Reduced Activation Ferritic/martensitic Research Articles

Manufacturing and testing of flat-type divertor mockup with advanced materials

During reactor operation, the divertor must withstand unprecedented simultaneous high heat fluxes and high-energy neutron irradiation. The extremely severe service environment of the divertor imposes a huge challenge to the bonding quality of divertor joints, i.e., the joints must withstand thermal, mechanical and neutron loads, as well as cyclic mode of operation. In this paper, potassium-doped tungsten (KW) is selected as the plasma facing material (PFM), oxygen-free copper (OFC) as the interlayer, oxide dispersion strengthened copper (ODS-Cu) alloy as the heat sink material, and reduced activation ferritic/martensitic (RAFM) steel as the structural material. In this study, a vacuum brazing technology is proposed and optimized to bond Cu and ODS-Cu alloy with the silver-free brazing material CuSnTi. The most appropriate brazing parameters are a brazing temperature of 940 °C and a holding time of 15 min. High-quality bonding interfaces have been successfully obtained by vacuum brazing technology, and the average shear strength of the as-obtained KW/Cu and ODS-Cu alloy joints is ∼268 MPa. And a fabrication route for manufacturing the flat-type divertor target based on brazing technology is set. For evaluating the reliability of the fabrication technologies under the reactor relevant condition, the high heat flux test at 20 MW/m2 for the as-manufactured flat-type KW/Cu/ODS-Cu/RAFM mockup is carried out by using the Electron-beam Material testing Scenario (EMS-60) with water cooling. This paper reports the improved vacuum brazing technology to connect Cu to ODS-Cu alloy and summarizes the production route, high heat flux (HHF) test, the pre and post non-destructive examination, and the surface results of the flat-type KW/Cu/ODS-Cu/RAFM mockup after the HHF test. The test results demonstrate that the mockup manufactured according to the fabrication route still have structural and interfacial integrity under cyclic high heat loads.

Effect of applied stress on the ductility of RAFM steel during long-term exposure in flowing Pb-17Li

The effect of applied stress on the ductility of reduced activation ferritic martensitic (RAFM) steel during long-term exposure in flowing Pb-17Li in the temperature interval 300–400 ℃ was investigated using tensile tests. The ductility of RAFM steels at a stress of 250 MPa decreases appreciably after exposure in following Pb-17Li at 400 ℃ for 1500 h. And a slight ductility loss occurred for the samples tested at 300 ℃, 350 ℃ and 400 ℃ for 500 h. Coarsening of martensitic laths occurred in RAFM steels owing to the thermal aging with a 250 MPa stress for 1500 h in liquid Pb-17Li. Due to the poor ductility of coarse laths, micro-cracks originated on the surface of necking zone during tensile test, which facilitates the penetration of liquid metal. With sufficient liquid Pb-17Li penetrated into cracks continuously, the surface tension of the newly formed fracture surface is declined by liquid metal in the crack tip. The fracture mode was altered from uniform to unstable crack propagation thus leading to a reduction of ductility. There was no depletion of Fe and Cr or Pb infiltration into the substrate for RAFM steels after exposure. The fracture mechanisms were proposed in terms of supply of liquid metal and micro-cracks caused by coarse laths. Accordingly, the reduction of ductility for RAFM steels after exposure in liquid Pb-17Li was the result of combined effect of applied stress and liquid metal.

Effect of austenitizing temperature on microstructure and mechanical properties evaluation of microalloyed low-carbon RAFM steel

The microstructure and mechanical properties of microalloyed low-carbon reduced activation ferritic/martensitic (RAFM) steel after austenitization at 950 °C, 1000 °C and 1050 °C for 0.5 h and tempering at 750 °C for 1.5 h were investigated using scanning electron microscopy, transmission electron microscopy, electron back scattering diffraction, X-ray diffraction, tensile tests, and impact tests. The grain size and lath width obviously increase because more M23C6 carbides dissolve after austenitization at 1050 °C. Besides, more fine MX particles would precipitate and result in higher dislocation density. Furthermore, the strengthening mechanisms at room temperature were also systematically discussed, the predominant strengthening mechanism changes from grain boundary strengthening and dislocation strengthening to dislocation strengthening when the austenitizing temperature increases from 950 °C to 1000 °C and 1050 °C. After taking the microstructure, strength, ductility, and toughness into thorough consideration, the optimum austenitizing temperature for microalloyed low-carbon RAFM steel is about 1000 °C.

Innovative Isostatic Processing Technologies for Defence and other Strategic Applications

Powder Metallurgy (P/M) involving Cold Isostatic Pressing (CIP) and Hot Isostatic Pressing (HIP) techniques is the best suited processing route for manufacture of critical components from ceramics and highly complex metallic materials. The CIP followed by sintering route was adopted for the development of fused silica radomes for application in high-speed target seeking missiles. On the other hand, the HIP, a single-step processing technique has been used for the development of First-Wall (FW) of Test Blanket Modules (TBMs) from India Specific Reduced Activation Ferritic Martensitic (INRAFM) steel. The FW is an important component required for International Thermo-nuclear Experimental Reactor (ITER) application.The details of innovative processes which have been established to overcome the specific technical challenges for the development of CIP and HIP technologies for manufacture of silica radomes and INRAFM steel first-wall respectively for intended applications are highlighted in this paper.

Influence of fatigue precracking and specimen size on Master Curve fracture toughness measurements of EUROFER97 and F82H steels

EUROFER97 and F82H steels are two leading reduced-activation ferritic-martensitic (RAFM) steels for fusion first wall and blanket applications. Exposure to the harsh environment of fusion reactors can result in severe degradation of fracture toughness. Thus, the post-irradiation evaluation of fracture toughness is critical for understanding the material behavior. Due to the space constraints of irradiation facilities and challenges in controlling a uniform irradiation condition for large size specimens, the development of small specimen test techniques (SSTT) is indispensable to evaluate the performance of irradiated materials. In this study, we evaluated specimen size effects on the Master Curve fracture toughness of EUROFER97 and F82H steels. A wide variety of specimens, including 0.5 T compact tension (C(T)) specimens, 0.16 T mini-compact tension (miniC(T)) specimens, and 1.65 mm miniature bend bar specimens, were tested. The testing methodology was based on the Master Curve method in the ASTM E1921 standard. No specimen size effect was observed in 0.5 T C(T) and 0.16 T miniC(T) specimens on the Master Curve reference temperature T0, while 1.65 mm miniature bend bar specimens yielded a higher T0Q. A strong effect of fatigue precracking on T0 for 0.5 T C(T) and 0.16 T miniC(T) specimens was observed, such that testing on specimens with skewed fatigue precrack fronts resulted in lower T0 than for specimens with ASTM standard qualified straight fatigue precrack fronts. The results highlight the importance of experimental quality control in developing SSTT for Master Curve fracture toughness testing. Lastly, we also evaluated and provided recommendations on the minimum number of specimens needed for each specimen type for yielding reliable T0Q values.

Microstructure and Mechanical Properties Evaluation of Microalloyed Low‐Carbon Reduced Activation Ferritic/Martensitic Steel

A novel method for reducing the amount of M23C6 carbides, increasing the amount of MX carbonitrides, and decreasing the coarsening rate of M23C6 carbides is put forward to improve the high‐temperature properties of reduced activation ferritic/martensitic (RAFM) steel. Scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, tensile tests, and impact tests are used to systematically investigate the microstructure evolution and mechanical properties of RAFM (RAFM‐0.1C) steel, low‐carbon RAFM (RAFM‐0.04C) steel, and minor boron and nitrogen microalloyed low‐carbon RAFM (RAFM‐CBN) steel. Some δ ferrite develops when C decreases from 0.1 to 0.04 wt% and subsequently disappears after 0.01 wt% N addition. The amount and size of M23C6 carbides and dislocation density decrease with the decrease of C (RAFM‐0.04C), while the amount of MX carbonitrides is 2‐3 times, and dislocation density is ≈2 times higher than that of RAFM‐0.04C steel after 0.01 wt% N addition (RAFM‐CBN). Compared with RAFM‐0.1C steel, the yield strength at room temperature and 550 °C slightly decreases for RAFM‐0.04C steel and considerably increases for RAFM‐CBN steel. The contributions of microalloy on strengthening mechanisms of RAFM steel at room temperature are also systematically revealed by combining the experimental and theoretical data.

Microstructure and Mechanical Properties of Autogeneous GTA Weldments of Reduced-Activation Ferritic/Martensitic Steel and the Effects of Alloying Elements

Five reduced-activation ferritic/martensitic (RAFM) steels containing different contents of Cr, Ta, and Ti were prepared and the weldability of these alloys were evaluated by autogeneous gas tungsten arc (GTA) welding. After welding, post weld heat treatment (PWHT) was performed at temperatures of approximately 730-750 ℃. The microstructures of all alloys after PWHT consisted of a tempered martensite matrix and two types of precipitates, namely, M<sub>23</sub>C<sub>6</sub> and MX. Both the tensile and yield strength of the GTA welds increased with an increase in Cr and Ta contents due to the enhanced solid solution and precipitation strengthening. An increase in Ti content promoted the precipitation of fine MX particles and thereby suppressed the precipitation of M<sub>23</sub>C<sub>6</sub> carbide. This improved the impact toughness without a considerable loss of strength.

Towards better understanding the question of helium-driven swelling in RAFM-ODS alloys

We irradiated three 9Cr alloys: Fe9Cr model alloy, reduced-activation ferritic/martensitic (RAFM), and advanced oxide dispersion strengthened ferritic (RAFM-ODS) alloys, with 100keV He ions at 723K to the peaked concentration of 6500 appm (low helium level) and 65000 appm (high helium level), respectively. The question of helium-driven swelling in Fe9Cr, RAFM, and RAFM-ODS alloys was investigated by multiple characterizations. Homogeneous helium bubbles, main in the form of VmHen (n < m), nucleated on different sinks, producing swelling behavior and dominating the microstructures of irradiated Fe9Cr, RAFM, and RAFM-ODS steels. The consequent swelling rate values in Fe9Cr, RAFM, and RAFM-ODS were ∼ 0.15% (1.33%), ∼ 0.058% (0.83%), and ∼ 0.048% (0.73%) in the low (high) levels, respectively. The RAFM-ODS steel exhibits better tolerance to helium bubble swelling and superior grain size stability than Fe9Cr and RAFM, which is manifested by the smaller average sizes and the lower number densities of bubbles. It can be attributed that the oxides in RAFM-ODS steel could provide more effective helium trapping sites that sequester the helium into smaller bubbles and away from the precipitate-matrix interfaces and grain boundaries. In addition, the existence of high density of oxide particles could also delay desorption peaks of helium atoms from RAFM-ODS steel compared to RAFM and Fe9Cr.

Large Component Simulation of an Inboard Sector of a Dual-Coolant Lead-Lithium Blanket

Advances in high-performance computing now enable the engineering evaluation of large blanket components from a systems perspective at the beginning of a normal design cycle. As an example, we discuss the computational fluid dynamics (CFD) involved in characterizing the thermal performance of an entire 22.5 toroidal sector of a dual-coolant lead-lithium blanket with particular attention to the integrated manifolding and flow distributions. The inboard sector is roughly 7 m tall, has 321 first-wall helium-cooled channels, and five parallel PbLi breeder channels complete with insulating SiC flow channel inserts. A finite volume model was developed with various degrees of mesh refinement from 36 to 187 million cells to perform the flow calculations on disparate fluids with conjugate heat transfer in the solid. Steady-state CFD calculations were performed using a realizable k-ε turbulence model. Simplifications include a uniform applied surface heat flux and a one-dimensional radial volumetric neutron heating profile. Constant material properties were used for the F82H reduced-activation ferritic martensitic (RAFM) steel walls, SiC flow channel inserts, and the PbLi breeder. Ideal gas behavior was assumed for the helium, which includes compressibility. Helium mass flows of 54 kg/s at 8 MPa and PbLi flows of 3 kg/s at 101 kPa supply the sector, both at an inlet temperature of 350°C. This early model is not optimized; however, it reveals important features not obvious in a conglomeration of smaller independent models. For example, all the manifolding was included to evaluate flow distributions throughout the full component. Submodels were only used to obtain the convective heat transfer coefficients (HTCs) inside the helium first-wall channels equipped with enhancement vanes that allow the first wall to handle 0.8 MW/m2 of plasma heat flux with the aim of keeping bulk RAFM steel temperatures near the creep-fatigue limit of 550°C. Average HTCs obtained from these detailed submodels were then used in the large model to predict thermal performance without incurring the meshing overhead from the thousands of internal vanes. Although much progress was made, initial results indicate that more must be done to further reduce hot spots on the first wall, minimize pressure drops, and provide optimal flow distributions.

A process optimization framework for laser direct energy deposition: Densification, microstructure, and mechanical properties of an Fe[sbnd]Cr alloy

Laser Direct Energy Deposition (DED) is a metal additive manufacturing technique with the ability to fabricate large and complex parts through deposition of metal powders. However, achieving high-density parts and targeted build heights using DED can be challenging due to the large number of highly sensitive process variables. This work proposes a robust fabrication parameter optimization framework to generate process maps for primary parameters in DED, including laser power, scan speed, mass flow rate, hatch spacing, and layer height. Simple single-track experiments were utilized to map out the parameter space, and a combination of geometric criteria for hatch spacing and layer height were proposed to determine parameter sets that achieve both targeted build heights and mitigate porosity formation. Using this framework, specimens with >99 % density and consistent mechanical properties were successfully fabricated over a wide range of process parameters for an Fe-9wt.%Cr (Fe9Cr) alloy, a surrogate for radiation damage-resistant reduced activation ferritic/martensitic (RAFM) steels. Processing these materials using DED is of particular interest in the development of plasma facing components for nuclear fusion applications. The microstructure and mechanical properties of as-printed Fe9Cr were characterized using optical and electron microscopy, X-ray diffraction, and uniaxial tensile tests. As-printed Fe9Cr displayed ~25 % elongation and ultimate tensile strengths of up to 475 MPa which is comparable to similar wrought alloys. The proposed framework will allow for accelerated DED parameter optimization for novel alloy systems, as well as open the possibility for local microstructure control while simultaneously mitigating defect formation.

Radiation-induced precipitates in ferritic-martensitic steels and their radiation resistance effects

Designing high-performance radiation-tolerant materials and understanding the atomistic mechanism of radiation resistance are important topics in nuclear energy structural materials research. In this study, we investigated the atomistic mechanism of the formation of rod-like precipitates in helium-irradiated ferritic-martensitic steels (F-M steels) by positron annihilation spectroscopy (PAS) and transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM/EDX). The results indicated that vacancies tended to accumulate near dislocation lines to form complexes in the reduced activation ferritic-martensitic (RAFM) steel. Moreover, Fe and Cr atoms diffused toward these complexes and eventually depleted therein, which was not conducive to the formation of rod-like precipitates. Conversely, impurity atoms such as C atoms were found to segregate near dislocations in the Y-bearing oxide dispersion strengthened (Y-ODS) steel. When Fe and Cr diffused toward dislocations, they exchanged positions with C atoms until Fe and Cr exceeded their saturation solubilities and precipitated on the glide plane. We also analyzed the distribution of vacancy defects and bubbles/voids in the four irradiated materials and discussed the radiation resistance mechanism of the precipitates. The results of this study are significant in demonstrating a microscopic mechanism of radiation-induced precipitates to swelling resistance in materials. The raw/processed data required to reproduce these findings cannot be shared online at this time due to technical limitations, but they are available upon contacting the corresponding author. Materials exposed to extreme environments have received great attention recently in the context of advanced energy systems, defense, and aerospace technologies. Specifically, radiation-damage-tolerant materials are in great demand for the safe operation and advancement of nuclear energy and accelerator systems. Designing high-performance radiation-tolerant materials and understanding the atomistic mechanism of radiation resistance are important materials science topics. In this study, we reported the contribution of a microscopic mechanism of radiation-induced precipitates to swelling resistance in helium-irradiated ferritic-martensitic (F-M) steels/alloys. Significant findings included the formation of large areas of rod-like precipitates, which exhibited swelling resistance effects in steels subjected to helium irradiation and a possible microscopic mechanism for their formation. • Dense rod-like precipitates were observed in samples irradiated with high-dose helium ions. • The Cr and C contents were important factors affecting the formation of rod-like precipitates. • We clarified the atomistic mechanism of the formation of rod-like precipitates in the steels. • High-density rod-like precipitates as "radiation-damage antibodies" reduced void swelling.

Modelling the creep curves of RAFM steel employing a dislocation density reliant model

The microstructure based creep modelling is one of the important tools to elucidate the creep deformation behavior with ongoing substructure evolution. Present research work embodied the development of hybrid creep model that combines a microstructure based creep model and continuum damage mechanics (CDM) approach for addressing the creep behavior of reduced activation ferritic-martensitic (RAFM) steel. By addition of these two methods, the model is capable to address the creep curves up to a large portion of tertiary creep regime. The model is capable of demonstrating the evolution of the microstructure based variables that are important to understand the microstructural degradation mechanisms during creep. The evolution of dislocation densities (mobile, dipole and boundary), glide velocity, dislocation mobility, mobile dislocation spacing and subgrain boundary energy/unit volume, with ongoing creep is discussed and the magnitude lies in the range of 6.37 × 1012-–1.19 × 1014 m−2, 4.66 × 109 – 4.38 × 1011 m−2, 5.91 × 1013 –2.86 × 1014 m−2, 3.34 × 10-12 − 3.99 × 10-12 m/s, 5.69 × 10-21-7.148 × 10-21 mPa-1s−1, 91.2–395 nm and 2.07 × 1015 - 2.42 × 1017 Jm−5, respectively, at the end of simulation. The simulated creep curves have shown a reasonable match with the experimental ones.

On the shrinkage of neutron irradiation-induced cavities in Eurofer97 steel upon heating

ABSTRACT By employing in situ transmission electron microscopy (TEM) observation and cluster dynamics (CD) simulations, we studied the annealing behaviours of neutron irradiation-induced cavities in reduced activation ferritic/martensitic (RAFM) steel Eurofer97 subjected to heating up to 550 °C. Experimental results showed that the cavities behaved differently from the general coarsening behaviours of cavities formed by ion implantations. Rapid shrinkage of some cavities was observed when heating temperatures exceeded 500 °C. Three types of cavity annealing behaviours were identified: (i) cavity remained stable; (ii) cavity shrank to a smaller size and then kept stable; (iii) cavity shrank and eventually disappeared. The CD simulations, which were based on the rate theory, well reproduced these experimental observations. It was revealed that the shrinkage course can be terminated by the dramatic rise of helium density (helium/vacancy ratio) inside the shrinking cavity. Cavity stopping shrinkage and being stable at high temperatures, without being annealed out, potentially suggests the modification in cavity coarsening kinetics driven by the conventional Ostwald ripening mechanism.

Microstructure and tensile properties of nano-sized ZrC particle strengthened RAFM steels

Reduced activation ferritic/martensitic (RAFM) steels containing ZrC nano particles (ZrC-RAFM steels) were prepared by a combination of spark plasma sintering (SPS) and hot-rolling of the mechanical alloyed powders. Moreover, the microstructure and tensile properties of the prepared ZrC-RAFM steels were investigated. The results show that the ZrC nano particles are uniformly dispersed in the RAFM steels, resulting in a significant decrease in grain size and the enhanced tensile properties of the steels. With the increase of ZrC content, the hardness and tensile strength of the steels are improved, and the mechanism of the improved properties is analyzed and discussed. The tensile strength of the 0.75ZrC- RAFM steel is 2069 MPa, which is 19.6% higher than that (1730 MPa) of the RAFM steel without ZrC. The conclusions provide researchers with the roles of ZrC-nanoparticle in micromorphology and tensile properties of the RAFM steel as potential cladding materials for fast reactors.

Electrodeposition of Functionally Graded Brazing Interlayers for Enhanced Joint Strength between Fusion Plasma-Facing Materials and Heat Sinks

In response to the significant plasma science and engineering breakthroughs witnessed over the past decade, the fusion research community has developed an ambitious roadmap to achieve large-scale, energy-positive fusion reactors that will enable economically competitive fusion electricity. Critical to the success of commercial fusion energy is the development of materials able to serve as plasma facing components (PFCs) that can withstand the unprecedented high heat, particle, and neutron fluxes within the fusion reactor. Tungsten is a leading candidate for the plasma-facing portion of these PFCs because of its high melting point, high sputtering resistance, and low tritium retention. Copper or copper-based alloys (e.g., copper-chromium-zirconium alloy C18150) and reduced activation ferritic/martensitic (RAFM) steels have been proposed for the heat sink materials behind the tungsten armor due to their high thermal conductivity. Brazing is considered a promising method for joining the tungsten armor and heat sink layer. However, the extreme mismatch in the coefficient of thermal expansion (CTE) between tungsten and these potential heat sink materials makes directly joining these two layers challenging. This thermal stress can be reduced by incorporating a joining layer of functionally graded materials between the tungsten and heat sink that imparts gradual changes in CTE. Copper/tungsten or copper/tungsten carbide composites are leading candidates for these functionally graded materials. For these composites, gradual changes in the CTE are achieved by varying the volume fraction of tungsten/tungsten carbide in the composite, from tungsten-rich at the tungsten armor to tungsten-poor at the heat sink.This talk will discuss the development of a scalable, electrochemical approach for fabricating interlayers with functionally graded CTE that can decrease the thermal mismatch between tungsten PFCs and copper or RAFM-based heat sinks. In this approach, the copper/tungsten or copper/tungsten carbide composite are electrodeposited onto the tungsten PFC. This component is then bonded to the heat sink via brazing. A key objective of the work is to achieve compositional control during electrodeposition through the use of highly tunable pulse and pulse-reverse electric fields. Engineering of the electric field enables control of the mass transport and crystallization phenomena in the deposition process and thus enables control of composite properties such as composition, porosity, and surface roughness. Results from electrodeposition trials and mechanical testing of brazed joints under ambient conditions as an initial screen for interlayer performance will be discussed, in anticipation of high-heat flux testing of joint performance under fusion-relevant conditions.

Effect of heat treatment on microstructure and mechanical property of ODS-CLAM-Zr steel

Oxide dispersion strengthening (ODS) is one of the effective methods to improve the high temperature strength of many reduced activation ferritic/martensitic (RAFM) steels, alloys and even intermetallic compounds. This manuscript is focused on the ODS experiments for the improvement of China low activation martensitic (CLAM) steel with Zr addition. The tempered ODS-CLAM-Zr samples were prepared by mechanical alloying (MA) and hot isostatic pressing (HIP) followed with different heat treatments (HTs). The results showed that the samples after HT had a typical bimodal grain structure consisting of residual ferrite and martensite structures. The inhomogeneous structure of the samples may be caused by MA, and the ultrafine grains had good thermal stability at 1150°C. Higher tensile properties were obtained under normalizing at 1050°C and tempering at 750°C. The coarsening of the rod-shape ZrO2 phase at the grain boundaries with the increase of temperature was the main reason for the decrease of the strength of the samples. This work provides a reasonable HT for tempering ODS martensitic steels.

Heterogeneous microstructure and anisotropic mechanical properties of reduced activation ferritic/martensitic steel fabricated by wire arc additive manufacturing

• Reduced activation ferritic/martensitic steel was fabricated by wire arc additive manufacturing. • The as-deposited microstructure exhibited the heterogeneous band characteristics. • Microstructure differences caused the periodical hardness and anisotropic tensile properties. • A potentially detrimental softening zone, was found at the heat-affected zone of the as-deposited RAFM steel. Reduced activation ferritic/martensitic (RAFM) steel is an iron-based alloy as a candidate structural material in fusion reactor. This paper evaluates the compatibility of RAFM steel as a choice material for wire arc additive manufacturing (WAAM). Two specimens of RAFM steels with high and low heat input were fabricated by WAAM. The effects of the heat input on the microstructure, microhardness and tensile properties of samples were investigated. The fusion boundaries are spaced uniformly in the whole sample. Three distinctive zones were present in the periodic region, including heat-affected zone (HAZ), columnar grains zone and fine-grained zone occurred alternatively. The HAZ was affected by the heat input. The fully γ-annealed top region consisted of epitaxial elongated grains without HAZ. The periodic pattern in microhardness along the building direction was found which was related to the periodic microstructure featured. The tensile properties presented anisotropic characteristics due to the heterogeneous microstructure. Further analysis indicated that the grain coarsening in the HAZ and C precipitates distributed at the grain boundaries caused substantial softened in the HAZ, resulted in the lower localized microhardness and tensile strength. Compared to the high heat input specimen, the low heat input specimen had smaller grain sizes, higher microhardness and tensile properties.

Evaluation of heat removal property in W/Cu/RAFM steel joint by using electron beam facility

• The durability of the W/Cu/JLF-1 joint sample fabricated by brazing was confirmed by performing steady-state and repetitive heat loading experiments. • The W/Cu/JLF-1 showed excellent heat removal performance under the steady-state heat loading. • A degradation of the heat removal performance was confirmed during the repetitive heat loading. • The delamination which could lead to the barrier of efficient heat transfer was confirmed after the repetitive heat loading of ∼6.0 MW/m 2 . A reliable technique for creating joints between tungsten (W) and Reduced Activated Ferritic/Martensitic (RAFM) steel with a pure-copper (Cu) interlayer (W/Cu/RAFM steel) has been explored for developing the heat removal component in the baffle and dome of a divertor. The first joint sample of W/Cu/RAFM steel was fabricated by using a brazing technique described in our previous work. In this work, to evaluate the heat removal property, a joint sample was subjected to a steady-state and a repetitive heat loading test with an electron beam facility (ACT2). In the steady-state heat loading test, a step-wised static heat load with ∼0.7, ∼1.0, ∼1.9, ∼2.7, ∼3.6, ∼4.4 and ∼6.0 MW/m 2 was applied to the joint sample. The temperatures on the W and RAFM steel showed a linear increasing tendency, as a function of the heat loading value. Also, in the repetitive heat loading test, a 30 s pulse width of 100 cycles was applied with a maximum heat loading value of ∼2.8 and ∼6.0 MW/m 2 . The obvious temperature excursion, while increasing the heat cycles, was confirmed at ∼6.0 MW/m 2 . After the heat loading, a major crack in the vicinity of the interface between the W and Cu was confirmed. This crack led to the degradation of the heat removal performance over ∼6.0 MW/m 2 .

Ambient, elevated temperature tensile properties and origin of strengthening in friction stir welded 6 mm thick reduced activation ferritic-martensitic steel plates in as-welded and post-weld normalised conditions

Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.

A multi-scaled process study of dissimilar friction stir welding of Eurofer RAFM steel to PM2000 ODS alloy

Both the reduced activation ferritic/martensitic (RAFM) steel and oxide dispersion strengthened (ODS) alloys have shown high potential applications in the nuclear industry. In this study, Eurofer RAFM steel and PM2000 ODS alloy in butt joint configuration was welded by friction stir welding (FSW), and a multi-scaled process study including procedure analysis, macro-/micro-structure, mechanical properties as well as deformation behavior was conducted. To obtain defect-free welds with equal strength and toughness matching, an intermediate rotation speed of 300 rpm was applied since it results in sufficient material intermixing both within the stirred zone (SZ) and along the SZ boundary. The SZ is composed of quenching martensite from the Eurofer steel and the recrystallized ferrite from PM2000, which shows significantly increased microhardness and excellent resistance to local deformation. As a result, strain localization occurs within the Eurofer steel during tensile testing. Additionally, a unique phenomenon, abnormal grain growth (AGG), was identified within the SZ of the as-welded joint. The underlying mechanism of AGG is related to the reduction of grain boundary pinning due to the dissolution of nanoparticles. The equiaxed ferrite nucleus with a similar orientation, surrounded by low-angle grain boundaries, gradually merge with each other through grain annexation, resulting in the finally coarsened grains. The reported study offers fundamental knowledge of FSW of RAFM steel to ODS alloy dissimilar combinations, promoting the usage of RAFM/ODS hybrid structures in future applications.