Plastic deformation of hafnium-oxygen alloys

The plastic flow kinetics of hafnium single crystals and polycrystals of a range of oxygen contents were investigated over the temperature range of 77 to 873 K employing the technique of incrementally changing the temperature and strain rate. It was found that τ*₀ ≈ 2.4 x 10⁻³ μ₀√Ci, where τ*₀ is the effective CRSS extrapolated to 0K, μ₀ the modulus at OK, and Ci the total interstitial content (0.05 to 1.16 at. pct). Using the theory of thermally activated deformation, it is concluded that overcoming local interstitial obstacles is rate controlling over the temperature range of 77 to 750 K and that the obstacles are characterized by a Gibbs activation free energy δG at 0K and zero effective stress of ∼1.7 eV (0.14 μ₀b³). Results for single crystals and polycrystals are in agreement using a Taylor factor of 3.5 for the ratio of tensile to shear stress. W. R. Tyson is on leave from the Physics Department of Trent University, Peterborough, Ontario, Canada.     

Plastic deformation in metallic glasses

A theory is presented for the plastic deformation of metallic glasses below their glass transition temperature. The theory is based on two modes of thermally activated shear transformations initiated around free volume regions under an applied shear stress. The regions are typically conceived to be about 5 atom diameters across. At high temperatures (0.6 T_g ≲ T ≲ T_g) the transformation is a diffuse rearrangement producing a relatively small local shear strain in a roughly spherical region. At low temperatures (0 < T ≲ 0.6 T_g) the transformation is in a narrow disk shaped region and resembles closely the nucleation of a dislocation loop. The theory is in good accord with experimental observations.Based on the theory, possible levels of flow dilatation have been computed from which rates of shear localization can be obtained. At low temperatures, very rapid shear localization is predicted which is in good accord with the observations reported in the literature and with recent cinematographic observations.     

Plastic deformation below worn surfaces

Large strains occur at and near the worn surface of a ductile material(e.g. ≈8 at a depth of 10 Μm after trepanning, ≈2.5 at 2.5 Μm below an abraded surface). The wear of such materials is thus controlled by their strain-hardening behavior. Measurements have been made of the strain and microhardness at points below the worn surface of copper-silver solder composite specimens. The results are consistent with a model of the abrasion process that suggests the strain below the surface should be proportional to the abrasive grit size and to the square root of the applied load, but the strain at the surface should be independent of these factors. A simple energy balance confirms the work done during wear is determined by the plastic deformation at and below the worn surface. The measured volume wear rate shows the linear dependence on load expected on the model of the abrasion process, but there is also some dependence on grit size at small grit sizes and possible reasons for this are discussed.     

Plastic deformation below worn surfaces

Large strains occur at and near the worn surface of a ductile material (e.g. ∼8 at a depth of 10 μm after trepanning, ∼2.5 at 2.5 μm below an abraded surface). The wear of such materials is thus controlled by their strain-hardening behavior. Measurements have been made of the strain and microhardness at points below the worn surface of copper-silver solder composite specimens. The results are consistent with a model of the abrasion process that suggests the strain below the surface should be proportional to the abrasive grit size and to the square root of the applied load, but the strain at the surface should be independent of these factors. A simple energy balance confirms the work done during wear is determined by the plastic deformation at and below the worn surface. The measured volume wear rate shows the linear dependence on load expected on the model of the abrasion process, but there is also some dependence on grit size at small grit sizes and possible reasons for this are discussed.     

Plastic deformation in Fe-Si alloys

A detailed study has been made of the compressive stress-strain behavior of polycrystal-line Fe-Si alloys containing from 0 to 25 at. pct Si. It is shown that the alloys containing less than 10 at. pct Si possess only short range atomic order and deform by slip or by twinning, depending upon temperature. At concentrations greater than 10 at. pct Si, the alloys possess long range atomic order. Deformation twinning is suppressed in this region, and the deformation occurs entirely by slip. Deformation by slip may occur either by the motion of perfect superlattice dislocations, or else by the movement of imperfect superlattice dislocations, with the subsequent generation of antiphase boundaries. The type of superlattice dislocation that prevails depends upon a critical combination of both composition and temperature.     

Plastic deformation kinetics of fine-grained alumina

The plastic deformation kinetics of a commercial fine-grained alumina with ∼300 ppm MgO and grain sizes from 1.4 to 2.9 μm were determined in tension at 1475 °C to 1600 °C and strain rates from 10⁻⁵ to 10⁻³ s⁻¹, employing stress relaxation (SR) as the principal test mode. The constants in the Weertman-Dorn (W-D) equation were determined and had the following values: A=2.9±0.5×10⁹, n=2.2±0.1, p=1.9±0.1, Q=492±3 kJ/mole, and threshold stress σ ₀=0. These constants are in accord with grain boundary sliding (GBS) accommodated by dislocation glide and climb with Al³⁺ ion lattice diffusion as the rate-controlling mechanism.     

Plastic and viscous deformation of metals

Deformation characteristics of tensile specimens of several alloys, including electrolytic copper, α-brass, and 304 stainless steel, have been studied by application of stress and measurement of change of length in a soft tensile machine. By means of experiments in which the stress rate is reduced suddenly from a positive value to zero and the strain rate measured, both during loading and during creep, it is found that permanent deformation consists of two components, a plastic component for which the strain rate is a function of stress and stress rate, and a viscous component which is functionally dependent on stress and temperature. Plastic deformation is relatively more evident at increasing stress rate but declines in importance through the series copper, a-brass, and stainless steel. As a consequence, for a fixed strain rate during loading, the initial creep rate is low in copper and little creep occurs; in stainless steel, however, the initial creep rate is nearly equal to the loading strain rate and creep is pronounced. The theory is not fully developed but is based on a competition between thermal and mechanical release of dislocation segments from obstacles or sources. Release produces a strain increment which may be small or large depending on the relative values of stress and structural resistance. Plastic deformation occurs when the applied stress is close to the mechanical threshold, mechanical release is relatively easy, and the strain consists, at a given strain rate, of a few large strain increments per unit time. For viscous flow the relative stress is low, thermal release easy, and the strain rate is composed of many small strain increments in each unit of time.     

Plastic deformation of Ni-Cr single crystals

Single crystals of nickel containing up to 21.5 at pct Cr have been deformed in tension over the temperature interval 78 to 623°K. The critical resolved shear stress of alloys containing up to 9 at. pct Cr in the region of the athermal plateau can be fitted into aC ^1/2 andC ^2/3 relationship whereC is the concentration of chromium. The observed magnitude of hardening, however is larger than that predicted by the theory of Fleischer or that of Labusch. Rapid solution hardening in alloys having 9 to 21.5 pct Cr arises due to the presence of short range order. The work hardening parameters are examined and the temperature and strain rate dependence of the stress at the onset of dynamic recovery has been used to evaluate the stacking fault energy as a function of chromium content.     

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

Plastic deformation and recrystallization of refractory compounds

Summary A study was made of the influence of plastic deformation on the properties of refractory compounds. The methods of x-ray diffraction analysis and microhardness measurement were employed,Translated from Poroshkovaya Metallurgiya, No. 2 (50), pp. 13–14, February, 1967.     

Plastic deformation of titanium at elevated temperatures

The deformation of iodide titanium single crystals containing 200 to 250 ppm O, was studied in compression at temperatures from 25° to 800°C. Reduction of about 5 pct along thec axis was accommodated almost entirely by \(\left\{ {11\bar 22} \right\}\) twinning from 25° to 300°C, and above 400°C by \(\left\{ {10\bar 11} \right\}\) twinning in combination with c+a slip. The stress for \(\left\{ {11\bar 22} \right\}\) twinning increased with increasing temperature, and twin formation was accompanied by a load drop, while the stress for \(\left\{ {10\bar 11} \right\}\) twinning decreased with increasing temperature and twinning was not accompanied by a load drop. Crystals reduced normal to thec axis deformed by a combination of prism slip and \(\left\{ {10\bar 12} \right\}\) twinning at 25°C and by prism slip alone above 500°C.     

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

Plastic deformation of hafnium under uniaxial compression

The plastic behavior of polycrystalline hafnium (Hf) was investigated over a range of strain rates under uniaxial compression. Hafnium exhibited considerable ductility and a moderately rate-sensitive plastic behavior. The stress-strain response consisted of initial yielding followed by parabolic hardening. Microstructural observations on quasistatically deformed specimens revealed that yielding occurred by dislocation activity and that hardening was dominated by twinning on {1012} planes and by slip/twin interactions. A considerable reduction in dislocation and twinning activity was observed in specimens deformed at high strain rates. Failure occurred by shear localization and void growth and coalescence within the shear bands. Measurement of the temperature rise during high strain rate deformation was also made. From these measurements, the fraction of work converted to heat as a function of strain was determined and found to decrease with increasing strain.     

Plastic deformation in polycrystalline Nb3Sn

A study of the high temperature plastic deformation of polycrystalline Nb₃Sn has been undertaken on hot isostatically pressed material having grain sizes in the 12 to 60 (μm range. Through compression testing and load-relaxation testing deformation has been studied over a strain rate range from 10⁻⁶to 10⁻²s and a temperature range from 1150 to 1650 °C. Plastic deformation can be observed in compression at 1400 °C and above and extensive deformation is possible at 1650°C. Except for the lowest strain rates at 1650 °C, load-relaxation stress-strain rate relationships are consistent with “power law creep”. Analysis of stress-strain rate-temperature relationships projects an activation energy for creep of very roughly 500 kJ/mol. Observations on yield point behavior and fracture mode transition are presented. A comparison to monocrystalline V₃Si behavior is made, and the role of the sub-structure during testing is considered.     

Plastic deformation in a multifunctional Ti-Nb-Ta-Zr-O alloy

Mechanisms for plastic deformation in the newly developed Ti-24 at. pct (Ta + Nb + V)-(Zr,Hf)-O alloys (Gum Metal) were investigated in relation to their unique properties. Transmission electron microscopy revealed that the microstructure after deformation was characterized by highly distorted crystal images, which are accompanied by numerous “giant faults.” Such plastic behavior implies that a large amount of elastic stain energy was stored discretely and hierarchically during cold working. Calculated elastic constants of the Ti-X (Nb,Ta,Mo,V) binary systems predicted that Young’s modulus in 〈001〉 and shear moduli along some directions including slip systems in a bcc crystal were extraordinary small. The low modulus not only well explains the highly distorted microstructure observed in the cold-worked specimens, but also signifies that ideal shear strength of the developed alloys is a very small value, which is close to the practical strength required for plastic deformation in the alloy. This implies that the giant faults observed in the deformed specimen were formed without the aid of dislocation glide.     

Plastic behavior of aluminum-killed steel following plane-strain deformation

Aluminum-killed steel sheets have been subjected to plane-strain prestrain in three ways: two-pass rolling, multi-pass rolling, and inplane, plane-strain tension. Subsequent uniaxial tensile tests were performed to evaluate the residual work-hardening behavior. The subsequent hardening curves depended primarily on the relative direction between major strain axes in the two deformation stages and very little on the specific prestrain procedure. These curves showed high initial yield stresses followed by a region of low (or negative) work hardening rate. This behavior contrasted with earlier results for 70/30 brass sheet, and a model of subsequent tensile behavior based on a strain-induced stress transient emerged. Formerly Staff Research Scientist, General Motors Research Laboratories     

Plastic deformation of metal particles in gas-dynamic spraying

The main stage of the gas-dynamic spraying process is the deformation and heating of particles upon impact with the substrate. A solution of the impact deformation problem by use of the finite element method is given. The model developed enables determination of the extent of particle deformation and the time and temperature of impact as functions of the material and particle properties and the initial velocity and temperature of particles. Ukraine State Metallurgical Academy, Dnepropetrovsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 10–15, July–August, 1998.