Volume Fraction Of Cavities Research Articles

Evidence for the coalescence of cavities in nickel irradiated with 500 keV 4He + ions

Transition electron microscopy has been used to study the effect of dose on the depth distribution of cavities (voids or bubbles) and dislocation damage in nickel irradiated at 500°C, with 500 keV 4He + ions. The diameter, number density and volume fraction of cavities were measured as a function of depth from micrographs taken from samples sectioned parallel to the direction of the incident beam. The results for the dose range 2 × 10 19 to 1 × 10 21 ions/m 2 show an increase in the average cavity diameter, number density, and volume fraction with increasing dose. A further increase in dose from 5 × 10 21 to 1 × 10 22 ions/m 2 shows a significant decrease in the number density (a factor of ∼40) and a corresponding increase in the average diameter (a factor of ∼8) of the cavities in the peak volume-swelling region. This observation is interpreted as evidence for the coalescence of cavities. The peak in the volume-swelling distribution occurs at depths ∼10% deeper than the calculated average projected- range of 500 keV 4He + ions in nickel.

On the role of grain-boundary migration during the creep of zinc

Constant engineering strain-rate tensile tests have been carried out in the temperature range 20 to 150‡C on high purity Zn, Zn-0.14 at. % Cu (alloy C) and Zn-0.16 at. % Al (alloy A) alloys. Measurements of angular distribution of orientations of grain boundaries have been used to study grain-boundary migration during deformation. Significant cavitation, with increasing propensity at higher test temperatures, occurred in the two alloys but not in pure Zn. A striking feature of the observations in pure Zn and alloy C, as the test temperature was raised, was the formation and subsequent decay of a diamond pattern of uncavitated grain boundaries, a majority of which were preferentially aligned at ∼ 45‡ to the stress axis. By comparison the changes in the angular distribution of grain boundaries was least marked in alloy A. Cavitation was observed in alloy C to maintain grain boundaries in the 45‡ orientation. At the test temperature of 150‡C alloy C, which was prone to the formation of diamond grain-boundary configuration, developed much larger volume fraction of cavities than alloy A. These results are discussed in terms of the different distribution coefficients of Cu and Al in Zn, the different rates of grain-boundary migration in pure Zn and the two alloys and the differences in the substructural features (cells) formed during high-temperature deformation.

The influence of polycrystal grain size upon the creep ductility of copper

A study of the relationship between grain size and creep ductility has been made on OFHC copper when the fracture process involves intergranular cavitation. Tests have been carried out at a constant nominal strain-rate at 10−2 hr−1 at temperatures of 350°, 425°, and 500°C. At all temperatures a peak in the grain size vs ductility plots is obtained at (approximately) 30 μm for 350°C, 60 μm for 435°C, and 150μm for 500°C. The work involved a metallographic study of the fracture process and it is deduced that the final stage of fracture (cavity linkage) controls ductility, rather than either the rate of nucleation or individual growth of cavities. At coarse grain sizes and low temperature the crack length seems to be critical and increasing grain size decreases ductility. At fine grain sizes and high temperature, failure is by a ductile void-sheet process and the volume fraction of cavities is controlling so that decreasing grain size decreases ductility.