G.J.C. Carpenter

The dilatational misfit of zirconium hydrides precipitated in zirconium

Abstract The unconstrained misfit strains, δ [uvtw] , associated with the formation of zirconium hydride precipitates in zirconium have been calculated with respect to directions [uvtw]Zr. Both the δ-hydride, (normally produced by slow cooling) and the γ-hydride (formed after fast cooling) are considered. The results are shown to be self-consistent and predict the correct volume change caused by the transformation.

The disordering of Zr3Al by 1 MeV electron irradiation

Abstract The effect of 1 MeV electron radiation on the L12 ordered intermetallic compound, Zr3Al, has been studied over the temperature range 130–775 K, using a high voltage electron microscope. At temperatures in the range 130–375 K, complete disorder was produced by irradiation to a dose of approximately one displacement per atom (dpa), independent of damage rate over the range 5 × 10−4 to 5 × 10−3 dpa/s. After irradiation to a few dpa at higher temperatures, 575–775 K, a steady-state was established which was characterized in part by an intermediate degree of long-range order which increased with irradiation temperature. By comparison with published results from ion irradiation experiments, it appears that the number of atomic displacements to cause complete disordering at low temperatures is independent of the nature of the damage events. At the higher temperatures, neither dislocation loops, dislocation networks nor voids were observed. Generally, the crystalline perfection was markedly reduced at all temperatures by irradiation to doses exceeding a few dpa.

The contribution of dislocation loops to radiation growth and creep of Zircaloy - 2

Abstract Annealed specimens of Zircaloy-2 have been examined using transmission electron microscopy after irradiation to various fluences up to 1 × 10 26 n · m −2 ( E > 1 MeV ) at 573 K. Measurements of the size and concentration of the radiation-induced defects show that they make a negligible contribution to radiation growth. An alternative mechanism is sug-gested, based on the bias interaction between interstitials and dislocations. A specimen of cold-worked and stress-relieved Zircaloy-2, creep tested at 138 MPa in a reactor at 573 K, was also examined. It was shown that the total number of point defects in the form of clusters was the same as in an unstressed, annealed specimen. It can be concluded that the stress-induced orientation of dislocation loops makes a negligible contribution to radiation creep of Zircaloy-2 at 573 K, regardless of the magnitude of the applied stress.

Dislocations generated by zirconium hydride precipitates in zirconium and some of its alloys

Abstract The character of dislocations emitted during the precipitation of γ-zirconium hydride in zirconium, Zircaloy-2, Zr-1% Al and Zr-1% Cr has been determined. Contrast experiments in the transmission electron microscope have shown that the dislocations possess Burgers vectors ( b ) of the type 1 3 a 2 0> . Of the three possible b s only one or two are observed associated with a given hydride needle, those giving a large component of b along a direction perpendicular to the needle. Dislocation generation is thought to result from the dilatational misfit associated with the hydride needles. The creation of dislocations with b s along the c- axis is more difficult and the misfit strain along this direction is not relieved by plastic deformation.

Calculations of irradiation growth in zirconium

Abstract Reaction rate theory is used to calculate the dimensional changes in zirconium that result from the annihilation of irradiation-induced point defects at internal sinks. The available data on point defect parameters and the microstructures produced by irradiation are reviewed in an effort to provide a reasonable, justifiable basis for the calculations. In accordance with the experimental evidence, the microstructure for the calculations is assumed to contain two main classes of sinks: dislocations with (a) -type Burgers vectors and grain boundaries. The growth strain results from a net flux of interstitials annihilating at edge dislocations and a corresponding vacancy flux arriving at the grain boundaries. The study includes a discussion of the sensitivity of the growth calculation to the expressions chosen for the sink annihilation probabilities, to the sink concentrations and to anisotropy of the microstructure. Finally, a comparison is made between calculated growth rates and experimental data for cold-worked Zircaloy-2, irradiated at 550 K. Two models are used to describe the behaviour of screw dislocations, which receive a net flux of vacancies. In model 1, the screw dislocations act as perfect sinks, forming dislocation helices, while in model 2, the excess vacancies do not annihilate at the screw dislocations but migrate by pipe diffusion to the grain boundaries. The absolute magnitude of the growth rate in cold-worked Zircaloy at 550 K, calculated using model 2, is much closer to experimental values than that obtained with model 1, which greatly underestimates the growth rate.

An in-situ study of the dissolution of γ-zirconium hydride in zirconium

Abstract The dissolution of γ-zirconium hydride has been examined in single crystal specimens of zirconium using a hot stage in a high voltage electron microscope. It was found that a significant proportion of the dislocations generated by the hydride needles during growth were not annihilated when dissolution occurred on heating to well above the solvus temperature. This is contrary to earlier work where similar experiments were carried out with thinner specimens at conventional accelerating voltages. Electron irradiation completely prevented annihilation of the hydride dislocations during dissolution. The results are discussed in relation to (a) repeated nucleation at the same sites during thermal cycling, (b) external shape changes (growth), (c) strain steps observed during thermal cycling under creep conditions, (d) positron annihilation experiments, and (e) the terminal solid solubility of hydrogen in zirconium.

In-reactor stress relaxation of selected metals and alloys at low temperatures

Sttess relaxation of bent beam specimens under fast neutron irradiation at 340 and 570 K has been studied for a range of materials, as follows: several stainless steels, a maraged steel, AISI4140, Ni, Inconel X-750, Ti, Zircaloy-2, Zr-2.5% Nb and Zr3 Al. All specimens were in the annealed or solution-treated condition. Where comparisons were possible, the creep coefficients derived from the stress relaxation tests were found to be consistent with other studies of irradiation-induced creep. The steels showed the lowest rates of stress relaxation; the largest rates were observed with Zr-Nb, Ti and Ni. For most materials, the creep coefficient at 340 K was equal to or greater than that at 570 K. Such weak temperature dependence is not easily reconciled with existing models of irradiation creep based on dislocation climb, such as SIPA or climb-induced glide. Rate theory calculations indicate that because the vacancy mobility becomes very low at the lower temperature, recombination should dominate point defect annealing, resulting in a very low creep rate compared to that at the higher temperature. It is shown that the weak temperature dependence observed experimentally cannot be accounted for by the inclusion of more mobile divacancies in the calculation.

Dislocation loop generation and irradiation growth in a zirconium single crystal

Abstract A zirconium single crystal was irradiated at 473–523 K and its growth measured. The irradiation damage was characterized at various fluences by transmission electron microscopy (TEM). The growth, an initial expansion in the c- axis direction followed by shrinkage and a net contraction of 0.03% at a fluence of 1.8 × 10 24 n/m 2 ( E > 1 MeV ), cannot be explained by the dislocation loops visible by TEM. An explanation for the measured growth is given in terms of defects that are too small to be seen in the electron microscope, i.e. point defects or very small defect clusters.

A study of electron irradiation damage in Zirconium using a high voltage electron microscope

Abstract Electron irradiation damage at 1 MeV has been studied in zircomium and Zircaloy-2, in the temperature range 315 to 725 K, using a high voltage microscope. At low doses, the damage takes the form of perfect prismatic dislocation loops of interstitial character, with Burgers vectors of the type 1 3 a 2 0> . The loops form in bands parallel to the trace of the basal plane, by a process of preferred nucleation. A significant proportion of the loops appear to nucleate on {1120} planes in a pure edge configuration and reorient during growth to reduce their energy. Continued irradiation causes loop growth and interactions, resulting in the formation of a dislocation network. The results are compared with data from neutron- and ion-irradiation studies.

Irradiation growth of zirconium single crystals

Abstract Irradiation growth of zirconium single crystals has been studied during neutron irradiation at 353 K and 553 K at fluences up to 2× 10 25 n/m 2 . The results may be summarized as follows: (a) there was an expansion parallel to the a -axis and a c -axis contraction; (b) the growth strains were small (~10 −4 ), (c) growth saturated at fluences less than ~5× 10 24 n/m 2 , (d) the growth behaviour was only weakly dependent on temperature for the range studied, (e) there was a calculated volume increase of the same order as the growth strain, and (f) single crystals prepared from stock of iodide and zone-refined purity showed similar growth behaviour at 553 K. The a -axis expansion is attributed to the annihilation of an excess of interstitials at a >-type dislocations and interstitial loops. The c -axis contraction may be caused by: (1) elastic relaxation around vacancies or small vacancy clusters, (2) non-linear elastic effects at the dislocation cores of small 〈 a 〉-type loops, or (3) sub-microscopic vacancy loops with 〈 c 〉-component Burgers vectors. Comparison with data from polycrystalline zirconium confirms that grain boundaries can play an important role in the irradiation growth of zirconium.