Evolution of radiation defects in nickel under low-temperature neutron irradiation

Neutron irradiation of pure nickel samples in an IBB-2M research reactor has been performed at a temperature of 305 K to damaging doses of 0.0015 and 0.15 dpa. Radiation defects formed in the material under irradiation have been investigated using transmission electron microscopy. It has been established that the main types of defects are vacancy clusters and interstitial dislocation loops. Sizes of vacancy clusters have been measured, and histograms of the cluster-size distribution have been constructed. It has been shown that, after irradiation with a dose of 0.15 dpa, the average cluster size is nearly half of that for samples irradiated with a dose of 0.0015 dpa. In the framework of the model of the migration of point defects, their evolution under irradiation has been analyzed. It has been shown that, at a temperature of 305 K, vacancies in nickel are immobile and migrating interstitials falling into clusters recombine with vacancies in them, which results in the exhaustion of clusters. The average life span of clusters has been calculated, and average concentrations of vacancies and interstitials under irradiation have been estimated.     

Ground state of the magnetization of nanowires

Analytical and numerical methods have been used to study the ground state of magnetization of a nanowire consisting of ferromagnetic crystallites that are coupled via exchange interactions. The random nature of the field of crystallographic anisotropy has been taken into account. It is shown that the magnetization of a nanowire is divided into weakly interacting regions of magnetization called blocks. Such characteristics, as the average size of a magnetic block, its effective anisotropy constant, and their dependence on the size of crystallites have been calculated analytically and simulated numerically in a two-angle approximation. The coordinate dependences of the magnetization dispersion at the edge and in the bulk of the magnet have been determined. It is shown that the functional dependence of the dispersion on the coordinate changes with moving away from the free edge of the nanowire.     

Resonance modes of two-layer exchange-coupled ferromagnetic film

Ferromagnetic and spin-wave resonances in two-layer exchange-coupled ferromagnetic films have been investigated numerically at different intensities of a magnetic field when it is directed in parallel or perpendicular to the film plane. Layers of the film have finite thicknesses and possess anisotropy of the easy-plane and easyaxis types. It has been shown that at a nonzero parameter of interlayer exchange coupling the dynamic component of magnetization upon ferromagnetic resonance is distributed nonuniformly across the film thickness. Its change has been described when the external magnetic field decreases from the saturating field to zero.     

Structural and phase states and irradiation-induced defects in Ni-Cr alloys

The structural and phase states in alloys of the Ni-Cr-Mo system which were induced by both heat aging and electron irradiation at elevated temperatures have been studied by the methods of measurement of residual resistivity and positron annihilation. Migration of irradiation-induced defects during irradiation at 300°C is shown to initiate processes of ordering or phase separation depending on the initial alloy microstructure and chromium content. It has been established that in the alloy with 32 wt % Cr the concentration of accumulated vacancy defects in the state of short-range ordering after irradiation with 5-MeV electrons to a dose of ～1.5 × 10^?4 dpa at 200°C is half as high as that in the state of long-range ordering with a homogeneous distribution of domains (to 10 nm in size) of the ordered Ni₂Cr phase in the matrix.     

On the theory of a supercritical state of ferromagnetic films with uniaxial perpendicular anisotropy and (001) single-crystal films with positive constants of magnetocrystalline anisotropy

A model of the magnetization distribution in thin ferromagnetic films that are in the so-called supercritical state has been considered. The model makes it possible to take into account the real distribution of the magnetization more precisely in ferromagnetic films than open and closed models. The theoretical results have been compared to the experimental data.     

Structural and magnetic properties of mechanically alloyed Fe–66 at%Al

A metastable, nanocrystalline Fe–66 at%Al solid solution, which is ferromagnetic at room temperature, was prepared from elemental powders by mechanical alloying using a high-energy ball mill. X-ray diffraction and Mössbauer studies were used to trace the evolution of microstructure and disorder in the system. Bulk magnetization studies showed that the system is ferromagnetic at room temperature. This has been attributed to the presence of a disordered Fe-rich phase that persists even after 30 h of milling. Thermomagnetic measurements showed that the ferromagnetic to paramagnetic transition occurs between 603 and 613 K for all the samples, irrespective of milling time.     

Plastic instability in polycrystalline metals after low temperature irradiation

The dose dependence of plastic instability behavior has been investigated for polycrystalline metals after neutron irradiation at low temperatures (<200 °C). The analyzed materials consist of 10 body-centered cubic (bcc), 7 face-centered cubic (fcc), and 2 hexagonal close packed (hcp) metals. It was found that the metals after irradiation showed necking at yield when the yield stress exceeded the true plastic instability stress, σ_IS, for the unirradiated material. It was also shown that σ_IS was almost independent of dose below a critical dose. The critical dose is called the dose to plastic instability at yield, D_C, because at higher doses the material shows necking at yield. The D_C values ranged from 0.002 to 0.2 dpa for bcc and hcp metals, except for a high purity iron, that had a D_C value of 6 dpa; whereas the fcc metals gave generally high values ranging from 0.1 to 40 dpa. It is attempted to explain the dose independence of the plastic instability stress by a straightforward shifting of tensile curves by the appropriate strain corresponding to the radiation-induced increase in yield stress. The dose independence of strain-hardening behavior suggests that radiation-induced defects and deformation-produced dislocations give similar net strain-hardening effects.     

Velocity of motion of domain walls with vortex structure in multilayer ferromagnetic films

A two-dimensional model of the magnetization distribution has been applied to investigating the influence of the layered structure of a ferromagnetic film in which the layers differ in saturation magnetization M _S on the velocity of the motion of domain walls with a vortex distribution of magnetization. Films with uniaxial planar magnetic anisotropy have been analyzed. It is established that, in three-layer films with values of M _S in the near-surface layers that exceed those of the internal layer in a wide range of the external field, the velocity of motion of the domain wall is higher than in a homogeneous film with the same average saturation magnetization.     

Irradiation Effects in Ni–17Mo–7Cr Alloy Bombarded with MeV Au Ions

Irradiation effects in Ni–17Mo–7Cr alloy have been systematically investigated by using 3 Me V Au ions at different fluences ranging from 8 9 1013cm-2to 2.3 9 1015cm-2,corresponding to doses of 1–30 dpa.The results indicated that sample microstrain increased gradually from 0.14 to 0.22% as dose increased from 0 to 30 dpa.Besides,the nanohardness of Ni–17Mo–7Cr alloy increased with irradiation dose until 10 dpa,and then,softening effect became dominant while further increasing dose to 30 dpa.After being irradiated at room temperature,the swelling rate of Ni–17Mo–7Cr alloy was found to be around 0.04% per dpa.These data are helpful in estimating the irradiation resistance of this newly developed Ni–17Mo–7Cr alloy in nuclear energy systems.     

Penetration of high-frequency electromagnetic field through bulk samples of La1? x Sr x MnO3 (0.33 ≤ x ≤ 0.60) manganites

Penetration of radio-frequency electromagnetic field through bulk samples of La_1−x Sr _x MnO₃ (0.33 ≤ x ≤ 0.60) manganites has been studied in the frequency range from 20 kHz to 30 MHz at temperatures from 300 to 400 K, which includes the temperature of the magnetic phase transition of some investigated compositions. It has been shown that, in spite of the differences in the static properties, the dynamic properties of the investigated compositions of these manganites are similar; the differences are mainly pronounced at frequencies higher than 1 MHz. In the ferromagnetic range, significant changes in the transmission coefficient have been observed for all compositions upon magnetization. The magnitudes of the initial dynamic magnetic permeability have been estimated. The changes in the transmission coefficient in the ferromagnetic region are mainly caused by changes in the dynamic magnetic permeability rather than by magnetoresistance. It is shown that in the paramagnetic temperature range the magnitude of the dynamic reversible permeability exceeds unity for all compositions, which supports the assumption on the retention of local magnetic ordering. Original Russian Text ? A.P. Nosov, A.B. Rinkevich, V.G. Vasil’ev, E.V. Vladimirova, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 106, No. 1, pp. 36–44.     

Characteristics of radiation porosity formed upon irradiation in a BN-600 reactor in the fuel-element cans of cold-deformed steel EK-164 (06Kh16N20M2G2BTFR)-ID c.d.

At present, it is the austenitic cold-deformed steel EK164 (06Kh16N20M2G2BTFR)-ID that is considered as a promising material for the achievement of a maximum damage (no less than 110 dpa) and maximum burnup (??15%). In this work, we have determined the characteristics of porosity formed upon irradiation in a BN-600 reactor to the maximum damaging dose of 77 dpa in the materials of fuel-element cans made of cold-deformed steel EK164-ID c.d. A comparison has been made with analogous characteristics obtained earlier using the standard material, i.e., the cold-deformed steel ChS68 (06Kh16N 15M2G2TFR)-ID c.d.     

Effect of the magnetic state on the damping properties of iron and nickel alloys

The dependences of the ΔEeffect and internal friction on the magnetic field have been investigated upon the magnetization and magnetization reversal of some polycrystalline ferromagnetic materials with different properties. It has been shown that for all the materials that have been investigated there is characteristic a nonmonotonic dependence of the magnitude of internal friction on the magnetic field both for the magnetization curves and for the major hysteresis loops. It is shown that in the sonic frequency range of elastic vibrations the basic mechanism of the formation of the magnetic peaks of internal friction in the materials investigated is magnetostrictive; the damping depends simultaneously on the instantaneous values of magnetostriction and magnetostrictive sensitivity.     

Asymmetric pinning of vortex domain walls in magnetic films in regions with lowered saturation magnetization

The asymmetric pinning of vortex domain walls in a region with a lowered saturation magnetization has been investigated in ferromagnetic films with the in-plane anisotropy. Numerical micromagnetic simulation within the two-dimensional model of the magnetization distribution has been used. The wall structure and the depinning fields in different directions have been investigated depending on the dimensions of the region with lowered saturation magnetization and the film thickness. The physical factors that determine the obtained regularities have been established.     

Deformation in metals after low-temperature irradiation: Part II – Irradiation hardening,strain hardening,and stress ratios

Effects of irradiation at temperatures ⩽200 °C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000 GPa/dpa, at the lowest dose of <0.0001 dpa and decrease with dose to a few tens of MPa/dpa or less at about 10 dpa. However, average irradiation-hardening rates over the dose range of 0 dpa−D_C (the dose to plastic instability at yield) are considerably lower for stainless steels due to their high uniform ductility. It is shown that whereas low-temperature irradiation increases the yield stress, it does not significantly change the strain-hardening rate of metallic materials; it decreases the fracture stress only when non-ductile failure occurs. Such dose independence in strain-hardening behavior results in strong linear relationships between the true stress parameters. Average ratios of plastic instability stress to unirradiated yield stress are about 1.4, 3.9, and 1.3 for bcc metals (and precipitation hardened IN718 alloy), annealed fcc metals (and pure Zr), and Zr-4 alloy, respectively. Ratios of fracture stress to plastic instability stress are calculated to be 2.2, 1.7, and 2.1, respectively. Comparison of these values confirms that the annealed fcc metals and other soft metals have larger uniform ductility but smaller necking ductility when compared to other materials.     

Temperature dependence of magnetic properties of a disordered Fe<Subscript>0.65</Subscript>Ni<Subscript>0.35</Subscript>

The method of calculation of average and local magnetic characteristics of transition metals at finite temperatures developed by the author and Grebennikov in previous works and based on the employment of a real band structure and spin fluctuations has been applied to a disordered invar alloy Fe_0.65Ni_0.35. The magnetization, Curie temperature, local magnetic moment, and uniform and local susceptibilities have been calculated for both ferromagnetic and paramagnetic states. The results are in agreement with experimental data in a wide temperature range. It has been shown that the effect of disorder in the filling of sites with Fe and Ni atoms on the magnetic properties of the Fe-Ni invar is insignificant.     

Experimental studies of the magneto-mechanical memory (MMM) technique using permanently installed magnetic sensor arrays

The magneto-mechanical memory (MMM) method, that is often referred to as the metal magnetic memory method, has been reported to be a non-destructive testing technique capable of quantifying stress concentrations and detecting defects in ferromagnetic materials. The underlying mechanism behind MMM has been explained in the literature, but the sensitivity to stress concentration has not been satisfactorily investigated. In this paper, both the normal and tangential components of the stress-induced MMM signal were measured by permanently installed magnetic sensor arrays on specimens made from three grades of L80 alloy steel and 20 other structural steels; tests were also carried out on a pipe made from the 4140-L80 steel. As expected, the stress history affects the MMM signal, but the experimental results show that significant irreversible change of magnetization always occurs only in the first cycle of loading regardless whether the deformation is purely elastic or partially plastic. If the peak stress level is increased at a given point during cycling, the immediately following next cycle acts as a new “first” cycle at that peak stress level and causes additional significant irreversible change of magnetization, but there is no evidence that plastic deformation might build up a cumulative magnetization. The MMM effect is very small in the steel samples tested, indicating that it will not be useful in field applications. In un-notched specimens the irreversible change in magnetization caused a proportional change in the measured external magnetic field on the order of only 5–10 A/m, while in the case of notched specimens the leakage field was on the order of 30–60 A/m.     

Field-dependent neutron depolarization study of the ferrite formation in medium-carbon steels

Neutron depolarization experiments have been performed on the ferrite and pearlite phase transformations of the medium-carbon C60 and C35 steels. The interaction of the polarized neutron beam with the ferromagnetic ferrite grains gives information on the mean magnetization and the magnetic correlation length. From these parameters the ferrite fraction and the mean ferrite grain size are determined in situ as a function of time and temperature during the phase transformations. The applied magnetic field was varied periodically in order to record a full hysteresis curve of the magnetization, which gives essential information on the microstructure of the ferromagnetic ferrite grains. The field dependence of the mean particle size during the early stages of the pearlite formation is a strong indication of multi-domain behavior, which is absent in the austenite–ferrite transformation and at the end of the pearlite formation.     

Magnetic and transport properties of YbRhIn and YbPdIn

Electrical resistivity, magnetization and specific heat have been measured on a single crystal of YbRhIn and polycrystals of LuRhIn and YbPdIn. It is confirmed that YbPdIn is a metal with a divalent Yb ion. For YbRhIn, the electrical resistivity increases with logarithmic temperature dependence with decreasing temperature and saturates at a constant value below 20 K, while that of the nonmagnetic reference LuRhIn shows typical metallic behavior. The magnetic susceptibility of YbRhIn indicates that the Yb ion is in the trivalent state. The irreversibility of the magnetization curves which is observed in the high pulsed magnetic field below 1.5 K suggests an increase of relaxation time in the magnetic system. The specific heat increases rapidly below 4 K with decreasing temperature down to 0.9 K and moves toward higher temperatures on application of a magnetic field. These facts suggest that some magnetic transition to either ferromagnetic or spin-glass state occurs at low temperature.     

Irradiation-induced hardening and softening of CLAM steel under Fe ion irradiation

The irradiation-induced hardening and softening of CLAM steel irradiated with 3.5 MeV Fe¹³⁺ ions at temperatures of 300 °C and 550 °C were investigated by nanoindentation tests in combination with microstructures. Irradiation- induced hardening occurred in the steel irradiated at 300 °C to doses of 0.46 dpa, 0.94 dpa, and 2.79 dpa. The hardening occurred at 300 °C is mainly attributed to the formation of irradiation-produced dislocation loops and a network of tangled dislocations in the irradiated steel samples. Significant hardening was found in the steel irradiated at 550 °C to 0.38 dpa. On the contrary, irradiation-induced softening occurred in the steel irradiated at 550 °C to both 0.76 dpa and 2.75 dpa. Irradiation-produced dislocation loops are not dominant effect on the irradiation hardening of the steel samples irradiated at 550 °C. The hardening and softening of the irradiated steel were explained in terms of the irradiation-produced defects and recovery process occurred during the irradiation.     

Influence of magnetostatic fields of a ferromagnetic substrate on the electrodeposition of nickel dendrites

The influence of the magnetic state of a ferromagnetic wire substrate on the morphology of deposited nickel obtained by electrolysis has been studied. It has been demonstrated using statistical analysis that in the course of nickel electrodeposition without external magnetic field the dimensional characteristics of the arising dendritic structures are affected significantly by the residual magnetization of the substrate. When in the course of Ni electrodeposition even a weak dc magnetic field is applied perpendicularly to the wire axis, nearly identical structures are formed; that is, the system “forgets” its previous magnetic state. The magnetic properties of the substrate were visualized using powder patterns.