hot working of aluminum alloys that are not in a fully annealed initial structural state can be analyzed quantitatively by equations such as ϵ ̇ = A 3 [ sinh αϐ] n 3 e −Q/RT , previously used to analyze the hot working of pure metals. However, the application of the analysis in this case is restricted with regard to deformation temperature in comparison with the application to a pure metal. The results for aluminum alloys indicate that the rate controlling deformation mechanisms for hot working are not exclusively associated with self diffusion over the complete working temperature range.

The low-temperature macro deformation of an epoxide resin

An epoxide resin was tested in tension between 77°K and 380°K at a strain rate of 3.3 × 10 −4 sec −1. Besides the usual mechanical characteristics, activation parameters, as determined in strain-rate change tests, are presented. Their dependence on temperature, stress and strain in demonstrated. Three different rate-controlling deformation mechanisms with total activation enthalpiees of ΔH 0 I < 1 eV, ΔH 0 II = 2–3 eV and ΔH 0 III > 4 eV are believed to operate, each in a distinct temperature region.

A re-evaluation of the Hall-Peteh analysis

Careful studies of the effects of grain size, temperature and strain rate on the lower yield stress of body-centred cubic transition metals have shown, through the application of rate theory, that changing anyone of these variables can change the rate-controlling deformation mechanism. Thus, theoretical models of the grain size dependence of yield stress based on one mechanism, such as have been developed to interpret the Hall-Petch equation, must be regarded as inadequate. A recent set of experimental results on niobium published by Szkopiak show that three different sets of Hall-Petch parameters, characteristic of different grain size ranges, can be obtained at room temperature. Each one of these is probably characteristic of a particular rate-controlling Liiders band propagation mechanism. There is probably a hitherto overlooked “inherent grain size effect” resulting from a variation in the width of the Luders front with grain diameter, which can account for at least part of the grain size dependence of yield stress. Résumé Des études minutieuses des effets de la taille du grain, de la température, de la vitesse de déformation sur la limite inférieure d'élasticité des métaux de transition de structure cubique centrée, par application des théories de l'activation, montrent que la variation d'une de ces variables peut modifier le mécanisme de déformation contrôlé par la vitesse. Par conséquent, les modèles théoriques de l'influence de la taille du grain sur la limite d'élasticité, fondés sur un seul mécanisme, tels qu'ils ont été développés pour interpréter l'équation de Hall-Petch, doivent être considérés comme inadéquats. Dne récente série de résultats expérimentaux sur le niobium, publiée par Szkopiak, montre que trois valeurs différen tes des parametres de Hall-Petch, caractéristiques d'un intervalle donné de taille de grains, peuvent être obtenues à température ambiante. Chacune de ces valeurs est probablement caractéristique d'un mécanisme particulier de propagation des bandes de Luders. Il existerait probablement un effet intrinsèque de la taille du grain, jusqu'ici ignoré, résultant de la varition de la largeur du front de Luders en fonction du diamètre du grain, ce qui peut expliquer en partie l'effet de la taille du grain sur la limite d'élasticité.

Dislocation glide controlled by linear elastic obstacles: A thermodynamic analysis

Abstract A general thermodynamic analysis is presented for the thermally activated motion of dislocations over linear elastic obstacles. Expressions for the activation parameters are derived both in the effective and the applied stress formulations and the generalized compatibility conditions satisfied by the respective parameters in the two conventions are proposed. It is shown that the analysis of Schoeck (1965) is in conflict with his assumptions, and that the formulae Schoeck proposed in the applied stress notation are only fortuitously correct. The problems involved in applying activation analysis to experimental results are considered, and various consistent methods for analysing experimental data are presented and compared. It is emphasized that analysis in terms of the effective stress is required in order to characterize the rate-controlling deformation mechanism.

High‐Temperature Creep Behavior of Polycrystalline SrZrO3

The high‐temperature creep behavior of sintered polycrystalline SrZrO3 containing 1.35 wt% Fe2O3 was investigated as a function of temperature, stress, grain size, and strain level over the ranges 1160° to 1275°C, 780 to 3110 psi, 0.45 to 2.04 μm, and 0.0014 to 0.014, respectively. A constant‐load 4‐point (pure bending) method was used to load the specimens. The creep rate is time‐dependent, decreasing exponentially with strain, i.e. , where the decay constant (β=118, measured at the 1560 psi stress level over the strain range 0.0014 to 0.014) is independent of temperature and grain size. No significant grain growth occurred during creep. The activation energy of 169±10 kcal/mol obtained for creep is relatively independent of temperature, stress, grain size, and strain level over the ranges investigated. The creep rate is directly proportional to the cube of the stress and the reciprocal of the grain size; this result is consistent with nonviscous creep theories based on dislocation generation and climb as the rate‐controlling deformation mechanism.

The creep behavior of W-5 re

Polycrystalline W-5 wt pct Re was creep-tested in tension from 1500° to 1900°C at stresses from 2500 to 10,000 psi in a vacuum of 10−8 torr. The steady-state strain rate was directly proportional to stress to the 5.5 power, and the apparent activation energy for creep was 104 kcal per mole. Dislocation substructure that developed during high-temperature deformation was studied by transmission electron microscopy. The total dislocation density was dependent on stress to the 2.1 power and was insensitive to temperature and strain. No subgrains were found in creep tested specimens. The rate-controlling deformation mechanism was ascribed to dislocation climb where the governing diffusion process was dislocation core diffusion. Comparison of creep data for tungsten, W-5 wt pct Re, and W-25 wt pct Re showed that W-5 wt pct Re alloy has significantly better creep properties than the other two materials.

Dependence of Flow Stress on Nonstoichiometry in Oxygen-Rich Uranium Dioxide at High Temperatures

The compressive flow stresses of UO2+x polycrystals and single crystals were measured between 900° and 16000°C. The stoichiometry was controlled by CO2/CO gas mixtures during the tests. The functional dependence of strength on stoichiometry was: where C and m are constant at a constant temperature and 0.001 ≤ x ≤ 0.25. In stoichiometry crystals there was a marked anistrophy in strength. The {100} (110) systems were much softer than the {110} (110) systems, and the latter seemed to determine the strength of polycrystals. The harder systems {110} (110) displayed a softer ones. Nonstoichiometry largely removed the anisotrophy in strength and temperature dependence. It is suggested that the rate-controlling deformation mechanism for polycrystals may be the thermally activated surmounting of the peierls barrier.

An investigation of the mechanism of thermally activated deformation in tantalum and tantalum-base alloys

The activation energy and activation volume of tantalum and tantalum-base solid-solution alloys were obtained from both constant stress (creep) tests and constant strain rate (tensile or compression) tests. These parameters were investigated as a function of stress, temperature and interstitial concentration. The agreement between the activation parameters obtained from creep and tensile or compression tests supports many assumptions that have been made in the past about the differential technique. The functional dependence of the activation energy with changes in modulus (produced by an alloying element) is in agreement with that predicted from calculations of the energy of formation of a double kink over the Peierls' stress barriers. Electron-transmission micrographs show a predominance of screw dislocations in samples deformed at large effective stresses. This observation is explained in terms of a larger Peierls' stress of the screw dislocations. It is concluded that experimental results support the double-kink mechanism as the rate-controlling mechanism of low-temperature deformation in b.c.c. metals.