Suppression Of Void Nucleation Research Articles

Irradiation temperature and dose rate dependence of void swelling in RAFM steel during irradiation: Reaction rate theory analysis

Reaction rate theory analysis has performed to investigate the irradiation temperature and dose rate dependence of void swelling of RAFM steel under a wide range of irradiation temperatures and dose rates (250 to 620 °C and 10−7 to 10−3 dpa/s) with focusing on nucleation behavior of voids. In the rate theory analysis, an improved nucleation model is employed where the stable nucleus size of voids is estimated by the vacancy flux into/from voids which depends on irradiation condition and void size. The calculation says that the void swelling strongly depends on irradiation temperature and dose rate, and there is one unique peak temperature for each dose rate. As dose rate increases, the peak temperature shifts to higher temperatures and the peak swelling decreases; this results from the enhancement and suppression of void nucleation. The calculated stable nucleus size of voids remarkably depends on irradiation conditions and varies widely from 5 to about 50: the nucleus size drastically increases at higher temperatures, while it decreases at higher dose rates. For the dose rate dependence of the peak temperature of void swelling, the calculation provides a good agreement with the experiment more than a conventional model.

Suppression of Void Nucleation in High-Purity Aluminum via Dynamic Recrystallization

The process of ductile fracture in metals often begins with void nucleation at second-phase particles and inclusions. Previous studies of rupture in high-purity face-centered-cubic metals, primarily aluminum (Al), concluded that second-phase particles are necessary for cavitation. A recent study of tantalum (Ta), a body-centered-cubic metal, demonstrated that voids nucleate readily at deformation-induced dislocation boundaries. These same features form in Al during plastic deformation. This study investigates why void nucleation was not previously observed at dislocation boundaries in Al. We demonstrate that void nucleation is impeded in Al by room-temperature dynamic recrystallization (DRX), which erases these boundaries before voids can nucleate at them. If dislocation cells reform after DRX and before specimen separation by necking, voids nucleation is observed. These results indicate that dislocation substructures likely plays an important role in ductile rupture.

Suppression of void nucleation in Sn3.0Ag0.5Cu/CU solder joint by rapid thermal processing

Electrodeposited (ED) Cu was prepared and exhibited a strong voiding propensity at the Cu3Sn/Cu interface in the subsequent Sn3.0Ag0.5Cu/Cu solder joint. However, we found that the rapid thermal processing treatment could significantly reduce the voiding propensity of ED Cu. After rapid annealing, the grain size of Cu was obviously increased, which was found to reduce the amount of effective vacancies and lower the driving force of void formation. Meanwhile, the ED Cu before and after rapid annealing was analyzed using X-ray photoelectron spectroscopy, and it can be found that there did exist additives remained in the Cu layer during electroplating process. However, after rapid annealing treatment, some residuals would evaporate, and the others would decompose and react with Cu, which would change the residuals from the free state to the “deactivated” state and make them lose their acting as the precipitate for the heterogeneous nucleation. Thus, the Cu3Sn/Cu interface with few voids was finally formed.

Development of a Plane Strain Tensile Geometry to Assess Shear Fracture in Dual Phase Steels

A geometrically modified sample capable of generating a triaxial stress state when tested on a standard uniaxial tensile frame was developed to replicate shear fractures observed during stretch bend tests and industrial sheet stamping operations. Seven commercially produced dual phase (DP) steels were tested using the geometrically modified sample, and the modified sample successfully produced shear fractures on a unique shear plane for all steels. For each steel, void densities were determined, based on metallographic analyses, as a function of imposed displacement. Microstructural properties of ferrite and martensite grain size, martensite volume fraction (MVF), retained austenite content, Vickers hardness, average nanoindentation hardness, average ferrite and martensite constituent hardness, and tensile properties were obtained in order to evaluate potential correlations with void data. A linear correlation was observed between Vickers hardness and the average nanoindentation hardness, verifying the ability of nanoindentation to produce data consistent with more traditional hardness measurement techniques. A linear relationship was observed between the number of voids present at 90% failure displacement and the martensite/ferrite hardness ratio, indicating that a decrease in relative hardness difference in a microstructure can suppress void formation, and potentially extend formability limits. The void population appeared independent of MVF, grain size, and tensile properties suggesting that constituent hardness may be a dominant parameter when considering suppression of void nucleation in DP steels.

Acoustic emission from particulate-reinforced metal matrix composites

A systematic study of the effect of microstructural parameters on the fracture behaviour of silicon carbide particle reinforced aluminium matrix composites has been carried out. Acoustic emissions have been monitored during tensile testing, giving the size and number of emmissions as a function of strain. This has been shown to be simply related to the rate of void nucleation at the reinforcing phase. Both particle fracture and particle/matrix decohesion mechanisms can be detected. Void nucleation was observed from the onset of plastic deformation and a linear relationship between damage initiation rate and strain was found. The rate of emission increased with reiforcing particle size and volume fraction but was independent of matrix alloy composition and heat treatment. These results show that the failure strain of particulate metal matrix composites is not controlled solely by the onset of void nucleation at the reinforcing phase. Local failure processes in the matrix are shown to promote void coalescence and dominate the ductility. However, suppression of void nucleation at the particles increases the ductility. It is suggested that a critical number of fractured particles is required before failure.

Influence of helium-generated interstitials in dual ion irradiations

Simulation of neutron-induced swelling by dual ion irradiation is influenced very strongly by the presence of injected interstitials due to self ions. Co-injected helium also generates extra interstitials, with its influence on void nucleation magnified by a synergism with those due to self ions. Low energy experiments are particularly affected since measurements have to be made where most ions come to rest. When the high helium/dpa ratios typical of fusion devices are employed the effect of helium-generated interstitials may dominate that of helium-as-gas. Co-injected helium therefore often appears to decrease swelling at a given damage level. The suppression of void nucleation by helium-generated interstitials decreases with increasing sink strength and is dominant at both low temperatures when recombination is high and at high temperatures when vacancy re-emission from vacancy clusters is significant. It is proposed that this behavior is an artifact of the simulation process and is atypical of the fusion reactor environment.

Suppression of void nucleation by injected interstitials during heavy ion bombardment

In a heavy ion irradiation the injected ions come to rest at the end of range as interstitials without a vacancy partner. These extra interstitials perturb the delicate balance of vacancy and interstitial flux to voids. It is shown that the void nucleation rate is drastically reduced by the injected interstitials whenever recombination is an important process. As a result, void nucleation is suppressed in ion-bombardment experiments below a characteristic threshold temperature in the region of the ion deposition. This leads to a void free zone near the end of range in low temperature ion-bombardment experiments. The results obtained are in qualitative agreement with earlier experimental observations.

Comments on the suppression of void nucleation by oversize solute atoms

Comments on the suppression of void nucleation by oversize solute atoms

The suppression of void nucleation by oversized solute atoms

The suppression of void nucleation by oversized solute atoms

Suppression of void nucleation by a vacancy trapping mechanism

A model for suppressing void nucleation in fast reactor environments is proposed in which the free vacancy concentration participating in void nucleation is reduced by vacancy trapping at oversize solute atoms. Calculations for void nucleation in an iron alloy containing 0.3 at −% of solute with a vacancy binding energy of 0.5 eV show reductions in void nucleation rate by a factor of 10 14 or more. The mechanism should not be subject to the instabilities of cold work recovery and precipitate overaging inherent in present methods of swelling reduction and since only small solute additions are required, the mechanism suggests a potential means for improving the swelling resistance of currrently available cladding materials.