Some observations on the deformation characteristics of bulk polycrystalline zirconium hydrides

The deformation behaviour of bulk polycrystalline zirconium hydrides in the composition range ZrH_1.27 to ZrH_1.66 has been investigated by compressive loading at temperatures between room temperature and 500° C. Single-phase -zirconium hydride is brittle below 100° C. Analyses of slip traces on specimens deformed at temperatures between 100 and 250° C have shown that the glide planes are {111} types. The deformation characteristics of and ( + ) alloys at temperatures between 100 and 500° C are consistent with the hydrogen vacancies in the -phase providing significant lattice friction to the movement of dislocations in the zirconium lattice of the hydride structure. The room temperature fracture stress of ( + ) alloys increases with the volume fraction of the -phase and this can be related to the resistance offered by platelets to the propagation of cleavage cracks in the matrix. In a ( + + ) alloy the resistance to crack propagation at room temperature is further increased by the soft -zirconium phase.     

Plastic deformation of polycrystalline zirconium carbide

The compressive yield strength of arc melted, polycrystalline zirconium carbide has been found to vary from 77 kg mm² at 1200C to 19 kg mm² at 1800C. Yield drops were observed with plastic strain-rates greater than 3×10^–3sec^–1 but not with slower strainrates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameterm which range from 6.5 to 1500C to 3.8 at 1800C, and the productm ^*(T) was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.     

Some observations on the deformation characteristics of titanium hydride

Deformation studies have been performed on bulk polycrystalline titanium hydride, of compositions between TiH_1.53 and TiM,_1.99, and at temperatures between –35 and 200° C. The yield stress increased with decreasing temperature and with increasing non-stoichiometric hydrogen vacancy content. The temperature dependence of the yield stress was enhanced with increasing vacancy content. Two-surface analysis of slip lines revealed that the slip plane was {1 1 1}. Strain-rate tests on TiH_1.65 demonstrated a considerable strain-rate dependence of both yield and fracture characteristics. The effects of test temperature, non-stoichiometric defect concentration and strain-rate on the deformation and fracture characteristics are shown to be strongly related to hydrogen ion mobility.     

Hysteresis effects on the high-temperature internal friction of polycrystalline zirconium

Hysteresis effects present on the high temperature internal friction of annealed polycrystalline zirconium are investigated in detail. It is shown that two internal friction maxima are present when the measurements are performed on heating. If a high enough temperature is reached, only one internal friction maxima is observed on cooling. Furthermore, when the temperature is not decreased below a certain value (critical temperature) only the lower temperature peak is present during a subsequent heating cycle. The critical temperature is strongly dependent on the grain size. Finally, both the hysteresis effects and the internal friction maxima are explained by relaxation mechanisms associated with grain boundary sliding and segregation of impurities to the grain boundaries.     

Some observations on the effect of specimen size upon the plastic deformation around deep notches

The purpose of this work was to investigate the effect of specimen thickness on the three-dimensional plastic deformation of notches. Notched bend specimens, made of lead as a model material, were tested. Transverse and longitudinal deformations at the notch root were measured using simple experimental techniques. The results provide a picture of the transition from the plane stress to the plane strain deformation of a notch root, which is discussed and interpreted. The shape which is assumed by the notch under the condition of plane plastic flow is shown to be a consequence of the plastic incompressibility of the material. Further, the formation of cracks at the notch root is discussed.     

X-Ray diffraction study of tensile deformation in a bulk polycrystalline Ag-30 at. % Cd alloy

X-ray diffraction techniques have been used to study plastic deformation in a polycrystalline Ag-30 at.% Cd alloy under tensile load. The positions and shapes of all (hkl) reflections were recorded using a parafocusing arrangement up to a maximum true strain of 0.265. The effects on the peak displacements caused by stacking faults and by macroscopic strains normal to the surface were distinguished. The longitudinal true stress in the surface layer evaluated by least square analysis was smaller than the macroscopic flow stress by an approximately constant amount over the whole range of strain (in accord with previous observations of a stress gradient near a free surface); the apparent rate of work hardening in the surface was equal to that for the specimen as a whole. The stacking fault probability was approximately a linear function of strain and attained a maximum value of 7×10^–3.Fourier analyses were performed on the profiles of (111) — (222) and (200) — (400) pairs of reflections. The effective particle sizes D _e(111) and D _e(100) and the estimated true domain size D decreased approximately inversely with increasing strain, tending to limiting values at high strains of 220, 150 and 300Å respectively. Similarly, the microscopic strains [_L ²_hkl ^*]^1/2 tended to limiting values at high mechanical strains. The twin fault concentration was found to be negligibly small. The particle size and microstrain parameters were compared with values for cold-worked filings of the alloy. A plot of against _L=50Å ²₁₁₁ ^* for the solid specimens and for the filings was linear and yielded a stacking fault energy for Ag-30 at. % Cd of 6.1 ergs/cm².This work was supported by the Office of Naval Research     

A calculation of the bulk modulus of polycrystalline materials

It is shown that the bulk modulus of a polycrystalline material, composed of cubic single crystals, is the same as that of the constituent single crystal. The bulk modulus of the aggregate is independent of the distribution of the individual single crystals. The same results apply also to other polycrystalline systems, whose constituent single crystals undergo a pure uniform contraction when subjected to hydrostatic pressure.     

Low-temperature deformation behaviour of polycrystalline copper

Low-temperature plastic flow in copper was investigated by studying its tensile and creep deformation characteristics. The dependence of the flow stress on temperature and strain rate was used to evaluate the thermal activation energy while the activation area was derived from the change-in-stress creep experiments. A value of 0.6 eV was obtained for the total obstacle energy both in electrolytic and commerical copper. The activation areas in copper of three selected purities fell in the range 1200 to 100 b². A forest intersection mechanism seems to control the temperature dependent part of the flow stress. The increase in the athermal component of the flow stress with impurity content in copper is attributed to a change in the dislocation density. The investigation also revealed that thermal activation of some attractive junctions also takes place during low-temperature creep. The model of attractive junction formation on a stress decrement during creep, yields a value of 45±10 ergs cm⁻² for the stacking fault energy in copper.     

Some observations on the bowl phenomenon

Previous studies suggest that performance of a series system may be enhanced if those station(s) having smallest mean operation time requirement(s) and/or smallest variability are placed in the middle of the line. There has been considerable confusion relative to whether this ‘bowl phenomenon’ has to do with the placement of means, variances, or some combination of the two. The results of the current study show that the phenomenon observed is not related to an imbalance in means. Rather, it is associated with an imbalance in absolute variability. Furthermore, it is suggested that the bowl effect vanishes altogether in cases where a minimal level of in-process buffer stock is provided.     

Effect of stress parameters on ratcheting deformation stages of polycrystalline OFHC copper

Engineering stress‐controlled ratcheting tests under different sets of stress amplitudes and mean stresses show that ratcheting deformation in polycrystalline OFHC copper occurs in three different stages. A plateau region with almost no accumulation of inelastic strain follows general ratcheting deformation during initial loading cycles. With breakdown of the plateau region inelastic ratcheting deformation occurs at an increasingly rapid rate. The effect of the stress amplitude on the ratcheting process is found to be more than mean stress effect. Reconstruction of the ratcheting curves clearly separates the conditions for stress‐controlled low cycle fatigue with zero mean stress and ratcheting with tensile mean stress.     

Some observations on the tearing of ductile materials

A relationship between tearing force and tensile properties is deduced by analogy with notched tensile failure and proves to be quite accurate in predicting the tearing forces of a wide range of materials. It is found that the elasticity modulus is a significant factor in the relationship and its role is investigated. A model of the tearing process is presented which seems to be a more accurate representation of the physical situation for many materials than those previously suggested. A simple analysis of the model leads to the relationship which is found to be experimentally reliable.     

An evaluation of rate-controlling obstacles for low-temperature deformation of zirconium

The low-temperature plastic flow of alpha-zirconium was studied by employing constantrate tensile tests and differential-stress creep experiments. The activation parameters, enthalpy and area, have been obtained as a function of stress for pure, as well as commercial zirconium. The activation area is independent of grain size and purity and falls to about 9b² at high stresses. The deformation mechanism below about 700° K is found to be controlled by a single thermally activated process, and not a two-stage activation mechanism. Several dislocation mechanisms are examined and it is concluded that overcoming the Peierls energy humps by the formation of kink pairs in a length of dislocation is the rate-controlling mechanism. The total energy needed to nucleate a double kink is about 0.8 eV in pure zirconium and 1 eV in commercial zirconium.     

Evidence for plastic deformation in the natural polycrystalline diamond,framesite

Relief-polished striations have been found within grains in polished sections of framesite, a naturally occurring polycrystalline diamond. Because the narrow zones represented by these surface striations are harder than any orientation of the matrix (as revealed by abrasion resistance) and because they etch preferentially, they have been interpreted as representing oriented deformation bands within the grains. It is concluded that the microstructure of framesite is the result of plastic deformation of diamond grains probably under conditions such that brittle fracture was inhibited.     

Effect of microstructure on temperature dependence of deformation behavior in polycrystalline CoNi-based superalloy

Journal of Materials Science - The initial microstructures with the unimodal and bimodal size distribution of γ′ precipitates (referred to as UD and BD, respectively) in CoNi-based...