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Effects of heavy ion irradiation on Zr-2.5Nb pressure tube alloy. I. Orientation dependent mechanical response

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Abstract

In this study, the orientation dependent hardness and creep properties of heavy ion irradiated Zr-2.5Nb pressure tube alloy are investigated by nanoindentation. The indentation tests are conducted along the axial direction (AD) and the transverse direction (TD) relative to the tube. TD samples demonstrate a dependence of the indentation size effect on irradiation damage, which is related to the decrease of the plastic zone size as irradiation damage increases. The hardness of AD and TD samples shows linear dependence on the square root of the irradiation damage density. The transition of the flow pattern from laminar to rotational flow happens in TD samples when the indentation is deeper than 1 μm; rotational flow is expected to be dominant after irradiation. AD samples exhibit laminar flow regardless of indentation depth or irradiation damage. The creep distance is increased for AD while it decreased for TD after irradiation. The creep process in the unirradiated materials and irradiated TD samples is found to be plasticity creep (dislocation glide plus climb). However, for AD samples, the mechanism is changed to power-law creep after 0.6 dpa irradiation. Both the hardness and creep results can be related to the anisotropic deformation mechanisms in the samples.

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... In Paper I, 1 we reported studies on heavy ion irradiated Zr-2.5Nb pressure tube alloy by instrumented nanoindentation. The hardness and indentation creep properties of transverse direction (TD) and axial direction (AD) oriented samples showed a different dependence on heavy ion irradiation. ...
... Damage calculations were carried out by using the Monte Carlo simulation code SRIM 2008, with a displacement energy of 40 eV specified for Zr. The Kinchin-Pease option of SRIM was used to determine the damage in terms of displacement per atom (dpa) as suggested by Stoller et al. 24 The damage profile is shown in Fig. 2 of Paper I. 1 The damage level was calculated by the following equation: 25 ...
... The peak broadening in TD samples in Fig. 2(b) indicates that it was caused by the irradiation induced dislocation loops. This is supported by the nanoindentation results reported in Paper I. 1 Indeed, in Paper I, the indentation hardness of TD samples increased following the irradiation damage. This was explained based on an irradiation hardening model where dislocation loops play the role of obstacles to the movement of prismatic <a> dislocations. ...
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Micro-pillars of 5 μm diameter and 5 μm height were made from textured Zr–2.5%Nb pressure tube material and were irradiated with 8.5 MeV Zr+ to simulate neutron irradiation. The yield stress, strain hardening exponent, and degree of strain localization of the micro-pillars when tested at 25 °C increased with ion irradiation and the extent of increase was largest in directions containing low (0 0 0 1) basal pole fraction. This suggests that Zr+ irradiation inhibits prismatic dislocation slip more than pyramidal slip in this material.
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Neutron irradiation data for the hardness and electrical resistivity of W and W–Re was obtained by JOYO, JMTR and HFIR irradiation experiments. The irradiation damage levels and temperature range were 0.15–1.0dpa at around 500–600°C. The effects of irradiation temperature, damage level and Re content on hardening and electrical resistivity are discussed. In the case of HFIR irradiated specimen, large irradiation hardening and an increase in electrical resistivity were observed, however, the trend for the electrical resistivity was different from previous work. The property change mechanism is discussed considering the irradiated microstructure and solute elements.
Article
Irradiation hardening and microstructure changes in Fe-Mn binary alloys were investigated after neutron irradiation at 290 degrees C and up to 0.13 dpa. Significant irradiation hardening comparable to that of Fe-1 at.%Cu alloy was observed in Fe-1 at.%Mn alloy. Manganese increases the number density of dislocation loops, which contributed to the observed irradiation hardening. Manganese serves as a nucleus of the loop by trapping interstitial atoms and clusters, preventing 1D motion of the loops.
Article
This chapter discusses the dislocations and indentations. Indentation hardness investigations of solids are usually carried out with indenters made of very hard materials, such as diamond, cubic boron nitride, sapphire and tungsten carbide. The indenter shapes may be broadly classified into two categories. These are (i) pointed indenters including cones and pyramids, and (ii) blunt indenters, such as spheres, paraboloids and ellipsoids. The indentation test thus provides a very simple, convenient and quite unique method for studying plasticity and its effects in any material. These effects can be indentation-induced phase transitions, dislocation mobility within the hinterland around the indentation, enhanced photoluminescence and cathodoluminescence, dislocation interactions giving rise to cracking and electrical charging, and others.
Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form: {ie1217-05} where {ie1217-06} is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m2)−4.3. The stress exponentn = 4.3 ± 0.3 and the activation energyQ = 2.05 ± 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.
Article
Humphreys' simple construction to aid understanding of the patterns of rotational plastic flow observed near undeformable particles in a ductile plastically sheared matrix can be generalised to predict flow under hardness indenters in crystalline metals. The consequences for internal stress distributions and polycrystalline plasticity are briefly indicated.
Article
A physically-based theoretical model is developed for describing the phenomenon of indentation creep over the whole temperature range, from 300 K to melting. In agreement with experimental data collected, the model predicts that most materials, including ceramics, exhibit indentation creep at temperatures down to 300 K. It is established that the principal mechanism causing indentation creep is dislocation glide plasticity. The dominance of this mechanism over the whole temperature range is due to the very high stresses involved in indentation creep. If, however, the grain size is small (typically less than 0.3-0.4-mu-m) indentation creep may be dominated by grain boundary (Coble) diffusive creep instead. The implications of these results in terms of the design, forming and application of the so-called "hard materials" is discussed.
Article
Effects of irradiation at temperatures ⩽200°C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000GPa/dpa, at the lowest dose of
Article
Constant load pyramidal indentation creep tests were performed to study the effect of Zr+ ion irradiation on the anisotropy of the local plastic deformation of Zr–2.5%Nb pressure tube material at 25°C. The ratio of the average indentation stress σindt=0 on the transverse normal (TN) plane relative to that on the axial-normal (AN) and radial-normal (RN) planes is 1.3 and 1.2 respectively. After Zr+ ion irradiation the ratio of σindt=0 on the TN plane relative to σindt=0 on the AN and RN planes is 1.04 and 1.08 respectively indicating that the anisotropy of the yield stress is decreased as a result of irradiation hardening. The relative change in indentation stress Δσˆ, as a result of irradiation damage, decreases with increasing resolved basal pole fraction in the indentation direction. This suggests that the Zr+ ion irradiation damage has a greater effect on blocking the movement of dislocations on prismatic slip systems compared to pyramidal slip systems in the Zr–2.5%Nb pressure tubing. The activation energy ΔG0 of the obstacles that limit the rate of dislocation glide during indentation creep at 25°C does not change with indentation direction but does increase with increasing levels of Zr+ ion irradiation damage.
Article
The microstructure of annealed crystal-bar zirconium, sponge zirconium and Zircaloy-2 and -4 have been analysed following neutron irradiation in EBR II over the temperature range of 644–710 K for neutron fluences up to 6– x 1025 n m−2 (). There is a correlation between measured high irradiation growth strains and the existence of vacancy dislocation loops. The concentration of these faulted 〈202̄3〉 dislocation loops is highest in alloyed or impure Zr. There is dissolution of Fe, Cr and Ni from intermetallic particles during irradiation. The amount of solute dissolution and secondary precipitation is dependent on the irradiation temperature and fluence and is most widespread for the Zircaloy irradiated to high fluences at high temperatures. Sn-rich precipitates are also observed in the Zircaloys and are the result of radiation-enhanced diffusion.
Article
The mechanism of irradiation hardening of Fe and Fe-Cr simple α-phase alloys was studied by means of tensile tests. Some commercial ferritic and martensitic steels were also examined.The irradiation hardening of Fe was explained by the presence of small dislocation loops formed at the sites of cascade damage collapse, while that of Fe-Cr alloys was thought to be due to Cr-rich precipitates formed at the same sites. This hardening mechanism suggests that there is a linear increase with Cr content in the athermal component of the irradiation hardening of Fe-Cr alloys. This mechanism will also contribute to the irradiation hardening of Fe-Cr ferritic and martensitic steels.
Article
Zirconium single crystals have been deformed in compression along the c-axis between 78 K and 1100 K. Plastic deformation occurs by {112̄2} twinning up to a temperature of 800 K, whereas twins of the type {101̄1} in conjunction with (c + a) slip have been observed to initiate plastic flow at temperatures above 800 K. The slip plane has been identified as {101̄1}. An increasing flow stress with temperature, high work-hardening rate and load drops are associated with {112̄2} twinning. A rapidly decreasing flow stress, high strain-rate sensitivity, low work-hardening rate and absence of yield drop characterize the region above 800 K where {101̄1} slip and twinning are observed. Localized recrystallisation in deformation bands occur above 973 K. The deformation behaviour is compared with that in other metals having the hep structure.
Article
The effect of nuclear radiation on the mechanical properties of copper was studied. It was found that the yield stress, which is substantially increased by the radiation, increases as the cube roet of the flux. A strong temperature dependence of the yield stress of irradiated copper was observed with the yield stress being given by a function similar to sigma = A - BT1/2 above 40 deg K. A Luders band with slip lines of very large step height was associated with the enhanced yield stress at small strains. At large strains the phenomenon of overshoot was observed. The annealing kinetics of the radiation hardness were also studied in the temperature range from 25 to 700 deg K. Little or no annealing was observed in the region below 80 deg K. In the region from 80 to 300 deg K, approximately 20% of the yield stress was recovered, with the remainder annealing in the range from 600 to 700 deg K. These results are discussed in terms of the possible mechanisms by which the hardening can occur. While by no means conclusive, these duta support a dislocation locking mechanism. On the other hand a very close analogy exists between radiation hardening and the hardening which arises from the addition of impurities, e.g., the hardening in alpha brass. The correlation between work hardening and radiation hardening appears to be quite small. (auth)
Article
The effects of some heat treatments on the hardness of Zr--2.5 wt% Nb, ; and the effects of neutron irradiation at 50 deg C and at 250 deg C on its ; tensile and impact properties are described. Post-irradiation damage recovery ; data are also presented. It is concluded that the alloy's mechanical properties ; both before and after irradiation are significantly better than those of the ; Zircaloys. The heat-treated Zr-Nb alloy appears to be metallurgically stable ; under irradiation. (auth);
Article
Oxide dispersion strengthened (ODS) steels are candidate materials for advanced electric energy and heat generation plants (nuclear, fossil). Understanding the degradation of mechanical properties of these alloys as a result of service exposure is necessary for safe design. For advanced nuclear applications combinations of temperature, irradiation and stress are important damage conditions. They are studied either with neutron irradiated samples (often highly active) or with ion-irradiated samples (irradiation damage often limited to only a few micrometer deep areas). High activity of samples and limited sample volume claim to subsized samples like nano-indentation, micro-pillar compression or thin strip creep testing. Irradiation hardening and irradiation creep were studied with these methods. Ferritic ODS steels with 19% chromium were investigated. The materials were studied in qualities differing in grain sizes and in sizes of the dispersoids. Irradiation was performed in an accelerator using He-ions. Irradiation damage profiles could be well analyzed with indentation. Yield stress determined with compression tests of single-crystal micropillars was well comparable with tension tests performed along the same crystallographic orientation. Irradiation creep of samples with different sizes of dispersoids revealed only a small influence of particle size being is in contrast with thermal creep but consistent with expectations from other investigations.