Reduced Activation Ferritic/martensitic Research Articles

Irradiation Effects on Tensile Properties of Reduced Activation Ferritic/Martensitic Steel: A Micromechanical-Damage-Model-Based Numerical Investigation

The tensile properties of reduced activation ferritic/martensitic (RAFM) steels are significantly influenced by neutron irradiation. Here, a mechanism-based model taking account of the typical ductile damage process of void nucleation, growth, and coalescence was used to study the temperature and irradiation effects. The elastic–plastic response of RAFM steels irradiated up to 20 dpa was investigated by applying the GTN model coupled with different work hardening models. Through a numerical study of tensile curves, the GTN parameters were identified reasonably and satisfying simulation results were obtained. A combination of Swift law and Voce law was used to define the flow behavior of irradiated RAFM steels. The deformation localization could be adjusted effectively via setting the nucleation parameter εn close to the strain where necking occurs. Because εn changed with uniform elongation, εn decreased with the testing temperature and rose with an irradiation temperature above 300 °C. The nucleation parameter fn increased with the testing temperature for RAFM steels before irradiation. For irradiated RAFM steels, fn barely changed when the irradiation temperature was below 300 °C and then it rose at a higher irradiation temperature. Meanwhile, the ultimate strength of the simulated and experimental curves showed good agreement, indicating that this method can be applied to engineering design.

Low cycle fatigue of EUROFER97 investigated for test blanket modules of ITER: An interlaboratory study

Several reduced-activation ferritic-martensitic (RAFM) steel grades have been proposed for structural applications in fusion reactors since the 1990s by several countries. The European reference RAFM steel EUROFER97 has been produced in four batches since 1998. It is intended for fabrication of Test Blanket Modules (TBM) for ITER as well as the blanket first wall and divertor cassettes of DEMO.RCC-MRx is the design code developed for high-temperature, research and fusion reactors by AFCEN. Its latest edition contains a provisional section dedicated to EUROFER97, aggregating properties of the first two batches, whereas a full qualification and final inclusion of a material requires three batches minimum. The completion of the code is required for the design assessment of TBM units in the frame of the nuclear licensing process.The EUROfusion project coordinates efforts to broaden the knowledge of EUROFER97 properties, which will be used for closing the database gaps in RCC-MRx. Low cycle fatigue (LCF) tests are a significant part of the EUROfusion test programme, since the material fatigue properties are indispensable for the component assessment owing to the cyclic nature of a tokamak operation.In this work we report the results of interlaboratory LCF tests on EUROFER97 batch 3 performed in a temperature range relevant for its application in ITER (RT–550 °C), wide selection of total strain ranges (0.3–1.5 %) and two values of strain ratio Rε (0 and –1). Along with the estimated fatigue life and cyclic behaviour, a fractographic analysis done with scanning electron microscopy (SEM) is presented.

Characteristics of oxide-dispersion strengthened alloys produced by high-temperature severe deformation

This study is to explore an economically attractive and technically feasible processing method for oxide-nanoparticle strengthened alloys for fusion reactor application. Despite many scientific merits of the advanced oxide-dispersion strengthened (ODS) alloys, such as the nanostructured ferritic alloy (NFA) 14YWT, the only viable production path for a high-quality NFA is the high-power mechanical alloying process. This process is often a multi-day high-speed ball milling of alloy powder with a small quantity of yttria (Y2O3) powder, followed by the milled-powder consolidation using extrusion or other methods and additional thermomechanical processing (TMP) for property control. This complex production path, including the low-temperature mechanical alloying in particular, has limited technical advancement toward the cost-effective and scale-up production of ODS alloy components. To overcome such a practical limitation, we proposed to explore alternative low-cost processing routes using traditional thermomechanical processing (TMP) method only. A series of continuous TMP cycles, which were designed to impose high-temperature severe plastic deformation (HT-SPD) conditions to the consolidated powder mixtures, were applied to achieve the effective distribution of oxide particles in nanograin structure and thus desirable mechanical properties. Since the reduced-activation ferritic-martensitic (RAFM) alloy powders (Fe-10Cr and Fe-14Cr alloys with various Y contents) are available in our inventory, we focused to utilize the new solid-state synthesis approach for controlling oxide (oxygen source) dissolution and nanoscale clustering in nanograin structure in those alloys. A combination of powder consolidation at 900 °C and continuous thermomechanical activation at 600 °C yielded two essential ODS alloy microstructure contents–nanograin structure and nanoparticle distribution–and thus demonstrated a good combination of strength and ductility.

Superior radiation resistance of ODS-RAFM steels revealed by Positron annihilation spectroscopy study

Oxide-dispersion-strengthened (ODS) reduced-activation ferritic/martensitic (RAFM) steels are considered as most promising structural materials for fusion reactors and other advanced nuclear systems due to its excellent irradiation resistance under high dose irradiations. However, the extreme complex synergistic effect between transmuted helium/hydrogen and displacement damage which most seriously affects the structure and mechanical properties of ODS-RAFM steels is very difficult to understand, especially the role of hydrogen still remains unclear. Taking advantage of very sensitive to small vacancy clusters of Positron annihilation spectroscopy (PAS) method, we conducted irradiation defect study on ODS-RAFM, RAFM and its corresponding model alloy Fe–9Cr and α-Fe to investigate their anti-swelling properties at the early stage of irradiation with hydrogen-helium synergism and the mechanism. Three distinct irradiation schemes including (1) single Fe ion beam, (2) simultaneous Fe and He (named Fe + He) ion beam, (3) subsequent H ion injection after Fe + He (named Fe + He/H) ion beam irradiation were performed. Hydrogen-helium synergistic effects on vacancy evolution within these steels were explored combining the first-principles calculation. The implantation of He was observed to significantly increase defect concentration among all four steels, while H exhibited interestingly complexity on affecting the evolution of helium bubbles. The H post-injection increased defect concentration obviously in the materials with abundant sinks, but improved poorly in ones with rare sinks. Through the systematic study of the interactions between H, He, and vacancies, it was revealed that even at the early irradiation stage, ODS-RAFM steels has already exhibited excellent irradiation resistance compared to other alloys across three irradiation schemes.

Hydrogen isotope permeation and retention behavior in the RAFM steel manufactured by laser powder bed fusion

Laser powder bed fusion (LPBF) has emerged as a promising additive manufacturing technique for producing complex components across various industries. This method is particularly instrumental in fabricating reduced activation ferritic/martensitic (RAFM) steel, a candidate structural material for the tritium breeding blanket. Hydrogen transport behaviors in the conventional RAFM steel have been largely investigated; however, they are not well understood in the LPBF-RAFM steel. In this work, we studied the hydrogen isotope permeation and retention behavior in the RAFM steel fabricated by LPBF, utilizing the deuterium gas driven permeation and thermal desorption spectroscopy experiments. The results show that the hydrogen isotope permeability of the LPBF-RAFM steel is close to the conventionally fabricated RAFM steel, but the diffusion coefficient is greatly lower and the retention is markedly higher in the LPBF-RAFM steel compared to the conventional RAFM steel. The large number density of microvoids and TiC precipitates in the LPBF-RAFM steel could be the predominant factors that result in the decrease in hydrogen isotope diffusion coefficient and the increase in the total amount of deuterium retention.

Microstructure and thermal stability of a structurally graded tungsten and reduced activation ferritic/martensitic steel joint

The leading design for plasma-facing high heat flux components in proposed fusion reactors involves joining plasma-facing tungsten tiles to underlying reduced activation ferritic-martensitic (RAFM) steel structures. Due to significant differences in the physical properties between W and steel, an effective method for joining them while preserving the mechanical and microstructural properties under service thermo-mechanical load is yet to be demonstrated. A transitional multilayer consisting of three layers (VCrTi, VCrAl, and FeCrAl) between W and steel is designed with the help of thermodynamic simulation and diffusion kinetics in an attempt to form solid solution bonding without forming brittle intermetallic phases. The solid solution bonding is desirable to mitigate the drastically different thermal expansion between W and steels. The multilayer structure was fabricated using spark plasma sintering (SPS). The microstructure of the bonded transition layers was analyzed after long-term annealing at 620 °C up to 1000h, through scanning electron microscope, energy dispersive spectroscopy and electron backscattering diffraction. Interdiffusion kinetics between layers and reliability of mobility database were also analyzed. This work provides a good understanding of thermal stability as well as the efficacy of multilayer functionally graded transition layer for joining W to ferritic/martensitic steel.

Hot compression bonding of a 9Cr oxide dispersion strengthened alloy and a 9Cr reduced-activation ferritic/martensitic alloy

An innovative method of hot compression bonding is proposed in this work for the joining of 9Cr oxide dispersion strengthened (ODS) alloy and 9Cr reduced-activation ferritic/martensitic (RAFM) alloy. The microstructural evolution of the bonding interface was investigated by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM). The results verify that the pinning effect of nano-oxides particles (NPs) in 9Cr ODS alloy significantly enhances its dynamic recrystallization (DRX) temperature and deformation resistance. Continuous DRX (CDRX) first occurred on the 9Cr RAFM alloy side, and the areas near the bonding interface were composed of recrystallized grains. With increasing strain, CDRX also showed up on the 9Cr ODS alloy side. Inevitable slight oxidation occurred at the bonding interface during the hot compression bonding (HCB) process, and the interfacial oxides transformed from initial coarse CrO to TiO and finally to Y-Ti-O nanoparticles with sizes comparable to pre-existing NPs dispersed in the 9Cr ODS alloy matrix. It is believed that interfacial oxide transformation and grain structure consistency contributed to the excellent interface healing of the two dissimilar alloy pieces. The effectiveness of the bonding was tested by tensile tests and fractography analysis, revealing that ideal metallurgical bonding could be achieved under a controlled strain level of 10 % at 800 °C followed by soaking at 1000 °C for 4 h.

Enhanced disperse Ta(C, N) precipitation in reduced activation ferritic/martensitic steels by optimized two-step austenitizing

High-density MX precipitates block the movement of dislocations during creep at high temperature and increase sink strength to point defects under irradiation. Therefore, the precipitation control of MX phase is crucial for the advancement of reduced activation ferritic/martensitic (RAFM) steels. In this paper, an optimized two-step austenitizing strategy was proposed to enhance Ta(C, N) precipitation in RAFM steels, and the temperature effects of the first and second austenitizing on the Ta(C, N) precipitation enhancement were studied. During the first austenitizing, Ta(C, N) particles with a small radius (∼12 nm) and a high number density (∼8.5 × 1019 m−3) precipitated by heterogeneous nucleation on the M23C6 phase boundaries retained by the comparatively low temperature (900 °C). Then in the second austenitizing, enhanced Ta(C, N) precipitates kept good size and thermal stability below 1050 °C. Compared to RAFM steels treated by conventional normalizing and tempering, the strength and creep performance were significantly improved by the pinning effect of the enhanced dispersed Ta(C, N) particles. Furthermore, the optimized two-step austenitizing strategy was validated in the other two RAFM steels, showing broad application prospects in RAFM and heat-resistant steels.

Impact of trace silicon on irradiation hardening and embrittlement of RAFM steel subjected to neutron irradiation

The content of silicon (Si) element in reduced activation ferritic/martensitic (RAFM) steels varies significantly among different candidates of fusion reactor structural materials. Si is not intentionally added to the RAFM steel but rather remains as a residue during the deoxidation process in steel fabrication. To control and reduce the Si content, removing processes and special treatments are required. The additional refining processes significantly increase the cost of steel fabrication. Currently, the role of residual or trace Si in neutron irradiation of steel remains unclear. In the presented study, two 9Cr-2 W RAFM steels with Si contents of 0.1 wt.% and 0.01 wt.% were fabricated and subjected to a neutron irradiation at 563 K with a neutron fluence of 4.8 × 1023m−2. The main results show that 0.1 wt.% Si can contribute to the suppression of irradiation hardening and embrittlement, as evidenced by the reduction in irradiation-increased hardness and yield strength, while maintaining preferable elongation. Microscopic analysis suggests that an appropriate content of Si helps refine the grains in RAFM steel, increases the ductile fracture region of the fracture surface, and effectively reduces the density of dislocation loops induced by neutron irradiation. The findings evidence that retaining a certain amount of trace Si should be an informed and rational choice in the design of RAFM steel.

Microstructure refinement and enhanced mechanical properties of wire arc additively manufactured RAFM steel via post heat treatments

Reduced activation ferritic/martensitic (RAFM) steel with high strength and neutron radiation damage resistance is deemed one of the promising structural materials for fusion blankets. In this study, RAFM steel tube was prepared through wire arc additive manufacturing, and the evolution of phase composition, microstructural features and mechanical properties during the as-built and heat treatment processes were investigated. The results show that the as-built state is predominantly composed of α-Fe, and the microstructure's morphology from bottom to top consists of alternating columnar grain regions, equiaxed coarse grain regions, and fine grain regions. Normalizing and tempering treatment eliminates the heterogeneous microstructure, resulting in the presence of M23C6 carbides enriched with Cr and W, as well as MX phases enriched with Ta and V. Partitioning heat treatment results in the formation of ultrafine martensitic, quenched martensite and bainitic microstructures. The mechanical properties of wire arc additively manufactured RAFM steel are influenced by variations in microstructure. The coarse-grained region formed in the as-built state represent weak links. Normalizing and tempering treatment reduces the strength while improving ductility. Partitioning heat-treated RAFM steel exhibited a high strength of 1.1 GPa and an elongation of approximately 10.9%. This study provides a method of combining additive manufacturing with heat treatment to enhance the strength and ductility of RAFM steel simultaneously.

Experimental determination of the surface rate constants of protium and deuterium in Eurofer97

The EUROFER reduced activation ferritic-martensitic (RAFM) steel is meant to be a firm candidate as structural material to be part of future fusion reactors. The correct definition of the transport parameters of hydrogen isotopes in these materials turns out to be a key issue in this field, as long as it can affect the operation of the reactor itself when quantifying the fuel inventory as well as the radiological safety of the whole facility.Traditionally the diffusion limited regime (DLR) of these kind of RAFM steels has been experimentally tested and values for the permeability and diffusivity have been published, showing some differences depending on the specific composition of the steel, the thermal treatment during the manufacturing processes, etc. However, these RAFM steels will have to operate also under conditions of pressure and temperature that will lead to surface limited regimes (SLR). In this case, there is much less information available and data for the adsorption and recombination constants is not so extensive.This work shows the results for a new set of experimental permeation measurements carried out within the EUROfusion program in order to determine the SLR. The surface rate constants of protium and deuterium were experimentally obtained for Eurofer97-3 at two different facilities (PermRIG at UPV/EHU and Thermoperm at CIEMAT), with consistent results that show a good agreement.

Deuterium retention and ammonia production from D-implanted 316 L stainless steel: insights for future fusion reactors

We present a systematic study that quantifies deuterium (D) retention and ammonia (ND3) production from 316 L stainless steel (SS316L) following the implantation of D ions in conditions similar to the ones expected in the ITER tokamak, i.e. with kinetic energy below 300 eV. Using Temperature Programmed Desorption (TPD) after deuterium ion implantation at 250 eV/D, we show that deuterium retention increases linearly with the D fluence up to 1021 D+m−2, with a retention probability of 18%. For higher D fluence, deuterium retention increases sub-linearly. Analysis of the TPD spectra evolution with varying storage time in vacuum after D implantation, shows that D retention is influenced by D diffusion into the bulk of SS316L. Subsequent to D ion implantation, we evidence the efficient production of ND3 molecules during TPD, between 400 K and 750 K, from the nitrogen present naturally in SS316L. Up to 21% of the D release during TPD can be found in ND3 molecules, indeed. The fraction of ND3 in the total D release depends both on the D ion fluence and the nitrogen concentration profile in the bulk. At least 7% of the D release is found in the form of ND3 molecules, even at a fluence of 2 × 1021 D+m−2 and for a natural N concentration bulk profile. Both N diffusion and D diffusion into the bulk appear to dictate the kinetics of ND3 production. Our findings of efficient production of ND3 in D-implanted austenitic 316 L stainless steel underline the need for similar studies on reduced-activation ferritic/martensitic (RAFM) steels that contain similar content of nitrogen and will be used in fusion reactor prototypes.

Annealing of hydrogen trap sites in displacement-damaged EUROFER

For future fusion power plants, the reduced activation ferritic/martensitic (RAFM) steel EUROFER is considered to be the primary choice as a structural material for plasma facing components (PFC) of the first wall, such as the blanket modules. Its exposure to high energy fusion neutrons will lead to creation of radiation-induced defects, which could act as trapping sites for hydrogen isotopes (HI) and consequently, strongly affect HI diffusion and retention. For predictive simulations of HI retention, the concentration of these trap sites needs to be known as a function of the temperature. In this work, EUROFER samples are irradiated by 20 MeV W-ions at 290 K to create displacement damage. Subsequently, they are exposed to deuterium (D) plasma at 370 K to decorate the created defects. Partial thermal desorption spectroscopy (TDS) is conducted in 50 K steps up to 620 K. D depth profiles are determined by nuclear reaction analysis (NRA) after each partial-TDS step. Following this analysis, the defects are again decorated with D and the D depth profiles are measured again by NRA to monitor the evolution of the concentration of trap sites. Most of the D is lost already at 470 K due to thermal de-trapping. Full recovery of the displacement damage is reached at 620 K.

Diffusion bonding of RAFM steels: Evolution of interfacial oxide layer with pressure and microstructure and mechanical property after post bonding heat treatment

The effect of elevating the bonding pressure from 10 to 20MPa(5MPainterval)at 1050?C for 60 minutes on the diffusion bonded joint of reduced activation ferritic/martensitic(RAFM)steel was studied. The results indicate that as the bonding pressure increases, the joint quality correspondingly improves. The oxide layer at the bonding interface underwent an evolution process from continuous to discontinuous and ultimately disappeared with the increase of bonding pressure. The optimal joint was achieved at the pressure of 20 MPa in this work. Considering the exploration of joint reliability in engineering applications, the optimal joint was subjected to a post bonding heat treatment(PBHT) of 750?C for 90 minutes. The diffusion bonded sample subjected to PBHT displays microstructural characteristics and tensile properties similar to those of the base metal. Notably, tensile fracture does not occur at the bonding interface, but in the base metal far from the interface. This work can provide a feasible method for the removal of oxides at the diffusion bonding interface of RAFM steel, and also contribute to the establishing and optimization of diffusion bonding processes, thereby improving the quality of diffusion bonded joint.

Comparison of electron-beam-welded joints manufactured by rolled and selective laser melted reduced activation ferritic/martensitic steel

As the core component of China Fusion Engineering Test Reactor (CFETR), the forming and manufacturing of the first wall faces severe challenges due to the constraints of materials, service environment and geometric structure. The microstructure and mechanical properties of welded joints obtained by rolled and laser selective melted (SLMed) reduced activation ferritic/martensitic (RAFM) steel were investigated in depth. The microstructure analysis results showed that the weld metal (WM) was composed of coarse lath martensite and δ-ferrite, while the microstructure were mainly coarse grains in heat affected zone (HAZ) which was easy to break. The average grains size of SLMed RAFM steel (S-RAFMs) was smaller than that of rolled RAFM steel (R-RAFMs), since the rapid melting and solidification of laser in the additive manufacturing process prevented grains to grow. The microstructure of R-RAFMs joint was relatively more uniform and there was no obvious texture and preferred orientation, while the WM of S-RAFMs joint had a weak cubic texture. The results of mechanical properties showed that the S-RAFMs joints exhibited higher strength (UTS = 697.46 MPa) and the R-RAFMs joints exhibited better plasticity (EL = 13.49 %) under the same tensile conditions. The tensile and impact fracture morphologies showed that the fracture mechanism of two joints was ductile-brittle mixed fracture, and the dimples of R-RAFMs joints were finer and denser. It was clearly observed that the hardness of WM (∼390–430 HV0.3) was higher than that of the base metal (BM) (∼190–230 HV0.3) from the hardness distributions along the width directions. The average hardness of BM in R-RAFMs joint (227 HV0.3) was lower than S-RAFMs joint (197.3 HV0.3), which was mainly related to the fine grain strengthening effect during SLM forming. The above results may provide theoretical guidance for the high-performance manufacturing of the first wall and promote the application of SLM and EBW technology in the field of nuclear fusion.

Nano-indentation deformation behavior of heavy ion irradiated reduced activation ferritic/martensitic (RAFM) steels

The Nano-indentation deformation behavior of the heavy ion irradiated RAFMs is investigated in the present work. Specimens of the CLF-1 steel were irradiated with 123.4 MeV 20Ne ion at both -50 °C and 400 °C, respectively, to achieve a mean displacement damage level of 0.15 dpa. A quasi-uniform distribution of the displacement damage was produced in the specimens by utilizing an energy degrader at the irradiation chamber. The results of nano-indentation experiments indicate that during the loading stage, the irradiated materials exhibit a lower strain rate, but during the holding stage after reaching the maximum load, the irradiated materials show a higher strain rate than the un-irradiated material. Transmission electron microscopy observation showed that the low-temperature irradiation produced a high density of irradiation-induced dislocation loops, which impeded the glide of dislocation lines during the loading stage. However, as the load increases gradually to the holding stage, the dislocation lines can overcome the pinning by the dislocation loops. The strain that did not occur due to the pinned effect of the dislocation loops in the loading stage was released during the holding stage, resulting in a higher strain rate in the irradiated specimen during the holding stage. Based on the Peierls mechanism, a quantitative relationship between the number density of the irradiation-induced dislocation loops and the strain rate during the holding stage was established. The calculated results agree well with the experimentally obtained strain rate during the holding stage.

Thermal hydraulic optimization on the First Wall of COOL blanket for CFETR

The supercritical carbon dioxide (S-CO2) cOoled Lithium-Lead (COOL) blanket is under development for China Fusion Engineering and Test Reactor (CFETR). The S-CO2 is adopted to cool the structural components, in which the key component First Wall (FW) dominates the total heat power due to the large plasma facing heat flux. In this paper, referring to the response of material temperature, coolant velocity and pressure drop, the thermal hydraulic optimization is firstly performed on the structure design (i.e. cross area and pitch of channel) using the single variable control method, and the optimization principle is that the outlet temperature remains as the same when the optimization variables are changed. Finally, the optimal structure design of FW is obtained, namely cross area of 12 × 12 mm2 and channel pitch of 5 mm. On basis of this, the comparative analyses are carried out using the different cooling system, structural material, i.e. Reduced Activation Ferritic Martensitic (RAFM) steel and Oxide dispersion-strengthened alloy (ODS) ferritic steel, as well as helium as coolant. Furthermore, the satisfied designs are obtained by referring to the constraints of coolant pressure drop and maximum temperature on the structural steel. Then, the thermal hydraulic designs of FW are continuously matching with the Power Conversion System (PCS) to ensure that the heat power removed by the S-CO2 on the primary side can be transferred to the secondary side. Finally, the achievable thermodynamic design are obtained, including the parameters of high operating pressure, turbine inlet temperature, split ratio as well as the thermoelectricity conversion efficiency. The results show that the high pressure of PCS should be larger than 25 MPa when the RAFM steel is used as the structure material, due to the limitation on the outlet temperature of FW. On the contrary, the ODS can raise the outlet temperature, which is beneficial for matching with the PCS, and the thermoelectricity conversion efficiency can reach up to 47.4 % at the high pressure of 25 MPa. The analyses results will provide useful guidance on the system design both for the Primary Heat Transfer System (PHTS) and PCS of CFETR.

Improving thermal conductivity of reduced-activation ferritic/martensitic steel using three-dimensional woven copper fiber

Reduced-activation ferritic/martensitic (RAFM) steel is a primary candidate for blanket modules in nuclear-fusion reactors. Blanket modules transfer thermal energy generated by nuclear-fusion reactions to the coolant; therefore, blanket materials must have high thermal conductivity. RAFM steel is suitable for fusion reactors owing to its high strength, toughness, and resistance to neutron irradiation; however, its thermal conductivity is lower than that of other metals. This study fabricates RAFM steel–copper composites using three-dimensional (3D) woven copper fibers with different volume fractions as a preform to improve the thermal conductivity of RAFM steel around the coolant pipes. The microstructural, mechanical, and thermal properties of the developed composites and the correlations between them are experimentally investigated. Finite element simulations were also conducted on the composites comprising 3D woven copper fibers to evaluate the effective thermal conductivity. The factors that improve the effective thermal conductivity of RAFM steel–copper composites are discussed through a comparison with copper-powder composites with comparable volumes. This study demonstrates the manufacturing of composites with excellent thermal and mechanical properties using various 3D woven structures as preforms.

Effect of nitrogen on microstructural evolution and mechanical properties of low carbon reduced activation ferritic/martensitic steel with boron microalloying

In this work, the microstructural evolution and mechanical properties of 0.01 wt% boron microalloying low carbon reduced activation ferritic/martensitic (RAFM) steel with various nitrogen content were studied. The results show that nitrogen could greatly reduce the dimension and volume fraction (fV) of M23C6 carbides and increase the fV of MX carbonitrides. Fine BN inclusions with the dimension of ∼200 nm could form with the addition of 0.01 wt% nitrogen, while clustered BN inclusions with the dimension of ∼6 μm could be found by 0.02 wt% nitrogen added. The higher nitrogen could result in the higher fV and number density of MX carbonitrides, which contribute to the higher dislocation density and dislocation strengthening. However, the strength and toughness of RAFM steel deteriorated by clustered BN inclusions.

Microstructure and mechanical properties of multi-component nano oxide-dispersion strengthened RAFM steel fabricated via laser powder bed fusion

Y4Zr3O12 multi-component nanoparticles prepared by sol–gel method were adsorbed on the surface of gas-atomized reduced activation ferritic/martensitic (RAFM) steel powder. This mixture powder was then utilized in the fabrication of oxide-dispersion strengthened (ODS) RAFM steel through laser powder bed fusion. The microstructure of the ODS RAFM steel comprised a combination of columnar ferrite grains and martensitic laths. The Y4Zr3O12 nanoparticles were homogeneously distributed in the matrix, with an average size of 36.3 nm and a high density of 2.49 × 1020/m3. The results of the mechanical properties tests revealed that the additively manufactured ODS RAFM steel exhibited a good balance of strength and elongation.