Metallographic studies of fatigue in 20% cold-worked zircaloy-2 containing zirconium hydride

The fatigue behaviour of unhydrided and hydrided 20% cold-worked zircaloy-2 reactor pressure-vessel tubing has been studied for fluctuating tension at room-temperature and 300 °C and for reversed torsion at room-temperature. At room-temperature, high concentrations of zirconium hydride markedly reduce the critical crack length. This effect is attributed to a lowering of the fracture toughness, since hydrides fracture in the path of crack propagation and promote matrix cleavage. At 300 °C, hydride particles constrained by the matrix are plastically deformed without fracturing; the toughness is probably unaffected by the presence of hydride.     

On stress relaxation of zirconium tubing

Stress relaxation offers a means for evaluating the deformation dynamics of metals. A previously suggested equation relating the stress rate and stress which avoids the indeterminacy of machine stiffness is applied to this type of experiments on zirconium tubes at room temperature and 400° C. The choice of zirconium tubes and temperatures is related with technological applications. Good agreement is found at room temperature and no agreement at 400°C. Parameters of this equation are discussed in terms of mobile and total dislocation densities.     

Void-swelling in cold-worked copper during HVEM irradiation

Void formation and growth in pure cold-worked copper and the parallel evolution in dislocation structure during HVEM irradiation have been investigated. Degrees of cold work in the range 10–90% rolling reduction and irradiation temperatures in the range 250–450°C have been covered. The cold-worked structure basically survives irradiation. Initially (for low degrees of cold work), void density and swelling rate increase with increasing degree of cold work; they then level off and eventually start decreasing with further cold work. The decrease in swelling rate cannot be explained in terms of a simple increase in dislocation density; one will have to consider the real, heterogeneous dislocation distribution.     

The dilatational misfit of zirconium hydrides precipitated in zirconium

The unconstrained misfit strains, $\text{δ}{}_{\mathrm{[uvtw]}}$, 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.     

Beryllium diffusion in zirconium

The diffusion coefficients of ⁷Be in both α and β phases of Zr, are reported. The temperature dependence of the diffusion coefficient in the α phase may be expressed by $\text{D}{}^{\mathrm{Be}}{}_{\mathrm{\alpha -Zr}}\text{= 0.33 exp(?31900/RT) cm}{}^2\text{/s}$. The measured values in the β phase are in agreement with previously literature reported data, which give a temperature dependence expressed by $\text{D}{}^{\mathrm{Be}}{}_{\mathrm{\beta -Zr}}\text{= 8.33 × 10}{}^{\mathrm{?2}}\text{exp(?31800/RT) cm}{}^2\text{/s}$. Be diffusion in Zr, which is consistent with an interstitial-like behavior, is analyzed in terms of the Anthony and Turnbull conditions, and atomic size criteria. It is concluded that the latter is a very important parameter when assessing the possibility of significant interstitial-like dissolution.     

Void swelling in zirconium

The void swelling behaviour of crystal bar zirconium, during electron irradiation in the High Voltage Electron Microscope, is described. Void growth is observed to occur during irradiation in the temperature range 625–770 K in specimens pre-injected with 100 atomic ppm helium. The results are incorporated into rate theory calculations to determine the dislocation “bias factor” for interstitials and to suggest a possible reason for the apparent resistance of zirconium to void swelling under in-reactor conditions.     

Silver diffusion in zirconium

The diffusion coefficients of Ag in both α and β phases of Zr are reported. The temperature dependence of the diffusion coefficients may be expressed by and and is strongly dependent on the phase transition. The general diffusion behaviour is analyzed in terms of the Hägg and the Anthony and Turnbull rules, indicating that Ag diffuses as substitutional in Zr.     

Swelling rate versus swelling correlation in 20% cold-worked 316 stainless steels

A quantitative correlation of the swelling rate versus swelling is presented. Swelling data of 20% cold-worked 316 stainless steels were analyzed using the power law swelling equation. The prolonged transient region with keeping the suppressed swelling rate was clearly demonstrated for the improved 316 stainless steels like PNC316 and 316Ti. Rate theory analyses lead to the role of precipitates as point defect sinks for retardation of the void growth.     

Nickel-ion bombardment of annealed and cold-worked type 316 stainless steel

Bombardment with high doses of 5 MeV nickel ions has produced swellings as high as 90% and 60%, respectively, in annealed and 20% cold-rolled Type 316 steels. The steels contained 15 ppm of cyclotron-injected helium. Swellings were determined by both transmission electron microscopy and by a step-height method that measures the total swelling integrated along the ion path. The swelling in annealed Type 316 has a pronounced peak in the vicinity of 625°C, which is about 155°C higher than the peak swelling temperature in-reactor. The magnitudes of the swelling, void densities and void sizes produced in annealed Type 316 by nickel ions and in-reactor at the respective peak swelling temperatures are similar and it is concluded that the nickel ion bombardments provide an acceptable simulation of in-reactor behavior. Using the high dose ion results to guide extrapolation of presently available EBR-II data to higher fluences leads to the prediction that the swelling of annealed Type 316 steel at the peak swelling temperature will reach 40% at $\text{2 × 10}{}^{\mathrm{p23}}\text{n/cm}{}^2\text{(E > 0.1 MeV)}$ in EBR-II core, and 70% at $\text{3 × 10}{}^{23}\text{n/cm}{}^2$. These fluences in EBR-II correspond to 155 and 230 dpa respectively. Twenty percent reduction by cold-rolling reduces the ion produced swelling by 35% at 625°C and by 50% at 575°C.     

The modified relaxation time function: A novel analysis technique for relaxation processes. Application to high-temperature molybdenum internal friction peaks

The modified relaxation time (MRT) function, which is based on a general linear viscoelastic formalism, has several important mathematical properties that greatly simplify the analysis of relaxation processes. In this work, the MRT is applied to the study of the relaxation damping peaks in deformed molybdenum at high temperatures. The dependence of experimental data from these relaxation processes with temperature are adequately described by a Havriliak-Negami (HN) function, and the MRT makes it possible to find a relation between the parameters of the HN function and the activation energy of the process. The analysis reveals that for the relaxation peak appearing at temperatures below 900 K, the physical mechanism is related to a vacancy-diffusion-controlled movement of dislocations. In contrast, when the peak appears at temperatures higher than 900 K, the damping is controlled by a mechanism of diffusion in the low-temperature tail of the peak, and in the high-temperature tail of the peak the creation plus diffusion of vacancies at the dislocation line occurs.     

Faulted loops in neutron-irradiated zirconium

Faulted loops have been observed in high-purity zirconium irradiated at 723 K to 1.3 × 10²⁵ neutrons/m² (> 0.1 MeV). The transmission electron microscopy characterization of these $\text{1}\text{6}$ 〈202̄3〉 faulted loops on (0001) is described in detail. It was found that the faulted loops were invariably vacancy in character although the coexisting population of perfect $\text{1}\text{3}$ 〈112̄0〉 loops was of a mixed interstitial/vacancy nature. The faulted loops were observed in specimens of only two out of five batches of high-purity zirconium irradiated in this experiment. Even in these two specimens, the presence of faulted loops was restricted to the 723 K irradiation temperature; at 673 K only perfect $\text{1}\text{3}$ 〈112̄0〉 loops were seen.     

Irradiation growth in deformed zirconium

Single crystals and polycrystals of pure zirconium were irradiated at 353 K and 553 K with fast neutrons to fluences up to 8.1 × 10²⁵ n/m² to determine the effects of various deformation-induced defect structures on irradiation growth. An a-axis single crystal that was swaged 18% grew linearly at a very high rate (˜1.4 × 10⁻²⁸ m²/n) as a result of the massive aligned twins and dislocations (with a-type and c-component Burgers vectors) introduced by deformation. Annealing the swaged crystal at 673 K for 1 h prior to irradiation resulted in a much lower growth rate due to the recovery of the dislocation structure generated by deformation. Annealing the swaged crystal at 823 K instead also retarded growth but to a lesser extent. Grain boundaries, caused by partial recrystallization, were found in the 823 K specimen only and were believed to be responsible for the observed effect. An a-axis crystal that was strained 5% in tension showed little change in length, suggesting that the effect of a small increase in a-type dislocations is relatively minor. Growth was suppressed in the polycrystalline zirconium specimens that were pulled 5%. This is attributed to radiation-enhanced stress relaxation. It also confirms that a high dislocation density is essential to giving a high growth rate in zirconium.     

In-reactor oxidation of crevices and cracks in cold-worked Zr-2.5 wt% Nb

Cracks responsible for heavy water leakage from cold-worked Zr-2.5 wt% Nb pressure tubes in units 3 and 4 of Pickering Generating Station were formed by delayed hydrogen cracking. High residual stresses combined with periods when the reactor was cold caused cracks to form the surfaces of which were oxidized during subsequent reactor operation. Analysis of fracture features, oxide thicknesses on crack faces, and the reactor thermal history indicates that the cracks initiated following the first reactor heating cycle. Experiments were done to confirm the time scale deduced from the oxide bands and to explain the presence of thicker oxide in cracks than on adjacent tube surfaces; oxidation in crevices was about six to nine times faster than on free surfaces. The enhancement is attributed to the concentration of LiOH (used to maintain coolant pH) by restricting flow conditions within the crevices.     

Mechanism of formation of zirconium sponge in zirconium production by the magnesothermic process

The mechanism of formation of zirconium sponge in the production of zirconium by the magnesothermic process has been investigated by introducing into the reducing agent a very soluble but not very volatile additive (aluminum, tin) acting as marked atoms. It was found that the growth of the zirconium sponge on the wall of the reaction crucible above the level of the molten bath is the result of the reaction of zirconium chloride vapor with molten magnesium as the latter ascends by capillary action through the previously formed sponge. The amount of zirconium chloride reacting with magnesium in unit time depends more upon the perimeter of the crucible than upon the surface area of the bath. To intensify the reduction of zirconium chloride by magnesium, it is recommended that partitions be placed in the crucible to act practically in the same way as the crucible wall.     

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

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 ($\text{b}$) of the type $\text{1}\text{3}\text{a <11}\text{2}\text{?}\text{0>}$. Of the three possible $\text{b}$'s only one or two are observed associated with a given hydride needle, those giving a large component of $\text{b}$ 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 $\text{b}$'s along the $\text{c-}\text{axis}$ is more difficult and the misfit strain along this direction is not relieved by plastic deformation.