Solute segregation along non-migrated and migrated grain boundaries during electron irradiation in austenitic stainless steels

Solute distribution and precipitation in the vicinity of the grain boundary in Type 316 steels were studied during electron irradiation up to about 50 dpa at temperatures from room temperature to 873 K. Undersized solute atoms, such as nickel, silicon and phosphorus, segregate toward the grain boundary, and oversized solutes, chromium and molybdenum, segregate away from the grain boundary during irradiation in the temperature range between 623 and 873 K. Enrichment of silicon and phosphorus along the grain boundary occurs after the irradiation at room temperature. The segregation of solute atoms increases with irradiation temperature except for silicon and phosphorus; the concentration of silicon and phosphorus along the grain boundary exhibits a maximum at 773 K. Remarkable depletion of chromium with enrichment of nickel, silicon and phosphorus occurs in the area swept by the migrating grain boundary. Massive M₂₃C₆ type carbide precipitates in front of the migrating grain boundary during irradiation in the temperature range from 723 to 873 K in the steels.     

α-Fe中刃型位错和富MnNi析出物相互作用的分子动力学模拟研究

本文采用分子动力学方法对α-Fe中1/2〈111〉{110}刃型位错和富MnNi析出物的相互作用进行了系统性研究。发现体系的临界剪切应力随析出物尺寸的增加而增大,随温度的升高而降低。富MnNi析出物和位错相互作用的临界剪切应力大于纯Cu析出物的。对于2 nm和3 nm富MnNi析出物,Mn原子对位错滑移的影响作用大于Ni原子的,Mn原子倾向于聚集到位错段,起拖拽位错运动作用。4 nm富MnNi析出物出现了体心立方相转变为面心立方和密排六方相现象,进一步提高了析出物对位错滑移的阻碍作用。随着温度的升高,Mn原子对位错段的拖拽作用减弱、析出物的相变程度降低,导致富MnNi析出物对应的临界剪切应力降低,呈较显著的温度依赖性。总而言之,相比于纯Cu析出物,富MnNi析出物表现出对位错滑移更强的钉扎作用,增强了基体的辐照硬化程度。     

Precipitates induced in austenitic stainless steel by nitrogen injection

Samples of solution-annealed Type 316 stainless steel were irradiated at 803 K with 1.1 MeV nitrogen ions up to 60 dpa at peak and the precipitates induced by the irradiation were examined by electron microscopy. The ratio of injected nitrogen concentration to dpa is estimated to be about 0.3 at%/dpa in an observed area. An ordered γ'-phase of nominally Ni₃Si is identified to form in the sample irradiated to 20 dpa; it shows superlattice spots in the electron diffraction pattern and enrichment of Si and Ni at the precipitate in EDS analysis. In the sample irradiated to 42 dpa, platelet precipitates of 18 nm in average diameter were observed with the number density of 7.4 × 10²¹/m³. The precipitate grows to an average diameter of 28 nm with little change in number density during irradiation from 42 to 60 dpa. The precipitate formed in the sample irradiated to 60 dpa was confirmed to be in the CrN phase with electron diffraction patterns and the spacing of moiré fringes.     

Formation of steady state size distribution of precipitates in alloys under cascade-producing irradiation

The effect of coherency loss on the development of precipitate size distribution under cascade-producing irradiation is considered. The nucleation of coherent precipitates, their growth followed by coherency loss and cascade-induced dissolution of large incoherent precipitates can occur simultaneously resulting in formation of a quasi-stationary size distribution of semicoherent precipitates. To describe this process we consider co-evolution of a mixed population of coherent, semicoherent and incoherent precipitates. Mathematically, the problem is formulated as a set of discrete rate equations of nucleation kinetics (the Master equation approach) which is also used for later stages of evolution. To solve the corresponding large set of equations (typically, more than 10⁵ equations) an efficient numerical method is developed. The simulation results obtained for material parameters and irradiation conditions typical for nuclear reactors show that the coherency loss affects considerably evolution of the precipitate population. Under certain irradiation conditions, both in solution-annealed alloys and in aged ones, the mean precipitate size and the number density during prolonged irradiation tend to steady state values, whereas the size distribution function of large precipitates narrows. The width of the quasi-stationary size distribution is controlled by cascade parameters. It was found that the asymptotic quasi-stationary state of the precipitate population may depend on initial state of the alloy.     

Annealing of neutron damage in graphite irradiated and stored at room temperature

Nimonic PE16, a gamma-prime Ni₃(Al,Ti) precipitate-strengthened alloy under consideration for fast reactor structural applications, has been neutron irradiated in three heat treatment conditions: solution treated, aged, and overaged. After irradiation at 600° C to 5.4 × 10²²n/cm² (E > 0.1 MeV), or 27 dpa, specimens were characterized for gamma-prime precipitate stability by transmission electron microscopy. The precipitate microstructures after irradiation reflected the influence of the preirradiation heat treatment; and indeed the precipitate particles present prior to irradiation remained stable. However, additional precipitation occurred during irradiation in each of the specimens examined. The in-reactor gamma-prime precipitation process decorated such microstructural features as voids, dislocations and carbide precipitates. Examples were found in the solution-treated condition where gamma prime in the form of Archimedes' screws had precipitated on climbing screw dislocations. The precipitation behavior observed is compared with predictions from existing models. It is concluded that models for solute diffusion to point-defect sinks and for Ostwald coarsening can account for the observations, but that the models for precipitate stability controlled by cascade dissolution during neutron irradiation do not.     

Precipitate dissolution by high energy collision cascades

The evolution of the microstructure during irradiation is now widely recognized due to: (1) radiation altered kinetic phenomena; (2) collisional processes by energetic cascades. In this paper, we investigate the specific nature of the collisional interaction between energetic cascades and precipitates. A new Monte Carlo-based computer program, TRIPOS, has been developed for the TRansport of Ions in POlyatomic Solids. The computer code utilizes standard nuclear and electronic energy loss formulas, and compares well with experimental data on particle reflection, penetration and sputtering. One of the unique features of the code is its applicability to problems involving multispecie media in multilayers of 3-dimensional configurations. The interaction of neutron-initiated high energy collision cascades is demonstrated to result in the partial dissolution of precipitates. However, the maximum precipitate size that may be completely destroyed by a high energy collision cascade is only a small fraction of the cascade size. Matrix atom implantation inside precipitates as well as preferential sputtering of light atoms from the surface of precipitates into the matrix is demonstrated to lead to changes in precipitate stoichiometry.     

Kinetics of radiation-induced precipitation at the alloy surface

Radiation-induced precipitation of a new phase at the surface of an alloy during irradiation at elevated temperatures was studied with the aid of a kinetic model of segregation. The preferential coupling of solute atoms with the defect fluxes gives rise to a strong solute enrichment at the surface, which, if surpassing the solute solubility limit, leads to the formation of a precipitate layer. The moving precipitate/matrix interface was accommodated by means of a mathematical scheme that transforms spatial coordinates into a reference frame in which the boundaries are immobile. Sample calculations were performed for precipitation of the γ'-Ni₃Si layer on Ni-Si alloys undergoing electron irradiation. The dependences of the precipitation kinetics on the defect-production rate, irradiation temperature, internal defect sink concentration and alloy composition were investigated systematically.     

Elevated-temperature tensile properties of irradiated 9 Cr-1 MoVNb steel

Normalized-and-tempered 9 Cr-1 MoVNb steel tensile specimens were irradiated in the Experimental Breeder Reactor-11 (EBR-11) at 390, 450, 500, and 550°C to ～2.1 and 2.5 × 10²⁶ neutrons/m² (> 0.1 MeV), which produced displacement damage levels of ～10 and 12 dpa, respectively. Tensile tests were conducted at the irradiation temperature and at room temperature. In addition to the irradiated specimens, as-heat-treated specimens and as-heat-treated specimens thermally aged at the irradiation for 5000 h were also tested.Thermal aging had no effect on the unirradiated tensile properties. Irradiation at 390°C increased the 0.2% yield stress and the ultimate tensile strength above those of the unirradiated control specimens. The ductility decreased slightly. After irradiation at 450, 500, and 550°C, the tensile properties were essentially the same as the unirradiated values. The hardening at 390°C was attributed to the dislocation and precipitate structure formed during the irradiation. The lack of hardening at 450°C and higher correlates with an absence of an irradiation-induced damage structure.     

Effect of the Cu-rich precipitates on thermal conductivity of RPV model steel

Cu-rich precipitates are the important influence factors for the irradiation embrittlement of the reactor pressure vessel model steels. The microstructure of the Cu-rich precipitates could be revealed by mechanical and magnetic properties. In this article, the effect of the Cu-rich precipitates on thermal conductivity was studied. The reactor pressure vessel (RPV) model steels were aged for different time at 500°C. The results show that the thermal conductivity of RPV model steel is first decreased and then increased during the experiment, with a minimum value at 48.33 ± 0.21 W·m^?1·K^?1 after being aged for 200 h. The changing thermal conductivity is decided by the synergistic effect of the following three factors: (1) the crystal structure transformation of Cu-rich precipitates, (2) the orientation relationship between the matrix and Cu-rich precipitates, (3) the content of Cu atoms in the matrix.     

Controlling the density distribution of SiC nanocrystals for the ion beam synthesis of buried SiC layers in silicon

The depth distribution of SiC nanocrystals formed during high-dose implantation of carbon ions into silicon at conditions suitable for the ion beam synthesis of buried SiC layers in silicon is studied in this paper. For implantation temperatures of 400–600°C and dose rates of 10¹² − 10¹³ C⁺/cm²s, SiC precipitates in crystalline silicon are observed to be of approximately equal size, independent of the depth position beneath the surface. Ballistic destruction of small precipitates and difficulties in precipitate growth are thought to be responsible for the observed narrow size distribution. The destruction of precipitates may lead to the simultaneous release of a superthreshold concentration of carbon atoms resulting in a carbon-induced amorphization of the silicon host lattice. The local reduction of the number density of SiC nanocrystals involved with this amorphization can be used to tailor discontinuous depth distributions of oriented SiC precipitates providing ideal starting conditions for the synthesis of well-defined single-crystalline SiC layers in silicon.     

Magnetic order in proton irradiated graphite: Curie temperatures and magnetoresistance effect

Defect induced magnetic order is a new phenomenon in material science that refers to the triggering and manipulation of magnetic order and magnetic moments in nominally non-magnetic materials by lattice defects and/or non-magnetic add atoms. A noticeable example of this effect is the magnetic order at room temperature produced by proton irradiation of graphite. In this work we have managed to increase the ferromagnetic signal by cooling the graphite samples down to 110 K during proton irradiation, diminishing in this way annealing effects. SQUID measurements of the magnetization show a fluence dependent Curie temperature. The longitudinal magnetoresistance shows an irreversible behavior similar to that found in ferromagnetic films indicating spin/domain reorientation effects. The observed magnetoresistance effects and Curie temperatures above room temperature are promising facts that may lead to useful carbon-based devices in the near future.     

Modification of the precipitate interface under irradiation and its effect on the stability of precipitates

Ni-alloys containing coherent γ'-précipitates of different lattice mismatch and an A1-Mg-Si alloy with semicoherent β' -precipitates were irradiated with self-ions of 25 0 keV energy. The microstructural changes as a function of irradiation dose, ranging from 2 to 398 dpa, and temperature, ranging from 323 to 998 K, were followed by transmission electron microscopy. It was found that irrespective of the initial interface, the alloy chemistry or the irradiation condition, the precipitate interface is considerably modified under irradiation. Further, the interfacial dislocations, evolved either during preaging or irradiation, obstructed the dissolution, and their loss marked the beginning of rapid dissolution starting from the surface of precipitates. Based on these observations, a hypothesis is proposed to account for the interfacial modification and to explain the stabilizing effect of the interfacial dislocations during the radiation-induced dissolution.     

Interstitial loop nucleation and growth in solution-treated type 316 stainless steel irradiated to low doses with 22 MeV C2+ and 46.5 MeV Ni6+ ions

The nucleation and growth of interstitial dislocation loops have been studied in solution-treated type 316 austenitic steel irradiated to low doses in the Harwell Variable Energy Cyclotron. Specimens have been irradiated with 46.5 MeV Ni⁶⁺ ions and 22 MeV C²⁺ ions, after room temperature pre-injection with 10 ppm helium and without helium pre-injection, at temperatures in the range 300–600°C. The effects of these irradiation variables on the interstitial loop populations produced are discussed. At low doses, where loop intersection is rare and dislocation network formation is minimal, the number of interstitial atoms stored in loops can give an indication of the swelling rate in circumstances where voids remain submicroscopic. It is shown that extrapolation of the low-dose swelling rates indicated by interstitial loop populations gives reasonable fit with experimentally determined high-dose void swelling values.     

Irradiation induced growth of CoSi2 precipitates in Si at 650°C: An in situ study

Crystalline Si samples were implanted at 350°C with 50 keV Co⁺ ions to a fluence of 10¹⁵ Co cm⁻². Small CoSi₂ precipitates were formed. We studied the precipitate growth, via in situ transmission electron microscopy, under irradiation with 100 keV Si ions at 650°C. We deduce the precipitate growth processes involved. Irradiation-induced (or enhanced) Ostwald ripening is the main growth mechanism. We also find an instability of the B-type precipitates, which leads to their transformation into A-type precipitates above a critical size. These preliminary results show that direct comparisons with kinetic Monte Carlo modelling of the precipitate growth is at hand.     

TEM investigations of MN nitride phases in a 9% chromium ferritic/martensitic steel with normalization conditions for nuclear reactors

The investigations on the precipitate phases in a 9%Cr ferritic/martensitic steel under different normalization conditions have been made by using a transmission electron microscope and an energy-dispersive X-ray spectroscopy. Hot-rolled steel samples were normalized at 1050-1200 °C for 1-2 h followed by an air cooling to room temperature. MN vanadium nitride precipitates with a plate-like morphology and a chemical formula of about (V_0.4Nb_0.4Cr_0.2)N have been observed at triple junctions, grain boundaries and within matrix in the steel samples normalized at 1050-1150 °C for 1-2 h, but they were dissolved out at 1200 °C within 1 h. Vanadium nitride is a stable phase at 1050 °C according to thermocalc prediction of equilibrium phases in the steel. With increasing normalizing temperature and time, there was no a striking change in the chemical composition of metallic elements in the MN phase, but a considerable increase in the size of the MN precipitate.     

Damage creation in ion irradiated Si1−xGex/Si structures

Strained SiGe/Si structures have been proposed as substrates for fabrication of high speed metal oxide semiconductor transistors. However, influence of strain and/or presence of Ge atoms on damage creation during ion irradiation have not been explored to a significant extent. In this study, Rutherford backscattering spectrometry (RBS) was used to characterize Si_1−xGe_x/Si structures irradiated by 140 keV He⁺ ions at room temperature. When compared with pure Si, strained samples show enhanced damage accumulation as a function of He fluence. Channeling angular scans did not reveal any specific configuration of displacements. Possible mechanisms for enhanced damage in strained Si are discussed.     

The effect of neutron irradiation on the precipitate distribution in adjusted uranium

Precipitates in α-uranium are thought to be very important in controlling the swelling that occurs in the reactor. Cross sections from the 30 mm diameter adjusted uranium fuel bars were cut and heat-treated to give a UAl₂ precipitate distribution peaking at about 30 nm diameter. Alternate sections were irradiated to 500 MWd/te at 673 K. Comparisons before and after irradiation show that a new precipitate population is formed during irradiation. Since new precipitates can form, mechanisms of swelling resistance based on the effects of aluminium in solution are no longer tenable.     

Ion induced structural modification and nano-crystalline formation of Zr–Al–Ni–Cu metallic glasses

The effect of the ion implantation on the phase transformation was studied for glassy and crystalline Zr₅₅Al₁₀Ni₅Cu₃₀ alloys, using Au⁺ ions with 500 keV. For the glassy metal surface, nano-crystalline precipitates were effectively formed in the amorphous matrix by 500 keV Au ion irradiation at a fluence of the about 10¹⁶ cm^?2. On the contrary, the long range ordering in the partly crystalline alloy was lost by the irradiation under the same condition. Moreover, the precipitation during the heat treatment near the crystallizing temperature was effectively suppressed in the ion implanted area. In the irradiated surface, the XPS valence band structure was drastically changed, while shifts of the binding energy were found in the core level electrons of Au 4f and Cu 2p, indicating a strong interaction between the implanted Au atoms and constituent atoms of the Zr-based alloy.     

Positron annihilation and nano-indentation analysis of irradiation effects on the microstructure and hardening of A508-3 steels used in Chinese HTGR

The irradiation and annealing behavior of Chinese A508-3 reactor pressure vessel (RPV) steel (0.04 wt% Cu) after 3 MeV Fe-ion irradiation ranging from 0.1 to 20 dpa at room temperature (called RTRPV) and high temperature (250?°C, called HTRPV) was studied by positron annihilation Doppler broadening (PADB) spectroscopy and nano-indentation hardness. PADB showed that the density of vacancy-type defects was higher for low-temperature irradiations. The higher hardness was found after high-temperature irradiation because of the formation of solute clusters during irradiation. Positron annihilation measurements revealed the interaction and clustering of vacancies with solute clusters which were introduced by Fe-ion irradiation. For both RTRPVs and HTRPVs, the positron defect parameter and positron diffusion length showed the recovery of the irradiation-induced defects. Total recovery was observed after annealing at 450 °C.     

Precipitate coarsening,redistribution and renucleaton during irradiation of Ni-6.35 wt.% Al

Precipitation during irradiation was investigated using 3.2-MV ⁵⁸Ni⁺ ions incident on Ni-6.35 wt.% Al at 650°C. The resultant γ' precipitate morphologies were observed using TEM. A radiation-enhanced coarsening regime is quantitatively demonstrated at doses ?20 dpa. The enhanced coarsening regime terminates by preferential interaction of dislocations with larger precipitates. Segregation of nickel to dislocations causes these large particles to dissolve and leads to renucleation of precipitates and un increase in the density of small particles. Consistent with this, maxima of γ' size as a function of dose are observed at 650 and 550°C.Biconvex lens shape, precipitate-free zones were observed and their growth kinetics were followed to 15 dpa. The zone diameter grows linearly at a rate of 0.21 nm/s which is consistent with dislocation loop growth. The precipitate-free zones are also caused by nickel segregation to dislocations, increase in size until maxima γ' mean size occurs, and then are obliterated due to γ' renucleation within the zone. The presence of a precipitate-free zone at the irradiated surface is confirmed. Mechanisms for the various phenomena are described.