A study of grain boundary sliding in copper with and without an addition of phosphorus

Copper will be used as a corrosion barrier in the storage of high level nuclear waste. In order to improve the creep fracture properties of the material it will contain 30-50 ppm of phosphorus, OFP copper as opposed to OF copper without P. It has been suggested that the phosphorus impedes grain boundary sliding in copper and recently a quantitative theory based on this idea has shown that there is no risk for creep-brittle fracture of OFP copper under waste storage conditions. In order to verify the basis of this theory grain boundary sliding has been investigated in copper with and without a P addition. The method has been to examine intentionally scratched surfaces of tensile specimens tension tested to plastic strains of 1%, 2% and 4% at 150 and 200 °C. After testing specimen surfaces have been examined in SEM and sliding distances have been measured as in-surface displacement of scratches. The results have been plotted as distribution functions where the fraction of slides smaller than a given value is plotted versus sliding distance. The result is that in most cases the distribution functions for OF and OFP copper overlap. In a small number of cases there is a tendency that less sliding has occurred in OFP copper. The overall conclusion is however that although there may be a slight difference between the materials with regard to grain boundary sliding it is not large enough to explain the observed difference in creep brittleness. Tension tests to fracture in the temperature range 100-200 °C show that the tensile properties of the two copper qualities are more or less identical until intergranular cracking starts in the OF copper. Then the flow stress decreases in comparison with OFP. It is suggested that at least part of the observed differences in creep strength between the two coppers may be due to the effect of intergranular cracking.     

Effect of thermo-mechanical processing on microstructure and creep properties of the foils of alloy 617

The effect of rolling and annealing on the microstructure and high temperature creep properties of alloy 617 were investigated. Two types of foil specimens with different thickness reductions were prepared by thermo-mechanical processing. Recrystallization and grain growth were readily observed at specimens annealed at 950 and 1100 °C. The uniform coarse grains increase resistance against creep deformation. The grain size effect in creep deformation was dominant up to 900 °C, while dynamic recrystallization effect became dominant at 1000 °C. Dynamic recrystallization was observed in all the creep deformed foils, even though some specimens had already been (statically) recrystallized during annealing. Steady state creep rates decreased with increasing annealing temperature in the less rolled foils. The apparent activation energy Q_app for the creep deformation increased from 271 to 361 kJ/mol as the annealing temperature increased from 950 to 1100 °C.     

Irradiation creep of SA 304L and CW 316 stainless steels: Mechanical behaviour and microstructural aspects. Part I: Experimental results

Solution annealed 304L (SA 304L) and cold work 316 (CW 316) austenitic stainless steel irradiation creep behaviour have been studied thoroughly. Irradiations were carried out in fast breeder reactors BOR-60 (at 330 °C, up to 120 dpa) and EBR-II (at 375 °C, up to 10.5 dpa), and in the OSIRIS mixed spectrum reactor (at 330 °C, up to 9.8 dpa). After an incubation threshold, the irradiation creep of the austenitic stainless steels is linear in stress and in dose. Creep appears to be athermal in this temperature range. A significant difference in the behaviour is measured between the creep of SA 304L and CW 316.In order to study the anisotropy of loop population, which would be the signature of a possible stress induced preferential absorption (SIPA) mechanism for irradiation creep, special attention was given to the measurement of anisotropy of loop distribution between the four families. The anisotropy induced by an applied stress has been shown to be in the range of the statistical scatter in the situation where no stress is applied. TEM microstructural analyses performed on this sample show slight difference between the microstructure of specimens deformed under irradiation and the microstructure of specimens irradiated without stress under the same irradiation conditions.     

Effect of alloying elements (Cu, Fe, and Nb) on the creep properties of Zr alloys

The thermal creep behaviors of Zr-based alloys containing Cu, Fe and Nb were investigated under constant load stress at temperatures of 280 and 330 °C, and a stress range of 100-140 MPa. To evaluate an alloying effect on a creep, Zr-based alloys were selected as the binary and ternary systems of Zr-0.3Cu, Zr-0.3Fe, Zr-0.5Nb-0.3Cu and Zr-0.5Nb-0.3Fe. The final annealing of these alloys was performed at 510 °C for 8 h to obtain a recrystallization structure for all the tested alloys. A microstructure characterization test was carried out for the samples before and after the creep test by using TEM, and the results were used to understand the creep mechanism. Creep tests were performed for up to 70 h, which showed a steady-state secondary creep rate in all the alloys. The value of the stress exponent was about 5.5 in all the alloys. The dislocation density was increased by increasing the applied stress, regardless of the alloy system. From the results of this study, it was revealed that the Nb as an alloying element showed the strongest effect on the creep resistance among the added alloying elements, and Fe was more effective than Cu from the viewpoint of creep resistance.     

Oxidation and creep failure of alloy 617 foils at high temperature

The microstructure of thermally grown oxides (TGO) and the creep properties of alloy 617 were investigated. Oxidation and creep tests were performed on 100 μm thick foils at 800-1000 °C in air environment, while the thickness of TGO was monitored in situ. According to energy dispersive X-ray (EDX) mapping micrographs observation, superficial dense oxides, chromia (Cr₂O₃), which was thermodynamically unstable at 1000 °C, and discrete internal oxides, alumina (α-Al₂O₃), were found. Consequently, the weight of the foil specimen decreased due to the spalling and volatilization of the Cr₂O₃ oxide layer after an initial weight-gaining. Secondary and tertiary creeps were observed at 800 °C, while the primary, secondary and tertiary creeps were observed at 1000 °C. Dynamic recrystallization occurred at 800 °C and 900 °C, while partial dynamic recrystallization at 1000 °C. The apparent activation energy, Q_app, for the creep deformation was 271 kJ/mol, which was independent of the applied stress.     

Irradiation creep of oxide dispersion strengthened (ODS) steels for advanced nuclear applications

Ferritic oxide dispersion strengthened steels with different microstructure were in-beam creep tested in a temperature range from 300 to 500 °C. Irradiation was by He-ions. Elongation was determined as a function of stress and irradiation damage rate. Damage was investigated by transmission electron microscopy. A thorough analysis of the loops developing during irradiation creep did not show any dependence of orientation or size on the direction of the applied stress. At 400 °C radiation induced segregation was found (most probably an iron aluminide) which had no effect on irradiation creep. No pronounced influence of microstructure or dispersoid size on the irradiation creep behavior was detected. Irradiation creep compliance of PM2000 with dispersoids of about 30 nm diameter were found to differ little from material with dispersoids of only 2-3 nm diameter. This is in contrast to thermal creep where dislocation-obstacle interactions are extremely important. An assessment of the technical relevance of irradiation creep in advanced nuclear systems is presented.     

High temperature design curves for high nitrogen grades of 316LN stainless steel

Nitrogen alloyed low carbon grade 316L(N) stainless steel (SS) is a major structural material for high temperature structural components of sodium cooled fast reactors. With a view to significantly enhance the high temperature mechanical properties of 316L(N) SS and thereby increase the design life of structural components from 40 years to 60 years, the influence of nitrogen content on the tensile and creep properties of this steel has been investigated. Four heats of 316LN SS with 0.07, 0.11, 0.14, and 0.22 wt.% nitrogen were used in this investigation. Tensile tests were carried out at various temperatures between room temperature and 850 °C. Creep tests were carried out at 650 °C at various stress levels in the range of 140-225 MPa. The maximum rupture life in these tests was 16,000 h. The tensile and creep data were analysed according to RCC-MR nuclear code procedures and the design curves have been generated. The tensile and creep strength of 316L(N) SS have been found to improve significantly by increasing the nitrogen content.     

Thermal stability of nano-structured ferritic alloy

The excellent tensile and creep strength and the potential for managing radiation damage make nano-structured ferritic alloys (NFAs) promising candidates for high-temperature applications in spallation proton, advanced fission and fusion neutron environments. The thermal stability of NFAs is critical for such applications, hence, this has been investigated in a series of aging experiments on MA957 at 900 °C, 950 °C and 1000 °C for times up to 3000 h. Optical and transmission electron microscopy (TEM) studies showed the fine scale grain and dislocation structures are stable up to 1000 °C. TEM and small angle neutron scattering (SANS) showed that the nm-scale solute cluster-oxide features (NFs), that are a primary source of the high strength of NFAs, were stable at 900 °C and coarsened only slightly at 950 °C and 1000 °C. Porosity that developed during high-temperature aging was minimal at 900 °C and modest at 950 °C, but was much larger after 1000 °C. Microhardness was basically unchanged after the 900 °C aging, and decreased only slightly (?3%) after aging at 950 °C and 1000 °C.     

Microstructural influence on high temperature creep flow of Zr-1%NbO alloy in near-α, (α + β), and β temperature ranges in a high vacuum environment

Uniaxial tensile creep tests were carried out at 650-1100 °C in a high vacuum environment on Zr-1%NbO tubes with various microstructures. The effect of microstructure on creep flow in the (α + β) temperature range is significant (the creep rate being modified by up to three orders of magnitude) under stresses lower than 10 MPa, that is, for stress values of one order of magnitude lower than those characteristic of prototypical Loss-of-coolant-accident (LOCA) conditions. Under stresses higher than about 20 MPa, this effect is much smaller. No transformation-induced plasticity was detected from anisothermal creep tests, once the creep strain was thoroughly taken into account to process experimental strain vs. time data.     

The effect of phosphorus on creep in copper

Pure copper with an addition of about 50 ppm phosphorus is the planned material for the outer part of the waste package for spent nuclear fuel in Sweden. Phosphorus is added to improve the creep ductility but it also strongly increases the creep strength. In the present paper the influence of phosphorus on the strength properties of copper is analysed. Using the Labusch-Nabarro model it is demonstrated that 50 ppm has a negligible influence on the yield strength in accordance with observations. For slow moving dislocations, the interaction energy between the P-atoms and the dislocations gives rise to an agglomeration and a locking. The computed break away stresses are in agreement with the difference in creep stress of copper with and without P-additions.     

Effect of ion irradiation and indentation depth on the kinetics of deformation during micro-indentation of Zr-2.5%Nb pressure tube material at 25 °C

Micro-indentation creep tests were performed at 25 °C on radial-normal samples cut from Zr-2.5Nb CANDU pressure tube material in both the as-fabricated condition and after irradiation with 8.5 MeV Zr⁺ ions. The average indentation stress, and hence the yield stress, was found to increase with decreasing indentation depth and with increasing levels of ion irradiation. The activation energy of the indentation creep rate and hence the, activation energy of the obstacles that limit the rate of dislocation glide, was independent of indentation depth but increased from ΔG₀ = 0.185 to 0.215 μb³ with increasing ion irradiation damage. The magnitude of the activation energy indicates that ion irradiation introduces a new type of obstacle into the microstructure which reduces the low temperature indentation creep rate of Zr-2.5Nb pressure tubes. This is supported by TEM images showing that Zr⁺ ion irradiation produces small, nanometer size, dislocation loops which act as obstacles to dislocation glide and thus influence both the yield stress and the activation energy of the low-temperature thermal creep of Zr-2.5Nb pressure tube material. These findings suggest that neutron irradiation will have similar effect upon yield stress and low-temperature thermal creep as the Zr⁺ ion irradiation since both create similar crystallographic defects in Zr-2.5Nb pressure tubes.     

High-temperature mechanical properties improvement on modified 9Cr-1Mo martensitic steel through thermomechanical treatments

In the framework of the development of generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9-12%Cr martensitic steels are candidate for these applications. Thermomechanical treatments including normalisation at elevated temperature (1150 °C), followed by warm-rolling in metastable austenitic phase and tempering, have been applied on the commercial Grade 91 martensitic steel in order to refine its microstructure and to improve its precipitation state. The temperature of the warm-rolling was set at 600 °C, and those of the tempering heat-treatment at 650 °C and 700 °C thanks to MatCalc software calculations. Microstructural observations proved that the warm-rolling and the following tempering heat-treatment lead to a finer martensitic microstructure pinned with numerous small carbide and nitride particles. The hardness values of thermomechanically treated Grade 91 steel are higher than those of the as-received Grade 91. It is also shown that the yield stress and the ductility of the thermomechanically treated Grade 91 steel are significantly improved compared to the as-received material. Preliminary creep results showed that these thermomechanical treatments improve the creep lifetime by at least a factor 14.     

Influence of prior dislocation structure on anisotropy of thermal creep of cold-worked Zr-2.5Nb tubes

Normally, creep anisotropy of hcp metals is thought to be controlled by the crystallographic texture. Here, we show that the creep anisotropy of cold-worked Zr-2.5Nb tubes is also very dependent on the anisotropic dislocation structures introduced by cold-work. The contribution of each slip system to the creep deformation of an individual grain orientation depends upon the activity of that slip system during prior cold-work. This conclusion is reached by comparing the self-consistant visco-plastic polycrystalline models with thermal creep tests performed on internally pressurized thin-wall capsules with different textures under a transverse stress of 300 MPa at 350 °C, where dislocation creep is the dominant operating mechanism. The non-uniform dislocation distributions prior to creep were derived by simulating the cold-work process of Zr-2.5Nb tubes from an Elasto-Plastic Self-Consistent (EPSC) model.     

Synthesis and characterization of PEFC membranes based on fluorinated-polymer-alloy using pre-soft-EB grafting method

Polymer electrolyte fuel cell (PEFC) membranes based on thin film of crosslinked perfluorinated polymer-alloys (RX-FA) have been fabricated by soft electron beam (soft-EB) grafting with styrene monomers using soft-EB irradiation under nitrogen atmosphere at room temperature (RT). The characteristic properties of styrene-grafted materials (GRX-FA) and sulfonated materials (SRX-FA) have been measured by differential scanning calorimetry (DSC) and FT-IR spectroscopy, ionic conductivity and so on. The glass transition temperatures (dry state) of all obtained SRX-FA were about 105 ± 1 °C, which are higher than Nafion^®. The ion exchange capacities of SRX-FA have been achieved about 3.3 meq/g (dry). The ionic conductivity of obtained SRX-FA has showed about 0.17 S/cm at 60 °C with relative humidity (RH) of ∼95%. The ionic conductivities of the obtained SRX-FA were higher than that of conventional perfluoro-sulfonic acid membranes (PFSA). Fabricated membrane electrode assemblies (MEAs) based on the obtained SRX-FA have shown encouraging performance in the PEFC, compared with the conventional PFSA. The power density of obtained MEAs based on the SRX-FA was about 330-340 mW/cm² under 500 mA/cm² at 60 °C operation. Moreover, the maximum power densities of obtained MEAs based on the SRX-FA shows about 630 mW/cm² at 60 °C. On the other hand, the power density at 500 mA/cm² and maximum power density of MEA based on Nafion^®112 were about 320 and 590 mW/cm² at 60 °C. Thus, the power density of the obtained SRX-FA was higher than that of conventional PFSA.     

Thermal and structural properties of low-fluence irradiated graphite

The release of Wigner energy from graphite irradiated by fast neutrons at a TRIGA Mark II research reactor has been studied by differential scanning calorimetry and simultaneous differential scanning calorimetry / synchrotron powder X-ray diffraction between 25 and 725 °C at a heating rate of 10 °C min⁻¹. The graphite, having been subject to a fast-neutron fluence from 5.67 × 10²⁰ to 1.13 × 10²² n m⁻² at a fast-neutron flux (E > 0.1 MeV) of 7.88 × 10¹⁶ n m⁻² s⁻¹ and at temperatures not exceeding 100 °C, exhibits Wigner energies ranging from 1.2 to 21.8 J g⁻¹ and a Wigner energy accumulation rate of 1.9 × 10⁻²¹ J g⁻¹ n⁻¹ m². The differential-scanning-calorimeter curves exhibit, in addition to the well known peak at ∼200 °C, a pronounced fine structure consisting of additional peaks at ∼150, ∼230, and ∼280 °C. These peaks correspond to activation energies of 1.31, 1.47, 1.57, and 1.72 eV, respectively. Crystal structure of the samples is intact. The dependence of the c lattice parameter on temperature between 25 and 725 °C as determined by Rietveld refinement leads to the expected microscopic thermal expansion coefficient along the c axis of ∼26 × 10⁻⁶ °C⁻¹. At 200 °C, coinciding with the maximum in the differential-scanning-calorimeter curves, no measurable changes in the rate of thermal expansion have been detected - unlike its decrease previously seen in more highly irradiated graphite.     

Creep rupture of a 9Cr1MoNbV steel at 500 °C: Base metal and welded joint

ASME Grade 91 steel base metal and a similar weld were tested under creep at 500 °C for rupture time up to 18,000 h. Creep failure of cross-weld specimens occurs in the weld metal at this temperature. No significant microstructural changes were observed after creep. Analysis of creep deformation of smooth creep bars, welded joints and slightly notched bars indicated an apparent creep stress exponent of 19. For the creep conditions considered, failure of the material can be explained by the viscoplastic instability of the specimens without significant damage development. This allowed to develop a simple analysis for time to failure prediction.     

Creep behaviour of δ-phase of U-Zr system by impression creep technique

In this study, the impression creep behaviour of δ-phase of U-50 wt.% Zr (U-72.29 at.% Zr) system was studied in the temperature range 525-575 °C at different stresses. The velocity of the punch at different stresses and temperatures were evaluated for the above alloy. The stress exponents and thermal activation parameters of the above alloy were determined. A power law behaviour is displayed with the stress exponents range from 6.5 to 7. The activation enthalpy for the δ-UZr₂ was found to be independent of stress with an average value of 106 kJ/mol.     

SIMS study of effect of Cr adhesion layer on the thermal stability of silver selenide thin films on Si

Effect of heat treatment on silver selenide films grown from diffusion-reaction of Ag and Se films on Cr-buffered Si substrates was investigated up to 400 °C. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the films. XRD patterns of the films showed stress assisted change in preferential orientation of the films upon annealing: the films annealed at 200 °C exhibited a strong orientation along (2 0 0) plane, which changed to (0 1 3) after annealing at 300 and 400 °C. Dynamic SIMS measurements showed that Cr is confined to the interface and that there is no diffusion of Cr into silver selenide.     

Evaluation of the thermal creep behaviors and microstructure of Zircaloy-4 strip

The thermal creep behaviors and the microstructure characteristics for the crept specimens of Zircaloy-4 strip were investigated under a constant load stress in the temperature range of 350-450 °C and a stress range from 110 to 230 MPa. A microstructure evaluation was carried out for the specimens before and after the creep test by using a TEM to understand the correlation between the creep mechanism and the microstructure. A variation of the crystal orientation with the creep deformation was investigated by using the electron back-scattered diffraction (EBSD) analysis. The stress exponent was in the range of 8-10 and the value of the stress exponent was varied by the applied stress. From the analysis of the stress exponent and a TEM microstructural observation of the crept specimens, the creep of Zircaloy-4 strip at a tested condition was mainly controlled by dislocations. From the EBSD results, it was observed that the crystal orientation in the crept sample strained about 10% was not changed as compared to the as-received sample.     

Study of electron beam curing process using epoxy resin system

Polymeric matrix composite (PMC) has been used in engineering applications instead of metal in the last few years, due to its corrosion resistance and excellent relation between tensile strength/density and elastic modulus/density. However, PMC materials cured by thermal process require high temperature and are time-consuming. The electron beam (EB) curing technology allows its use at room temperature and reduced curing times, and this is one of the main advantages over thermal technology. The aim of this work is to investigate electron beam curable epoxy formulations to use in filament winding processes to produce composite material with similar or better properties than thermal curable composites. The study has been made with commercial epoxy resins and cationic initiators. The epoxy resin samples were irradiated for few minutes with total dose of 150 kGy. The glass transition temperatures (T_g) were determined by dynamic mechanical analyzer (DMA) and the result was 137 °C. The thermal process was carried out in a furnace following three steps: 4 h at 90 °C, increasing temperature from 90 °C to 130 °C during 4 h and 12 h at 130 °C. The total process time was 20 h. The T_g of this sample was 102 °C.