Jarir Aktaa

Microcrack propagation and fatigue lifetime under non-proportional multiaxial cyclic loading

Abstract Non-proportional multiaxial fatigue tests of tubular specimens were performed under purely alternating strain-controlled loading. Different loading paths with different phase shifts were applied. With increasing phase shift at the same equivalent load, the lifetime was found to increase. For lifetime prediction a model based on the Manson–Coffin law was developed. By including the hydrostatic loading part, it was possible to compare the results of the multiaxial fatigue tests with uniaxially ascertained results. To obtain more information about the microcrack behaviour under multiaxial non-proportional loading, sonic emission studies and fractographic analyses were performed. The results suggest a discontinuous microcrack propagation. Motivated by the good agreement between these observations and some microcrack propagation models known from literature, a simplified model was proposed for micro and short crack propagation. This model which is based on the J-integral range ΔJ yields a quite good agreement between the experimentally observed and the calculated lifetimes.

High temperature tensile properties of oxide dispersion strengthened T91 and their correlation with microstructural evolution

Abstract In the present work, high temperature deformation behavior of oxide dispersion strengthened T91 was investigated and linked to the corresponding microstructure. First, tensile properties are presented and discussed in terms of yield strength, tensile stress and total elongation as a function of temperature. The results are compared to the matrix material and other ODS alloys. Second, transmission electron microscopy was applied to as received and deformed tensile test specimens. It is shown that the Y2O3 particle diameter increases slightly upon deformation at elevated temperatures. Additionally, distinctive coarsening of M23C6 carbides occurs at prior austenite grain boundaries. At temperatures above 500°C, dislocations are straight and pile up at grain boundaries due to thermally activated climbing. Oxide dispersion strengthened T91 provides high strength due to strong particle/dislocation interactions and good toughness properties.

Unified modelling of time dependent damage taking into account an explicit dependency on backstress

A new continuum damage model is presented. It describes the time dependent damage of metallic materials at high temperatures in a unified manner, without a subdivision into creep and fatigue damage parts. Taking into account the dependence of damage evolution on the deformation history, a dependency of the damage rate on an internal variable, the backstress, is incorporated. The model is formulated first with a scalar damage variable for uniaxial load cases. Some ideas on a multiaxial extension are discussed. For application, the new model is coupled with the viscoplastic deformation model proposed by Chaboche. Low-cycle fatigue tests with the Ni-base alloy Nicrofer 5520 Co are considered, and the results are presented. The influence of the deformation model on the results of prediction of the new damage model is discussed.

Quantitative TEM Investigations on EUROFER 97 Irradiated up to 32 Dpa

The objective of this work is to evaluate the microstructure of the neutron-irradiated reduced activation ferritic/martensitic (RAFM) steel EUROFER 97. For this purpose irradiation induced defects like defect clusters, dislocation loops, voids/bubbles and precipitates are identified by transmission electron microscopy (TEM) and quantified in size and volume density. Emphasis is put on analyzing the influence of the irradiation dose and neutron fluxe on the evolution of size and density of the defects at irradiation temperatures between 300 and 335 °C. A first sample irradiated to a dose of 31.8 dpa was analyzed. The irradiation was carried out in the BOR 60 fast reactor of JSC “SSC RIAR” in Dimitrovgrad, within the framework of the ARBOR-1 irradiation program. To study the dose dependence in a next step the results will be compared to quantitative data on samples irradiated to a dose of 15 dpa. The obtained quantitative data will be used for correlation of the changes in the microstructure to the changes in the mechanical properties and will serve as an input for models describing this correlation.

Development of Functionally Graded Tungsten/EUROFER Coating System for First Wall Application

Abstract Reduced activation Ferritic/Martensitic (RAFM) steels, e.g. EUROFER are to be used as structural material for the First Wall (FW) of future fusion power plants. The interaction between plasma and FW, especially physical sputtering will limit the FW lifetime under normal operation. Therefore tungsten coating is selected to protect the FW due to its very low sputtering yield and low activation. However, the mismatch in thermo-physical properties between tungsten and EUROFER can lead to large residual thermal stresses and even failure. To overcome the issue of erosion a protective tungsten coating with a functionally graded (FG) tungsten/EUROFER layer (FG tungsten/EUROFER coating system) on EUROFER substrate will be developed and optimized. Non-linear finite element simulations are performed to predict optimal parameters of the coating system. Thereby the potential of the FG-layer in reducing inelastic strains and improving lifetime is demonstrated, and the investigated thickness of FG-layer is suggested. Based on the simulation results samples are fabricated by vacuum plasma spraying (VPS) with three different thicknesses of FG-layer. The microstructural observations revealed that the coating system has fine gradation and variable thickness as designed, low porosity, as well as a sound interface. Berkovich and Vickers hardness identify basic properties of those layers.

Fabrication of Tungsten-Vanadium Hybrid Material with Sufficient Toughness for High-Temperature Applications by Diffusion Bonding

Abstract The design of fusion plasma-facing components is challenging, as their materials have to meet rigorous requirements in terms of low activation and high-temperature strength. At the same time, sufficient ductility is required even in the low-temperature range. Unfortunately, these properties are not found in conventional materials. To solve this problem, a hybrid material that combines the high strength of one material with the high ductility of the other material was developed. This paper presents the hybrid material, which consists of thin tungsten and vanadium layers. This hybrid material was produced by means of diffusion bonding at relatively low temperature in a vacuum chamber. Microstructural investigations and nanoindentation tests indicated no cracks, no delamination, and no brittle intermetallic phases along the bond interfaces. Investigations of the mechanical properties of the hybrid material by instrumented Charpy impact tests revealed a relatively low ductile-to-brittle transition temperature (DBTT) at 124°C (compared to the DBTT of polycrystalline tungsten of >441°C) with an absorbed Charpy impact energy of 4.53 J [kleinst (KLST)-specimen]. Additionally, the tested Charpy impact specimens were found to be not fractured thoroughly even at room temperature.

A combined microtensile testing and nanoindentation study of the mechanical behavior of nanocrystalline LIGA Ni–Fe

Abstract In this work, we studied nanocrystalline LIGA Ni-2.6 at.% Fe and Ni-5.6 at.% Fe with an average grain size of 10 nm. Microtensile samples were produced by the LIGA process including direct current electrodeposition. Microstructures and mechanical properties were investigated in the as-deposited state as well as after annealing at different temperatures. Results from tensile testing were compared to nanoindentation experiments with a particular emphasis on the strain rate sensitivity of the alloys. The Ni – Fe alloys were confirmed to be suitable LIGA materials for applications that require high hardness combined with microstructural stability and low internal stresses. For both alloys with 10 nm grain size, tensile yield strength and corresponding hardness values of the order of 2 GPa and 6 GPa, respectively, were found. After annealing at moderate temperatures (200 °C), strength and hardness increased although some grain growth was observed.

Impact of pulsed operation on lifetime of DEMO blanket

The impact of pulsed operation on thermo-mechanical fatigue and creep-fatigue lifetimes of DEMO blanket is evaluated. Non-linear finite element (FE) analyses are performed for the current design of the helium cooled pebble bed test blanket module (HCPB-TBM), and assessed predicting the cyclic lifetime by evaluating the damage evolution obtained in the component for the candidate structural material. Therewith the influence of the pulse duration on the cyclic lifetime and hence full operation lifetime of the HCPB-TBM is investigated and evaluated considering the gain in operation time due to pulse durations longer than that specified for ITER. For very long pulse durations the gain in operation time is reduced due to creep-fatigue and consequently optimal pulse duration exists.

Low cycle fatigue properties of reduced activation ferritic/martensitic steels after high-dose neutron irradiation

This paper focuses on the low cycle fatigue (LCF) behaviour of reduced activation ferritic/martensitic steels irradiated to a displacement damage dose of up to 70 dpa at 330–337 °C in the BOR 60 reactor within the ARBOR 2 irradiation programme. The influence of neutron irradiation on the fatigue behaviour was determined for the as-received EUROFER97, pre-irradiation heat-treated EUROFER97 HT and F82H-mod steels. Strain-controlled push–pull loading was performed using miniaturized cylindrical specimens at a constant temperature of 330 °C with total strain ranges between 0.8% and 1.1%. Comparison of the LCF behaviour of irradiated and reference unirradiated specimens was performed for both the adequate total and inelastic strains. Neutron irradiation-induced hardening may have various effects on the fatigue behaviour of the steels. The reduction of inelastic strain in the irradiated state compared with the reference unirradiated state at common total strain amplitudes may increase fatigue lifetime. The increase in the stress at the adequate inelastic strain, by contrast, may accelerate fatigue damage accumulation. Depending on which of the two effects mentioned dominates, neutron irradiation may either extend or reduce the fatigue lifetime compared with the reference unirradiated state. The results obtained for EUROFER97 and EUROFER97 HT confirm these considerations. Most of the irradiated specimens show fatigue lifetimes comparable to those of the reference unirradiated state at adequate inelastic strains. Some irradiated specimens, however, show lifetime reduction or increase in comparison with the reference state at adequate inelastic strains.

Selective Laser Sintering as Manufacturing Process for the Realization of Complex Nuclear Fusion and High Heat Flux Components

Abstract The development of fabrication technologies for ITER and DEMO Blanket concepts is an activity followed by the KIT since a long time. A variety of fabrication technologies has been developed and qualified in strong collaboration with industry. Besides the standard technologies, an activity has been launched to explore the capabilities of generative fabrication procedures such as Laser Beam Melting (LBM) and Selective Laser Sintering (SLS). To manufacture demonstrator parts for Blankets by LBM /SLS, EUROFER (a Reduced Activation Ferritic Martensitic/RAFM steel applied e.g. in ITER) has been produced as powder metallurgical product. With this material, test parts have been realized. The test program started with solid parts and simple geometries used for extraction of specimen for material qualification purpose. Later, more complex parts were fabricated to investigate the feasibility of hollow and double walled structures and components with internal channel structures. Finally, blanket relevant part segments (e.g. for the Stiffening Plates) with meandering cooling channel structures and Flow Channel Insert segment demonstration parts for the EU Helium Cooled Pebble Bed and the Dual Coolant Lithium Lead Breeder Blanket concepts for DEMO have been fabricated. First preliminary qualification activities have been concluded using test procedures applied e.g. for the qualification of welding seams such as Tensile – and Charpy tests, macro- and micro structure investigation or hardness measurement. The findings have been compared to standard material properties of EUROFER in order to quantify the fabrication results. Material properties of ~ 80% and more, compared to standard rolled EUROFER with comparable heat treatment history could be demonstrated in case of Tensile- and Yield- strength, total strain after fracture as well as energy consumption in Charpy tests. Also the joining of generatively fabricated sub-components together with conventionally fabricated EUROFER parts by Electron Beam welding has been investigated in order to test the option of the fabrication of hybrid components. These hybrid components are intended to combine parts with straight channels fabricated by Electrical Discharge Machining together with generative fabricated parts with complex structures of cooling channels (e.g. nested U-shaped flow paths) which cannot be realized using standard machining technologies. This technical note reports the first promising qualification results of generatively fabricated EUROFER parts. Also the weldability of generative fabricated parts and conventionally fabricated EUROFER has been demonstrated. Preliminary qualification results of the welding are shown, and possibilities for experimental qualifications are discussed.