Ion-irradiated Stainless Steel Research Articles

A robust nano-indentation modeling method for ion-irradiated FCC single crystals using strain-gradient crystal plasticity theory and particle swarm optimization algorithm

Addressing the challenge of identifying an appropriate set of material and irradiation parameters for accurate simulation models using crystal plasticity finite element method (CPFEM), this study proposes a novel two-stage method for nano-indentation modeling of ion-irradiated face-centered cubic (FCC) materials. It includes implementing the strain-gradient crystal plasticity (SGCP) theory with irradiation effects and the calibration of simulation parameters using the particle swarm optimization (PSO) algorithm with experimental data. The proposed method consists of two stages: establishing CPFEM without irradiation effects in stage 1 and modeling irradiation effects based on CPFEM in stage 2. Modeling the nano-indentation test of ion-irradiated stainless steel 304 (SS304) using real experimental data is conducted to evaluate the efficiency of the proposed method. The accuracy of the calibration method using PSO is verified through comparisons between simulation and experimental results for force-indentation depth and hardness-indentation depth relationships under both unirradiated and irradiated conditions. Moreover, effect of ion-irradiation on the mechanical behavior during the nano-indentation of single crystal SS304 is also examined to demonstrate that the proposed method is a powerful approach for nano-indentation modeling of ion-irradiated FCC single crystals using SGCP theory and the PSO algorithm.

Investigation and correlation between surface modifications and field emission properties of laser-induced silicon plasma ion irradiated stainless steel

Laser-induced silicon (Si) plasma has been used as a source of ion irradiation for modifications in surface, structural and field emission properties of Stainless Steel (SS). Nd:YAG (532 nm, 10 ns) laser at irradiance of 15 GW/cm2 was used to generate Si-plasma ions which were detected by solid-state nuclear track detector (CR-39) and Faraday Cup (FC). Both the energy and fluence of Si ions were measured by FCs. In response to a stepwise increase in the number of laser pulses from 3000 to 12000, the ion fluence varies from 4.6 × 1014 to 18.3 × 1014 ions/cm2 with constant energy of 30 keV. Fourier Transformation Infrared (FTIR) spectroscopy analysis revealed that irradiated silicon makes the stretch band with oxygen Si–O–Si. X-Ray Diffraction (XRD) analysis confirmed the identification of new phase of Si (111) with an anomalous trend in crystallite size, dislocation line density and induced stresses in response to the irradiation with various Si ion fluences. Optical microscopy analysis showed the formation of pits, voids and tracks. Scanning electron microscope (SEM) analysis revealed the formation of nanoscale surface features including pores, craters, embedded particulates and protruded disk-like structures with multiple ablative layers at the fluence of 4.6 × 1014 to 13.7 × 1014 ions/cm2. At the highest ion fluence of 18.3 × 1014 ions/cm2, the flake/flower-like morphology is observed. Field emission (FE) properties were studied under ultra-high vacuum condition in a parallel plate configuration using planar virgin SS as an anode and structured SS as a cathode. The Fowler–Nordheim plots were obtained from I–V characteristics to evaluate the turn-on field, field enhancement factor β and a maximum current density ranging 1.5–4.5 V/µm, 5008–12807 and 96–454 nA/cm2, respectively. The variation in the FE properties is attributed to the different morphological features at varying silicon ion fluences.

Dynamic Interaction between Dislocation and Irradiation-Induced Defects in Stainless Steels during Tensile Deformation

A series of in-situ transmission electron microscopy (TEM) observations during tensile deformation were conducted on the ion-irradiated stainless steel. The jerky motion of dislocations appeared, and dislocations were pinned and depinned at the defects through the in-situ TEM observation. The jump distance traveled by dislocation was measured and discussed as the mean interval of defects interacting with the dislocation motion. Microstructural information of irradiation defects such as obstacle interval was obtained by TEM and atom probe tomography (APT), and the type of pinning site was identified. It was found that Frank loops and black dots were irradiation defects that strongly interacted with dislocations. It was suggested that solute atom clusters act as weak obstacles for dislocations in the dynamic interaction behavior with dislocation motion.

Comparative Study of Two Nanoindentation Approaches for Assessing Mechanical Properties of Ion-Irradiated Stainless Steel 316

Nanoindentation is a commonly used method to measure the hardness of surfaces with thin layers, and is especially useful in studying the change in mechanical properties of ion irradiated materials. This research compares two different methods of nanoindentation to study the changes in hardness resulting from ion irradiation of SS316 alloy. The samples were irradiated by He2+ ions at beam energies of 1, 2, and 3 MeV, respectively. The first method involves the indentation of the irradiated surface perpendicular to it using the continuous stiffness mode (CSM), while the second applies the indents on an oblique surface, accessing an inclined cross-section of the irradiated material. Finite element modelling has been used to further illuminate the deformation processes below the indents in the two methods. The hardness profiles obtained from the two nanoindentation methods reveal the differences in the outcomes and advantages of the respective procedures, and provide a useful guideline for their applicability to various experimental conditions. It is shown through an in depth analysis of the results that the ‘top-down’ method is preferable in the case when the ion irradiation energy, or, equivalently, the irradiated depth is small, due to its greater spatial resolution. However, the oblique cross section method is more suitable when the ion irradiation energy is >1 MeV, since it allows a more faithful measurement of hardness as a function of dose, as the plastic field is much smaller and more sensitive to local hardness values.

Atom probe tomography analysis of radiation-induced solute clustering in austenite stainless steels

ABSTRACTVarious types of defects are produced by the irradiation of energetic particles onto a structural material. The large number of mobile vacancies and self-interstitial atoms during irradiation induce defect fluxes and the diffusion of solute atoms in the matrix. The preferential interaction between the solute atoms and radiation-induced defects leads to the enrichment/depletion or clustering of the solutes at defect sinks. In the current work, atom probe tomography (APT) was used for the analysis of radiation-induced solute clustering in ion-irradiated austenite stainless steel 316. Quantitative analysis of the localised clustering of chemical elements was implemented and a parameter selection procedure was proposed. The number density and size distribution of Si clusters in APT specimens irradiated at various temperatures were examined. At high temperature, the number density of the clusters decreased and their size increased. The localized Si atoms in variously shaped defects were clearly identified. The APT method was demonstrated to be suitable for identifying defect structures and for the quantitative analysis of clustering in irradiated specimens.

B23-P-16Multi-layer Method combined with Nano-indentation, FIB and XTEM for Nano-hardness Measurement

Nano-indentation is one of important method for measuring precise hardness in thin films with nano-level, where an average value could be led in depth direction. When the hardness is distributed as the function of depth in nanometer scale, we could not get precise hardness. Recently, our group has proposed a new method, named as “multi-layer method” combined with FIB and XTEM [ 1 ]. In this study, the depth dependence of hardness in an ion-irradiated stainless steel was evaluated by the sectioning of damaged region and subsequent nano-indentation. The range of plastically deformed region by the nano-indentation was supplementary investigated by XTEM. When the indentation depth was the critical indentation depth, hc , which was derived from the Nix-Gao plot [ 2 ], the dislocation structure was observed from the specimen surface to right below the bottom of the ion-irradiated region, as shown in figure. To verify the depth dependence of hardness, the “multi-layer method' was applied in this measurement. In this case, the nano-indentation was measured for four different thicknesses after sectioning by FIB. As the result, we got successively reasonable values of local hardness, H L , as a function of depth and damage level; 278, 333, 372 and 408kg/mm 2 for 0, - 0.6, -1.2 and - 1.8 μm from top surface, respectively.

B23-P-15The Microscopic Observation and Shape Memory Performance of Fe-Mn-Si-Cr-Nb Shape Memory Alloys

Nano-indentation is one of important method for measuring precise hardness in thin films with nano-level, where an average value could be led in depth direction. When the hardness is distributed as the function of depth in nanometer scale, we could not get precise hardness. Recently, our group has proposed a new method, named as “multi-layer method” combined with FIB and XTEM [1]. In this study, the depth dependence of hardness in an ion-irradiated stainless steel was evaluated by the sectioning of damaged region and subsequent nano-indentation. The range of plastically deformed region by the nano-indentation was supplementary investigated by XTEM. When the indentation depth was the critical indentation depth, hc, which was derived from the Nix-Gao plot [2], the dislocation structure was observed from the specimen surface to right below the bottom of the ion-irradiated region, as shown in figure. To verify the depth dependence of hardness, the “multi-layer method” was applied in this measurement. In this case, the nano-indentation was measured for four different thicknesses after sectioning by FIB. As the result, we got successively reasonable values of local hardness, HL, as a function of depth and damage level; 278, 333, 372 and 408 kg/mm2 for 0, 0.6, 1.2 and 1.8 μm from top surface, respectively. [1] H. Oka, Y. Sato, N. Hashimoto, S. Ohnuki, J. Nuclear Materials, 462 (2015) 470 474 [2] W. D. Nix, H. Gao, J. Mech. Phys. Solids 46 (1998) 411. Figure: XTEM for deformation zone due to Nano-indentation. 2.5 0

Depth distribution of Frank loop defects formed in ion-irradiated stainless steel and its dependence on Si addition

Although heavy ion irradiation is a good tool to simulate neutron irradiation-induced damages in light water reactor, it produces inhomogeneous defect distribution. Such difference in defect distribution brings difficulty in comparing the microstructure evolution and mechanical degradation between neutron and heavy ion irradiation, and thus needs to be understood. Stainless steel is the typical structural material used in reactor core, and could be taken as an example to study the inhomogeneous defect depth distribution in heavy ion irradiation and its influence on the tested irradiation hardening by nano-indentation. In this work, solution annealed stainless steel model alloys are irradiated by 3MeV Fe2+ ions at 400°C to 3dpa to produce Frank loops that are mainly interstitial in nature. The silicon content of the model alloys is also tuned to change point defect diffusion, so that the loop depth distribution influenced by diffusion along the irradiation beam direction could be discussed. Results show that in low Si (0% Si) and base Si (0.42% Si) samples the depth distribution of Frank loop density quite well matches the dpa profile calculated by the SRIM code, but in high Si sample (0.95% Si), the loop number density in the near-surface region is very low. One possible explanation could be Si’s role in enhancing the effective vacancy diffusivity, promoting recombination and thus suppressing interstitial Frank loops, especially in the near-surface region, where vacancies concentrate. By considering the loop depth distribution, the tested irradiation hardening is successfully explained by the Orowan model. A hardening coefficient of around 0.30 is obtained for all the three samples. This attempt in interpreting hardening results may make it easier to compare the mechanical degradation between different irradiation experiments.

Atomistic Analysis Of Radiation-Induced Segregation In Ion-Irradiated Stainless Steel 316

Abstract Stainless steel (SS) is a well-known material for the internal parts of nuclear power plants. It is known that these alloys exhibit radiation-induced segregation (RIS) at point defect sinks at moderate temperature, while in service. The RIS behavior of SS can be a potential problem by increasing the susceptibility to irradiation-assisted stress corrosion cracking. In this work, the RIS behavior of solute atoms at sinks in SS 316 irradiated with Fe4+ ions were characterized by atom probe tomography (APT). There were torus-shaped defects along with a depletion of Cr and enrichment of Ni and Si. These clusters are believed to be dislocation loops resulting from irradiation. The segregation of solutes was also observed for various defect shapes. These observations are consistent with other APT results from the literature. The composition of the clusters was analyzed quantitatively almost at the atomic scale. Despite the limitations of the experiments, the APT analysis was well suited for discovering the structure of irradiation defects and performing a quantitative analysis of RIS in irradiated specimens.

Evaluation of multi-layered hardness in ion-irradiated stainless steel by nano-indentation technique

Depth dependence of hardness in ion-irradiated 316 stainless steel was evaluated by the sectioning of damaged region and subsequent nano-indentation. The range of plastically deformed region by the nano-indentation was supplementarily investigated by transmission electron microscopy. When the indentation depth was the critical indentation depth, hc, which was derived from the Nix–Gao plot, the dislocation structure was observed from the specimen surface to right below the bottom of the ion-irradiated region. To verify the depth dependence of hardness, the “multi-layer model” was introduced in this study. The multi-layer model is based on the following assumptions; (1) the ion-irradiated region can be divided into sub-layers having their own local hardness, HL; (2) the hardness can be the product of f and HL in each sub-layer where f is the volume fraction of a deformation zone; and (3) the deformation zone can be a hemisphere. Eventually, through the sectioning and following nano-indentation, HL in each sub-layer was experimentally evaluated. Further the correlation between the displacement damage and the irradiation hardening ΔH in this study agreed with that of neutron irradiation experiments.

Micromechanistic origin of irradiation-assisted stress corrosion cracking

A multi-scale study of the micromechanics of dislocation–grain boundary interactions in proton and ion-irradiated stainless steels is presented. Interactions of dislocation channels with grain boundaries result in slip transfer, discontinuous slip without or with slip along the grain boundary. The presence of the irradiation damage enhances the importance of the magnitude of the resolved shear stress on the slip system activated by the grain boundary to transfer slip across it. However, the selected slip system is still determined by the minimization of the grain boundary strain energy density condition. These findings have implications for modelling the mechanical properties of irradiated metals as well as in establishing the mechanism for disrupting the grain boundary oxide, which is a necessary prerequisite for irradiation-assisted stress corrosion cracking.

Observation and rate theory modeling of grain boundary segregation in Σ3 twin boundaries in ion-irradiated stainless steel 316

Radiation-induced segregation (RIS) is the phenomenon of compositional change at point defect sinks in alloys irradiated at a moderate temperature. Owing to the potential relevance of RIS by way of the susceptibility of structural materials to irradiation-assisted stress corrosion cracking, basic research on austenitic stainless steels used in nuclear reactors has been carried out in recent years. In this work, commercial stainless steel 316 specimens were irradiated with Fe ions, and the resulting changes in Cr and Ni compositions were characterized using transmission electron microscopy and energy-dispersive X-ray spectroscopy. The samples with various grain boundary orientations, including the special Σ3 orientation, were analyzed. The ledges of a few special Σ3 twin boundaries showed significantly higher RIS compared to the coherent regions. The RIS behavior of a parallel twin pair was observed, and two profiles of RIS were found in them. The inner twins in multi-twins showed considerably lower RIS compared to the outer twins. For the calculation of RIS, time-dependent differential equations based on the rate theory were established and numerically integrated. An additional variable, representing the sink strength of the grain boundary, was introduced in the differential equations, and the concentration profiles of the Σ3 twins were calculated. The calculated results were in good agreement with the experimental results.

Effects of hydrogen on interaction between dislocations and radiation-induced defects in austenitic stainless steels

Effects of solute hydrogen on slip deformation behavior and dislocation–defect interaction were investigated by using Fe ion-irradiated stainless steels. Types 304 and 316 stainless steel (304SS and 316SS) specimens irradiated to 10dpa with 2.8MeV Fe ions were strained in tension to 2% plastic strain at 573K in 3 or 75MPa hydrogen and 1atm argon gas atmospheres, and then slip line spacing and height measurements and cross-sectional microstructure observations were conducted. Slip concentration into dislocation channels due to irradiation was suppressed in the presence of solute hydrogen because solute hydrogen is suggested to promote sweeping of dislocation loops by dislocation glide. Such effects of solute hydrogen would mitigate slip localization in neutron-irradiated stainless steels. The effects of solute hydrogen were not observed for 316SS specimens, probably due to insufficient hydrogen concentration, because hydrogen-carrying dislocations could more readily cross-slip in steels with high stacking fault energies resulting in less accumulation of solute hydrogen at dislocation–defect interaction points.

In situ and tomographic observations of defect free channel formation in ion irradiated stainless steels

The effects of heavy-ion irradiation on dislocation processes in stainless steels were investigated using in situ irradiation and deformation in the transmission electron microscope as well as post mortem electron tomography to retrieve information on the three-dimensional dislocation state. Irradiation-induced defects were found to pose a strong collective barrier to dislocation motion, leading to dislocation pileups forming in grain interiors and at grain boundaries. The passage of multiple dislocations along the same slip plane removes the irradiation defects and leads to the eventual formation of a defect-free channel. These channels are composed of densely tangled dislocation networks which line the channel-matrix walls as well as residual dislocation debris in the channel interiors. The structures of the dislocation tangles were found to be similar to those encountered in later stages of deformation in unirradiated materials, with the exception that they developed earlier in the deformation process and were confined to the defect free channels. Also, defect free channels were found to widen through both source widening as well as complex cross-slip mechanisms.

Influence of crystal orientation on hardness and nanoindentation deformation in ion-irradiated stainless steels

The influence of crystal orientation on hardness and the range of plastic deformation caused by nanoindentation was investigated in a solution annealed type 316 stainless steel irradiated with Fe2+ ions. The hardness was a function of grain orientation and was correlated with the Taylor factor averaged over three normal directions of the contact surface of the Berkovich indenter. The transmission electron microscope observations of the deformation microstructure under the indentations showed that the range of plastic deformation reached up to 10 times the indent depth for unirradiated material and depended on the orientation relation between the contact surface of the indenter and the slip directions. The range of plastic deformation decreased as the damage structure developed in ion irradiation.

Characterization of deformation structure in ion-irradiated stainless steels

The effects of strain rate and type of stainless steels on the deformation behavior of irradiated stainless steels were investigated. Type 316 and 304 stainless steels irradiated with 200 keV He + ions up to 20 dpa at 300 °C were deformed at 10 −4 and 10 −7/s at 300 °C. The deformation changed from uniform dislocation tangling to dislocation channeling and twinning, and slip step spacing widened due to prevention of dislocation gliding by dislocation loops and cavities. The deformation mode was the same in both stainless steels deformed at both strain rates. The loop and cavity density in type 304 stainless steel was smaller than that in type 316 stainless steel. However, dislocation gliding was more effectively prevented in the case of faster deformation and in type 304 stainless steel when dislocation loops were the dominant microstructure. The difference became small at higher doses when cavities were the dominant microstructure.

Positron beam studies of void swelling in ion irradiated Ti-modified stainless steel

The void swelling behavior of heavy ion irradiated D9 steel has been investigated using variable low energy positron beam. The normalized defect-sensitive S-parameter shows up a large increase in the depth region corresponding to the maximum radiation damage as a function of irradiation temperature. From the variation of S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced and the results are discussed.

Interaction between Dislocation Sliding and Damage Structure in Ion-irradiated Stainless Steels

ABSTRACTThe interaction between dislocation sliding and damage structure in ion-irradiated austenitic stainless steels was investigated. Solution annealed type 316 and 304 stainless steels (316SS and 304SS) were irradiated with 2.8 MeV Fe2+ ions at 300 °C up to 10 dpa and tensiled to 2% plastic strain at 300 °C. Dislocations moving from unirradiated matrix were prevented due to the interactions with the damage structures consisted of dislocation loops and voids in the damage region. The prevention of dislocation movements by the damage structures became strong in 304SS compared in 316SS; probably due to lower stacking fault energy in 304SS. The prevention of dislocation movements was weak for Fe ion-irradiated specimens in which the increase in shear strength calculated from the size and number density of the defects was small compared to He ion-irradiated specimens.

Positron annihilation lifetime spectroscopy on heavy ion irradiated stainless steels and tungsten

Radiation effects have been studied in the modified 316L stainless steel and the commercially available stainless steel and tungsten by the heavy ion irradiation simulation and positron annihilation lifetime techniques. The measured positron lifetime data show that the radiation resistant property of stainless steels is much better than that of tungsten, and the modified 316L stainless steel possesses the best radiation resistant property among them. The present experimental results demonstrate that the commercially available stainless steel is better than the commercially available tungsten for a beam window material of an ADS spallation neutron source system, and the modified 316L stainless steel is the best among the three.

Radiation-induced segregation in multicomponent alloys: Effect of particle type

The problem of irradiation-assisted stress-corrosion cracking (IASCC) in reactor cores is currently being addressed using different types of particle irradiation (electrons, protons, and heavy ions) to study the effect of neutron damage. The effect of radiation damage of greatest interest to the IASCC problem is the segregation of impurities or redistribution of alloying elements in the vicinity of the grain boundary. Differences among the types of irradiation include particle type, temperature, dose, and dose rate. The different particle type results in different “effective” displacement rates due to fundamental differences in the radiation damage state. The effect of displacement efficiency is incorporated into existing models for radiation-induced segregation and compared with experimentally determined values. Comparisons between theory and experiment were made using neutron-,proton-, and ion-irradiated stainless steels. Results showed that the measured chromium depletion profile is generally narrower and shallower than that calculated from theory. Agreement between experiment and model was better in almost all cases when the particle efficiency was taken into account. Agreement was best for proton-irradiated steels, and less so for neutron and heavy ion-irradiated steels. The origins of this difference are probably due to spatial and depth resolution of the scanning transmission electron microscopy and Auger electron spectrometry measurement techniques, respectively, the lack of knowledge of the material parameters in modeling, and the uncertainty in the true displacement rate. Nevertheless, beyond 2 nm from the grain boundary, the shape of the experimental and calculated profiles agree reasonably well.