Helium Precipitation Research Articles

POSSIBLE FAST-REACTOR CANNING MATERIAL STRENGTHENED AND STABILIZED BY DISPERSION

The high-temperature irradiation embrittlement of steels and nickel alloys is probably due to helium precipitation in grain boundaries. It would be desirable to retain the helium as fine bubbles evenly distributed within the grains. A fine dispersion in the metal matrix may act as nucleation site for helium bubbles and it is proposed to produce by powder-metallurgy techniques afine dispersion of a ceramic oxide in a steel matrix. Ferritic chromium steel was chosen because it is known to be less susceptible to embrittlement than austenitic alloys and in this case the oxide dispersion must also improve the high-temperature mechanical properties.Methods of preparation are discussed and results of preliminary tensile tests performed on iron containing alumina, magnesia, and titania dispersions indicate the feasibility of the proposed solution.

The effect of concentration upon helium precipitation in irradiated copper boron alloys

The lattice parameter of copper is proportional to the concentration of dissolved interstitial helium up to approximately 0.03 at %. When the concentration of helium is increased to 0.08–0.1 at %, the gas precipitates to form bubbles during irradiation at temperatures in the range 75° C–100° C. During annealing at 500° C, the lattice parameter contracts owing to the formation of helium-vacancy complexes, at a rate which is proportional to the concentration of helium. Subsequent expansion of the lattice, caused by the precipitation of these complexes to form bubbles, also occurs at a rate which is proportional to helium concentration. The total time required for gas precipitation is inversely proportional to the helium content and this behaviour is associated with an increasingly fine scale of gas bubble nucleation. The average size and spacing of bubbles formed during the complete precipitation of helium increase with decreasing gas concentration. The bubble density is related to the number of dislocations generated by the annealing of displacement defects caused by fast neutron and alpha particle collisions. However, some homogeneous nucleation may also occur in materials with a high gas concentration. As the gas concentration of helium increases, a rapid initial contraction of the lattice develops during the first two minutes of annealing at 500° C. It is suspected that many of the helium atoms formed during irradiation are closely associated with potential vacancy sources such as jogs on dislocations, or clusters of vacancies caused by displacement collisions. At temperatures above those at which the vacancies become mobile, a proportion of these helium atoms enter substitutional solution very rapidly because the diffusion distances are short.

Precipitation of Helium along Dislocations in Aluminum

Helium was generated in an Al-0.1 at. % Li alloy by transmutation of the Li6 atoms during neutron irradiation. During subsequent heating of the specimens, the helium appeared to precipitate in the form of small ``bubbles.'' On using a dislocation etchant it was found that (a) the regions of largest dislocation density also contained the highest cavity density, (b) in some regions each dislocation etch pit was associated with a cavity, and (c) in other regions a series of cavities appeared to delineate the track of a dislocation. It is believed that the helium precipitates along the dislocations in the form of continuous or semicontinuous cylindrical cavities.

Radiation-enhanced helium precipitation in copper

Abstract The effect of neutron irradiation on the precipitation of helium atoms in copper has been studied at temperatures up to 425°c. At 160°c end 300°c no effect was observed. At 425°c the irradiated sample exhibited considerable precipitation around grain boundaries as compared to an unirradiated sample held at the same temperature. It is concluded that the additional vacancies introduced during the irradiation enhance the precipitation process. The resultant voids are equal in size to those obtained at higher temperatures in the absences of radiation.