C. J. Beevers

An investigation of the nucleation of creep cavities by 1 MV electron microscopy

Abstract Under constant strain rate conditions (~10 −3 hr −1 ) at 873 K intergranular cavities were observed by optical microscopy to form in a copper base alloy of grain size 16 /gmm average diameter at a tensile strain of about 0.2. Transmission electron microscopy at 1 MV demonstrated that cavity nucleation was associated with grain boundary particles. In particular, it was observed that cavities nucleated on one side of the particles and that the cavities were polyhedral in the very early stages of growth. The relationship between the particle size and the size of the cavity nucleated at the particle is discussed as is the frequency of cavities as a function of particle dimensions and inter-particle spacing. Constant load tests were carried out on material of 530 μm. grain size to determine the relationship between cavity nucleation and grain boundary sliding. It was established that a critical sliding displacement existed for nucleation (54 ± 5 × 10 −10 ° m). This nucleation criterion is discussed in terms of the build-up of a dislocation network, the effective length of the dislocation pile-up at a particle and the various models for cavity nucleation by grain boundary sliding.

Some observations on the deformation characteristics of titanium hydride

Deformation studies have been performed on bulk polycrystalline titanium hydride, of compositions between TiH1.53 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 TiH1.65 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.

The relationship between intergranular cavitation and superplastic flow in an industrial copper base alloy

Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 μm) Cu-2.8% Al-1.8% Si-0.4% Co alloy when tested at temperatures ≥500° C. High voltage electron microscopy revealed that the cavities could be nucleated at twin boundary/grain boundary intersections. The maximum elongation occurs at a higher temperature than that of the maximum strain-rate sensitivity and this is explained in terms of grain-boundary migration, at the higher temperature, which restricts the cavitation process. This explanation was put forward on the basis of texture analysis which was used to study the deformation characteristics at the temperatures of maximum elongation and strain-rate sensitivity. The final fracture mode is shown to change with test temperature: (i) at 400° C no cavitation occurs and fracture is by ductile rupture, (ii) at 500 to 550° C cavitation occurs and fracture is by the interlinkage of voids by an intergranular void sheet (IVS) mechanism and (iii) at 800° C grain growth occurs and fracture occurs by the propagation and interlinkage of grain-boundary cracks along the grain boundaries.

Anisotropy of grain boundary sliding in zinc bicrystals examined in terms of a dislocation climb-glide mechanism

Abstract An examination of grain boundary sliding in symmetrically oriented bicrystals of high purity zinc has shown that sliding in the absenoe of transcrystalline deformation exhibits a strong dependence on the direction of shear along the boundary plane. This behaviour is explained in terms of a basic mechanism for sliding based upon the movement of lattice dislocations along the boundary by a combination of climb and glide. The mechanism involves dislocation emission from the sliding boundary and can account for observations concerning slide hardening, sub-structure formation in the grain boundary vicinity and the rats controlling process for sliding.

The stability of sapphire whiskers in nickel at elevated temperatures

Composites of 1 to 20 vol % sapphire whiskers contained in a nickel matrix were annealed in vacuum in the temperature range 1100 to 1400° C for times up to 1000 h. After annealing, whiskers and alumina particles were extracted from composites and examined by optical and electron microscopy. The change of the aspect ratio distribution of whiskers during annealing was determined and related to the theory of tip ovulation. Pronounced spheroidization of whiskers occurred. This was only partly due to ovulation from the tips of whiskers by interfacial diffusion. Ostwald ripening made a significant contribution to the extent of spheroidization. For sapphire whiskers in nickel, ovulation times and interfacial diffusion constants were determined. The activation energy for the growth of ovoidal particles increased from 35 to 110 kcal during spheroidization. This is believed to indicate that the rate controlling process changes from interfacial diffusion to bulk matrix diffusion as the rate of tip ovulation decreases during annealing.

The effect of inclusions on the stress distribution in solids

The stress distribution existing in and around hard inclusions in solids subjected to a uniaxially applied stress has been investigated. Two-dimensional models containing inclusions of various shapes and orientations have been analysed by means of the photoelasticity technique, and the influence of elastic modulus, shape, and orientation of inclusions on the stress concentrations produced has been examined further by a mathematical treatment.

The prediction of creep fracture from intergranular damage measurements in a copper alloy

Abstract Tests on an industrial copper alloy containing grain-boundary precipitates were carried out at 923 K under constant load conditions and in a vacuum of 6.7 × 10−1 Pa. Three types of creep test were performed: (a) specimens were held at stresses in the range 9–20 MPa for a period of 15 h; (b) tests at 11 and 15MPa were conducted for various times up to fracture; (c) specimens were taken to fracture at stresses in the range 11–39 MPa. Grain-boundary damage was observed to occur by the formation and interlink age of grain-boundary cavities. The interlinkage of the cavities was observed to occur by a grain-boundary tearing process. Quantitative metallographic examination revealed that the extent of cavitation damage increased with stress and time in Test Series A and B, and in Test Series C the rate of damage accumulation increased with increase in stress. From these observations an approach to predict the time to rupture was developed. The prediction of the time to rupture is also discussed in terms ...

Some electron microscope observations of alumina whisker/nickel composites

Alumina whiskers separated from Thermokinetic Fibres sapphire wool mat type 1A have been incorporated into nickel by roll-bonding and by hot-pressing. The roll-bonded composites containing up to 5 vol % whiskers were successfully electrothinned to allow transmission electron microscope examination of alumina whiskers contained in the nickel matrix. In the hot-pressed composites which contained 20 vol % whiskers, examination of the whiskers was only possible after extraction from the composite. The roll-bonding process resulted in whisker break-up but the whisker segments retained their mono-crystalline form. The whiskers in these composites also retained extensive dislocation tangles in the adjacent nickel matrix after annealing at temperatures up to 1400° C. The whiskers extracted from the as-hot-pressed composites had a monocrystalline form and gave no indication of spheroidisation. In hot-pressed samples annealed at 1400° C for up to 100 h the whiskers exhibited evidence of spheroidisation. In some instances the whiskers developed strong crystallographic morphologies with faces parallel to the <11¯20> directions of the approximated close-packed hexagonal lattice ofα-alumina.