Boundary Cavities Research Articles

Radiation-induced strengthening and embrittlement in aluminum

Commercially pure aluminum (1100 grade) in the annealed condition was irradiated to fast fluences in the range 1 to 3 × 1026 neutron/m2 (E > 0.1 MeV) and to similar thermal fluences at temperatures of 318 to 328 K, and was then tensile tested at temperatures between 298 and 643 K. This irradiation doubled the ultimate tensile strength and more than tripled the flow stress at all test temperatures. The work hardening exponent was severely reduced and there was a large loss in ductility. Strengthening is shown to be due to a fine precipitate of transmutation-produced silicon and an associated dislocation structure. At test tempera-tures below about 423 K the fracture mode was transgranular. Above 473 K grain boundary cavities were observed, the fracture mode become predominantly intergranular, and ductil-ity was further reduced. Unirradiated specimens containing cyclotron-injected helium showed no change in strength but displayed a loss in ductility at elevated temperatures. Concurrently holes were formed on the grain boundaries. Embrittlement in the neutron-irradiated specimens arises from two sources. One is through the defect structure which reduces the work-hardening exponent. The other is an additional effect at elevated temper-atures involving grain boundary failure by cavity growth and coalescence. Helium encour-ages cavity nucleation, the degree of cavitatlon increasing with increasing tensile strength.

The sintering of grain boundary cavities in uranium dioxide

The final stage sintering of uranium dioxide has been investigated in the temperature range 1623–1973 K where the residual porosity is situated mainly on grain boundaries. The shape of the sintering curve is quantitatively predicted from a knowledge of the cavity distribution and from the sintering rate of an individual cavity described by the Hull and Rimmer (1959) equation. The effective grain-boundary diffusion coefficient is estimated from these curves and is shown to be in reasonable agreement with the grain-boundary diffusion coefficient of uranium measured by radio-tracer methods. The effect of entrapped gas appears to be of negligible importance since cavities were observed to sinter to closure; neither did grain growth strongly affect sintering rates, since cavities remained on grain boundaries.

The effect of grain boundary cavities on vacancy diffusion

Abstract The effects of cavities on vacancy diffusion processes have been studied by the annealing of quenched-in dislocation loops and helices. In material containing thermodynamically stable grain boundary cavities, growth of the loops and helices was prevented as the excess vacancies present could be absorbed by the cavities. Precipitation of a second phase was also affected by the presence of cavities. The results indicate that cavities become thermodynamically stable at the onset of tertiary creep.

On the distribution of cavities during creep

Abstract The true and apparent angular distributions of grain boundary cavities have been measured for OFHC copper deformed over a range of creep rates at 415°C. No effect of strain rate was observed, the frequency of cavity formation being highest in all cases for grain boundaries which are at right angles to the tensile stress direction. The apparent distribution of cavitated boundaries was shown to give a good representation of the true distribution. This was also the case when the frequency of cavitation on inclined boundaries was increased by a prior compressive strain.

The stability of grain boundary cavities in copper

Direct evidence was obtained that gas diffuses into cavities formed in copper either by high temperature fatigue or creep. The gas pressure inside the cavity builds up and reduces the tendency for the cavity to collapse during isothermal annealing. Sintering was observed when the cavitated specimens were annealed in a hydrostatic pressure of 2000 lb/in 2 and this sintering was reversed by subsequent annealing in a vacuum.

On the mechanisms of tertiary creep in face centred cubic metals

Using a constant stress machine, the compressive creep properties of a copper-aluminium alloy and a dilute nickel alloy have been studied. This data has been compared with the results of tensile creep tests also carried out under constant stress. It is shown that a stage of accelerating creep in compression (in the absence of recrystallisation) is associated with non-homogeneous deformation due to barrelling of the specimen after extensive deformation. The tertiary stage observed in tensile creep is accounted for by the increase in recovery rate throughout this stage. Also, cavities present on the grain boundaries do not cause an acceleration of the creep rate under compressive creep conditions even when the cavities continue to grow in compression. It is concluded that the thermodynamic stability of grain boundary cavities is an important factor in determining their effect on the creep rate.

Stability of Grain Boundary Cavities in Copper

DURING both creep and fatigue at high temperatures cavities form on the grain boundaries of polycrystalline copper and cause a decrease in density. Under creep conditions cavity nuclei grow on those boundaries experiencing a normal tensile stress by the diffusion of vacancies from the grain boundary to the cavity surface1. For high temperature fatigue it has been proposed that an excess concentration of lattice vacancies is formed2; the vacancies then reach the cavity surface by lattice diffusion3. It is axiomatic that cavities formed by either high temperature creep or fatigue should shrink by losing vacancies when the stress is removed. The driving force to collapse a spherical cavity is 2γ/r where γ is the surface energy of the cavity and r is the cavity radius. In the absence of stress the time ts required to collapse a cavity of radius r is given by where kT has its usual significance, a is the cavity spacing (∼ 10−3 cm), Dg is the grain boundary atomic diffusion coefficient (∼10−8 cm2/sec for Cu at 400° C), δZ is the thickness of the grain boundary available for diffusion (∼ 10−7 cm) and Ω is the atomic volume (1.2 × 10−23 cm3). For copper γ ∼ 1,500 ergs/cm2. Thus a cavity of radius 5 × 10−5 cm in copper should collapse in about 5 h at 400° C.

The growth of grain boundary cavities during high temperature fatigue

Abstract The variation of the grain boundary self-diffusion coefficient with applied stress and the production of vacancies in the lattice are applied to the case of cavity growth in high temperature fatigue with particular reference to the case of magnesium at 400°c. It is shown that cavity growth should increase with increasing testing frequency and that there is a critical condition for growth to take place. An outline is given of a theoretical estimate of times to fracture and it is shown that the time of testing, rather than the total number of cycles, becomes the important endurance parameter. Predictions of the model are compared with experimental findings.

Observations on the oxidation of beryllium in carbon dioxide at 600–700°

The oxidation behaviour of beryllium sheet and extruded discs prepared from two sources of powder and by different fabrication routes has been examined and compared. Oxidation appeared to occur in at least three stages; (a) thin film formation, (b) non-uniform film thickening, and (c) localized intergranular penetration. Stage (c) has been observed only in specimens with weight gains >0·1 mg/cm 2. Extruded materials in disc form had greater resistance to oxidation than sheet at all temperatures. All materials have been found to exhibit recrystallization during prolonged exposure at 600–700° and considerable grain growth has been observed in sheet. Discs prepared by direct extrusion of Brush powder exhibited extrusion faults and the oxidation behaviour has been related to these faults. Similar discs prepared from Pechiney electrolytic flake and which contained 2·8% oxygen were more resistant to moist carbon dioxide at 700° than at 600°. It is suggested that the relief of surface stresses in the metal may be an important factor which contributes towards localized breakdown of the protective film. Intergranular penetration may be initiated by the formation of grain boundary cavities due to diffusion of vacancies during recrystallization or surface stress relief at exposure temperatures.