Structural materials are key elements in the safe and reliable operation of machine and plant components. However, the harsh operating conditions found in power plants are at the limits of what current materials can withstand. Metallic materials remain widely used for structural applications, with the implementation of nanofeatures allowing their use in more extreme service conditions. The most significant improvement is in their creep and stress rupture properties, obtained through the inclusion of nanosized obstacles. These features act as sinks for helium, a by-product of radiation exposure in reactors. Nanosized grains may help to improve strength and radiation resistance, but their stability at elevated temperatures requires further enhancement. Nanosized powders are another option.

Embrittlement by helium was investigated in a lamellar TiAl alloy under two conditions:(a) Specimens were implanted to various amounts of helium up to 762 appm at temperatures from 630 degrees C to 1000 degrees C and some of them subsequently creep-tested at the same temperature under stresses from 150 to 300 MPa. The microstructure and fracture surfaces of creep-deformed and non-creep-deformed specimens were then studied by transmission electron microscopy (TEM) and by scanning electron microscopy (SEM), respectively.(b) Specimens were implanted to various amounts of helium at a low temperature (150 degrees C) and post-implantation annealed at elevated temperatures for TEM studies.Embrittlement was revealed by reduction in time- and strain-to-rupture and by a transition in fracture surface from ductile to an inter-lamellar appearance. Embrittlement occurred above a critical He concentration, which decreased from about 10 appm at 700 degrees C to below 6 appm at 900 degrees C. TEM showed that embrittlement could be associated to reaching a critical bubble diameter of about 5 nm. Bubble diameters increased with increasing temperature ranging in high-temperature implanted specimens from about 3 nm (630 degrees C) to 20 nm (1000 degrees C) and in post-implantation annealed ones from 1.2 nm (600 degrees C) to 2.2 nm (900 degrees C), respectively. With increasing temperature, the bubble distribution grew less homogenous with a lower density of larger bubbles situated preferentially at interfaces and sinks. This was ascribed to a change in bubble nucleation mode from homogeneous di-atomic nucleation at lower temperatures to multi-atomic nucleation at sinks at higher temperature. (C) 2012 Elsevier B.V. All rights reserved.

Irradiation creep and microstructural changes of two ferritic ODS steels with 12% and 14% Cr have been studied by homogeneously implantation with helium under uniaxial tensile stresses from 40 to 300 MPa. The maximum dose was about 1.2 dpa (5000 appm-He) with displacement damage rates of 1 × 10 −5 dpa/s at a temperature of 300 °C. Irradiation creep compliances were measured to be 4.0 × 10 −6 dpa −1 MPa −1 and 10 × 10 −6 dpa −1 MPa −1 for 12 and 14Cr ODS, respectively. Subsequently, microstructural evolution was studied in detail by TEM observations, showing dislocation loops and bubbles distributed homogenously in the matrix. Some bubbles were attached to ODS particles. Finally, the effects of Cr content on irradiation creep and microstructural changes are discussed, including earlier results of a 19Cr ODS and a PM2000 ferritic steel.

In the present paper, irradiation creep results of an intermetallic TiAl alloy and two ferritic oxide dispersion strengthened (ODS) steels are summarized. In situ irradiation creep measurements were performed using homogeneous implantation with α- and p-particles to maximum doses of 0.8dpa at displacement damage rates of 2–8×10−6dpa/s. The strains of miniaturized flat dog-bone specimens were monitored under uniaxial tensile stresses ranging from 20 to 400MPa at temperatures of 573, 673 and 773K, respectively. The effects of material composition, ODS particle size, and bombarding particle on the irradiation creep compliance was studied and results are compared to literature data. Evolution of microstructure during helium implantation was investigated in detail by TEM and is discussed with respect to irradiation creep models.

Titanium aluminide (TiAl) alloys exhibit high specific strength, low density, good oxidation, corrosion, and creep resistance at elevated temperatures, making them good candidate materials for aerospace and automotive applications. TiAl alloys also show excellent radiation resistance and low neutron activation, and they can be developed to have various microstructures, allowing different combinations of properties for various extreme environments. Hence, TiAl alloys may be used in advanced nuclear systems as high-temperature structural materials. Moreover, TiAl alloys are good materials to be used for fundamental studies on microstructural effects on irradiation behavior of advanced nuclear structural materials. This article reviews the microstructure, creep, radiation, and oxidation properties of TiAl alloys in comparison with other nuclear structural materials to assess the potential of TiAl alloys as candidate structural materials for future nuclear applications.

International collaborations like the Generation IV initiative have the aim to create the technical basis for design and operation of advanced nuclear plants. Materials data shall be created in joint international materials projects. Data will be aggregated in databases like the “Generation IV materials handbook”. Mechanical data, but also microstructural information and information concerning materials production shall be included. This information will be used to create or amend code rules, to provide a basis for life-time analysis, damage assessments and for safety analyses. Such considerations need not only raw materials data but also tools for data analysis and evaluation. Multiscale modeling, establishing constitutive equations, development of advanced life-time prediction methods, quantitative correlation of mechanical properties with microstructure, quantification of environmental effects, tools for non destructive evaluation and condition based monitoring etc. are important analysis techniques needed for safe design and operation of advanced plants. These needs led the author to ask the question if current databases could not be enlarged by data evaluation and methods tools which could even end some day in the availability of web-based design codes and safety analyses. The database could also be used as a web-based discussion and development space. It could become then a powerful tool for knowledge management The paper will discuss this concept on basis of some examples.

In 1999, an international collaborative initiative for the development of advanced (Generation IV) reactors was started. The idea behind this effort was to bring nuclear energy closer to the needs of sustainability, to increase proliferation resistance, and to support concepts able to produce energy (both electricity and process heat) at competitive costs. Six reactor concepts were chosen for further development: the sodium fast reactor (SFR), the very-high-temperature gas-cooled reactor (VHTR), the lead or lead-bismuth cooled liquid metal reactor, the helium gas-cooled fast reactor, the molten salt reactor (MSR), and the super critical water reactor. In view of sustainability, the Generation IV reactors should not only have superior fuel cycles to minimize nuclear waste, but they should also be able to produce process heat or steam for hydrogen production, synthetic fuels, refinery processes, and other commercial uses. These reactor types were described in the 2002 Generation IV roadmap. Different projects around the world have been started since that time. The most advanced efforts are with reactors where production experience already existed. These reactors include the SFR and the VHTR. The other reactor types are still more in a design concept phase. This chapter briefly describes the six Generation IV concepts and then provides additional details, focusing on the two near-term viable Generation IV concepts. The current status of the applicable international projects is then summarized. These new technologies have also created remarkable demands on materials compared with light water reactors (LWRs). Higher temperatures, higher neutron doses, environments very different from water, and design lives of 60 years present a real engineering challenge. These new demands have led to many exciting research activities and to new Codes and Standards developments, which are summarized in the final sections of this chapter.

The creep properties of an intermetallic alloy Ti–46Al–2W–0.5Si (at%) including strain rate and time to fracture were investigated in vacuum using helium-implanted and non-implanted samples, at a temperature of 1073K and a stress of 200MPa. The implantation was performed using 24MeV He-ions, homogeneously implanting the samples with up to 1333appm (atomic parts per million) helium. The size and location of helium bubbles were determined with transmission electron microscopy (TEM). Samples implanted with helium content above 10appm exhibited strong helium embrittlement, reducing both the time to fracture and the elongation at fracture. The corresponding critical helium bubble size rc was determined to 10nm.

Relaxation processes of dislocation systems are studied by two-dimensional dynamical simulations. In order to capture generic features, three physically different scenarios were studied and power-law decays found for various physical quantities. Our main finding is that all these are the consequence of the underlying scaling property of the dislocation velocity distribution. Scaling is found to break down at some cut-off time increasing with system size. The absence of intrinsic relaxation time indicates that criticality is ubiquitous in all states studied. These features are reminiscent to glassy systems, and can be attributed to the inherent quenched disorder in the position of the slip planes.

Simulation of subgrain growth during recovery is carried out using two-dimensional discrete dislocation dynamics on a hexagonal crystal lattice having three symmetric slip planes. To account for elevated temperature (i) dislocation climb was allowed and (ii) a Langevin type thermal noise was added to the force acting on the dislocations. During the simulation, a random ensemble of dislocations develop into subgrains and power-law type growth kinetics are observed. The growth exponent is found to be independent of the climb mobility, but dependent on the temperature introduced by the thermal noise. The in-depth statistical analysis of the subgrain structure shows that the coarsening is abnormal, i.e. larger cells grow faster than the small ones, while the average misorientation between the adjacent subgrains remains nearly constant. During the coarsening Holt's relation is found not to be fulfilled, such that the average subgrain size is not proportional to the average dislocation spacing. These findings are consistent with recent high precision experiments on recovery.