Reduced Activation Ferritic Martensitic Steel Research Articles

Interfacial plasticity mediated by lath boundaries in reduced-activation ferritic/martensitic steels

• The onset lath boundary sliding sensitively depends on the microscopic structure of the interfacial planes. • The lath boundary sliding is mediated by the motion of dislocation arrays lying on the interfacial plane. • For predominant lath boundary sliding: - The Burgers vector of the interfacial dislocations must lie on the interfacial plane. - The interfacial plane must be parallel to the common slip planes in the BCC structure, i.e., {110}. In the hierarchically-arranged crystallographic structure of reduced-activation ferritic/martensitic (RAFM) steel, the smallest but the most abundant microstructural unit, i.e., the lath, plays an important role in dislocation plasticity. Due to the multi-scale complexity in the lath-martensitic microstructure, miniaturized mechanical characterization at very fine scale is required to understand the deformation mechanism associated with laths. In this study, uniaxial micro-compression tests combined with rigorous crystallographic analysis were performed to figure out the plastic deformation mechanism of lath boundary sliding in RAFM steels. These experimental results were further interpreted via molecular dynamics simulations to discover the underlying dislocation mechanisms. We found that the amount of lath boundary sliding is controlled by the crystallographic orientation of the lath boundary plane, the direction of Burgers vectors of the interfacial dislocations, and the magnitude of resolved shear stress on the lath boundary plane. Also, the effect of normal stress on the lath boundary plane was investigated.

Modification of Surfaces of Reduced-Activation Ferritic–Martensitic Steels upon Irradiation by Pulsed Deuterium Plasma with Parameters Typical for Peripheral Plasma Disruption

Modification of Surfaces of Reduced-Activation Ferritic–Martensitic Steels upon Irradiation by Pulsed Deuterium Plasma with Parameters Typical for Peripheral Plasma Disruption

Identification of Retained Austenite in 9Cr-1.4W-0.06Ta-0.12C Reduced Activation Ferritic Martensitic Steel

Reduced activation ferritic martensitic (RAFM) 9Cr steels, which are candidate materials for the test blanket module (TBM) of nuclear fusion reactors, are considered to be air hardenable. However, alloy composition and the processing conditions play a significant role during the transformation of austenite to martensite/ferrite on cooling. The presence of retained austenite is known to influence the mechanical properties of the steel. Identifying very low amounts of retained austenite is very challenging though conventional microscopy. This paper aims at identifying a low amount of retained austenite in normalized 9Cr-1.4W-0.06Ta-0.12C RAFM steel using synchrotron X-ray diffraction and Mossbauer spectroscopy and confirmed by advanced automated crystal orientation mapping in transmission electron microscopy. Homogeneity of austenite has been understood to influence the microstructure of the normalized steel, which is discussed in detail.

Comparison experiment on the sputtering of EUROFER, RUSFER and CLAM steels by deuterium ions

In this work, the RAFM steels EUROFER, RUSFER and CLAM, along with a reference pure Fe sample, were all exposed to the same source of deuterium and analyzed using the same techniques, allowing a direct comparison of the experimental results. A 200 eV/D mass-selected deuterium ion beam at the SIESTA facility was used to bombard the samples to a fluence of 5 × 1024 Dm−2 at 450 K. The surface morphology of the samples was investigated with Scanning Electron Microscopy. The sputter yield of the samples was determined by weight-loss measurements and confirmed by measurements of the eroded depth. The near-surface enrichment of W and Ta was investigated via Energy-dispersive X-ray spectroscopy, X-ray Photoelectron Spectroscopy and Rutherford Backscattering Spectrometry. Grain-orientation-dependent sputtering was studied with Confocal Laser Scanning Microscopy and Electron Backscatter Diffraction. Lastly, Nuclear Reaction Analysis and Thermal Desorption Spectrometry were employed to analyze deuterium retention in all the samples. The erosion behavior of all three steels under deuterium bombardment was confirmed to be similar. The measured sputter yield was comparable for all three steels, and significantly lower than that of pure Fe. Likewise, all steels develop a needle-like surface morphology under the given exposure conditions and a W- and Ta-enriched layer in the range of few nanometers, while the Fe sample remained smooth. Retained deuterium amounts were also comparable among the steel samples, and were overall larger than the retention measured for the pure Fe sample.

Corrosion Kinetics of Reduced Activation Ferritic Martensitic Steel Jlf-1 in Liquid Metal Sn

Corrosion Kinetics of Reduced Activation Ferritic Martensitic Steel Jlf-1 in Liquid Metal Sn

Microstructure and Tensile Properties of Rafm Steel Strengthened by Zrc-Nanoparticle Dispersion

Microstructure and Tensile Properties of Rafm Steel Strengthened by Zrc-Nanoparticle Dispersion

Characterization and qualification of neutron radiation effects – Summary of Japan-USA Joint Projects for 40 years –

The Joint Projects under the Japan-USA Fusion Cooperation Program started in 1981 and has continued for more than 40 years. In the Joint Projects, although a wide range of fusion materials and engineering issues were covered, neutron radiation effects on fusion reactor materials have always been the major research emphases, and the neutron irradiation facilities in the US were jointly used by Japanese and US researchers. Japanese test facilities including neutron and charged particle irradiation facilities were complementarily used.The initial focus of the Joint Projects was on fundamental fusion neutron radiation effects and irradiation correlation. Systematic comparison of fission and fusion radiation effects in comparable damage levels and the effects of transmutation-induced helium were investigated. The collaboration was then focused on the effect of dynamic irradiation effects in variable conditions. In addition to the relatively fundamental studies, the Joint Projects contributed largely to development of candidate materials such as RAFM steels, vanadium alloys, SiC/SiC composites, and tungsten alloys, through a mechanism-oriented approach. The Joint Projects also covered issues specific to materials application to fusion blankets and plasma-facing components, including neutron radiation effects such as tritium retention and permeation of neutron-irradiated plasma-facing materials. Various irradiation technologies were developed and applied to the irradiation experiments, including those for in-situ testing.Considering that high energy neutron sources, such as A-FNS and IFMIF-DONES, now have high viability, the research supporting the neutron source programs is essential. The knowledge obtained through the Joint Projects is valuable and should be advanced for this purpose. To this end, it is of urgent necessity to launch an international scientific program accumulating knowledge of fusion neutron radiation effects, including their fundamental aspects.

Effect of heat treatment on microstructures and properties of electron beam welding CLF-1 steel

Vacuum electron beam welding was adopted for the reduced activation ferritic martensitic steel (CLF-1 steel) with a thickness of 32 mm, and a well-formed butt joint without metallurgical defects was obtained under the optimum technological parameters. After welding, the specimens were subjected to high temperature tempering (710 °C/120 min/AC (air cooling), abbreviation HTT) and incomplete annealing (840 °C/120 min/AC, abbreviation IA) heat treatment, and the microstructure and mechanical properties of the as-welded state and two different heat treatment states were compared. The results showed that the as-welded weld metal (WM) was composed of coarse lath martensite, and the heat affected zone (HAZ) was composed of a large amount of massive martensite and dispersed fine MX and M23C6. Compared with the welded state, the martensite structure of HTT state WM and HAZ were refined, nano-scale M23C6 was precipitated along the boundary of tempered martensite in WM and nano-scale MX was precipitated in tempered martensite lath structure, both of which played a role of dispersion strengthening. Compared with the HTT state, the martensite structure of the WM and HAZ in the IA state became coarser, and both M23C6 and MX tend to grow together. The fracture position of the tensile specimens of as-welded state, HTT state and IA state welded joints were all located on the base metal (BM), and the tensile strength decreased successively, while the yield strength and elongation increased successively. The average impact absorbed energy of the BM, as-welded state, HTT state and IA state welds were 236 J, 42 J, 241 J and 51 J, respectively. The impact performance of HTT state welds surpassed the BM.

Atomistic Study on the Energetic and Mechanical Behaviors of Helium Bubble Nucleation and Growth in RAFM Steel

Helium produced by neutron irradiation is a crucial inducement to bring about the property of deterioration of structural materials served in a fusion reactor. To investigate the nucleation and growth behavior of helium bubbles in reduced activation ferritic/martensitic steels, which comprise one of the most promising candidate structural materials, the Molecular Statics method and the Metropolis Monte Carlo algorithm are combined to investigate the energetic and mechanical behaviors of HenVm clusters in α-Fe. The simulation results show that the vacancy and helium atom binding energy are inclined to reach a saturation state, i.e., 4.0 eV for the vacancy and 2.4 eV for the helium atom; however, the binding energy of self-interstitial atoms decreases to minus values at high helium-to-vacancy (He/V) ratios. The crossover of the binding energy curve of the helium and vacancy indicates that the equilibrium He/V ratio is 1.68 during the nucleation of helium bubbles. Meanwhile, the dissociation energy analysis indicates that the stable He/V ratio of the clusters is 1.3 at high temperatures. Moreover, the pressure analysis of the HenVm clusters indicates that the He/V ratio corresponding to their mechanical equilibrium state varies from 0.50 to 0.65 at 0 K. Furthermore, the analysis combined with the relevant experimental data of helium density in helium bubbles indicates that the actual He/V ratio of helium bubbles in the served materials is closely relevant to the irradiation condition, such as helium production rate, temperature, etc. The investigation results in this paper contribute to elucidate the microscopic process of helium bubble nucleation and growth and provides the energetic and mechanical parameters of small-sized helium bubbles with different sizes for large-scale simulation studies.

Enhanced D retention in RAFM steel caused by D bubbles formed inside Cr-rich surface layer

To identify the cause for an enhanced D retention in RAFM (reduced-activation ferritic/martensitic) steels with decreasing incident D fluence, ϕD, CLAM steel samples were exposed to D plasma at a low ϕD ∼ 3 × 1023 m−2 in the PISCES-A linear plasma device. At this low ϕD, a Cr-rich surface layer with a thickness of ∼ 10 nm still remains, confirmed with cross-sectional EDX (energy dispersive x-ray spectroscopy) elemental mapping. TEM (transmission electron microscopy) observations of a plasma-exposed surface reveal the formation of D bubbles inside the Cr-rich surface layer, while no D bubbles are seen in an unexposed surface. Furthermore, depth profiling using GDOES (glow discharge optical emission spectroscopy) detects D in the Cr-rich surface layer. These results demonstrate that the D retention at low ϕD is enhanced by D atoms trapped in bubbles created inside the Cr-rich surface layer.

Development of RAFM steel for nuclear applications with reduced manganese, silicon and carbon content

In this work, we investigate an alternative route for the production of reduced activation ferritic-martensitic (RAFM) steels with reduced contents of manganese and carbon. Alternation of the chemical composition coupled with various heat treatments as well as quench & rolling procedures are applied in an attempt to reduce the DBTT without altering the nominal strength and ductility. The obtained experimental results on the mechanical properties and microstructural analysis are compared to the reference European RAFM steel i.e. Eurofer97. It is demonstrated that an alternation of the conventional heat treatment of the newly developed low Mn/C steel allows the production of a material with a ductile to brittle transition temperature as low as −133 °C and strength comparable to that of Eurofer97. On the other hand, the performed quench & rolling procedure does not lead to any noticeable reduction of the DBTT, albeit it significantly increases the tensile strength without compromising the ductility in the relevant operational temperature range.

The kinetics of dynamic recrystallization and construction of constitutive modeling of RAFM steel in the hot deformation process

The dynamic recrystallization (DRX) behavior of austenite in reduced activation ferritic/martensitic steel was investigated by isothermal compression tests and constitutive modelling. The flow stress decreases with deformation temperature increasing and strain rate decreasing. The constitutive equations were constructed based on the analysis by Arrhenius equations. Based on the dynamic material model, the hot processing map was constructed. In the stability region, the power dissipation efficiency (η) increases with increasing strain, and the softening is dominated by DRX when η grows to a specific value. However, η decreases as strain increases further after the complete evolution of DRX, indicating coarsening of DRXed grains. As the strain increases, the processing stability region shifts to the low temperature and high strain rate region. A novel recrystallization kinetics model was developed by considering the characteristics of the recrystallization velocity with strain increasing. It can reflect the "slow-rapid-slow" growth trend of recrystallization volume fraction (XDRX). The relationship between XDRX and D¯ (average grain size of austenite) was constructed, which indicates that D¯ decreases with strain increasing after the onset of DRX. When XDRX is greater than 50%, the decrease rate of D¯ slows down until the steady-state is reached. Although strain increase further, the grain size remains constant. This suggests that the expected D¯ can be obtained by controlling XDRX during hot deformation.

Enhancement of mechanical properties by repeated heat treatment in reduced activation ferritic/martensitic steel with Ta and Ti

The effects of heat treatment conditions on the microstructures and mechanical properties of Ta and Ti added reduced activation ferritic/martensitic (TaTi-RAFM) steel were investigated in this study. The authors attempted to control the precipitation of MX particles and M23C6 carbide, which critically affects the mechanical properties of RAFM steels, by repetition of normalizing and tempering, and the resultant change of microstructure and mechanical properties was examined. Double-normalizing was effective for improving the ductile-brittle transition temperature (DBTT) in TaTi-RAFM steel. Moreover, the DBTT was further reduced by double-tempering in double-normalized TaTi-RAFM steel. This improvement of the DBTT by double-normalizing or double-normalizing and double-tempering results from an increased fraction of MX precipitate and refinement of M23C6 carbide, which is confirmed by a quantitative analysis based on more than ten SEM micrographs with high magnification. The tendency of DBTTs in variously heat-treated specimens was consistent with the tendency of fracture strengths calculated using the modified Griffith model with maximum size of M23C6 carbide, which is vulnerable to fracture.

Irradiation hardening and ductility loss of Eurofer97 steel variants after neutron irradiation to ITER-TBM relevant conditions

Ten Eurofer97 steel variants, produced by non-standard fabrication-processing routes and modified alloying chemistries, were studied by neutron irradiations in the high flux isotope reactor. The irradiations were performed to ITER-TBM relevant conditions of ∼255–350 °C, 2.94–3.24 dpa. We quantified the irradiation-induced degradation of the steels using mechanical property tests. All the steels suffered from irradiation hardening, where a significant increase in Vickers microhardness and yield stress (σYS) occurred, accompanied with severe loss of tensile elongation. The extent of hardening was material dependent. For Tirr = 300±30 °C, most steels showed σYS increase in the range of ∼30% to as high as ∼66%, except for a low temperature tempered steel with σYS increase below 15%. Despite large losses in elongation, most failures were ductile. Significant post-necking ductility was retained with reduction in area (RA) between 65–75%, but <50% for low temperature tempered steels. The ultimate tensile stress to yield stress (σUTS/σYS) ratios decreased significantly after irradiation, highlighting irradiation-induced strain hardening capacity reduction. No major effect of irradiation on the plastic instability stress (σPIS) and true fracture stress of the steels was observed. By comparing the tensile stresses in true stress units and with literature, the results suggest that RAFM steel designing should target materials with a large separation between σPIS and σYS, to ensure the materials can maintain large work hardening and uniform deformation capability after irradiation. The tensile data of the steels additionally revealed a compelling evidence of an inverse trend between the change in RA and increase in σYS of the neutron irradiated Eurofer97 type steels.

Power exhaust concepts and divertor designs for Japanese and European DEMO fusion reactors

Concepts of the power exhaust and divertor design have been developed, with a high priority in the pre-conceptual design phase of the Japan–Europe broader approach DEMO design activity (BA DDA). Common critical issues are the large power exhaust and its fraction in the main plasma and divertor by the radiative cooling (P rad tot/P heat ⩾ 0.8). Different exhaust concepts in the main plasma and divertor have been developed for Japanese (JA) and European (EU) DEMOs. JA proposed a conventional closed divertor geometry to challenge large P sep/R p handling of 30–35 MW m−1 in order to maintain the radiation fraction in the main plasma at the ITER-level (f rad main = P rad main/P heat ∼ 0.4) and higher plasma performance. EU challenged both increasing f rad main to ∼0.65 and handling the ITER-level P sep/R p in the open divertor geometry. Power exhaust simulations have been performed by SONIC (JA) and SOLPS5.1 (EU) with corresponding P sep = 250–300 MW and 150–200 MW, respectively. Both results showed that large divertor radiation fraction (P rad div/P sep ⩾ 0.8) was required to reduce both peak q target (⩽10 MW m−2) and T e,i div. In addition, the JA divertor performance with EU-reference P sep of 150 MW showed benefit of the closed geometry to reduce the peak q target and T e,i div near the separatrix, and to produce the partial detachment. Integrated designs of the water cooled divertor target, cassette and coolant pipe routing have been developed in both EU and JA, based on the tungsten (W) monoblock concept with Cu-alloy pipe. For year-long operation, DEMO-specific risks such as radiation embrittlement of Cu-interlayers and Cu-alloy cooling pipe were recognized, and both foresee higher water temperature (130 °C–200 °C) compared to that for ITER. At the same time, several improved technologies of high heat flux components have been developed in EU, and different heat sink design, i.e. Cu-alloy cooling pipes for targets and RAFM steel ones for the baffle, dome and cassette, was proposed in JA. The two approaches provide important case-studies of the DEMO divertor, and will significantly contribute to both DEMO designs.

A Joining Process between Beryllium and Reduced-Activation Ferritic-Martensitic Steel by Plasma Sintering.

To investigate the growth kinetics of the reaction layer and mechanical strength of joined materials, we joined beryllium and reduced-activation ferritic–martensitic steel (F82H) by plasma sintering under various conditions and characterized the joined region. Scanning electron microscopy revealed that the thickness of the reaction layer increased with an increase in the joining time and temperature. Line analyses and elemental mapping using an electron microprobe analyser showed that the reaction layer consists of Be–Fe intermetallic compounds, including Be12Fe, Be5Fe, and Be2Fe, with small amounts of chromium and tungsten. Owing to the time and temperature dependence of the reaction-layer thickness, the layer growth of Be–Fe intermetallic compounds obeys the parabolic law, and the activation energy for the reaction-layer growth was 116.2 kJ/mol. The bonding strengths of the joined materials varied inversely with the thickness of the reaction layer.

Dislocation loop coarsening and shape evolution upon annealing neutron-irradiated RAFM steel

Neutron-irradiation induced prismatic dislocation loops contribute to the degradation of materials’ mechanical properties. Recent post-irradiation annealing (PIA) investigations on neutron-irradiated reduced-activation ferritic-martensitic (RAFM) steel “EUROFER97” have revealed an almost complete recovery of the mechanical properties. To identify underlying recovery mechanisms, thick-foil annealing experiments on neutron-irradiated EUROFER97 were carried out. It is observed that PIA at 550 °C results in a reduction of initial dislocation loops density with a simultaneous increase in their mean size. This is linked to the dislocation loops shrinkage/annihilation as well as their coarsening/merging phenomena. Moreover, a clear coarsening of the quasi-circular 〈100〉 loops into their rectilinear versions and simultaneous shrinkage/annihilation of the ½<111> loops/clusters are observed. The coarsening phenomenon is a non-conservative climb process, which is discussed in terms of the movement of jogs whilst considering the change in the iron's elastic anisotropy with temperature and loops lowest energy directions. Furthermore, the observed interaction of the coarsened 〈100〉 loops and thereafter their merging to form large loops of various geometries are presented and discussed. Lastly, a rationalization justifying pronounced microstructural transition from dislocation loops dominated to dislocation tangles dominated microstructure with continued annealing is presented. These thick-foil PIA results complement our previous works and provide complete description of the dislocation loops annealing behaviour in neutron-irradiated EUROFER97.

In-situ TEM investigations of dislocation loop annealing kinetics in neutron-irradiated 9%Cr RAFM steel

Post-irradiation annealing (PIA) is beneficial in recovering degraded mechanical properties of irradiated materials due to the evolution of irradiation-induced defects. To understand the underlying responsible mechanisms, isothermal PIA experiments were carried out in this work on a neutron-irradiated 9%Cr reduced-activation ferritic-martensitic steel (EUROFER97) using in-situ transmission electron microscopy. In general, PIA at 600 °C led to an appreciable reduction of the dislocation loop density and increment in the loops mean size. This is found mainly associated with the vacancy-mediated loops shrinkage phenomenon. The driving force for this process is loop's curvature force. Nevertheless, the spontaneous evolution of the loops neighboring microstructure substantially alters their annealing kinetics. Therefore, in this study, loops’ annealing kinetics in EUROFER97 is discussed in terms of various scenarios originating in terms of the presence or absence of the local external sources and sinks. In addition, influence of loops type, alloying elements segregation and free surface are also discussed.

Microstructure and high-temperature mechanical properties of RAFM steel processed by friction stir processing combined with tempering heat treatment

In this study, the microstructure and high-temperature mechanical properties of a 9Cr-1.5W reduced activation ferritic/martensitic (RAFM) steel processed by friction stir processing (FSP) combined with a tempering treatment were investigated. During the FSP, lath martensite microstructures containing needle-like M3C and nano-sized MX precipitates were formed in the steel owing to the severe plastic deformation and high cooling rate. The steel subjected to FSP and the tempering treatment at 700 °C for 60 min (FSP-T60) had a fine-grained microstructure consisting of ferrite and martensite, and a large number of M23C6 precipitates formed at the grain boundaries and within the grains. The FSP-T60 steel showed an increase of ∼100 MPa in the tensile strength at 600 °C and a high elongation of 26% as compared to the steel prepared via traditional hot rolling, normalizing, and tempering. The FSP-T60 steel also exhibited improved creep resistance with a creep rupture time of 4523 h at 600 °C/210 MPa. The increase in the strength and ductility of the steel can be attributed to its refined grain structure and the high density and dispersion of the M23C6 precipitates. Meanwhile, the homogeneous distribution and refinement of M23C6 carbides could reduce the growth rate of the Laves phase during creep, thus extending the formation time of the creep cavities and cracks and further increasing the creep rupture time of the steel.

The dislocation-based fatigue deformation mechanism of a RAFM steel under multi-axial loadings

Reduced Activation Ferritic/Martensitic (RAFM) steel is a candidate blanket material for a fusion reactor, which should have outstanding performance under extreme environments containing complex thermal and stress damage during operation. To date, the properties under a single-axial loading type of tensile, creep and fatigue have been extensively reported, while the multi-axial loadings were rarely studied. Here, the multi-axial fatigue behaviors of a RAFM steel were investigated in an axial-torsional cyclic tester with controlled strain and different loading paths. The results show that the fatigue behavior of the RAFM is strongly determined by the shear-stress condition, where the lifetime under the pure torsional loading is only 16% of the uniaxial cyclic loading one. The fatigue life of the specimens under multi-axial loadings with a phase angle of 0° or 180° are comparable to that of uniaxial tension-compression one. However, it was decreased by ∼28% as the loading angle changed to 90° with a circular path. Besides, we found the fatigue softening under the proportional multi-axial loading while additional hardening in the non-proportionally multi-axial cyclic loading related to the dislocation activities during loadings. Under the proportional-multiaxial loading and uniaxial tension-compression loading and torsion loading, the high density of dislocation was rearranged and annihilated by the cyclic shear stress, while the tempered martensite lath structure was mainly retained. However, under the non-proportional multi-axial loading, dislocations transform the lath structure into a finer cell structure, improving the resistance upon fatigue deformation.