Investigation of damage in Pt,Al, and Al-0.1 at%Mg produced by d-Be and fission-reactor neutrons using positron-annihilation

Specimens of Al, Al-Mg, and Pt were irradiated using the U.C. Davis d-Be neutron source which has a spectrum peaking at ～14 MeV. Lifetime and Doppler-broadening measurements have been made on these specimens in the as-irradiated state, for three fluences, and for subsequent anneals to 500°C. Saturation of positron trapping at the higher fluences was observed in the Pt specimen, with the average trap size increasing after annealing through stage III (vacancy migration). Surviving traps after the 500°C anneal are presumed to be voids. The Al and Al-Mg showed some trapping introduced by the room-temperature irradiation, but complete recovery is seen at 500°C. These trapping sites are assumed to be small vacancy clusters or small dislocation loops. Some loops in very low concentration were observed near grain boundaries in the as-irradiated specimens of Al by TEM.     

Ni3Al（0.98at%B）中硼原子扩散行为的正电子湮没研究

用正电子湮没技术研究了Ｎｉ３Ａｌ（０．９８ａｔ％Ｂ）合金经高温均匀化处理后，在空冷、炉冷、水冷条件下硼原子的扩散行为，结果表明，炉冷处理的合金中扩散至晶界的硼原子最多，并有微量硼化物在晶界处析出：水冷处理时扩散至晶界的硼原子最少，为使既有较多的硼原子扩散至晶界，又无硼化物在晶界析出，宜采用高温均匀化空冷的热处理工艺。     

Analysis of irradiation growth in zirconium-base alloy

Irradiation induced growth phenomena in zirconium-base alloy have been investigated on the basis of the numerical results obtained by dynamical-method computer simulations. Point defect trapping by solute atoms causes transient growth strain which is recoverable during temperature cycling. The growth strain change of cold-worked Zircaloy-2 irradiated at 553 K can be explained by a dislocation climb and a net inflow of vacancies into grain boundaries. The effects of various parameters such as grain size are also discussed.     

Phase-field modeling of gas bubbles and thermal conductivity evolution in nuclear fuels

A phase-field model was developed to simulate the accumulation and transport of fission products and the evolution of gas bubble microstructures in nuclear fuels. The model takes into account the generation of gas atoms and vacancies, and the elastic interaction between diffusive species and defects as well as the inhomogeneity of elasticity and diffusivity. The simulations show that gas bubble nucleation is much easier at grain boundaries than inside grains due to the trapping of gas atoms and the high mobility of vacancies and gas atoms in grain boundaries. Helium bubble formation at unstable vacancy clusters generated by irradiation depends on the mobilities of the vacancies and He, and the continuing supply of vacancies and He. The formation volume of the vacancy and He has a strong effect on the gas bubble nucleation at dislocations. The effective thermal conductivity strongly depends on the bubble volume fraction, but weakly on the morphology of the bubbles.     

腐蚀疲劳断裂前氢与缺陷相互作用的正电子湮没研究

用正电子湮没谱和加热释氢研究了低合金高强钢人工海水腐蚀疲劳断裂前经若干小时疲劳的样品。实验结果表明,腐蚀疲劳早期阶段产生的位错、空位团以及钢中晶界是氢的捕获位,其捕获氢量随疲劳时间增加。氢与空位相互作用使空位团尺寸增加,数目增多,氢与位错相互作用促使位错增殖且随疲劳时间增加。晶界与氢相互作用能随疲劳时间、晶界捕获氢量增加而降低。碳化物沿晶析出促进了晶界捕获氢。较低的晶界-氢结合能有利于氢沿晶运动,促使预裂缝沿晶发展。     

Are the stacking faults in Cu—Zn—Al able to trap positrons

The phase transformation of Cu-23at.%Zn-11at.% al Al alloy from about 5℃ to 25℃ has been studied by means of the positron annihiation and transmission electron microscope.It is shown that the stacking faults in the alloy have no remarkable effect on the trapping of positrons.     

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements

Doppler-broadening measurements of the electron–positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference material. The positron lifetime technique has been used to detect the presence of positron traps in all the samples. The change in the high-momentum part of the annihilation line due to positron trapping is illustrated. The measured data are in a good qualitative agreement with recent theoretical calculation and constitute the most complete measurement series, up to now, to establish a future data-base for positron annihilation spectroscopy.     

Surface cracking on Σ3, Σ9 CSL and random grain boundaries in helium implanted 316L austenitic stainless steel

The relationship between surface cracking at grain boundaries and the grain boundary nature in helium implanted 316L austenitic stainless steel was investigated by in situ annealing in a high-voltage electron microscope, and by SEM and TEM observations. The nucleation and growth of helium bubbles at a random grain boundary was observed during annealing up to 973 K. After annealing, surface cracking was observed at the random grain boundaries and some coincidence site lattice (CSL) boundaries because of the formation and rupture of the helium bubbles at these grain boundaries. At the faceted CSL boundaries, surface cracking occurred only on one boundary facet plane. This indicates that the twin boundary and pure tilt Σ9 CSL boundary show the highest resistance to cracking because of their low boundary energies.     

Large molecular dynamics simulations of collision cascades in single-crystal, bi-crystal, and poly-crystal UO2

Classical molecular dynamics simulations have been carried out to study the primary damage due to α-decay self-irradiations in single-, bi-, and poly-crystal UO₂ matrices. In all the cases no amorphization has been found, only the creation of few point defects is observed. However, in all grain boundary systems numerous point defects are created along the interfaces. Furthermore, cascade morphologies depend strongly on the grain boundary structure. For symmetrical tilt grain boundaries with small misorientation angles (lower than 20°) the structure at the grain boundaries is composed of edge dislocations, whereas for higher misorientation angles is formed by Schottky defects. The grain boundary structure in the poly-crystal is found to be highly disordered. For the last two systems, cascades seem stopped by the interfaces unlike those with edge dislocation grain boundaries. These types of interface act like sink which traps moving atoms.     

Modeling and simulation of uniformity effects in Cd/sub 1-x/Zn/sub x/Te gamma-ray spectrometers

Monte Carlo simulations of pulse height spectra for Cd/sub 1-x/Zn/sub x/Te detectors are used to investigate the effect of variations in alloy composition and carrier drift lengths on energy resolution. The results, which are based on a simple phenomenological model, show that these nonuniformities can have significant detrimental effects on spectrometer performance. For the case of Bridgman-grown material, the orientation of the growth axis relative to the detector axis is shown to be an important consideration, especially for crystals which come from the heel end of a boule, where the composition gradient due to zinc segregation is large. Other effects which we have simulated include growth striations, zinc segregation at grain boundaries, and trapping by inclusions and grain boundaries; each of these effects is detrimental to energy resolution. We conclude that material nonuniformity is a major obstacle to achieving statistically limited energy resolution in cadmium zinc telluride detectors.