Size Of Microvoids Research Articles

The relation between changes in ductility and in ductile fracture topography: Control by microvoid nucleation

Changes in microvoid size on ductile fracture surfaces under varying experimental conditions can arise in several ways. An analysis is developed here for the case in which an altered density of microvoid nuclei, expressed as the volume fraction of nucleation of microvoids, is the sole reason for size changes. The analysis is based on the concept that an altered nucleation density can be regarded as an alteration in the effective volume fraction of nuclei. The analysis is shown to agree with the form of available experimental data. Although more detailed experiments are necessary before a complete comparison can be made, the physical rationale presented appears qualitatively correct, and selfconsistent.

A comparison of microvoid sizes in nickel base alloys tested in air and in the presence of hydrogen

A comparison of microvoid sizes in nickel base alloys tested in air and in the presence of hydrogen

On impurity-induced vacancy clustering in nickel

A reaction-rate theory model of impurity-induced vacancy clustering and micro-void formation has been used to interpret the positron annihilation data of Dlubek et al. (1979) on the recovery of cold-worked Ni-Sb alloys. The calculated mean microvoid sizes agree well with the experimental data. The model predicts the time and temperature-dependent microvoid density for a given impurity concentration.

Annealing of Defects in Irradiated Niobium

Results obtained by means of positron annihilation for defect annealing in Nb irradiated by 7 MeV-electrons are present. The lifetime spectra for initial, irradiated and annealed samples were obtained using a standard ORTEC apparatus with time resolution 300 ps (FWHM). 22NaCl on mylar foil was used as a positron source. Analysis of the experimental results allows concluding about the sample state after irradiation and the defect evolution during annealing. Monovacancies and interstitials trapped by impurities and some small vacancy clusters are present after irradiation. The interstitials release from impurities in the range 25-70°C, migrate and annihilate with monovacancies and vacancy clusters thus reducing the whole defect number without relative defect concentration changing (τ2 is constant, I2 and K decrease). Releasing and migration of vacancies occur above 170°C (τ2 increases with microvoid size growth).

Impurity-induced vacancy clustering in cold-worked nickel

Isochronal and time-linear annealing of pure nickel and dilute NiSb-alloys (0.03 and 0.1 at.%Sb) cold rolled to different thickness reductions have been studied by positron lifetime, angular correlation and Doppler-broadening measurements. The results show that free vacancy migration occurs at the stage III around 80 degrees C. A small amount of Sb impurities induces three-dimensional vacancy clustering during the stage III. These microvoids grow continuously with further heating. After an annealing at 335 degrees C they become visible in the transmission electron microscope. The average microvoid size as a function of annealing temperature is estimated. The microvoids anneal out at 450 degrees C during the recrystallisation.

Microvoids in pitch-based and lignin-based carbon fibres as observed by X-ray small-angle scattering.

X-ray small-angle observation on some lignin-based and pitch-based carbon fibres is discussed with reference to the results of high-resolution electron microscopy, which have already been reported separately.The results showed that the sizes of microvoids, as well as of the solid phase around them, increased with increase of heat-treatment temperature and that stretching of the fibre promoted their further increase. So far as the pitch-based carbon fibres are concerned, the microvoid size distribution proved to be almost independent when the sizes are normalized with the length of the radius of gyration; the shapes of the microvoids were found to be well approximated with oblate ellipsoids for the non-stretched fibres and with prolate ellipsoids for the stretched one. The microvoids in the lignin-based fibre were composed of two groups: one was almost similar to that of the pitch-based carbon fibres heat-treated at the same temperature, while the other was much larger in size and more complicated in their distribution.

The effect of microvoid size on positron annihilation characteristics and residual resistivity in metals

Abstract The annihilation characteristics of a positron trapped in spherical clusters of 1., 50 vacancies in Al and Mo are calculated. The results are found to depend sensitively on the void size up to a radius of about 5 A, beyond which the size dependence disappears. The residual resistivity is shown to be nearly independent of the microvoid size during initial stages of vacancy clustering. The sizes of the microvoids in electron irradiated Mo as a function of annealing temperatures are evaluated from recent experimental positron lifetime results.

VOIDS IN NYLON 6 WET SPINNING FIBERS AND THEIR EFFECTS ON DYE AND WATER ABSORPTION BEHAVIOR

Wet spun filaments (WSF) prepared by using phenol (Ph) or formic acid (FA) as solvent were studied in comparison with melt spinning fiber (MSF) and chemicals-treating specimen (CTS) of MSF by Ph, FA or I2.Porosity measured by density and X-ray crystallinity is zero for MSF and CTS, but is 1 for WSF (Ph) and 28 for WSF (FA). Microvoid content estimated from small angle X-ray scattering power is relatively low for WSF (Ph) and very high for WSF (FA). Furthermore, WSF (Ph) contains smaller microvoid, while WSF (FA) contains larger ones. When WSF is drawn, the microvoid content decreases for Ph but increases for FA, and the smaller portion of size of microvoid increases for both Ph and FA. These behaviors can be explained by deformation of microvoid.Dye absorption was measured by using Orange II on Di (measured indirectly from dye bath) and on Dd (measured directly by dissolving dyed sample). For MSF (Di-Dd) is nearly zero and Dd is fairly small, while for WSF both values of (Di-Dd) and Dd are relatively high, and (Di-Dd) decreases and Dd increases by drawing. These dye absorption behaviors can be explained as (Di-Dd) being related to larger microvoids and Dd to smaller microvoids.Moisture regain is same for MSF and for WSF at low humidities ( 60%) its values are larger for WSF than for MSF. After drawing, the moisture regain increases for WSF (Ph) and decreases for WSF (FA) except for 100% RH. These water absorption behaviors were compared with dye absorption behaviors and discussed in the relation to the sub-microvoid.

The effects of particles on fracture processes in magnesium alloys

The influence of coherent and noncoherent particles on the mechanism of ductile fracture has been studied in two magnesium base alloys: Mg-3.7 pct Th-0.4 pct Zr and Mg-0.6 pct Zr. The ternary alloy, was tested in the solution treated condition, and as a function of aging time at the temperature of peak hardening, 325°C; the binary alloy was tested in the solution treated condition and after hydriding to form e-ZrH2 particles. Fractographic studies were carried out on smooth-bar and notched tensile samples after testing at −196°, 25°, and 210°C. Fractographic features observed in the solution treated alloys were explainable in, terms of the operative slip systems in magnesium. Features observed in aged Mg−Th−Zr and hydrided Mg−Zr were related to particle size, morphology, and distribution, with emphasis on dimple-particle relation-ships. Coherent, plate-like particles (250A long) produced dimples in Mg−Th−Zr, and did larger, noncoherent particles. However, many cases of dimple formation without associated particles also were noted. Microvoid sizes observed by light microscopy were 20X dimple sizes revealed by electron fractography. When submicroscopic voids (<0.5μ) associated with cracked particles were present, lateral growth of these voids did not occur prior to fracture. It was concluded that microvoid coalescence as classically pictured, was not the failure mechanism in these alloys nor was it the mechanism of formation of dimples.