Embrittlement Kinetics Research Articles

Oxygen Embrittlement Kinetics at 500–600 °C of the Ti–6Al–4V Alloy Fabricated by Laser and Electron Powder Bed Fusion

Oxygen Embrittlement Kinetics at 500–600 °C of the Ti–6Al–4V Alloy Fabricated by Laser and Electron Powder Bed Fusion

Damage mechanisms and embrittlement kinetics in a β-rich Ti 17 alloy after long term aging

The Ti 17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) alloy is used in the first stages of engine compressors, at temperatures up to 450°C. However, its use in structural parts such as engine pylons is more prospective and raises the question of damage tolerance. As the engine pylon environment involves prolonged exposures to high temperatures, the impact of aging on mechanical properties has also to be taken in account.In this work, the consequences of a long-term aging (up to 10 000 h at 450°C) on the mechanical resistance of a Ti 17 alloy are examined at different scales, from macroscopic fatigue crack growth tests to in-situ tensile tests performed on micro samples in a SEM. The detrimental effect of aging on the mechanical properties of the Ti 17 alloy was then discussed in the light of damage mechanisms analysis and embrittlement kinetics.

Contribution of Hardening Mechanism to VVER-1000 RPV Welds Flux Effect

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Nanostructure Self-Organization as Cause of Wavelike Radiation Embrittlement of the RPV Steel

Influence of neutron irradiation on reactor pressure vessel (RPV) steel degradation are examined with reference to the possible reasons of the substantial experimental data scatter and furthermore-nonstandard (non-monotonous) and oscillatory embrittlement behavior. In our glance this phenomenon may be explained by presence of the wavelike recovering component in the embrittlement kinetics. In our opinion controversy in the estimation on neutron flux on radiation degradation impact may be explained by presence of the wavelike component in the embrittlement kinetics. Therefore flux effect manifestation depends on fluence level. At low fluencies radiation degradation has to exceed normative value, then approaches to normative meaning and finally became sub normative. As a result of dose rate effect manifestation peripheral RPV’s zones in some range of fluencies have to be damaged to a large extent than situated closely to core. Moreover as a hypothesis we suppose that at some stages of irradiation damaged metal have to be partially restored by irradiation i.e. neutron bombardment. Nascent during irradiation nanostructure undergo occurring once or periodically evolution in a direction both degradation and recovery of the initial properties. According to our hypothesis at some stage(s) of metal nanostructure degradation neutron bombardment became recovering factor. As a result oscillation arise that in tern lead to enhanced data scatter. In this case we have to consider irradiation as a recovery factor. Disclosure of the steel degradation oscillating is a sign of the steel nanostructure cyclic self-recovery transformation as it take place in self-organization processes.

Vibratory Processes in Vessel Steel Damage Kinetics in Nuclear Reactors

The possible reasons for nonstandard embrittlement and significant variance of the experimental data in the study of the impact of neutron irradiation on the properties of reactor vessel materials are analyzed. The authors assume on the basis of experimental data and analysis of foreign research that the fluence dependence of the degree of radiation embrittlement can be of nonmonotonic as well as cyclic. The observation of vibratory processes in the embrittlement kinetics of vessel steel attests periodic transformation of structure and change in the properties of the metal. It is expected that the defect structure formed as a result of neutron bombardment undergoes transformation once or periodically in the direction of losses as well as restoration of the initial properties. The mentioned data shape the search for methods of regulating the radiation resistance of materials.

On the kinetics of transgranular particle embrittlement during simulated carburizing in steel containing grain-refining additions of aluminum and niobium plus aluminum

The kinetics of toughness degradation resulting from transgranular particle embrittlement are evaluated as a function of composition and processing history for simulated carburizing operations in air-melt steel containing grain-refining additions of aluminum and aluminum plus niobium. The kinetics of particle embrittlement are inherently linked to the ripening of AlN precipitates after extended austenitization in steel containing carbon contents representative of both the case and core of a carburized component. Embrittlement in steel containing AlN occurs with an activation energy similar to the value for aluminum diffusion in austenite, although an AlN volume fraction effect on the embrittlement kinetics is manifested as decreases in activation energy with decreases in the [Al]/[N] ratio of steel. In contrast, the presence of niobium substantially retards the kinetics of particle embrittlement in steel containing 120–200ppm N. Observations of AlN precipitates coated with Nb(C,N) indicate that the decreases in embrittlement kinetics are related to a reduction in the potential for AlN ripening during austenitization.

Radiation annealing of radiation embrittlement of the reactor pressure vessel steel

Influence of neutron irradiation on RPV steel degradation are examined with reference to the possible reasons of the substantial experimental data scatter and furthermore - nonstandard (non-monotonous) and oscillatory embrittlement behavior. In our glance this phenomenon may be explained by presence of the wavelike component in the embrittlement kinetics. We suppose that the main factor affecting steel anomalous embrittlement is fast neutron intensity (dose rate or flux), flux effect manifestation depends on state-of-the-art fluence level. At low fluencies radiation degradation has to exceed normative value, then approaches to normative meaning and finally became sub normative. Data on radiation damage change including through the ex-service RPVs taking into account chemical factor, fast neutron fluence and neutron flux were obtained and analyzed. In our opinion controversy in the estimation on neutron flux on radiation degradation impact may be explained by presence of the wavelike component in the embrittlement kinetics. Therefore flux effect manifestation depends on fluence level. At low fluencies radiation degradation has to exceed normative value, then approaches to normative meaning and finally became sub normative. Moreover as a hypothesis we suppose that at some stages of irradiation damaged metal have to be partially restored by irradiation i.e. neutron bombardment. Nascent during irradiation structure undergo occurring once or periodically transformation in a direction both degradation and recovery of the initial properties. According to our hypothesis at some stage(s) of metal structure degradation neutron bombardment became recovering factor. As a result oscillation arise that in tern lead to enhanced data scatter.

Self-Recovering Section of RPV Steel Radiation Embrittlement Kinetics as Indication of Material Smart Behavior

Influence of neutron irradiation on reactor pressure vessel (RPV) steel degradation are examined with reference to the possible reasons of the substantial experimental data scatter and furthermore -nonstandard (non-monotonous) and oscillatory embrittlement behavior. In our glance this phenomenon may be explained by presence of the wavelike recovering component in the embrittlement kinetics. We suppose that the main factor affecting steel anomalous embrittlement is fast neutron intensity (dose rate or flux), flux effect manifestation depends on state-of-the-art fluence level. At low fluencies radiation degradation has to exceed normative value, then approaches to normative meaning and finally became sub normative. Data on radiation damage change including through the ex-service RPVs taking into account chemical factor, fast neutron fluence and neutron flux were obtained and analyzed. In our opinion controversy in the estimation on neutron flux on radiation degradation impact may be explained by presence of the wavelike component in the embrittlement kinetics. Therefore flux effect manifestation depends on fluence level. At low fluencies radiation degradation has to exceed normative value, then approaches to normative meaning and finally became sub normative. As a result of dose rate effect manifestation peripheral RPV’s zones in some range of fluencies have to be damaged to a large extent than situated closely to core. Moreover as a hypothesis we suppose that at some stages of irradiation damaged metal have to be partially restored by irradiation i.e. neutron bombardment. Nascent during irradiation structure undergo occurring once or periodically transformation in a direction both degradation and recovery of the initial properties. According to our hypothesis at some stagem(s) of metal structure degradation neutron bombardment became recovering factor. Therefore self-recovering section of RPV steel radiation embrittlement kinetics is an indication of material smart behavior. As a result oscillation arise that intern lead to enhanced data scatter. Disclosure of the steel degradation oscillating is a sign of the steel structure cyclic self-recovery transformation as it take place in self-organization processes. This assumption has received support through the discovery of the similar “anomalous” data in scientific publications and by means of own additional experiments. Data obtained stimulate looking-for ways to management of the structural steel radiation stability (for example, by means of nano-structure modification for radiation defects annihilation intensification) for creation of the intelligent self-recovering material. Expected results: - radiation degradation theory and mechanisms development, - more adequate models of the radiation embrittlement elaboration, - methods and facility development for usage data of the accelerated materials irradiation for forecasting of their capacity for work in realistic (practical) circumstances of operation, - search of the ways for creating of the stable under neutron irradiation self-recovery smart materials.

Evolution of weld metals nanostructure and properties under irradiation and recovery annealing of VVER-type reactors

The results of VVER-440 steel Sv-10KhMFT and VVER-1000 steel SV-10KhGNMAA investigations by transmission electron microscopy, scanning electron microscopy, Auger-electron spectroscopy and mechanical tests are presented in this paper. The both types of weld metals with different content of impurities and alloying elements were studied after irradiations to fast neutron (E>0.5MeV) fluences in the wide range below and beyond the design values, after recovery annealing procedures and after re-irradiation following the annealing. The distinctive features of embrittlement kinetics of VVER-440 and VVER-1000 RPV weld metals conditioned by their chemical composition differences were investigated. It is shown that the main contribution into radiation strengthening within the design fluence can be attributed to radiation-induced precipitates, on reaching the design or beyond design values of fast neutron fluencies the main contribution into VVER-440 welds strengthening is made by radiation-induced dislocation loops, and in case of VVER-1000 welds – radiation-induced precipitates and grain-boundary phosphorous segregations. Recovery annealing of VVER-440 welds at 475°C during 100h causes irradiation-induced defects disappearance, transformation of copper enriched precipitates into bigger copper-rich precipitates with lower number density and leads to almost full recovery of mechanical properties followed by comparatively slow re-embrittlement rate. The recovery annealing temperature of VVER-1000 welds was higher – 565°C during 100h – to avoid temper brittleness. The annealing of VVER-1000 welds leads to almost full recovery of mechanical properties due to irradiation-induced defects disappearance and decrease in precipitates number density and grain-boundary segregation of phosphorus. The re-embrittlement rate of VVER-1000 weld during subsequent re-irradiation is at least not higher than the initial rate.

Thermal Embrittlement of a Lean Grade of Duplex Stainless Steel: Alloy 2003

Duplex stainless steels are desirable for use in power generation systems due to their attractive combination of strength, corrosion resistance, and cost. However, thermal embrittlement at intermediate temperatures (~475°C) via α-α' phase separation limits upper service temperatures for many applications. The development of low Cr and Ni equivalent lean grade alloys potentially increases the upper service temperature of these alloys by delaying the onset of α-α' phase separation. The present work assesses the thermal stability of a relatively new lean grade of duplex stainless steel, alloy 2003. In this paper, alloy 2003 has been compared to the most widely used duplex alloy, 2205, through a series of isothermal agings between 260°C and 538°C for times between 1 and 10,000 hours. The thermal stability of these alloys was primarily characterized by changes in microhardness. The microhardness data were fit to a JMA-type equation to quantify embrittlement rates and predict microstructural changes out to 50 years. Additionally, as-received specimens were characterized with the scanning electron microprobe to quantify the chemistry within the ferrite grains relative to the bulk material. Alloy 2003 was shown to be much more resistant to thermal embrittlement than alloy 2205. For 50 years of service at 288°C, it is predicted that alloy 2003 components will have a change in microhardness of about 25 HK where alloy 2205 components would increase approximately 175 HK, which indicates significant embrittlement. These findings show that lean grade alloys will have a greater service temperature range than standard grades. However, additional data, characterization, and modeling are needed to better predict embrittlement kinetics over component lifetimes.

Nondestructive Characterization and Evaluation of Embrittlement Kinetics and Elastic Constants of Duplex Stainless Steel SAF 2205 for Different Aging Times at 425°C and 475°C

In this work, an experimental study was carried out to evaluate the potential of the ultrasonic technique, with ultrasonic velocity and attenuation measurements, to assess the heat aging effects on duplex stainless steel SAF 2205, at temperatures of 425°C and 475°C for time periods of 12 h, 24 h, 50 h, 100 h and 200 h, as well as in the as received state of the material. Velocity measurements were calculated for both longitudinal and transversal waves. The elastic constants, Young’s modulus and shear modulus, of the material were computed from the relationship between longitudinal and transversal velocities. For the ultrasonic attenuation, only longitudinal waves were considered. Despite the large scatter measurements, both ultrasonic velocity and attenuation increased with the heat aging time, particularly at 475°C. Thus, it may be concluded that the technique used is promising and provides relevant contributions to an accurate characterization of materials and evaluation of their mechanical properties in a non-destructive manner.

Relationship Between Grain Boundary Segregation of Antimony and Temper Embrittlement in Titanium-Doped Nickel-Chromium Steel

The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed. It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary. Besides, the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony-induced embrittlement kinetics in the nickel-chromium steel. Based on these, the activation energy and frequency factor of diffusion of antimony-vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.

Long term aging of various duplex stainless steels between 250 °C and 400 °C – relationship between toughness measurements and metallurgical parameters

Embrittlement of duplex stainless steels after aging below 500 °C is a well documented phenomenon, usually attributed to the unmixing of the ferritic Fe-Cr solid solution, either by spinodal decomposition or by nucleation and growth of Cr-rich α′ phase, depending on aging temperature. Additional precipitation taking place during aging, like NiSiMo-rich G phase or Cu-e, are also known to participate to embrittlement, making the composition dependence of embrittlement kinetics even more intricate. In this study, long term aging treatments, up to 2 years at intermediate temperature (250–400 °C), have been performed for various duplex stainless steels grades containing different alloying contents in Cr, Mo, Si, Ni and N. Impact toughness has been measured after each aging condition, and an apparent activation energy could be deduced for the various grades. This quantity is a good indicator of embrittlement resistance. Fine microstructural characterizations have been performed in the ferrite: composition before aging was determined by EPMA, and precipitation state before and after aging was observed on TEM. The influence of alloying elements on the embrittlement phenomenon is analyzed on the basis of these results and of driving force calculations performed on Thermo-Calc ® .

Embrittlement of Zr-based bulk metallic glasses

Embrittlement of Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 and Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature T g for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub- T g annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above T g. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (< T g) is sufficient to embrittle the BMG significantly. Above T g, plasticity is retained for annealing far beyond the relaxation time but, instead, embrittlement is caused by crystallization. The magnitude of the decrease in Poisson’s ratio is insufficient to explain the severe embrittlement within the framework of a critical value as previously suggested.

Radiation Embrittlement of Cr–Ni–Mo and Cr–Mo RPV Steels

Abstract Radiation embrittlement kinetics of Cr–Mo and Cr–Ni–Mo steels, used for manufacturing VVER-440 and VVER-1000 RPV steels, respectively, are under consideration. Radiation embrittlement of VVER-440 and VVER-1000 RPV steels exposed to irradiation for 17–22 years is analyzed. Extremely high levels of thermal aging of VVER-1000 RPV steels at 320°C is pointed out. The neutron flux effect problem and the problem of accelerated radiation embrittlement of welds with high nickel contents are emphasized. The effect of phosphorus, copper, silicon, manganese, and nickel contents on radiation embrittlement of VVER-440 and VVER-1000 RPV steels is studied using the Russian surveillance database and representative results of research programs. The trend curves for VVER-440 and VVER-1000 RPV steels are proposed. The mechanisms of radiation embrittlement of Cr–Ni–Mo and Cr–Mo RPV steels are supposed.

Monitoring of Radiation Embrittlement of the First and Second Generation of VVER RPV Steels

Abstract The analysis of the current condition on the prediction of radiation embrittlement for VVER RPV materials of the first and second generation has been carried out. The radiation lifetime of a definite type of pressure vessel is determined by various parameters caused by operational features and the chemical composition of pressure vessel’s metal. The emphasis on re-irradiation embrittlement kinetics is given for the first generation of VVER RPV steels (VVER-440/179 and VVER-440/230 grades). The technique for the first generation RPV materials monitoring, based on sampling and studies of metal from the pressure vessel’s inner surface, is considered. The current program of cutting out templates, for the inner surface of a RPV wall, and their re-irradiation in surveillance channels of VVER-440 units with full core and dummy assembles is described. The new approaches to re-irradiation embrittlement models are analyzed. The basic problems of VVER-440/213 and VVER-1000 metal monitoring are analyzed. The representativity of VVER-440/213 and VVER-1000 RPV surveillance programs is discussed. The necessity of updating of the current standard reference dependences for the prediction of radiation embrittlement of RPV materials is shown. The most urgent questions of reactor dosimetry for VVER pressure vessel materials have been covered. The lifetime extension problem for the first and second generation of VVER units is considered.

Study on Temper Embrittlement of Phosphorus in Steel 12Cr1MoV

According to the non-equilibrium grain-boundary segregation (NGS) kinetics curve of phosphorus, a series of Charpy impact tests was performed in an industrial steel 12Cr1MoV, at solution temperature 1050oC, with isothermal holding temperature 540oC, on specimens of different holding time (at the segregation process, the critical time, and the desegregation process respectively). The DBTT values were measured and a temper embrittlement kinetics curve was given. The reverse temper embrittlement NGS mechanism for steel 12Cr1MoV was proposed.

Radiation embrittlement kinetics of the first generation of VVER-440 RVPs after post-irradiation annealing

A new approach for estimation of re-irradiation kinetics of VVER-440 reactor pressure vessel steels is presented. Effect of gamma flux on re-irradiation kinetics is discussed. Comparision of predicted and observed values of re-irradiation ductile-to-brittle transition termperature shifts for VVER-440 reactor pressure vessel core welds is provided.

Assessment of irradiation response of WWER-440 welds using samples taken from Novovoronezh unit 3 and 4 reactor pressure vessels

The results of the study on Novovoronezh unit 3 and 4 (NV NPP-3 and 4) reactor pressure vessel (RPV) radiation embrittlement measured using subsize impact specimens (5×5×27.5 mm 3) fabricated from samples taken from the corresponding RPV walls are presented. The post-irradiation annealing effectiveness and the embrittlement kinetics of Novovoronezh unit 3 and 4 RPV welds under re-irradiation are discussed. Ductile-to-brittle transition temperatures (DBTT) obtained using standard Charpy (TT 10×10) and subsize impact (TT 5×5) specimens of trepans cut out from Novovoronezh unit 2 RPV are compared. A new relation between TT 10×10 and TT 5×5 has been developed.

Kinetic Lattice Monte Carlo Simulations of Cascade Aging in Iron and Dilute Iron-Copper Alloys

AbstractNeutron hardening and embrittlement of pressure vessel steels is due to a high density of nm scale features, including copper-manganese-nickel rich precipitates and what are generally believed to be defect cluster-solute complexes. It has been postulated that the sub nanometer defect cluster-solute complexes form directly in displacement cascades. Cluster-complexes that are thermally unstable mediate the effect of flux on embrittlement kinetics. Larger cluster-complexes, that are relatively thermally stable for irradiation times up to 1 Gs, cause embrittlement in low copper steels. Robust characterization of these two types of so-called matrix defects has been an elusive goal. In this work, Kinetic Lattice Monte Carlo (KLMC) simulations of the long term evolution of the vacancy-rich cascade core regions were carried out for both pure iron and dilute iron-copper alloys at the nominal irradiation temperature of 563°K up to times when the vacancy clusters completely dissolve. Energetics were based on lattice embedded atom method potentials. Special time scaling and pulse annealing techniques were used to deal with the enormous range of inherent time scales involved, viz., rapid free vacancy jumps to slow emission from large complexes. Three-dimensional clusters rapidly form, containing a wide range of vacancies, as well as copper atoms in alloys. Small complexes are very mobile and growth takes place primarily by coalescence. The vacancy clusters ultimately dissolve at times from less than 0.1 to more than 100 MS. These simulations support the hypotheses that cascade cluster- complexes constitute both thermally stable and unstable matrix defect features.