Sputtering and surface state evolution of Bi under oblique incidence of 120 keV Ar ions

The sputtering and surface state evolution of Bi/Si targets under oblique incidence of 120 keV Ar⁺ ions have been investigated over the range of incidence angles 0° ? θ_i ? 60°. Increasing erosion of irradiated samples (whose surface thickness reduced by ∼3% at normal incidence up to ∼8% at θ = 60°) and their surface smoothing with reducing grain sizing were pointed out using Rutherford backscattering (RBS), atomic force (AFM) and X-ray diffraction (XRD) techniques. Measured sputtering yield data versus θ_i with fixed ion fluence to ∼1.5 × 10¹⁵ cm⁻² are well described by Yamamura et al. semi-empirical formula and Monte Carlo (MC) simulation using the SRIM-2008 computer code. The observed increase in sputter yield versus incidence angle is closely correlated to Bi surface topography and crystalline structure changes under ion irradiation.     

Large-scale molecular dynamics simulation of sputtering process with glancing-angle Ar cluster impact on 4H-SiC

Large-scale molecular dynamics simulations with two Ar₆₈₈ cluster impacts on a 4H-SiC surface are performed to investigate the mechanism of lateral sputtering caused by two clusters collisions. The two Ar clusters are composed of 688 atoms each (referred as Ar₆₈₈) which are described by a simple Lennard-Jones potential. The initial velocities of both clusters are 2.55 × 10⁴ m/s when the acceleration voltage is 100 keV. The computational volume is 30 nm × 30 nm × 16 nm, which is constructed by 1444608 4H-SiC atoms. At 0.8 ps after the impact from the first Ar cluster on the 4H-SiC surface, a second argon cluster with predetermined incident-angle collides with 4H-SiC surface at a distance of one “diameter” away from the center of the first impact where the term “diameter” refers to the diameter of the footprint of the first impact on 4H-SiC. The incident-angle of the second argon cluster was set at 0°, 60°, or 80° for three different trials. Consequently, in each case the crater formed by the first cluster showed signs of being smeared out by the impact of the second cluster. Especially at the incident-angle of 80° the effects of surface modification were clearly noticeable.     

Probing stray surface electric patch fields using Rydberg atoms

The stray electric “patch” fields present at a Au(1 1 1) surface are investigated by studying the ionization of Rydberg atoms incident at near-grazing angles. Measurements of the threshold conditions required to observe the resulting ions are used to estimate how large such stray fields can be. The data show that the stray fields can be sizeable, as large as ∼10³ V cm⁻¹ 100 nm from the surface and ∼20 V cm⁻¹ 500 nm from the surface, and illustrate the potential of Rydberg atoms for detecting and characterizing surface electric fields.     

Differential cross-section measurements for the C(d, p0)C reaction and applications to surface analysis of materials

This work involves surface analysis by nuclear techniques, which are non-destructive, and computer simulation. The “energy analysis” method for nuclear reaction analysis is used. Energy spectra are computer simulated and compared to experimental data, giving target composition and concentration profile information. Measured values are presented for the differential cross-section of the ¹²C(d, p₀)¹³C reaction in the deuteron energy range 0.81-2.07 MeV for laboratory detection angles of 165° and 135°, using self-supported two-layered targets consisting of high purity thin films of typically 13 μg/cm² natural carbon and 65 μg/cm² gold. The error in the absolute differential cross-section values is generally ∼6%. The method, using these values, is successfully applied to determination of uniform concentration profiles of ¹²C, along considerable depths, for a thick flat target of high purity pyrolitic graphite. It is characterised a thin surface film of carbon on a thick flat quartz target. Uniform concentration profiles of ¹⁶O are also obtained from (d, p) and (d, α) reactions.     

LiMgPO4:Tb,B OSL phosphor - CW and LM OSL studies

In the adjoining paper, the authors have proposed LiMgPO₄:Tb,B (LMP) OSL phosphor as a potential alternative to α-Al₂O₃:C for dosimetry applications. This calls for developing an understanding on TL and OSL aspects of this highly sensitive LMP phosphor. CW and LM OSL processes were therefore studied experimentally and kinetic analysis was carried out using theoretical models. Continuous wave (CW) OSL curve for LMP phosphor was found not to follow single decaying exponential implying that the CWOSL curve does not follow first order kinetics. Under pre-readout annealing at 125, 200 and 300 °C for 10 s, the nature of decay profile was unaffected and same holds true for optically bleached CWOSL curves. From linearly modulated (LM) OSL studies, it was found that the shape/geometrical factor μ_g was ∼0.72 ± 0.03 for wide range of doses (up to 12 Gy studied) and peak position “t_m” was also independent of dose. The μ_g was found to be unaffected with pre-readout annealing at 125, 200 and 300 °C for 10 s and optical bleaching, however it was found that peak position “t_m” shifted towards higher side in time with increase of optical bleaching. Dose dependence tests were also carried out for LMOSL curves and it was found that peak position “t_m” was independent of dose, which is typical characteristic of curves following first order kinetics. Hence LM-OSL curve might be mixture of more than one component.Further from CW and LM OSL studies, it was also found that the individual contribution from first, second and third TL peak toward OSL is ∼33%, ∼45% and ∼22%, respectively. Traps beyond 350 °C were found not to contribute towards OSL when stimulated using blue LEDs. In the present paper, the CW and LM OSL processes for LMP phosphor were studied experimentally and their kinetic analysis was carried out.     

Improved wear resistance of Al-15Si alloy with a high current pulsed electron beam treatment

A hypereutectic Al-15Si alloy (Si 15 wt.%, Al balance) was irradiated by high current pulsed electron beam (HCPEB). The HCPEB treatment causes ultra-rapid heating, melting and cooling at the top surface layer. As a result, the special “halo” microstructure centering on the primary Si phase is formed on the surface due to interdiffusion of Al and Si elements. The composition of the “halo” microstructure is distributed continuously from the center to the edge of the “halo”. Compared to an untreated matrix, the remelted layer underneath the surface presents single contrast because of the compositional homogeneity after HCPEB treatment. The thickness of the remelted layer increases slightly from 4.4 μm (5 pulses) to 5.6 μm (25 pulses). HCPEB treatment broadens and shifts the diffraction peaks of Al and Si. The lattice parameters of Al decreases due to the formation of a supersaturated solid solution of Al in the melted layer. Through analysis of Raman spectra and transmission electron microscopy (TEM), the amorphous Si (a-Si) and nanocrystalline Si are formed in the near-surface region under multiple bombardments of HCPEB. The relative wear resistance of a 15-pulse sample is effectively improved by a factor of 9, which can be attributed to the formation of metastable structures.     

Effects of stress on radiation hardening and microstructural evolution in A533B steel

Bent specimens of A533B steel (0.16 wt% Cu) were irradiated at 290 °C to 1 dpa with 6.4 MeV Fe³⁺ ions. Calculated tensile stresses at the irradiated surface were set to 0, 250, 500 and 750 MPa. The specimens were subjected to hardness measurements, transmission electron microscopy (TEM) observations and three-dimensional atom probe (3DAP) analysis. The radiation-induced hardening decreased with increasing stress to 500 MPa which was near the yield strength. TEM and 3DAP results showed that well-defined dislocation loops and solute clusters were formed. The diameter of dislocation loops increased and the number density decreased when the stress was applied, whereas the diameter and number density of solute clusters decreased. The hardening was mainly attributed to solute cluster formation. Application of tensile stress would control hardening by suppressing the solute cluster nucleation and growth.     

Influence of hot rolling and high speed hydrostatic extrusion on the microstructure and mechanical properties of an ODS RAF steel

An argon gas atomized, pre-alloyed Fe-14Cr-2W-0.3Ti (wt.%) reduced activation ferritic (RAF) steel powder was mechanically alloyed with 0.3wt.% Y₂O₃ nano-particles in an attritor ball mill and consolidated by hot isostatic pressing at 1150 °C under a pressure of 200 MPa for 3 h. In the aim to improve its mechanical properties the ODS steel was then submitted to a thermo-mechanical treatment (TMT): hot rolling (HR) at 850 °C or high speed hydrostatic extrusion (HSHE) at 900 °C, followed by heat treatment (HT).Transmission electron microscopy (TEM) observations of the ODS alloys after TMT and heat treatment revealed the presence of elongated grains in the longitudinal direction, with an average width of 8 μm and an average length of 75 μm, and equiaxed grains, a few microns in diameter, in the transverse direction. Two populations of oxide particles were observed by TEM: large Ti-Al-O particles, up to 250 nm in diameter, usually located at the grain boundaries and small Y-Ti-O nanoclusters, about 2.5 nm in diameter, uniformly distributed in the matrix. Charpy impact tests revealed that the HSHE material exhibits a larger upper shelf energy (5.8 J) than the HR material (2.9 J). The ductile-to-brittle transition temperature of both alloys is relatively high, in the range of 55-72 °C. Tensile mechanical properties of both ODS alloys were found satisfactory over the full range of investigated temperatures (23-750 °C). The HSHE material exhibits better tensile strength and ductility than the HR material. These results indicate that HSHE can be considered as a promising TMT method for improving the mechanical properties of ODS RAF steels.     

Thermal stability of nano-structured ferritic alloy

The excellent tensile and creep strength and the potential for managing radiation damage make nano-structured ferritic alloys (NFAs) promising candidates for high-temperature applications in spallation proton, advanced fission and fusion neutron environments. The thermal stability of NFAs is critical for such applications, hence, this has been investigated in a series of aging experiments on MA957 at 900 °C, 950 °C and 1000 °C for times up to 3000 h. Optical and transmission electron microscopy (TEM) studies showed the fine scale grain and dislocation structures are stable up to 1000 °C. TEM and small angle neutron scattering (SANS) showed that the nm-scale solute cluster-oxide features (NFs), that are a primary source of the high strength of NFAs, were stable at 900 °C and coarsened only slightly at 950 °C and 1000 °C. Porosity that developed during high-temperature aging was minimal at 900 °C and modest at 950 °C, but was much larger after 1000 °C. Microhardness was basically unchanged after the 900 °C aging, and decreased only slightly (?3%) after aging at 950 °C and 1000 °C.     

Proton sputtering from polycrystalline Al surface interacting with highly charged ions at grazing incidence angle

Sputtering processes of protons from a polycrystalline Al surface interacting with Ar^q+ (q = 3-14) ions at a grazing incidence angle (∼0.5°) were investigated. The intensity of protons (I_H) detected in coincidence with scattered Ar atoms was measured as a function of q. I_H saturated at q ? 10, although it increased rapidly with q at 3 ? q ? 8. The angular distribution of protons with low kinetic energy (?2 eV) began to deviate from the cosine distribution and assumed a rather flat equidistribution as q increased. To analyze the sputtering processes of protons at the grazing incidence angle, a modified model of the “above-surface potential sputtering model” was proposed by considering image acceleration of projectile ions.     

Study of the structural modifications induced by He implantation in cubic zirconia

This article deals with the study of the structural modifications induced in yttria-stabilized zirconia implanted with low-energy (30 keV) He ions. For this purpose, three complementary analysis techniques, namely RBS/C, XRD and TEM, were used. After implantation at 5 × 10¹⁶ cm⁻² (∼4 at% to ∼1.7 dpa), it is found that the disorder level is weak, and the damage likely consists in small interstitial-type defects and helium-vacancy clusters. These defects induce a tensile strain gradient in the direction normal to the implanted crystal surface. This weak damage indicates a strong mechanical resistance of the zirconia matrix against He implantation.     

CFD modeling and thermal-hydraulic analysis for the passive decay heat removal of a sodium-cooled fast reactor

In this study, a pool-typed design similar to sodium-cooled fast reactor (SFR) of the fourth generation reactors has been modeled using CFD simulations to investigate the characteristics of a passive mechanism of Shutdown Heat Removal System (SHRS). The main aim is to refine the reactor pool design in terms of temperature safety margin of the sodium pool. Thus, an appropriate protection mechanism is maintained in order to ensure the safety and integrity of the reactor system during a shutdown mode without using any active heat removal system. The impacts on the pool temperature are evaluated based on the following considerations: (1) the aspect ratio of pool diameter to depth, (2) the values of thermal emissivity of the surface materials of reactor and guard vessels, and (3) innerpool liner and core periphery structures. The computational results show that an optimal pool design in geometry can reduce the maximum pool temperature down to ∼551 °C which is substantially lower than ∼627 °C as calculated for the reference case. It is also concluded that the passive Reactor Air Cooling System (RACS) is effective in removing decay heat after shutdown. Furthermore, thermal radiation from the surface of the reactor vessel is found to be important; and thus, the selection of the vessel surface materials with a high emissivity would be a crucial factor for consideration in safety design. This study provides future researchers with a guideline on designing safety measures for the fourth generation of the fast reactors with no particular reference to any specific manufacturer.     

Grain refinement of T91 alloy by equal channel angular pressing

We report on the grain refinement of modified 9Cr-1Mo ferritic-martensitic steel (T91) by equal channel angular pressing, a severe plastic deformation method. Microstructural refinement depends on processing temperature (300-700 °C) and extrusion strain (1.2-2.3). The average grain size has been refined by over an order of magnitude down to 300 nm, accompanied by a hardness increase of up to 70%. The refined microstructure undergoes little grain growth or softening up to 500 °C. At an annealing temperature of 700 °C or higher, significant softening occurs as a result of grain growth.     

Dynamic observations of heavy-ion damage in Fe and Fe-Cr alloys

We give an overview and summary of recent in-situ heavy-ion irradiation experiments on Fe and Fe-Cr alloys carried out on the Argonne IVEM Facility at irradiation temperatures up to 500 °C. Several new and unexpected observations were made. At low doses the contrast of new irradiation-induced dislocation loops sometimes developed over time intervals as long as 0.2 s, many orders of magnitude longer than expected for a process of cascade collapse. In addition at temperatures ?300 °C, “hopping” of 1/2<1 1 1> loops was induced by the ion or electron beams, especially in UHP Fe. At high doses complex microstructures developed in all materials, involving the formation of large interstitial loops. At 300 °C and RT these loops had Burgers vectors of type b = 1/2<1 1 1> and large shear components. At 500 °C only edge loops with b = <1 0 0> were produced.     

Synergistic effects of implanted helium and hydrogen and the effect of irradiation temperature on the microstructure of SiC/SiC composites

The microstructure of near-stoichiometric fiber SiC/SiC composites implanted with He and H ions was studied at implantation temperatures of 1000 and 1300 °C. The average size of He bubbles in the CVI SiC matrix decreases with increasing concentration of implanted H ions. Moreover, the number density of He bubbles increases with increasing irradiation temperature and amount of implanted H. At the irradiation temperature of 1000 °C, He bubbles were mainly formed at grain boundary within the matrix. On the other hand, He bubbles were formed both at grain boundaries and within grains at the irradiation temperature of 1300 °C. The average size of He bubbles at grain boundaries was much larger than within the grain. The average size of He bubbles in the fiber was smaller than that in the matrix in all cases.     

Localization by TEM and EELS of deuterium trapping sites in CFC exposed to plasma irradiation in Tore Supra

Carbon fiber composite (CFC) Sepcarb^® N11 is used in the tokamak Tore Supra as plasma-facing components. To investigate the fuel retention capability of this material, a mobile sample holder was used to expose CFC N11 samples to direct irradiation by the scrape-off layer plasma of Tore Supra at fluences up to 1 × 10²⁵ m⁻². Deuterium (D) elemental mapping using nuclear reaction analysis for the most-exposed CFC sample showed that D retention occurs at depths greater than 8 μm due to the presence of deep (>3.5 μm) local retention sites. In this work, combining transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS), we describe at a high spatial resolution where and how D atoms are trapped in these sites. TEM experiments performed on thin cross-sections of the plasma-modified surface show evidence of the presence of a 3.5 μm-thick deuterated amorphous carbon layer deposited on the CFC surface. We show that specific localized retention sites correspond to the filling of relatively large (∼3 μm.) and deep (at least 3 μm below the initial CFC surface) cracks between fibres and matrix by the deuterated amorphous carbon layer.     

The annealing behavior of Cu nanoparticles in Ar-ion implanted spinel

Magnesium aluminate spinel crystals (MgAl₂O₄ (1 1 0)) deposited with 30 nm Cu film on surface were implanted with 110 keV Ar-ions to a fluence of 1.0 × 10¹⁷ ions/cm² at 350 °C, and then annealed in vacuum condition at the temperature of 500, 600, 700, 800 and 900 °C for 1 h, respectively. Ultraviolet-visible spectrometry (UV-VIS), scanning electron microscopy (SEM), Rutherford backscattering (RBS) and transmission electron microscopy (TEM) were adopted to analyze the specimens. After implantation, the appearance of surface plasmon resonance (SPR) absorbance peak in the UV-VIS spectrum indicated the formation of Cu nanoparticles, and the TEM results for 500 °C also confirmed the formation of Cu nanoparticles at near-surface region. In annealing process, The SPR absorbance intensity increased at 500 and 700 °C, decreased with a blue shift of the peak position at 600 and 800 °C, and the peak disappeared at 900 °C. The SPR absorbance intensity evolution with temperature was discussed combined with other measurement results (RBS, SEM and TEM).     

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.     

The Influence of lithium environment on tensile behavior and microstructure of V-4Cr-4Ti

Thermal tests exposed V-4Cr-4Ti in static liquid lithium at 700 and 800 °C for 250, 500, and 1000 h. Post-exposure examination included chemical analysis of interstitial impurities in V-4Cr-4Ti to monitor impurity transfer, and tensile tests at room temperature and 500 °C. Microstructures were characterized by room-temperature electrical resistivity measurements and transmission electron microscopy. Oxygen was not depleted from V-4Cr-4Ti when nitrogen pickup occurred during lithium exposures. In spite of a significant increase in interstitial impurity concentration, the matrix interstitial solute content was reduced due to precipitation. Plate-shaped precipitates in the matrix and globular precipitates at grain boundaries were formed during lithium exposures at 700 °C, while only globular precipitates were observed at grain boundaries at 800 °C. Increases in strength, decreases in ductility, and reduced dynamic strain aging resulted. Ductility remained high after 1000 h exposures at both 700 and 800 °C.     

Grain growth and crack formation in NiO thin films by swift heavy ion irradiation

NiO thin films grown on Si(1 0 0) substrates by electron beam evaporation and sintered at 700 °C, were irradiated by 120 MeV Au⁹⁺ ions. Though irradiation is known to induce lattice disorder and suppression of crystallinity, we observe grain growth at some fluences of irradiation. Associated with the growth of grains, the films develop cracks at a fluence of 3 × 10¹² ions cm⁻². The width of the cracks increased at higher fluences. Swift heavy ion irradiation induced atomic diffusion and strain relaxation in nanoparticle thin films, which are not in thermodynamic equilibrium, seem to be responsible for the observed grain growth. This phenomenon along with the tensile stress induced surface instability lead to crack formation in the NiO thin films.