[en] In most of alloys, the surface composition is different from bulk one. This phenomenon, called 'surface segregation' have drawn up to now much attention in this case of alloys which have reached thermodynamic equilibrium in the solid solution. Using a kinetic model including bulk and surface driving forces, we study segregation phenomenon during dissolution and precipitation kinetics, in the case of the Fe-Cu alloy. Within a mean field approximation, we point out the dissolution modes for Fe/Cu and Cu/Fe deposit. If the substrate surface energy is lower than the deposit one (case of Fe/Cu deposit) the substrate element climbs through the deposit to reach the surface and forms a layer of the substrate element floating on the deposit. In the case of thick deposit (typically 10 monolayers), a competition between two layer by layer dissolution modes leads to a wide range of behaviours, depending on temperature and deposit thickness. Furthermore, the major part of the concentration profiles obtained during kinetics is at local equilibrium in a region near the surface. In the second part of this work, we study the surface influence during phase separation kinetics in thin layers using Monte-Carlo simulations. A surface directed spinodal decomposition occurs, leading to the appearance of a Cu-rich layer at the surface, which goes toward the layer's core with time. This process is linked with bulk precipitation in layer's core, and leads to different behaviours depending on average concentration and layer thickness. (authors)

[en] Hardfacing alloys are weld deposited to provide a wear resistant surface for structural base materials. Commercial low cobalt hardfacing alloys are being evaluated to reduce plant activation levels. Since hardfacing alloys typically must be resistant to cracking to assure adequate in service performance, fracture toughness is a critical material property. Fracture toughness (K_IC) measurements of Fe base, Ni-base, and Co-base hardfacing were performed in accordance with ASTM E399-90 procedure in an effort to identify a tough cobalt-free alternative. Reduced scatter in K_IC data was observed for the Fe base hardfacing, and the 95% lower bound K_IC values were generally higher than the Ni-base Hardfacing alloys. Preliminary crack growth data obtained during precracking indicate that the Ni-base hardfacing possess better fatigue crack growth resistance. However, none of the Fe-base or Ni-base hardfacing have K_IC values that are comparable to the reference Co-base hardfacing. The test specimens were machined from thick (0.5 inches) weld deposits, and the microstructures of the test specimens are compared with the more prototypic, thinner deposits. Microstructural and fractographic examinations are used to characterize the fracture mechanisms and delineate the operative toughening mechanisms. Crack deflection and crack bridging toughening mechanisms are shown to be relevant for most of the commercial hardfacing

[en] Tensile property measurements and fractographic analysis of S-65C beryllium are reviewed. Tests were performed on specimens oriented in the longitudinal and transverse directions with respect to the direction of vacuum hot pressing. Specimens were tested in air at RT, 100 C, 200 C, 300 C, 415 C and 500 C at an initial strain rate of 1.1 x 10(sup -4) per second. Ductility of the material was strongly affected by the test temperature, exhibiting a peak ductility at 300 C. The material displayed a yield point phenomenon which was most pronounced at this same temperature. Scanning electron microscopy was performed on the resulting fracture surfaces and observations are reported

[en] A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow-surface flaws. Shallow-flaw fracture toughness of RPV material has been shown to be higher than that for deep flaws, because of the relaxation of crack-tip constraint. This report describes the preliminary test results for a series of cruciform specimens with a uniform depth surface flaw. These specimens are all of the same size with the same depth flaw. Temperature and biaxial load ratio are the independent variables. These tests demonstrated that biaxial loading could have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials. Through that temperature range, the effect of full biaxial (1:1) loading on uniaxial, shallow-flaw toughness varied from no effect near the lower shelf to a reduction of approximately 58% at higher temperatures

[en] The fatigue behavior at heat affected zone, HAZ, of TIG zirconium weldment in boiling nitric acid has been studied to evaluate with respect to the reliability of commercial nuclear fuel reprocessing equipment made of zirconium. The crack growth rate was measured as a function of the stress intensity factor both in boiling nitric acid and in air at room temperature. The fracture morphology was examined with a scanning electron microscope. The crack growth rate in boiling nitric acid was ten times faster than that in air at room temperature. The fracture surface formed in nitric acid showed both the brittle striation and quasi-cleavage type facet. Comparing with same testing results of base metal, these results of weld metal and the HAZ showed the SCC susceptibility similar to that of base metal. (author)

[en] Precipitation strengthening of nickel was investigated using ion-implantation alloying and nanoindentation testing for particle separations in the nanometer range and volume fractions extending above 10O/O. Ion implantation of either oxygen alone or oxygen plus aluminum at room temperature was shown to produce substantial strengthening in the ion-treated layer, with yield strengths near 5 GPa in both cases. After annealing to 550''C the oxygen-alone layer loses much of the benefit, with its yield strength reduced to 1.2 GP but the dual ion-implanted layer retains a substantially enhanced yield strength of over 4 GPa. Examination by transmission electron f microscopy showed very fine dispersions of 1-5 nm diameter NiO and y-A1203 precipitates in the implanted layers before annealing. The heat treatment at 550''C induced ripening of the NiO particles to sizes ranging from 7 to 20 nm, whereas the more stable -A1203 precipitates were little changed. The extreme strengthening we observe is in semiquantitative agreement with predictions based on the application of dispersion-hardening theory to these microstructure

[en] Austenitic stainless is used for the structural material of liquid metal reactor (LMR) because of good mechanical properties at high temperature. Stainless steel having more resistant to temperature by adding minor element has been developing for operating the LMR at higher temperature. Of many elements, nitrogen is a prospective element to modify type 316L(N) stainless steel because nitrogen is the most effective element for solid solution and because nitrogen retards the precipitation of carbide at grain boundary. Ti, Nb, and V are added to improve creep properties by stabilizing the carbides through forming MC carbide. Testing techniques of tensile, fatigue, creep, and creep-fatigue at high temperature are difficult. Moreover, testing times for creep and creep-fatigue tests are very long up to several tens of thousands hours because creep and creep-fatigue phenomena are time-dependent damage mechanism. So, it is hard to acquire the material data for designing LMR systems during a limited time. In addition, the integrity of LMR structural materials at the end of LMR life has to be predicted from the laboratory data tested during the short term because there is no data tested during 40 years. Therefore, the effect of elements on mechanical properties at high temperature was reviewed in this study and many methods to predict the long-term behaviors of structural materials by simulated modelling equation is shown in this report. (author). 32 refs., 9 tabs., 38 figs

[en] The yield and maximum strengths of an irradiated series of isotopically tailored ferritic alloys were evaluated using the shear punch test. The composition of three of the alloys was Fe-12Cr-1.5Ni. Different balances of nickel isotopes were used in each alloy in order to produce different helium levels. A fourth alloy, which contained no nickel, was also irradiated. The addition of nickel at any isotopic balance to the Fe-12Cr base alloy significantly increased the shear yield and maximum strengths of the alloys, and as expected, the strength of the alloys decreased with increasing irradiation temperature. Helium itself, up to 75 appm over 7 dpa appears to have little effect on the mechanical properties of the alloys

[en] In the Canadian Nuclear Fuel Waste Management Program, it is proposed that nuclear fuel waste be placed in a durable container and disposed of in a deep underground vault. Consideration of various disposal-container designs has identified either titanium or copper as the material suitable for constructing the container shell. As part of the R and D program to examine the structural integrity of the container, creep tests are being conducted on commercially pure titanium and oxygen-free copper. This report presents the preliminary data obtained. It also describes the evaluation of various constitutive equations to represent the creep curves, thus providing the basis for extrapolation of the creep behaviour over the design lifetime of the container. In this regard, a specific focus is placed on equations derived from the 0-Projection Concept. Recognizing that the container lifetime will be determined by the onset of tertiary creep leading to creep rupture, we present the results of the metallographic examination of creep damage. This shows that the tertiary stage in titanium is associated with the formation of transgranular cavities within the region of localized necking of the creep specimens. In contrast, creep damage in copper is in the form of intergranular cavities uniformly distributed throughout the gauge length. These results are analyzed within the context of the extant literature, and their implications for future container design are discussed. (author)

[en] The austenitic steels in a hot chlorinated medium present a rupture which is macroscopically fragile, discontinuous and formed with crystallographic facets. The interpretation of these facies crystallographic character is a key for the understanding of the stress corrosion damages. The first aim of this work is then to study into details the micro fractography of 316 L steels mono and polycrystals. Two types of rupture are observed: a very fragile rupture which stresses on the possibility of the interatomic bonds weakening by the corrosive medium Mg Cl₂ and a discontinuous rupture (at the micron scale) on the sliding planes which is in good agreement with the corrosion enhanced plasticity model. The second aim of this work is to search for controlling the stress corrosion by the mean of a pre-strain hardening. Two types of pre-strain hardening have been tested. A pre-strain hardening with a monotonic strain is negative. Indeed, the first cracks starts very early and the cracks propagation velocity is increased. This is explained by the corrosion enhanced plasticity model through the intensifying of the local corrosion-deformation interactions. On the other hand, a cyclic pre-strain hardening is particularly favourable. The first micro strains starts later and the strain on breaking point levels are increased. The delay of the starting of the first strains is explained by a surface distortion structure which is very homogeneous. At last, the dislocations structure created in fatigue at saturation is a planar structure of low energy which reduces the corrosion-deformation interactions, source of micro strains. (O.M.)