Serrated flow behavior in AL6XN austenitic stainless steel

Serrated flow behavior of the AL6XN austenitic stainless steel has been investigated at different temperatures and strain rates. The results show the serrated flow, peak/plateau in flow stress and negative strain rate sensitivity appearing in tensile deformation of the AL6XN steel at 773-973 K and 3.3 × 10⁻⁵-3.3 × 10⁻³ s⁻¹ (excluding 873 K, 3.3 × 10⁻⁵ s⁻¹), suggesting the occurrence of dynamic strain aging (DSA). The activation energy for type-A and -(A + B) serrations was calculated to be 304 kJ/mol and diffusion of substitutional solutes, such as chromium and molybdenum is considered as the mechanism of serrated flow. TEM observations further revealed a typical planar slip mode in the regime of DSA of the deformed AL6XN steel.     

Impact deformation behaviour of Ti-6Al-4V alloy in the low-temperature regime

The impact response of Ti-6Al-4V alloy is investigated using a compressive split-Hopkinson pressure bar at strain rates of 1.0 × 10³ s⁻¹, 3.0 × 10³ s⁻¹ and 4.3 × 10³ s⁻¹ and temperatures of −150 °C, 0 °C and 25 °C, respectively. It is shown that for a constant temperature, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, while the activation volume decreases. Meanwhile, for a constant strain rate, the activation volume increases with increasing temperature, while the flow stress, work hardening rate and strain rate sensitivity decrease. Scanning electron microscopy (SEM) observations reveal that the fracture surfaces are characterised by a transgranular dimpled structure, indicating that Ti-6Al-4V alloy has excellent ductility. The density of the dimples increases with an increasing strain rate or increasing temperature. Transmission electron microscopy (TEM) observations show that the dislocation density increases with an increasing strain rate, but decreases with an increasing temperature. The linear correlation between the square root of the dislocation density and the true stress confirms the existence of a Bailey-Hirsch type relationship. Finally, the strengthening effect observed at higher strain rates and lower temperatures is attributed to a greater dislocation density.     

Strain rate dependence of impact properties of sintered 316L stainless steel

This paper uses a material testing system (MTS) and a compressive split-Hopkinson bar to investigate the impact behaviour of sintered 316L stainless steel at strain rates ranging from 10⁻³ s⁻¹ to 7.5 × 10³ s⁻¹. It is found that the true stress, the rate of work hardening and the strain rate sensitivity vary significantly as the strain rate increases. The flow behaviour of the sintered 316L stainless steel can be accurately predicted using a constitutive law based on Gurson’s yield criterion and the flow rule proposed by Khan, Huang and Liang (KHL). Microstructural observations reveal that the degree of localized grain deformation increases, but the pore density and the grain size decrease, with increasing strain rate. Adiabatic shear bands associated with cracking are developed at strain rates higher than 5.6 × 10³ s⁻¹. The fracture surfaces exhibit ductile dimples. The depth and density of these dimples decrease with increasing strain rate.     

Characterization of hot deformation behavior of Zr-1Nb-1Sn alloy

The hot deformation behavior of β-quenched Zr-1Nb-1Sn was studied in the temperature range 650-1050 °C and strain rate range 0.001-100 s⁻¹ using processing maps. These maps revealed three different domains: a domain of dynamic recovery at temperatures <700 °C and at strain rates <3 × 10⁻³ s⁻¹, a domain of dynamic recrystallization in the temperature range 750-950 °C and at strain rates <10⁻² s⁻¹ with a peak at 910 °C and 10⁻³ s⁻¹ (in α + β phase field), and a domain of large-grain superplasticity in the β phase field at strain rates <10⁻² s⁻¹. In order to identify the rate controlling mechanisms involved in these domains, kinetic analysis was carried out to determine the various activation parameters. In addition, the processing maps showed a regime of flow instability spanning both α + β and β phase fields. The hot deformation behavior of Zr-1Nb-1Sn was compared with that of Zr, Zr-2.5Nb and Zircaloy-2 to bring out the effects of alloy additions.     

Liquid metal embrittlement of an austenitic 316L type and a ferritic-martensitic T91 type steel by mercury

The susceptibility to liquid metal embrittlement (LME) of 316L and T91 steels by mercury has been studied at room temperature. A dedicated experimental device using center crack tension (CCT) specimens was built. We developed a specimen preparation procedure that must be rigorously applied in order to investigate the embrittling effect of Hg. The high strength ferritic-martensitic steel of type T91 is embrittled by Hg at room temperature over a large range of crosshead speeds, between 6.67 × 10⁻⁷ and 6.67 × 10⁻³ m s⁻¹. More surprisingly, the austenitic steel of type 316L is also embrittled by Hg between 1.67 × 10⁻⁸ and 2.5 × 10⁻⁴ m s⁻¹. The fracture of the T91 and 316L CCT specimens in contact with Hg occurs by shear band decohesion over the above-mentioned range of crosshead speeds.     

Deformation mode maps for tensile deformation of neutron-irradiated structural alloys

The deformation microstructures of neutron-irradiated nuclear structural alloys, A533B steel, 316 stainless steel, and Zircaloy-4, have been investigated by tensile testing and transmission electron microscopy to map the extent of strain localization processes in plastic deformation. Miniature specimens with a thickness of 0.25 mm were irradiated to five levels of neutron dose in the range 0.0001-0.9 displacements per atom (dpa) at 65-100 °C and deformed at room temperature at a nominal strain rate of 10⁻³ s⁻¹. Four modes of deformation were identified, namely three-dimensional dislocation cell formation, planar dislocation activity, fine scale twinning, and dislocation channel deformation (DCD) in which the radiation damage structure has been swept away. The modes varied with material, dose, and strain level. These observations are used to construct the first strain-neutron fluence-deformation mode maps for the test materials. Overall, irradiation encourages planar deformation which is seen as a precursor to DCD and which contributes to changes in the tensile curve, particularly reduced work hardening and diminished uniform ductility. The fluence dependence of the increase in yield stress, ΔYS = α(?t)ⁿ had an exponent of 0.4-0.5 for fluences up to about 3 × 10²² n m⁻² (∼0.05 dpa) and 0.08-0.15 for higher fluences, consistent with estimated saturation in radiation damage microstructure but also concurrent with the acceleration of gross strain localization associated with DCD.     

Liquid metal embrittlement susceptibility of ferritic-martensitic steel in liquid lead alloys

The susceptibility of the ferritic-martensitic steels T91 and EUROFER97 to liquid metal embrittlement (LME) in lead alloys has been examined under various conditions. T91, which is currently the most promising candidate material for the high temperature components of the future accelerator driven system (ADS) was tested in liquid lead bismuth eutectic (LBE), whereas the reduced activation steel, EUROFER97 which is under consideration to be the structural steel for fusion reactors was tested in liquid lead lithium eutectic. These steels, similar in microstructure and mechanical properties in the unirradiated condition were tested for their susceptibility to LME as function of temperature (150-450 °C) and strain rate (1 × 10⁻³-1 × 10⁻⁶ s⁻¹). Also, the influence of pre-exposure and surface stress concentrators was evaluated for both steels in, respectively, liquid PbBi and PbLi environment. To assess the LME effect, results of the tests in liquid metal environment are compared with tests in air or inert gas environment. Although both unirradiated and irradiated smooth ferritic-martensitic steels do not show any or little deterioration of mechanical properties in liquid lead alloy environment compared to their mechanical properties in gas as function of temperature and strain rate, pre-exposure or the presence of surface stress concentrators does lead to a significant decrease in total elongation for certain test conditions depending on the type of liquid metal environment. The results are discussed in terms of wetting enhanced by liquid metal corrosion or crack initiation processes.     

Thermal and structural properties of low-fluence irradiated graphite

The release of Wigner energy from graphite irradiated by fast neutrons at a TRIGA Mark II research reactor has been studied by differential scanning calorimetry and simultaneous differential scanning calorimetry / synchrotron powder X-ray diffraction between 25 and 725 °C at a heating rate of 10 °C min⁻¹. The graphite, having been subject to a fast-neutron fluence from 5.67 × 10²⁰ to 1.13 × 10²² n m⁻² at a fast-neutron flux (E > 0.1 MeV) of 7.88 × 10¹⁶ n m⁻² s⁻¹ and at temperatures not exceeding 100 °C, exhibits Wigner energies ranging from 1.2 to 21.8 J g⁻¹ and a Wigner energy accumulation rate of 1.9 × 10⁻²¹ J g⁻¹ n⁻¹ m². The differential-scanning-calorimeter curves exhibit, in addition to the well known peak at ∼200 °C, a pronounced fine structure consisting of additional peaks at ∼150, ∼230, and ∼280 °C. These peaks correspond to activation energies of 1.31, 1.47, 1.57, and 1.72 eV, respectively. Crystal structure of the samples is intact. The dependence of the c lattice parameter on temperature between 25 and 725 °C as determined by Rietveld refinement leads to the expected microscopic thermal expansion coefficient along the c axis of ∼26 × 10⁻⁶ °C⁻¹. At 200 °C, coinciding with the maximum in the differential-scanning-calorimeter curves, no measurable changes in the rate of thermal expansion have been detected - unlike its decrease previously seen in more highly irradiated graphite.     

Deuterium retention in sintered boron carbide exposed to a deuterium plasma

Depth profiles of deuterium up to a depth of 10 μm have been measured using the D(³He,p)⁴He nuclear reaction in a resonance-like technique after exposure of sintered boron carbide, B₄C, at elevated temperatures to a low energy (≈200 eV/D) and high ion flux (≈10²¹ m⁻² s⁻¹) D plasma. The proton yield was measured as a function of incident ³He energy and the D depth profile was obtained by deconvolution of the measured proton yields using the program SIMNRA. D atoms diffuse into the bulk at temperatures above 553 K, and accumulate up to a maximum concentration of about 0.2 at.%. At high fluences (?10²⁴ D/m²), the accumulation in the bulk plays a major role in the D retention. With increasing exposure temperature, the amount of D retained in B₄C increases and exceeds a value of 2 × 10²¹ D/m² at 923 K. The deuterium diffusivity in the sintered boron carbide is estimated to be D = 2.6 × 10⁻⁶exp{−(107 ± 10) kJ mol⁻¹/RT} m² s⁻¹.     

Behaviour of chromium steels in liquid Pb-55.5Bi with changing oxygen content and temperature

The behaviour of protective oxide layers on P122 steel and its welds and of ODS steel in liquid Pb_44.5Bi_55.5 (LBE) is examined under conditions of changing temperatures and oxygen concentrations. P122 (12Cr) and its welded joints are exposed to LBE at 550 °C for 4000 h with oxygen concentrations of 10⁻⁶ and 10⁻⁸ wt% (p(O₂) = 8.1 × 10⁻²³ bar and 5.2 × 10⁻²⁷ bar) which change every 800 h. It is found that like in case of constant oxygen concentration of 10⁻⁶ wt% a protective spinel layer (Fe(Fe_1−xCr_x)₂O₄) was maintained on P122 and also on its welded joint. Two experiments with exposure times of 4800 h are conducted on ODS steel, both with temperatures changing from 550 to 650 °C and back every 800 h, one experiment with 10⁻⁶ the other with 10⁻⁸ wt% oxygen in LBE. Both experiments show strong local dissolution attack after 4800 h which is in agreement with the behaviour of ODS in LBE at a constant temperature of 650 °C. However, dissolution attack is less in LBE with 10⁻⁸ wt% oxygen (p(O₂) = 3.0 × 10⁻²⁵ bar).     

The effect of stress levels on the coefficient of thermal expansion of a fine-grained isotropic nuclear graphite

Due to the fluctuation and non-uniform distribution of temperature within the core structure of high-temperature gas-cooled reactors (HTGRs), the thermal expansion behavior of graphite materials plays an important role in the design of graphite components, especially of large-scale components. In the present paper, in order to investigate the influence of stress levels on the coefficient of thermal expansion (CTE) of IG-110 graphite, the strain gauge method was used to measure the CTE on the cylindrical specimens under a series of loads applied using a universal tensile testing machine. In addition, a more precise measurement using a thermal dilatometer was employed to validate the tests using the strain gauge method. A good agreement has been obtained between the experimental results using these two methods. The results show that when the specimens were under compressive loads, the CTE along the loading direction of the specimens increased and that along the perpendicular direction decreased, with more changes in the former. The absolute changes of the CTE in the two directions increased with increasing applied load. When graphite specimens were subjected to a compressive load of 40 MPa, the axial CTE of specimens sectioned along the radial direction of the graphite brick as it is installed in the core structure increased from 4.13 × 10⁻⁶ to 5.35 × 10⁻⁶ K⁻¹, while the axial CTE of specimens sectioned along the vertical direction increased from 3.97 × 10⁻⁶ to 5.58 × 10⁻⁶ K⁻¹. Moreover, the residual change of the CTE, which was caused by the permanent residual strain after unloading, was observed. The change of the CTE with stress levels should be considered in the stress analysis and life prediction of the nuclear graphite components.     

Mechanism of dynamic strain aging and characterization of its effect on the low-cycle fatigue behavior in type 316L stainless steel

Low-cycle fatigue tests were carried out in air in a wide temperature range from 20 to 650 °C with strain rates of 3.2 × 10⁻⁵–1 × 10⁻² s⁻¹ for type 316L stainless steel to investigate dynamic strain aging (DSA) effect on the fatigue resistance. The regime of DSA was evaluated using the anomalies associated with DSA and was in the temperature range of 250–550 °C at a strain rate of 1 × 10⁻⁴ s⁻¹, in 250–600 °C at 1 × 10⁻³ s⁻¹, and in 250–650 °C at 1 × 10⁻² s⁻¹. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It seems to be reasonable to infer that DSA is caused by the pipe diffusion of solute atoms through the dislocation core. Dynamic strain aging reduced the crack initiation and propagation life by way of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening, respectively.     

Structural characterization of buried nitride layers formed by nitrogen ion implantation in silicon

The synthesis of buried silicon nitride insulating layers was carried out by SIMNI (separation by implanted nitrogen) process using implantation of 140 keV nitrogen (¹⁴N⁺) ions at fluence of 1.0 × 10¹⁷, 2.5 × 10¹⁷ and 5.0 × 10¹⁷ cm⁻² into 〈1 1 1〉 single crystal silicon substrates held at elevated temperature (410 °C). The structures of ion-beam synthesized buried silicon nitride layers were studied by X-ray diffraction (XRD) technique. The XRD studies reveal the formation of hexagonal silicon nitride (Si₃N₄) structure at all fluences. The concentration of the silicon nitride phase was found to be dependent on the ion fluence. The intensity and full width at half maximum (FWHM) of XRD peak were found to increase with increase in ion fluence. The Raman spectra for samples implanted with different ion fluences show crystalline silicon (c-Si) substrate peak at wavenumber 520 cm⁻¹. The intensity of the silicon peak was found to decrease with increase in ion fluence.     

Ion-beam irradiation effects on reactively sputtered CrN thin films

The present study deals with CrN/Si bilayers irradiated at room temperature (RT) with 120 keV Ar ions. The CrN layers were deposited by d.c. reactive sputtering on Si(1 0 0) wafers, at different nitrogen partial pressures (2 × 10⁻⁴, 3.5 × 10⁻⁴ and 5 × 10⁻⁴ mbar), to a total thickness of 240-280 nm. The substrates were held at room temperature (RT) or 150 °C during deposition. After deposition the CrN/Si bilayers were irradiated up to fluences of 1 × 10¹⁵ and 1 × 10¹⁶ ions/cm². Structural characterization was performed with Rutherford backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (XTEM) and grazing angle X-ray diffraction (XRD). For the highest nitrogen pressure (5 × 10⁻⁴ mbar) a pure stoichiometric CrN phase was achieved. The results showed that Ar ion irradiation resulted in the variation of the lattice constants, micro-strain and mean grain size of the CrN layers. The observed microstructural changes are due to the formation of the high density damage region in the CrN thin film structure.     

Effect of dynamic strain aging on the leak-before-break analysis in SA106 Gr.C piping steel

The characteristics of dynamic strain aging (DSA) on material properties used in leak-before-break (LBB) analysis were discussed. Using these material data, the effect of DSA on the LBB analysis was estimated through the evaluation of leakage-size crack and flaw stability in SA106 Gr.C piping steel. Also, the results were represented as a form of ‘LBB allowable load window'. In the DSA temperature region, the leakage-size crack length was smaller than that at other temperatures and it increased with increasing tensile strain rate. In the results of flaw stability analysis, the lowest instability load appeared at the temperature corresponding to the minimum J–R curve which was caused by DSA. The instability load depended on the loading rate of J–R data, and decreased with increasing tensile strain rate at the plant operating temperature. These are due to the strain hardening characteristic and strain rate sensitivity of DSA. In the ‘LBB allowable load window', the LBB allowable region at the temperature and loading conditions where DSA occurs was decreased by about 30% compared with that in other conditions.     

Study of silver diffusion in silicon carbide

Diffusion of silver in 6H-SiC and polycrystalline CVD-SiC was investigated using α-particle channeling spectroscopy and electron microscopy. Fluences of 2 × 10¹⁶ cm⁻² of ¹⁰⁹Ag⁺ were implanted with an energy of 360 keV at room temperature, at 350 °C and 600 °C, producing an atomic density of approximately 2% at the projected range of about 110 nm. The broadening of the implantation profile and the loss of silver through the front surface during vacuum annealing at temperatures up to 1600 °C was determined. Fairly strong silver diffusion was observed after an initial 10 h annealing period at 1300 °C in both polycrystalline and single crystalline SiC, which is mainly due to implant induced radiation damage. After further annealing at this temperature no additional diffusion took place in the 6H-SiC samples, while it was considerably reduced in the CVD-SiC. The latter was obviously due to grain boundary diffusion and could be described by the Fick diffusion equation. Isochronal annealing of CVD-SiC up to 1400 °C exhibited an Arrhenius type temperature dependence, from which a frequency factor D_o ∼ 4 × 10⁻¹² m² s⁻¹ and an activation energy E_a ∼ 4 × 10⁻¹⁹ J could be extracted. Annealing of 6H-SiC above 1400 °C shifted the silver profile without any broadening towards the surface, where most of the silver was released at 1600 °C. Electron microscopy revealed that this process was accompanied by significant re-structuring of the surface region. An upper limit of D < 10⁻²¹ m² s⁻¹ was estimated for 6H-SiC at 1300 °C.     

Effect of swift heavy ion irradiation on structural, optical and electrical properties of Cd2SnO4 thin films

Transparent conducting cadmium stannate thin films were prepared by spray pyrolysis method on Corning substrate at a temperature of 525 °C. The prepared films are irradiated using 120 MeV swift Ag⁹⁺ ions for the fluence in the range 1 × 10¹² to 1 × 10¹³ ions cm⁻² and the structural, optical and electrical properties were studied. The intensity of the film decreases with increasing ion fluence and amorphization takes place at higher fluence (1 × 10¹³ ions cm⁻²). The transmittance of the films decreases with increasing ion fluence and also the band gap value decreases with increasing ion fluence. The resistivity of the film increased from 2.66 × 10⁻³ Ω cm (pristine) to 5.57 × 10⁻³ Ω cm for the film irradiated with 1 × 10¹³ ions cm⁻². The mobility of the film decreased from 31 to 12 cm²/V s for the film irradiated with the fluence of 1 × 10¹³ ions cm⁻².     

Radiation tolerance of Cu/W multilayered nanocomposites

Magnetron sputtered Cu/W multilayer samples with individual layer thicknesses from 2.5 to 50 nm were irradiated by 50 keV He⁺ ions at ion fluences from 7 × 10²⁰ to 6 × 10²¹ m⁻² at room temperature. Evolution of the interfacial structure during irradiation is monitored by X-ray diffraction and cross-sectional transmission electron microscopy. Moreover, radiation responses on the individual layer thickness and He⁺ ion irradiation fluence are revealed. The highly morphological stability of the multilayered structure suggests that the interfacial structure and grain boundary can serve as sinks for radiation-induced defects.     

Determination of fatigue curve parameters at cyclic strain limited loading according to the mechanical characteristics of power energy structural materials

Over 280 structural materials used in the Russian nuclear power energy industry were tested at Kaunas University of Technology, as commissioned by St. Petersburg Central Research Institute of Structural Materials in the period of 1970-2000. Alloyed structural steels, stainless steels and metals of their welded joints with different types of thermal treatment were under research in the conditions of symmetric low cycle tension-compression (N ≤ (1-2) × 10⁴) at room and elevated (200-350 °C) temperatures. During these experiments the characteristics of monotonic tension, cyclic stress-strain curves and low cycle fatigue curves, which are expressed by linear dependence of the total strain ? on the number of cycles N in co-ordinates log ? − log N, parameters C_1exp and m_1exp were determined. The range of cycle strain ? when this low cycle fatigue curve may be used is determined. The definition of the approached values of low cycle fatigue curves parameters C_1cal and m_1cal at room temperature by the mechanical characteristics of alloyed structural steels and their weld metals, stainless steels and their weld metals is analyzed in this work. The new relationships ln C_1exp − m_1exp among low cycle fatigue curves parameters, obtained by analyzing experimental data of 22-48 materials in each group, are determined. These relationships were used for determination of dependencies of low cycle parameters C_1cal and m_1cal by the mechanical characteristics of analyzed groups of materials. From theoretically and experimentally determined equations by using calculated values C_1cal and m_1cal dependencies for revision of Coffin-Manson fatigue curves parameters C and m were proposed. These parameters for the cyclically hardening or softening materials are calculated by estimating the equivalent value of the cycle plastic strain range. The proposed analytical dependencies make it possible to more exactly calculate the number of cycles N ≤ N₀ (N₀ = 10⁶) before the fatigue crack initiation for the structural materials of analyzed groups under cyclic strain limited loading by using mechanical characteristics of materials.     

Thermal expansion studies on Inconel-600 by high temperature X-ray diffraction

The lattice thermal expansion characteristics of Inconel-600^® have been studied by high temperature X-ray diffraction (HT-XRD) technique in the temperature range 298-1200 K. Altogether four experimental runs were conducted on thin foils of about 75-100 μm thickness. The diffraction profiles have been accurately calibrated to offset the shift in 2θ values introduced by sample buckling at elevated temperatures. The corrected lattice parameter data have been used to estimate the instantaneous and mean linear thermal expansion coefficients as a function of temperature. The thermal expansion values estimated in the present study show a fair degree of agreement with other existing dilatometer based bulk thermal expansion estimates. The lattice parameter for this alloy at 300 K is found to be 0.3549(1) nm. The mean linear thermal expansivity is found to be 11.4 × 10⁻⁶ K⁻¹.