B. Russell

The behaviour of helium gas in irradiated copper-boron alloys

The behaviour of helium in neutron-irradiated copper-boron alloys was investigated in the temperature range between 300° C and 800° C. It was found that bubbles nucleate preferentially upon dislocations which are created by the annealing of displacement damage. After gas precipitation is complete, the bubbles grow by migration and coalescence, at such a rate that the average bubble radius is proportional to t 1 14, where t is time. Helium causes solution-hardening and the strength of an alloy containing bubbles is inversely proportional to the bubble spacing. One atomic per cent of helium causes a 3 μohm · cm increase in electrical resistivity. The changes which occur in the resistance during annealing are complicated and contributions arise from the concentration of helium in solution and the changing distribution of bubbles. An activation energy of 0.98 ± 0.1 eV was obtained for the first stage of annealing during which all of the gas leaves solution to form bubbles.

A preliminary investigation of helium gas precipitation in irradiated copper-boron alloys

Changes which occur in the lattice parameter of a neutron irradiated copper-boron alloy during the early stages of annealing, have been measured. After irradiation at 100° C, the lattice parameter is larger than that of the unirradiated alloy. During annealing, the lattice parameter decreases until it becomes smaller than that of the unirradiated alloy, and then increases. After complete annealing, the lattice parameter is equal to that of the original untreated alloy. Initially the helium atoms, which are formed by an (n, α) reaction from B10, occupy interstitial positions. During annealing, each helium atom captures two vacancies and enters substitutional solution, before migrating to form gas bubbles. This process causes the lattice to contract. As bubbles form, helium leaves solution and the concentration of heliumvacancy complexes is reduced. There is a corresponding expansion of the lattice parameter. The activation energy for the first stage is 0.76 ± 0.1 eV. The activation energy for the second stage is 0.96 ± 0.1 eV, which is possibly the energy required to move a helium atom-vacancy complex through the lattice.

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

Abstract 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.

Spontaneous grain boundary swelling in irradiated copper-boron alloys

The degeneration of grain boundaries during the annealing of copper-helium alloys has been investigated. The grain boundary bubbles are stable at temperatures below 700° C, but at higher temperatures they grow spontaneously by absorbing vacancies generated in the grain boundaries. Resistivity measurements are particularly sensitive to grain boundary degeneration and can be used to investigate the process in its earliest stages. Increasing the temperature after bubbles have reached an equilibrium size causes breakaway growth of grain boundary bubbles at temperatures below those at which it would normally occur. Gas concentrations as low as 46 ppm are sufficient to cause spontaneous grain boundary degeneration at 800° C. The ultimate failure of grain boundaries is caused by the growth of bubbles until they touch and the conditions which contribute to this growth in the absence of an applied stress are examined.

The behaviour of radiation-induced gas in irradiated aluminium-lithium alloys

The behaviour of radiation-induced gas in neutronirradiated aluminium-lithium alloys was investigated in the temperature range 200-600 °C. It was found that gas bubbles nucleate homogeneously during neutron irradiation at 75 °C. These gas bubbles grow by a migration and coalescence mechanism under the influence of a driving force provided by a non-equilibrium of vacancies. Experimental evidence has been obtained for the conservation of lattice strain energy during the growth process so that the internal pressure, P, of the bubble is balanced by the surface tension forces γ such that P = 2γ/r. The time dependence of the growth process is highly temperature-dependent, varying from r ∝ t at 600 °C to r ∝ t at 200 °C, where r is the mean bubble radius and t the annealing time. Small bubbles were observed to persist after annealing at temperatures of 500 °C and below, indicating that not all bubbles take part in the growth process at these temperatures and gas resolution is not an operative process. Under conditions of high vacancy concentration, the bubbles developed major (111) type faces and minor (100) type faces and denuded zones surrounded grain boundaries.

A theory of grain boundary degeneration in materials which contain an inert gas

Abstract A theory is proposed in which the lenticular growth of grain boundary bubbles and the resultant force generated across the boundary by the gas within them are considered, in order to explain spontaneous cracking at high temperatures. It is proposed that most of the inert gas which leaves solution to precipitate in grain boundary bubbles enters them in the plane of the boundary. A non-equilibrium shape results and the forces due to gas pressure gradually exceed the restraint imposed by surface tension effects. The driving force for this mechanism of non-equilibrium growth is derived from the chemical potential gradients for vacancies between the grain boundary and the bubble surface. The theory predicts a minimum temperature for spontaneous bubble growth which is comparable with experimental observation. The effect of an applied stress is also considered and an equation which predicts the time to rupture as a function of stress and temperature is derived.

The effect of radiation induced gas on the stress rupture properties of an aluminium-lithium alloy

The changes in stress-rupture properties of an aluminium-lithium alloy induced by the presence of inert gas bubbles after neutron irradiation have been investigated in the temperature range of 175 °C to 300 °C. A bubble distribution with a mean diameter of 47 A at an average spacing of 1400 A produces a decrease in the minimum creep rate of two orders of magnitude below that for the bubble-free material under the same test conditions. For very fine bubble distributions and distributions which contain a narrow size range of large bubbles, a breakaway to a very high stress-sensitivity at low stresses occurs. This is absent in alloys which contain stronger bubble obstacle arrays which include a wide range of bubble sizes. All inert gas concentrations reduce creep ductility of aluminium-lithium alloy but the severity of embrittlement increases markedly with gas concentration and testing temperature. The existence of a critical bubble size for breakaway growth of grain boundary voids is implied by the experimental evidence. For a given bubble distribution a critical stress exists below which grain boundary degeneration does not occur. As the gas concentration is increased, the fracture mechanism tends increasingly towards inter-granular failure.

The stress-rupture characteristics of copper and alloys containing boron and helium

Abstract The stress-rupture proporties of OFHC copper, Cu/0.04 wt % B and heat-treated Cu/0.05 at % 4He alloys are compared. The activation energy for the process of fracture is 2.17 ± 0.2 eV in all three materials which is close to that for volume diffusion in copper. At temperatures below a critical temperature, the stress rupture properties are independent of the presence of gas in the grain boundary voids and depend only on the void distribution. The only apparent effect of gas is to alter the size and distribution of voids within the grain boundaries. The distribution of helium bubbles can be controlled by heat treatment and the stress-rupture properties of Cu-B alloys can bo improved at low stresses by introducing a coarse distribution of bubbles. However, the number of voids in the grain boundaries increases with stress even when helium bubbles are present. At temperatures above a critical temperature, the pressure of gas within a bubble produces an internal stress, in addition to the applied stress, which decreases the time required for rupture. This additional stress arises when the stress duo to pressure exceeds the restraint caused by surface tension during lenticular bubble growth. The common assumption that stressed grain boundaries, in which void growth occurs, act as vacancy sources is not supported by the experimental evidence. Rather, it appears that vacancies flow towards the boundaries from the surrounding matrix.

The dependence of the tensile properties of irradiated aluminium-lithium alloys upon gas bubble size and distribution

Abstract The effect of inert gas bubbles on the tensile properties of neutron irradiated aluminium-lithium alloys was examined in the temperature range 103 to 523 °K. An analysis of the macroyield strength of alloys containing various size distributions of gas bubbles at 223 °K showed that bubbles are weak obstacles to dislocation movement with breaking angles which vary from 132° for 150 A diameter bubbles to 144° for 50 A bubbles. Below 50 A diameter the breaking angle decreases rapidly and becomes 174° for bubbles approximately 15 A in diameter. The temperature dependence of the macro-yield stress for alloys irradiated to thermal neutron doses of 1.2 × 10 19 nvt and less showed that the presence of small bubbles caused an additional friction stress. Higher irradiation doses produced dislocation locking, possible due to the formation of jogs on glide dislocations. Ductility was severely reduced at high temperatures by the presence of inert gas bubbles in the grain boundaries. The ductility of irradiated material at low temperatures increased significantly and material which contained bubbles became more ductile than the unirradiated alloy. During post-irradiation annealing, which results in the coarsening of an originally fine distribution of small bubbles, the macro-yield stress increases to a maximum value and then decreases.