Research on splitting phenomenon of isochronal martensitic transformation in T91 ferritic steel

T91 steel is a representative type of ferritic heat-resistant steel currently used in power plant components, and is a potential candidate for structural steel in nuclear reactors. The isochronal martensitic transformation behaviors during continuous cooling after austenitization in T91 ferritic steel were systematically investigated by high-resolution dilatometry and microstructure observation. The splitting phenomenon of martensitic transformation is accompanied with the precipitation of needle-like M₃C particles, which is suppressed by rapid cooling after austenitization. The appearance of this splitting is ascribed to the concentration gradient caused by the consumption of alloy element in process of the formation of M₃C. This concentration gradient results in the appearance of wide martensitic laths ahead of the generation of normally narrow laths. These two types of martensitic laths possess different M _s (martensitic start transformation) temperatures, which are attributed to the splitting transformation phenomenon.     

Mössbauer study of martensitic transformation in Xe ion-irradiated type 304 austenitic stainless steel

Foil specimens of type 304 stainless steel have been irradiated with Xe⁺ ions in the range of 100–400 keV and 1×10²⁰–1×10²¹ ions/m² to elucidate the dynamics of the ion-induced martensitic phase transformation in stainless steel. It has been clearly shown by depth selective conversion electron Mössbauer spectroscopy (DCEMS) that the ion-induced martensitic phase in type 304 stainless steel has grown from the surface to a depth dependent both on the ion energy and the fluence of the Xe⁺ ions. Especially, we observed by means of DCEMS that the extension of the martensitic phase into the interior of stainless steel has been induced with increasing ion energy. It is concluded from these results that the depth distribution of the ion-induced martensitic phase is stress-induced by the formation of the highly pressurized Xe⁺ inclusion in type 304 stainless steel.     

Dislocation density crystalline plasticity modeling of lath martensitic microstructures in steel alloys

A three-dimensional multiple-slip dislocation density-based crystalline formulation, specialized finite-element formulations and Voronoi tessellations adapted to martensitic orientations were used to investigate large strain inelastic deformation modes and dislocation density evolution in martensitic microstructures. The formulation is based on accounting for variant morphologies and orientations, retained austenite and initial dislocation densities that are uniquely inherent to martensitic microstructures. The effects of parent austenite orientation and retained austenite were also investigated for heterogeneous fcc/bcc crystalline structures. Furthermore, the formulation was used to investigate microstructures mapped directly from SEM/EBSD images of martensitic steel alloys. The analysis indicates that variant morphology and orientations have a direct effect on dislocation density accumulation and inelastic localization in martensitic microstructures, and that lath directions, orientations and arrangements are critical characteristics of high strength martensitic deformation and behavior.     

Special features of the localized plastic deformation and fracture of high-chromium steel of the martensitic class

The deformation behavior of high-chromium steel (0.4%C–0.6%Si–0.55%Mn–12.5%Cr) of martensitic structure upon quenching and of sorbitic structure upon high-temperature tempering has been investigated. Each of the states is shown to be represented by a particular stress-strain curve. The stress-strain curve for the steel in the martensitic state consists of a single linear-hardening stage, whereas in the sorbitic state, it exhibits a three-stage deformation pattern. The plastic flow of the examined material in the two states has been found to be of a localized character. The evolution of localized-strain center distributions follows the law of plastic flow, i.e., it depends on the deformation stages in the stress-strain curve. The fracture process is determined by the kinetics of the localized-strain centers in the final (prefracture) deformation stage in the stress-strain curve.     

Cavity swelling of RAFM steel under 792 MeV Ar-ions irradiation at 773 K

China reduced-activation ferritic/martensitic steel is irradiated at 773 K with 792 MeV Ar-ions to fluences of 2.3×10²⁰ and 4.6× 10²⁰ ions/m², respectively. The variation of the microstructures of the Reduced-activation ferritic/martensitic (RAFM) steel samples with the Ar-ion penetration depth is investigated using a transmission electron microscope (TEM). From analyses of the microstructure changes along with the Ar-ions penetrating depth, it is found that high-density cavities form in the peak damage region. The average size and the number density of the cavities depend strongly on the damage level and Ar-atom concentration. Swelling due to the formation of cavities increases significantly with an increased damage level, and the existence of deposited Ar-atoms also enhances the growth of the average size of the cavities. The effect of atom displacements and Ar-atoms on the swelling of the RAFM steel under high energy Ar-ion irradiation is discussed briefly.     

Orientation relationship in Eurofer martensitic steels

Both TEM and SEM/EBSD orientation measurements are carried out on a Eurofer97 martensitic steel. The influence of the prior austenitic grain size is studied using dedicated heat treatments. The intra laths misorientation is estimated by TEM. SEM/EBSD orientation mapping enable to study the actual orientation relationship (OR) between the parent austenitic phase and the martensitic phase. Neither the Nishiyama–Wasserman nor the Kurdjumov–Sachs OR is able to account for both the misorientation angle distributions, the pole figure and the misorientation axes measured. The mixed OR recently proposed by Gourgues et al. (Electron backscattering diffraction study of acicular ferrite, bainite, and martensite steel microstructures, Mater. Sci. Tech. 16 (2000), pp. 26–40.) and Sonderegger et al. (Martensite laths in creep resistant martensitic 9–12%Cr steels – Calculation and measurement of misorientations, Mater. Characterization (2006), in Press.) seems to be able to account for most of these results. Based on this OR, a new angular criterion is proposed to detect blocks of laths.     

Martensitic transformation and physical properties of ‘steel–TiNi’ bimetal composite,produced by explosion welding

The aim of this work is an investigation of structure and martensitic transformation in bimetal composite ‘TiNi–stainless steel’ produced by explosion welding. The results have shown that the mixture of chemical elements is observed in very narrow intervals of 6 µm close to the joint – 2 µm from the TiNi side and 4 µm from the steel one. Micro-hardness distribution in the vicinity of the joint is non-monotonic in the interval of 60 µm. Connection of stainless steel and TiNi plates by explosion welding leads to a dramatic change of martensitic transformation kinetics. Temperatures and the temperature interval of phase transformation increase strongly and heat transformation decreases. Annealing at 500°C for 2 h of bimetal composite decreases the interval of micro-hardness variation and partially recovers kinetics of phase transitions.     

高Co-Ni二次硬化钢的电子结构及合金化行为

在紧束缚框架下用Ｒｅｃｕｒｓｉｏｎ方法计算了高Ｃｏ－Ｎｉ二次硬化钢的电子结构，替位和间隙固溶元素的局域态密度通过总态密度的积分计算了体系的费米能及原子间相互作用的键级积分，讨论了替位和间隙溶元素对合金网物理性质的影响。     

Measurement of the magnetic moment in a cold worked 304 stainless steel using HTS SQUID

The magnetic properties of stainless steel have been investigated using a radio frequency (RF) high-temperature superconductivity (HTS) SQUID (Superconducting QUantum Interference Device)-based susceptometer. The nuclear grade 304 stainless steel is nonmagnetic at a normal condition but it changes to a partially ferromagnetic state associated with martensitic transformation under a plastic deformation. The magnetic moment of the 304 stainless steels was increased with an increasing cold work rate, and decreased with an increasing annealing temperature. The change of mechanical properties such as yield strength and ultimate tensile strength (UTS) are also analyzed in terms of deformation-induced martensitic transformation.     

Residual stresses in biaxially fatigued austenitic stainless steel sample of cruciform geometry

A specifically designed cruciform-shaped austenitic stainless steel AISI 321 sample was subjected to ex-situ biaxial tension-compression cycling to establish ferromagnetic martensitic phase conversion under the action of plastic deformation. The time-of-flight neutron diffraction technique was employed for in-plane residual stress determination in this sample for both the austenitic and martensitic phases. The 2D data enabled determination of the macro-, micro-, hydro- and deviatoric contributions to the total phase stresses.     

JLF-1钢高温循环变形后硬度与微观结构的数值关系

高温循环变形是结构材料性能降级的主要原因之一.用透射电子显微镜对低活化铁素体/马氏体钢——JLF-1钢低周疲劳样品的微观结构进行了分析,并测试了循环变形前后此钢显微维氏硬度的变化.为了掌握JLF-1钢性能在高温循环变形中的变化机理,依据位错理论,用最小二乘法对高温循环变形后的显微维氏硬度与微观结构进行了回归计算,得到了此钢显微维氏硬度与板条尺寸、位错胞尺寸、位错密度的数值关系. 关键词： 低活化铁素体/马氏体钢 循环变形 微观结构 显微维氏硬度     

超临界水冷堆燃料包壳材料的辐照损伤研究进展

超临界水冷堆（SCWR）是第四代核电站的主力堆型之一,高温、高压、超临界水环境下的辐照损伤问题是其燃料包壳材料面临的最大挑战。SCWR燃料包壳候选材料主要包括锆合金、奥氏体不锈钢、铁素体/马氏体不锈钢、镍基合金、ODS合金五大类,奥氏体不锈钢是最有希望的候选材料。介绍了近年来在这个领域国际上的主要研究进展。作者所在团队也对多种SCWR的候选材料进行了辐照损伤研究,包括:镍基合金C-276和718、铁素体/马氏体钢P92、奥氏体不锈钢AL-6XN和HR3C。对AL-6XN的氢离子辐照实验发现,辐照产生的缺陷主要是间隙型位错环,伯格斯矢量为1/3<111>,在较高剂量（5～7 dpa）辐照下,出现空洞肿胀。在氢滞留的影响下,位错环有着独特的演化规律,总结提出了位错环的四阶段演化过程。The Supercritical Water-cooled Reactor (SCWR) is one of the prior Generation IV advanced reactors. Irradiation damage is one of the key issues of fuel cladding materials which will suffer serious environment, such as high temperature, high pressure, high irradiation and supercritical water. The candidate materials contain zirconium alloys, austenitic stainless steels, ferritic/martensitic stainless steels, Ni-base alloys and ODS alloys. Austenitic stainless steels are the most promising materials. This paper summarized the international researches on irradiation effects in fuel cladding materials for SCWR. The group of authors also has done many researches in this field, including nickel-base alloy C-276 and 718, ferritic/martensitic steel P92 and austenitic stainless steel AL-6XN and HR3C. In AL-6XN austenitic stainless steels irradiated by hydrogen ions, dislocation loops were the dominant irradiation defects. At higher irradiation dose (5～7 dpa), the voids were found. All the dislocation loops were confirmed to be 1/3<111> interstitial type dislocation loops, and four evolution stages of dislocation loops with hydrogen retention were suggested.     

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Thermal diffusivity of reduced activation ferritic/martensitic (RAFM) steel CLF-1 has been determined by the modified photoacoustic piezoelectric (PAPE) technique. Firstly, the modified PAPE theoretical model is introduced and the experimental condition with high measurement precision are identified; Secondly, the experimental system is set up and calibrated by investigating the thermal diffusivity of nickel; Finally, the thermal diffusivity of RAFM steel CLF-1 is measured by the modified PAPE method. The results show that the RAFM steel CLF-1 has good thermal diffusion properties and is a fusion reactor structural material with excellent heat conductivity. The modified PAPE technique can determine the thermal diffusivity of RAFM steel CLF-1effectively, and provides an effective way to investigate the thermal-physical property of fusion reactor structural material.     

Bulk and Surface Quenching of Structural Steel: Morphological Analysis of the Structure

Structures formed in medium-carbon low-alloy steels during bulk quenching from furnace heating and surface quenching initiated by a low-power high-current electron beam are investigated by the methods of diffraction electron microscopy. The influence of the carbon concentration, initial austenite grain size, and cooling rate on the morphology of martensitic crystals and self-tempered carbide particles, long-range stress fields inside a packet and plates, and their dislocation substructure is analyzed. The temperature intervals for the formation of high-temperature plate martensitic crystals and packet (lath) martensite are estimated. It is demonstrated that the structure formed at ultrahigh heating and cooling rates is determined mostly by the morphology of martensite in the initial steel samples.     

快重离子引起的辐照效应

用PAT、CEMS、SEM和TEM分析观测了HIRFL提供的能量为几十MeV到几百MeV的碳离子在Ni、马氏体时效不锈钢、HT—9铁素体钢和316L不锈钢中引起的辐照效应,包括辐照引起的偏析、脱溶、相变和肿胀,以及辐照肿胀随辐照温度的变化关系,研究了510MeV的碳离子在高纯Ni中的损伤分布,并给出了95MeV的Ar离子模拟宇宙射线在宇宙飞行器中引起数字半导体器件“软故障”的最近结果。 The radiation effects were studied by means of PAT, CEMS, SEM and TEM in nickel,martensitic ageing stainless steel, ferrite steel HT-9 and stainless steel 316L induced by carbon ionswith energy of tens of MeV to hundreds of MeV delivered by HIRFL. It includes segregate,preciptale, phase change and void swelling, as well as temperature-relativity of the irradiationswelling. The damage distribution was also studied in nickel irradiated by 510MeV C~(6+). And the recentresults of Ar ions ...     

Microstructure,Mechanical Properties and Fracture of EP-823 Ferritic/Martensitic Steel After High-Temperature Thermomechanical Treatment

Russian Physics Journal - The paper investigates the microstructure and mechanical properties of the EP-823 ferritic/martensitic steel containing 12 wt.% chromium after the high-temperature...     

Microstructure evolution of T91 steel after heavy ion irradiation at 550 ℃

Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr⁺ ions at 550 ℃. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a₀/2<111> type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a₀<100> type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.     

Crystal plasticity-based constitutive modelling of irradiated bcc structures

A constitutive crystal plasticity model is proposed and developed for the inelastic deformation of irradiated bcc ferritic/martensitic steels. Defects found in these irradiated materials are used as substructure variables in the model. Insights from lower length- and time-scale simulations are used to frame the kinematic and substructure evolution relations of the governing deformation mechanisms. Models for evolution of mobile and immobile dislocations, as well as interstitial loops (formed due to irradiation), are developed. A rate theory-based approach is used to model the evolution of point defects generated during irradiation. The model is used to simulate the quasi-static tensile and creep response of a martensitic steel over a range of loading histories.     

Influence of reactive stresses on thermodynamics and kinetics of martensitic transitions in single crystals of the shape memory Cu-Al-Ni alloy

The heat of the β′₁-β₁-martensitic transition in single crystals of the alloy Cu-13.5 wt % Al-4.0 wt % Ni was studied under conditions of a constrained shape memory deformation and emergence of reactive stresses. The experiments were performed with samples bended in the form of a clamp. The sample was put into a continuous stainless steel ring, and this construction was placed in the capsule of a differential calorimeter. It was found that, with an increase in the magnitude of preliminary bend deformation, the maximum in the heat release (or heat absorption) curves decreases noticeably and its position shifts to higher temperatures. It was revealed that the latent heat of the transition decreases by a factor of 2 and 3 during heating and cooling, respectively. It was assumed that the observed effects are related to the influence of reactive stresses on the parameters of martensitic transitions. A quantitative analysis of the data obtained was performed in terms of the theory of diffuse martensitic transitions taking into account both the thermodynamic and kinetic factors.     

Atom probe microanalytical studies of some commercially important steels

The FIM atom probe has been used to study the atomic scale distribution of chemical elements in several important ferrous materials. Microanalysis of patented and drawn pearlitic steel wires reveals the partitioning of alloying elements between the carbide and ferrite phases. Studies of a similar material in the martensitic state show the progressive redistribution of carbon atoms during the martensite ageing process. Accurate phase analysis of a duplex stainless steel confirms and extends previous X-ray microanalytical studies, and reveals details of alloy element partititoning between austenite and ferrite.