Maps of deformation-induced structures in neutron-irradiated metals and alloys

Using dislocation kinetic equations, maps of deformation-induced structures observed in neutron-irradiated metals and alloys are theoretically grounded. The critical strains and radiation doses for the transition from cellular and chaotic dislocation structures to heterogeneous channel-like deformation structures are found, and the relations of the critical strains and radiation doses to the kinetic coefficients that determine the evolution of the density of dislocations and radiation defects in irradiated materials are established. These relations are used to quantitatively analyze the effect of irradiation on the strength and deformation properties of Ni and martensitic steel A533B available in the literature. The critical conditions for the appearance of irradiation embrittlement in irradiated materials are considered.     

Temperature characteristics of the mechanical properties and of the dislocation structure of molybdenum-rhenium alloys

The temperature characteristics of the yield strength and of the dislocation structure of supersaturated molybdenum-rhenium alloys were studied over the 77–673°K temperature range. A lesser dependence of σ_0.1 on the temperature was found to result from twinning. The mobility of screw dislocations was found to become much lower due to alloying with rhenium. An analysis of the peculiarities in the dislocation structure of the alloys under consideration here (plane pileup, screw dipoles, singularities of interaction between glissile dislocations) has led to the conclusion that the lower mobility of screw dislocations is a consequence of the lower energy of the packing defect in molybdenum alloyed with rhenium. The lattice resistance to a movement of screw dislocations (τ_p) at room temperature has been estimated from the distance between dislocations in screw dipoles and found to be τ_p≥16 kgf/mm².     

Analysis of the parameters of a brittle-ductile transition during impact loading of neutron-irradiated bcc metals and alloys

The impact toughness (fracture energy-temperature) curves of neutron-irradiated bcc metals and alloys, including structural alloys applied in nuclear power, are theoretically analyzed. The analysis is based on the stress-strain curves of these metals and alloys with allowance for the effect of temperature and irradiation on their parameters. The energy of ductile fracture of smooth and notched (Charpy) specimens during both static and impact loading is shown to substantially depend on the uniform strain and its temperature and radiation-hardening dependences. As a result of this analysis, the dependence of the critical brittle-ductile transition temperature on the radiation dose is established. Theoretical relations obtained for the transition parameters are illustrated with experimental data for martensitic steels.     

Magnetic properties of tetragonal uranium compounds I. the U2N2Z-ternaries

In the present work we report the magnetic behaviour of the tetragonal ternaries of the U₂N₂Z-type (Z=Sb, Bi and Te). Magnetic susceptibility measurements performed in a wide temperature range (4.2–1000 K) have shown, that they are ferromagnetically ordered with Curie temperatures 166, 154 and 71 K for U₂N₂Sb, U₂N₂Bi and U₂N₂Te, respectively. For all the investigated compounds the κ^-1_M(T) function in the temperature range above T_C can be expressed as (A/T + B)^-1 - λ.Magnetization measurements were carried out up to magnetic field strengths of 140 kOe in the temperature range from 4.2 to the respective T_C. It follows from these measurements that σ(T)_H and σ(H)_T for U₂N₂Sb and U₂ N₂Bi are typical as for normal ferromagnets. On the other hand, U₂N₂Te exhibits, unexpectedly, two distinct maxima on the σ(T) curves up to fields of 4.5 kOe; one at 45 K and the other one at 71 K. Previous neutron diffraction studies of this compound have shown that the magnetic moments of uranium atoms at 4.2 K are titled by about 20° from the basal plane.The results obtained are interpreted in terms of the crystal-field interaction of the 5f² electrons of the U⁴ ion. In consequence a γ_5t doublet is expected to be the ground-state crystal field level in the U₂N₂Z-type as well as in many other ternary uranium tetragonal compounds. However, in the case U₂N₂Te, the singlet-singlet-doublet “band” as a ground system is postulated.     

On stage V recovery in VIa b.c.c. metals

Abstract

A resistometric study at 4.2 °K of the high-temperature recovery spectrum of neutron-irradiated molybdenum as a function of fast fluence and of cold-worked molybdenum as a function of the heating program, yields evidence against vacancy migration in the 0.31 T_m stage for VIa b.c.c. metals.     

The temperature dependence of the quenching of Na-D-doublet by N2 and H2O in flames of 1500–2500 K

The temperature dependence of the quenching cross section of the Na-D-doublet by N₂ and H₂O molecules has been measured in flames at temperatures from 1500 to 2500 K. The nitrogen and water cross sections are temperature independent. Their values were found to be σ_N2 = (22±2) Ų and σ_H2O = (2.2±0.3) Ų. Our value of the nitrogen cross section is about half of the value measured at about 400 K in vapour bulbs. A possible explanation for this difference is given.     

Measuring the Temperature Coefficient of the Surface Tension of Metals in the Solid State

Shortcomings of the zero creep compensation used in measuring surface tension σ_T of metals in the solid state are analyzed, especially its low sensitivity in measuring the temperature coefficient. A device for directly measuring σ_T with a high degree of accuracy and without the flaws of zero creep compensation is proposed.     

Solid-solution strengthening of intermetallic compounds in the region of anomalous thermal behavior of the deforming stress

Strong solid-solution strengthening of intermetallic materials in the region where the deforming stress increases anomalously with temperature is shown to be caused by the stimulating influence of solution atoms on dislocation locking via the Kear-Wilsdorf mechanism. The dependence of the deforming stress on temperature, solution concentration, strain rate, and other parameters is described in terms of a simple model in which the thermally activated kinetics of cross slip of dislocations is modified by solution atoms. The proposed theory is compared with available experimental data for a (Ni_{1 ? c} Fe_c)₃Ge solid solution.     

Fatigue in precipitation hardened materials: a three-dimensional discrete dislocation dynamics modelling of the early cycles

Three-dimensional dislocation dynamics (DD) fatigue simulations in precipitation hardened metals is a major challenge in terms of numerical development. Several precipitate configurations have been investigated with an original treatment of precipitate–dislocation interactions and a parallelized DD code. In grains containing single-sized shearable particles (r _p?=?160?nm), strain is localized in clear bands where the precipitates are totally sheared-off. The fatigue behaviour involves an initial hardening followed by severe cyclic softening and significant surface slip irreversibility. In the presence of large single-sized particles (r _p?=?400?nm), the persistent slip band (PSB) structure is accompanied by highly reversible surface displacements. Slip dispersion originates from Orowan loops that have little effect on the mechanical response. The mechanical behaviour of a bimodal distribution is intermediate between the two above cases with the above microstructural features coexisting within the same grain. Unlike in the monomodal large-particle case, where all the particles retain their initial strength, some of the large particles of the bimodal distribution undergo a significant strength reduction.     

单晶铝在拉压循环过程中形成的两种位错组态与成因

研究了［１１０］和［１００］取向高纯铝单晶在６×１０^－4拉压疲劳应变振幅条件下的应力σ_m和内耗Ｑ^－１的变化，对不同阶段的位错组态作了详细的透射电子显微镜观察，并利用滑移几何的观点予以解释 关键词：     

Low-temperature deformation of nanocrystalline niobium

The strain characteristics of nanocrystalline niobium are measured in the temperature range 4.2–300 K. It is shown that the development of a strong local deformation with clearly delineated macroscopic slip bands occurs at 4.2 K and 10 K. The thermal effects at a stress jump observed upon transition of the sample (or a niobium strip placed close to the sample) from the superconducting state to the normal state are estimated. It is demonstrated that the temperature dependence of the yield point σ_s(T) can be divided into three portions: two portions (T<10 K and T>70 K) with a slight change in σ_s and the third portion with a strong dependence σ_s(T). The strain characteristics of polycrystals with nano-and larger-sized grains are compared with those of single crystals.     

Thermoluminescence of α-Al 2O 3 by neutron irradiation at low temperature

Thermoluminescence (TL) mechanisms of neutron-irradiated α-Al₂O₃ at 20 K is reported. The TL glow curves of neutron-irradiated and γ-ray-irradiated α-Al₂O₃ were observed. The TL emission bands near 340, 430, 530 and 694 nm were observed in the neutron-irradiated α-Al₂O₃. The γ-ray-irradiated α-Al₂O₃ only showed the TL emission line nearly at 694 nm, corresponding to the R lines of α-Al₂O₃:Cr³⁺. Therefore, the first three emission bands are related to the atomic displacement defects as F-type centers caused by neutron irradiation.     

Die Verfestigungserholung von plastisch verformten Steinsalzeinkristallen

Cylindrical rock salt single crystals have been plastically deformed by compression in the [001]-direction at room temperature to shear stresser τ _E of 200 N/cm² and 350 N/cm², respectively. Isochronal annealing experiments reveal, that workhardening recovers at >300° C. The characteristic annealing temperature was found between 400° C and 450° C. At 600° C the residual workhardening still amounts to 15–20%. The isochronal reduction of screw dislocation density between 400 and 600° C shows qualitatively the same behaviour as recovery of workhardening. From the isothermal annealing curves of the samples deformed to 200 N/cm² the activation energy for recovery of workhardening was found to be about 1 eV. Assuming that the kinetics of recovery can be explained by processes distributed in activation energy, an approximate spectrum of activation energies (with a maximum arising at ～1 eV) has been evaluated. The results show that recovery of workhardening after low deformation (stage I of the stress strain curve) is mainly due to the dislocations.     

Effect of neutron-irradiation on optical properties of SiO2-Na2O-MgO-Al2O3 glasses

Silica based glasses are used as nuclear shielding materials. The effect of radiation on these glasses varies as per the constituents used in these glasses. Glasses of different composition of SiO₂-Na₂OMgO-Al₂O₃ were made by melt casting techniques. These glasses were irradiated with neutrons of different fluences. Optical absorption measurements of neutron-irradiated silica based glasses were performed at room temperature (RT) to detect and characterize the induced radiation damage in these materials. The absorption band found for neutron-irradiated glasses are induced by hole type color centers related to non-bridging oxygen ions (NBO) located in different surroundings of glass matrix. Decrease in the transmittance indicates the formation of color-center defects. Values for band gap energy and the width of the energy tail above the mobility gap have been measured before and after irradiation. The band gap energy has been found to decrease with increasing fluence while the Urbach energy shows an increase. The effects of the composition of the glasses on these parameters have been discussed in detail in this paper.      

Defect aggregates in neutron-irradiated β-Si3N4

Abstract

Microstructural analysis of the defect aggregates formed in bulk samples of polycrystalline β-Si₃N₄ neutron-irradiated to a dose of ～2.0 × 10²⁶n/m² at temperatures of 1100 K and 925 K has been carried out. This study has shown that the defect aggregates formed are faulted dislocation loops lying on the {1010} planes with a Burgers vector of b ? 1 /10<1125>. The vector is non-rational but corresponds to the insertion of an extra layer of [SiN4] tetrahedra on the {10l0} planes plus an additional shear in the loop plane. The formation of these loops is dependent upon the temperature of irradiation. In the sample irradiated at 1100 K their formation is additionally dependent upon whether or not a particular grain contains pre-existing c-axis dislocations. If no c-axis dislocations are present then independent nucleation of the loops is apparent; if there are pre-existing c-axis dislocations then the loops form from an apparent dissociation between the arcs of the irradiation-induced helical c-axis dislocation. In the sample irradiated at 925 K only independent nucleation of the loops occurs, regardless of whether or not there are any pre-existing c-axis dislocations in the grains.     

Microstructural analysis of neutron irradiated titanium and rhenium

The defect structure in α-titanium and rhenium irradiated with neutrons at 0.4T _m (T _m = absolute melting temperature) has been analyzed using transmission electron microscopy. In rhenium, the vacancies agglomerate into voids whereas in titanium, vacancy dislocation loops lying on the prism planes are the only vacancy type defects observed. In both metals, dislocation segments and network fragments are the main evidence of interstitial type defects. The presence of dislocation loops rather than voids in titanium irradiated at this temperature is an anamalous result when compared to results on other similarly irradiated pure metals. Possible explanations for the preferential formation of loops rather than voids in the titanium are discussed.     

Investigation of the effect of gamma rays on electrical properties of chlorinated poly (vinyl chloride)

Dielectric constant, dielectric loss and AC conductivity were measured, in the frequency range 100 Hz to 5 MHz in chlorinated poly (vinyl chloride) (CPVC) before and after exposure to gamma irradiation at doses between 5.0 KGy and 50.0 KGy. The frequency dependencies of ε′, ε″ and σ_AC at 30 °C were investigated. A relaxation peak in the dielectric loss and a corresponding step in the dielectric constant have been observed, in the frequency ranges 10³ Hz to 10⁴ Hz. The dielectric constant ε′, dielectric loss ε″ and AC conductivity σ_AC are also found to increase at heating up to 100 °C. In addition the effect of gamma irradiation on the frequency dependencies of ε′, ε″ and σ_AC was measured at room temperature. The gamma irradiation leads to an increase in the efficiency of soft segments. Furthermore, the DC electrical conductivity of both the irradiated and non-irradiated samples was investigated. The induced electrical conductivity and the activation energy were measured, at various temperatures, as a function of gamma dose. It was found that the gamma radiation has a definite effect on the DC conductivity of the CPVC polymer.     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

Mechanism of formation of cellular dislocation structures during propagation of intense shock waves in crystals

The mechanism of formation of a cellular dislocation structure in face-centered cubic (fcc) metal crystals subjected to shock compression at strain rates \(\dot \varepsilon \) > 10⁶ s^?1 has been considered theoretically within the dislocation kinetic approach based on the kinetic equation for the dislocation density (dislocation constitutive equation). A dislocation structure of the cellular type is formed in the case of a two-wave structure of the compression wave behind its shock front (elastic precursor). It has been found that, at pressures σ > 10 GPa, the dislocation cell size Λ _c depends on the pressure σ and the density ρ _G of geometrically necessary dislocations generated at the shock front according to the relationship Λ _c ～ ρ _G ^?n ～ σ^?m , where n = 1/4–1/2, m = 3/4–3/2, and m = 1, for different pressures and orientations of the crystal. It has been shown that, in copper and nickel crystals with the shock loading axis oriented along the [001] direction, the cellular structure is not formed after reaching the critical pressures σ _c equal to 31 and 45 GPa, respectively.     

Modelling the yield point phenomena during deformation at elevated temperatures: case study on Inconel 600

The discontinuous yield behaviour (DYB) of Inconel 600 was studied during hot compression tests at temperatures in range of 850–1150°C and strain rates of 0.001–1?s^?1. The yield point phenomena were observed in the temperature range of 850–1000°C and strain rates of 0.001–0.1 s^?1. The DYB was modelled by considering the evolution of dislocation density at the early stages of yielding. The opposite effects of dislocation multiplication, dislocation interaction (work hardening) and dynamic recovery (DRV) were considered. It was shown that the dislocation multiplication and DRV result in flow softening, while the dislocation interaction leads to work hardening. The model was established in a way to consider the effects of various microstructural evolutions on the σ(ε) function. The discontinuous flow curves were fitted by the developed model with acceptable precision. The variations of material constants with temperature and strain rate were found physically meaningful. The dislocation multiplication parameter was determined at various temperatures and strain rates. It was concluded that the rate of dislocation multiplication increases as temperature rises or strain rate declines. Accelerated dislocation multiplication leads to less drop in yield stress between the upper and lower yield points.