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Effects of long-term thermal aging on elevated temperature deformation behaviors of wrought 316LN stainless steel by small punch test
Abstract
The deformation behaviors of as-received and thermal aged wrought 316LN austenitic stainless steel (ASS) were investigated at 350 °C by small punch test (SPT) in order to evaluate the effects of long-term thermal aging on the elevated temperature deformation behaviors of the wrought ASS. The results showed that long-term thermal aging at 400 °C had obvious influences on the density and structure of dislocations, hence affecting the deformation mechanism and mechanical behaviors. There were many planar slip bands and tangled dislocations in the as-received wrought 316LN ASS, while, a low-density dislocation multiples, coplanar dislocation arrays, stacking faults and extended dislocations were formed due to the microstructure recovery during the long-term thermal aging. Correspondingly, both dislocations slip and deformation twinning played an important role in plastic deformation in the as-received wrought 316LN ASS, while interaction of dislocations and stacking faults inclined to dominate the plastic deformation process of the thermal aged 316LN ASS under the elevated temperature. As a result, long-term thermal aging resulted in the change of the mechanical properties of the wrought 316LN ASS. The hardness and SPT maximum load of 316LN ASS had a slight increase, while the SPT total energy exhibited a reverse trend with increasing aging time, indicating that the 316LN ASS showed a tendency of hardening after the long-term thermal aging.
... It provides an efficient mean of estimating the local mechanical properties of materials when the large volumes of standard specimens cannot be extracted from the components, and now, it has been considered as a quasi-nondestructive technique and experienced wider application. In addition, SPT is considered as a direct method to investigate the deformation and fracture behavior of materials by characterizing the morphologies of the sample after SPT [29,31,32]. ...
... A special SP-SCC fixture, which mainly consists of an upper and a lower drawbar, a lower die, a punch and a ceramic ball, as shown in Fig. 2, was developed for the SSRT method. Similar fixture has been reported in our previous research [32]. In order to prevent corrosion and the occurrence of a galvanic couple, the main part of the fixture was made of 316 L ASS, especially, the punch was made of the nickel-based alloy GH4169, and the ceramic ball was made of ZrO 2 . ...
... These residual strains were attributed to the uneven shrinkage and expansion caused by rapid heating and rapid cooling during weld fabrication. In general, the plastic deformation process of 316LN ASS was dominated by dislocations slip and deformation twinning at high temperature [32,46], interestingly, a large number of slip bands entangled with the surrounding dislocations were found in the HAZ, indicating that nonuniform shrinkage and expansion caused obvious plastic deformation in the HAZ. The appearance of high-density dislocations led to hardening tendency in HAZ. ...
... Recently, the 3D printing technology [1], robot software organization [2,3], self-healing display screen [4], wearable electronic materials and battery expansion deformation are promoted [5,6]. These materials have a common deformation of porous materials deformation, followed by deformation related to temperature [7][8][9][10]. The theory and model of these problems are still in the exploratory stage, and there are still many areas to be improved in traditional numerical methods. ...
... Before solving the deformation of the 2D thin plate, we need to give the equation for the bending of the plate. As for the fourth-order bending differential equation of thin plate, this follows in Equation (10). ...
Elastic materials include metal plates, rubber, foam, airbags and so on, which have a good buffer effect, toughness and strong recovery ability. In this paper, the deformation and thermal diffusion of 2D and 3D thin plates are studied. Two models are established for the deformation of 2D thin plates. The bending deformation equation of rectangular and circular plates is derived, and the semi-analytical solution of the deflection function w(x,y) is found through the Fourier series approximation in the polar coordinate. The consistencies of the numerical solution and the theoretical solution are verified by numerical method. Then, we find that the factors affecting the deformation are related to the Young’s modulus, load, plate length and deformation factor α of the material. In a separate temperature physics field, we establish a heat conduction model of 2D graphene film. Three numerical schemes of the transient heat conduction equation of FDM-FEM are given. In contrast, this paper uses the implicit Euler method to discrete the time term. Furthermore, we compared the difference between the adiabatic condition and the convection condition by the graphical method and the curve trend. The results show that the temperature near the adiabatic boundary is higher. Finally, we proposed a 3D dynamic thermal–mechanical coupling model (3D-DTMCM) that has been established. A laser heating monocrystalline silicon sheet with periodic motion formula is given. The temperature radiation of the laser heat source has Gaussian distribution characteristics. Our proposed model can dynamically determine Young’s modulus with a variable temperature. The numerical results show that the higher the temperature is, the higher the strain energy density of the plate is. In addition, the deformation amplitude of the plates in the coupling field is larger than that in the single mechanical field. Finally, we also discussed the stress field distribution of mixed cracks under high temperature and high load. Our research provides theoretical support for the deformation of different plates, and also reflects the value of the coupled model in practical applications.
... Regarding material conditioning and processing, the SPT has been reportedly used for studying the effects of neutron-ion irradiation [14], exposure to hydrogen embrittlement [15], thermal ageing [16,17], annealing [18], hot isostatic pressing [19], and thermal cutting [20]. These thermal ageing, annealing, hot isostatic pressing, and thermal cutting studies are related to the field of heat treatment. ...
Studying the effect of quench and tempering heat treatments on steel, more specifically screening the effect of the austenitizing, quenching, and tempering conditions on mechanical properties, can be extremely material- and time-consuming when standard tensile testing specimens are employed. Jominy bar end quench testing has been used as a standard method to reduce the resources that are required for this type of screening. Jominy bar testing by itself shows, though, the limitation of yielding only hardness and microstructure as a result. In the last few years, the small punch test (SPT) standard has been developed. This technique can obtain an estimation of tensile mechanical properties with miniaturized specimens, which can be dissected from Jominy bars. The paper proposes a new testing methodology for screening the outcome of heat treatment conditions by combining the Jominy bar testing and SPT. Quench and tempering of API 5L X65Q pipe steel is used as a case study to describe the proposed methodology. The ability of the Jominy with SPT to detect variations in the mechanical properties produced by heat treatments is shown. This methodology can be directly applied as a high-throughput testing approach in the optimization of heat treatments.
... Meanwhile, there is currently a scarcity of studies into the development of precipitates in 316 LNSS at high temperatures. Furthermore, the current focus of 316 LNSS application performance research is on the organization's longterm thermal aging process at 300-400 • C base material and weld temperatures [16][17][18], with no mention of the effect of brief aging at 750 • C on its structure and attributes. Because carbides coarsen at grain boundaries in austenitic steel with a high Cr content, they are unfavorable for mechanical properties [19,20]. ...
The microstructure development of 316 LN austenitic stainless steel (316 LNSS) during the aging process is investigated in this article. The thermal aging processes were conducted at 750 °C with different periods ranging from 50 to 500 h. The metallographic results show that the coherent and incoherent twins were present in the original 316 LNSS grains, but dwindled as the aging period increased. After 50 h of aging, many fine, dispersed particles precipitated from the matrix, which were identified as M23C6 by energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM). Additionally, the impact toughness and Brinell hardness (HBW) changed during the aging, which was closely related to the effects of dispersion strengthening and solution strengthening. A negatively linear relationship between Brinell hardness and Charpy impact energy was established, which could be utilized to predict the degree of thermal embrittlement.
This work investigates the physical, chemical, and mechanical properties of coir-coconut husk powder reinforced low-density polyethylene composites exposed to an aggressive (acidic) environment. The composites were prepared by the melt mixing moulding method. The fabricated composites were immersed in an acidic environment of pH 2.2 for a period of 27, 54, and 81 days. Two variants of samples were prepared namely: unreinforced samples (C1) and reinforced samples (C2). The unreinforced polymer samples (C1) were obtained before ageing (C1-0d), and after ageing (C1-27d, C1-54d, and C1-81d), and also, the reinforced polymer samples (C2) were obtained before ageing (C2-0d), and after ageing (C2-27d, C2-54d and C2-81d). Hardness, X-ray diffraction (XRD), friction, wear, and water absorption studies were carried out on the samples. From the experimental results, after immersion in the acidic solution for 27, 54, and 81 days, the hardness values of samples C1 (acid-aged variants) increased by 12.3, 19.4, and 31.5% with hardness values of 16.53 Hv, 18.0 Hv, and 21.7 Hv, respectively. For C2 samples, the hardness value increased by 6.50% and 17.3% after immersion in the acidic solution for 27 and 54 days with increments from 35.47 Hv (hardness value for C2-0d sample) to 37.93 Hv and 42.87 Hv, respectively. The crystallinity of the LDPE polymer- C2-54d sample, indicates its increasing strength since the intermolecular bonding is more significant in the crystalline phase. The study concluded that acid ageing has a noticeable influence on the physico-chemical and mechanical properties of coir-coconut husk powder reinforced low-density LDPE composites.
Tailoring nanoscale defect structures for desirable deformation behaviors is crucial to designing and optimizing the mechanical properties of alloys. Distinguishing from the predominant toughening mechanisms (e.g., mechanical twinning and deformation-induced phase transformation), here we report an unusual stacking-fault-mediated deformation in equiatomic FeNiCoCr high-entropy alloy (HEA) by controllably introducing helium nanobubbles with high pressures of ∼2.5-4.7 gigapascals. Using in situ transmission electron microscopy nanomechanical testing, we demonstrate that highly pressurized helium nanobubbles can not only increase the strength by serving as dislocation obstacles but also enhance the strain hardening capacity and accommodate considerable plasticity via facilitating the multiplication and interaction of interwoven stacking faults. Through atomistic simulations, we reveal that high helium pressures contribute to reducing the nucleation energy of partial dislocations at the nanobubbles surface, which enhances dislocation nucleation rates and offers sustainable stacking fault sources for retaining ductility. Our results provide a novel design strategy for tuning deformation mechanisms of HEAs via introducing highly pressurized helium nanobubbles, which may open up avenues towards the facile tailoring of mechanical responses in micro/nanoscale HEA components.
The small punch test (SPT) was developed for situations where source material is scarce, costly or otherwise difficult to acquire, and has been used for assessing components with variable, location-dependent material properties. Although lacking standardization, the SPT has been employed to assess material properties and verified using traditional testing. Several methods exist for equating SPT results with traditional stress–strain data. There are, however, areas of weakness, such as fracture and fatigue approaches. This document outlines the history and methodologies of SPT, reviewing the body of contemporary literature and presenting relevant findings and formulations for correlating SPT results with conventional tests. Analysis of literature is extended to evaluating the suitability of the SPT for use with additively manufactured (AM) materials. The suitability of this approach is shown through a parametric study using an approximation of the SPT via FEA, varying material properties as would be seen with varying AM process parameters. Equations describing the relationship between SPT results and conventional testing data are presented. Correlation constants dictating these relationships are determined using an accumulation of data from the literature reviewed here, along with novel experimental data. This includes AM materials to assess the fit of these and provide context for a wider view of the methodology and its interest to materials science and additive manufacturing. A case is made for the continued development of the small punch test, identifying strengths and knowledge gaps, showing need for standardization of this simple yet highly versatile method for expediting studies of material properties and optimization.
The effect of thermal aging on the intergranular stress corrosion cracking (IGSCC) susceptibility of Type 310S stainless steel was investigated. Samples were solution-annealed (baseline), sensitized (short-term aging), and thermally-treated (long-term aging) to form Cr-rich M23C6 carbides and sigma (σ) phase precipitate particles, respectively, on the grain boundaries. The thermal-aging heat treatments represent two limiting exposure conditions expected in-service for a fuel cladding in the Canadian Generation IV (supercritical water-cooled reactor) design concept and were considered to better understand the risk factors associated with an in-service IGSCC damage mode. The sensitized treatment exhibited the highest relative degree of sensitization (measured by double loop electrochemical potentiokinetic reactivation testing in 2 M H2SO4 + 0.01 M KSCN) and IGSCC susceptibility (measured by slow strain rate testing [SSRT] in a hot alkaline solution). The relative susceptibility reflects the controlling role played by Cr-depleted zones, which were only observed in the sensitized (short-term aging) material. The relatively large σ phase grain boundary particles in the thermally-treated (long-term aging) material preferentially cracked during SSRT, suggesting an apparent IGSCC susceptibility at the sample surface. However, the absence of Cr-depleted zones in this material prevented IGSCC from occurring, despite preferential cracking in the σ phase grain boundary particles. The dominant fracture mode in the hot alkaline solution was transgranular mixed-mode cracking. The implications of the results are discussed within the context of risk factors associated with an in-service IGSCC susceptibility in supercritical water.
The ferrite of thermally aged CF3M duplex stainless steels has been studied at the atomic scale. Accelerated ageing of ingots has been performed at 350 °C in laboratory up to 200,000 h (>20 years). Spatial and chemical evolution of the microstructure of the ferrite has been characterised by 3D atom probe. In addition, micro-hardness has been performed on the same samples. The results obtained have been compared to the microstructural and mechanical characteristics of ferrite of the same ingots aged at 325 °C (close to service temperature) and to the ferrite of an elbow steel aged on-site. This work has shown that: (i) accelerated ageing at 350 °C anticipates the on-site ageing at 323 °C, (ii) the linear relationship found between micro-hardness measurements and the variation V (defined as the integral of the difference between the Cr frequency distribution of the aged sample and the corresponding binomial distribution characteristic of a random solution with the same concentration) is still valid after 200,000 h of ageing at 350 °C, (iii) the activation energy is the same for both spinodal decomposition and G-phase precipitation and finally (iv) the coarsening of G-phase particles has no influence on the relationship between ferrite micro-hardness and the V parameter.
True stress vs. true strain responses of Cu–6 wt.% Al and Cu–12 wt.% Mn alloys are presented. While Cu–6 wt.% Al alloy shows the typical mechanical response of low stacking fault energy alloys, the Cu–12 wt.% Mn alloy behaved similarly to medium to high stacking fault energy alloys. These findings clearly show that while short-range ordering triggers slip planarity, it has a minor effect on total dynamic recovery of these copper alloys.
High temperature low cycle fatigue tests at various strain amplitudes (±0.5%, ±0.6%, ±0.8%, ±1.0%) and strain rates (0.5%/s, 0.05%/s, 0.005%/s) were performed at 550°C. The stress decomposition method and microstructure characterization were used to analyze cyclic hardening and softening behavior. Results showed that increased back stress and the combined action of back and friction stresses were responsible for the pronounced cyclic hardening at the early and late stages of initial hardening, respectively. The decrease in friction stress resulted in cyclic softening until final fracture. Moreover, the evolution behavior of dynamic strain aging (DSA) activity was discussed, which showed a clear dependence on loading cycles and directions. Specifically, the DSA activity was firstly attenuated and then gradually intensified from the late stage of cyclic hardening, and it was stronger in the compressive plastic deformation regime of hysteresis loops than in the tension part.
Discontinuous plastic flow due to dynamic strain aging (DSA) in a Fe–13Cr–3.4Mn–0.47C metastable stainless steel was studied by uniaxial tensile tests in the temperature range from 20 °C to 500 °C. A complex picture of the discontinuous flow and its disappearance in an intermediate temperature interval was interpreted in terms of an existing model, which had to be modified. Different mechanisms of DSA were proposed for different temperature intervals. DSA at temperatures up to 200 °C was attributed to deformation-induced martensitic transformation within glide bands. Immobilization of dislocations at martensite/austenite boundaries and emission of fresh dislocations into the adjacent austenite was considered as a major process in this temperature regime. Rapid diffusion of carbon from martensite to the martensite/austenite boundaries was considered as an additional pinning mechanism contributing to discontinuous plastic flow. Serrations on the deformation curves at temperatures between 200 °C and 300 °C were associated with dislocation pinning due to carbon diffusion in austenite. This regime of DSA did not involve high diffusivity martensite. In spite of an increase in the diffusivity of carbon in the temperature range of 350-450 °C, a serration-free regime was observed there. The absence of intermittent dislocation pinning/unpinning events responsible for serrations implied continuous solute drag by carbon atoms according to the Cottrell mechanism. Serrations reappeared at 500 °C and were attributed to dislocation pinning by substitutional chromium and manganese atoms.
Physically based constitutive equations incorporating the key microstructural mechanisms e.g. dislocations, grain size, etc, have been used widely to predict the stress-strain behaviour of alloys at plastic and viscoplastic conditions. This enables an accurate prediction of the formed geometry as well as the final underlying microstructures. However, these physically based constitutive equations have not been practically validated due to the lack of systematic experimental data at microscopic scale. This leads to a large number of unknown constants required to be determined through various optimization algorithms. The aim of this paper is to provide direct and systematic experimental data by revealing the dislocation (geometrically necessary one) density and grain size evolution of AA6082, which is a widely used high-strength aluminium alloy for automobile structural panels, as functions of strain, strain rate and temperature, and is the first-time using Electron Back Scattering Diffraction (EBSD) technique to visualize the microstructures during the hot deformation. The evolution of the dislocation accumulation during the hot tensile deformation at 300, 450, 530 ˚C using various strain rates (i.e. 1/s, 0.1/s, 0.01/s) was achieved. EBSD maps were analysed on samples submitted to a true strain level of ∼0.1 and ∼0.3 under each condition. These maps cover >3000 grains and enable to capture the statistical nature of the geometrically necessary dislocation densities during hot deformation. Despite the rapidly plateaued flow stress curves at high temperatures, a continuously increased average GND density was observed in AA6082 with the imposed true strain levels under all conditions. Dislocation channel structures were observed in the hot deformed samples. Dynamic recrystallization was also observed, which coupled with the GNDs and affects the hardening behaviour of the flow stress-strain curves. This work is the first study, using EBSD, to visualize the high temperature and high strain rate induced dislocation distributions over a relatively large area, providing valuable data that may be used for subsequently improving and calibrating the physically based material models.
In sensitized (700°C for 10 h) AISI 316L stainless steel samples attacked by an HF solution, coincidence site lattice (CSL) ∑7 grain boundaries (GBs) always show corrosion while ∑5 GBs never do. This distinct corrosion behaviour can be related to the distinct precipitation behaviour of the different GBs, which itself depends on the GB characters. It is found that ∑5 GBs, although being curved both macro- and microscopically, neither show facets nor precipitates. The amount of segregation is enhanced in sensitized ∑5 GBs compared to solution-annealed ones. In contrast, ∑7 GBs show clear faceting in both the solution-annealed and sensitized state, with one facet orientation showing strong segregation while the other not. GB energy anisotropy is applied to explain the faceting phenomenon. The formation of two kinds of precipitates, i.e., C14 Laves phase and M23C6 carbide, holding responsible for two distinct morphologies of corrosion, are believed to be both linked to intensive faceting and the anisotropic segregation of ∑7 GBs. For the carbide, a preferred {111} orientation out of all ∑7 facets measured by trace analyses is presumed to facilitate its nucleation.
Type 316LN stainless steel (SS) with varying nitrogen content (0.07, 0.14 and 0.22 wt.%) was thermally aged at 873 K and 923 K for 20,000 h. The microstructures of thermally aged steels revealed secondary phases such as M23C6 carbides and Fe-Cr-Mo intermetallics at all the nitrogen contents and Cr2N nitrides additionally at 0.14 and 0.22 wt.% N. The amount of precipitates increased with increase in nitrogen content and aging temperature. The effect of thermal aging on the tensile properties of 316LN SS was evaluated using automated ball indentation (ABI) technique at 298 K. The flow curves of the steels aged at 873 K were lower than those obtained under unaged condition. Whereas due to ageing at 923 K, the flow stress response gradually increased above the unaged condition with increasing nitrogen content. The ultimate tensile strength varied similarly at 873 K aging condition for all nitrogen levels and was observed to recuperate upon aging at 923 K, except for 0.07 wt.% N. Yield strength was not strongly affected by thermal ageing, except the considerable increase in 316LN SS with 0.22 wt.% N aged at 923 K. The addition of nitrogen generally imparted interstitial solid solution strengthening to 316LN SS. The higher the nitrogen content and ageing temperature, the higher is the precipitation strengthening due to ageing, which was reflected in the tensile behavior. Transmission electron microscopy examination of ABI deformed zone revealed dislocation-precipitate interactions, precipitate-shear band interaction and pile-up of dislocations at precipitates, influencing the tensile properties.
Duplex stainless steels (DSS) suffer from thermal aging embrittlement after long-term service in the temperature range of 280-320°C, leading to severe deterioration of mechanical properties and corrosion resistance. It is now well known that the typical microstructural evolutions of the thermally aged DSS are the spinodal decomposition and the G-phase precipitation in ferrite. This review provided a comprehensive analysis of the microstructure, mechanical characteristics, and corrosion resistance of the long-term thermally aged DSS. The principles for how the microstructural evolutions affect the performance of the thermally aged DSS were also highlighted. Some controversial issues such as the formation mechanism of G-phase and its role in the evolutions of material properties were discussed. In addition, a comprehensive review of the recovery behaviors of thermal aging embrittlement was also provided. At last, the limitations of present researches and future research directions of the practical engineering application requirements were summarized.
The Small Punch Test (SPT) has demonstrated tremendous potential for high throughput evaluation of mechanical response of metal sheets. However, its application has thus far been limited to the estimation of plastic material properties such as yield strength and ultimate tensile strength. This paper demonstrates new analysis protocols for extracting the uniaxial stress-strain curves from the SPT measured load-displacement curves. This is accomplished by first establishing a correlation between the SPT measurements and corresponding finite element (FE) simulations that take as input the uniaxial stress-strain response of the material. After establishing this correlation, it is shown that the correlation can be inverted to estimate the uniaxial stress-strain response from the SPT load-displacement measurement on a new material sample.
Deformation of single phase and two phase materials can be heterogeneous depending on the grain size distribution, orientation of the grains, amount and arrangement of the second phase and phase transformation during plastic deformation and twins. The deformation heterogeneity requires generation of geometrically necessary dislocations (GND) to maintain contact of the deforming grains and phases. Depending on the amount and character of deformation at the microscopic length scale, the evolution of the GND density changes. The present paper discusses the buildup of the GND density in different microstructural configurations using, single phase ferritic interstitial free (IF) steel, dual phase (DP) steel containing ferrite and martensite and austenitic stainless steel prone to deformation induced martensitic transformation and twining and whether GND density can be used as a descriptor of the deformation characteristics of the materials. The estimation of GND content using electron backscatter diffraction (EBSD) is recognized. The application potential of the present study is in understanding the in-grain deformation behavior of single phase IF steel, two phase DP steel, transformation induced plasticity assisted austenitic stainless steel to assess the forming of the steels for automobile applications. It should also facilitate the validation of strain gradient plasticity dependent damage model.
The evolution of geometrically necessary dislocation (GND) distribution in the deformed 6061 alloy is studied with electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). In dynamic recrystallized state, the average GND density is 8.70 × 1014 m−2 with grain size ~30 μm. In annealed state, the average GND density decreases by an order of magnitude, from 6.81 × 1015 to 7.32 × 1013 m−2 (annealing temperature from 200 to 500 °C) accompanied by increasing grain size. Moreover, the sample has greatest ductility with lowest GND density. According to the tendency of dislocations in TEM, GND gradually migrate from near (200) plane to near (220) plane with increasing annealing temperature. Additionally, GND mainly concentrate on the grain boundaries in the deformed state, and the GND distribution is more homogenous after annealing. It is shown that the GND density is related to the grain boundary type and misorientation angle between adjacent grains. The relationship between average GND density and grain size is approximately parabolic.
We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite-ferrite and ferrite-martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.
Cast duplex stainless steels (CDSS) were investigated in order to understand the mechanical properties of their individual constituents, ferrite and austenite, after thermal aging at 475 °C for up to 2000 h, using nanoindentation and micropillar compression. Nanoindentation experiments indicate that the hardness of ferrite continuously increases with increasing aging time, while after annealing heat treatment the hardness drops to slightly higher than that of the as-received condition. The hardness in austenite did not change significantly with different aging conditions. The results of micropillar compression on both ferrite and austenite, with [1 1 1] orientation, reveal similar trends to the results from nanoindentation. After the long-term thermal aging process, spinodal decomposition takes place, as well as G-phase precipitates forming in the ferrite phase. Then, after annealing, the spinodal decomposition products were reduced and only the G-phases were left in the ferrite phase. Hardening in ferrite is mostly caused by the spinodal decomposition but also related to G-phase precipitates. The role of the G-phase can be explained via precipitation hardening theory, i.e., the small G-phases block the movement of dislocations throughout the ferrite lattice.
Two austenitic stainless steels (SS) were designed to exhibit close to the largest possible difference in stacking fault energy (SFE) while staying within specifications. Neutron diffraction and scanning electron microscopy electron backscatter diffraction analysis were used to quantify stacking fault widths and deformation twinning, respectively, during room temperature straining. While the low-SFE SS exhibits much wider stacking faults (19 nm) throughout deformation as compared to the high-SFE SS (12 nm), the difference in twinning is less pronounced, with a slightly lower stress (50–100 MPa lower) and lower strain (5–10%) for twinning onset in the low-SFE SS.
https://authors.elsevier.com/a/1XZX84r9TDtBrp
Ferritic-austenitic cast duplex stainless steels (CDSS) undergo significant thermal aging embrittlement in service in light water nuclear reactors. Cast CF–3 and CF–8 duplex stainless steels have been laboratory aged for up to 17,200 h at four different temperatures (280 °C, 320 °C, 360 °C, 400 °C) in order to characterize the evolution of microstructure and mechanical properties. Standard Charpy V-Notch (CVN) impact testing and tensile testing have been conducted to measure the progression of embrittlement of the steels. Concurrently, nanoindentation measurements were performed to measure the hardening of the individual ferrite and austenite phases. Atom probe tomography (APT) analysis of the constituent phases is used to investigate the relationship between spinodal decomposition of the ferrite and the mechanical property evolution during aging. The effects of the local microstructure and property changes in the ferrite phase on the overall bulk mechanical property degradation of these steels are discussed.
Thermal aging behavior of stainless steel weld overlay cladding aged at 400 °C for 10,000 h was investigated. The results showed that the precipitates along the dislocation and within ferrite matrix contained the face-centered cubic structural G-phase and silicide phase. Silicide phase, supposed to be (Fe, Mn)3Si phase, preferred to form with G-phase along the dislocations and within ferrite matrix. Silicide phase showed a cube-on-cube relationship with ferrite phase and G-phase. Heterogeneous distribution of elements within Ni-Mn-Si clusters was responsible for silicide phase formation during thermal aging process, and the corresponding precipitation behavior discrepancy at different forming sites (dislocations, ferrite matrix and phase boundary) was discussed.
In the last few decades there is a continuous demand for characterization of mechanical properties of metals and alloys using small specimens. This is especially true in the nuclear industry due to the limited number of irradiated standard specimens and strict safety regulations. One common method for small specimen testing is the Small Punch Test (SPT) method. In a previous publication by the authors, it was demonstrated that the accuracy in estimation of material yield and ultimate stress from classical analysis of SPT experiments deteriorates as specimen thickness decreases below t0 = 300 µm. As a result, the classical equations for analysis of the SPT need to be corrected to be applicable to non-standard thin specimens. In the current study the finite element method incorporating a ductile damage model was used to investigate the SPT method applied to two very different representative materials with thickness values in the range of t0 = 100 − 500 µm. The effect of SPT system setup on the load displacement curves was also examined. The thorough theoretical study enabled the formulation of novel correction functions for yield stress estimation which are independent of the material stress-strain response. An additional correction function for estimation of the ultimate stress based on strain energy and specimen thickness is also proposed. The proposed correction functions were validated using Tensile and SPT experiments on t0 = 100, 200 µm thick SS316 L (Stainless Steel, grade 316 L). It is demonstrated that by applying the novel correction functions, a very good estimation of the yield and ultimate stress can be obtained from analysis of the SPT experiments.
Structural characterization of dislocations and dislocation reactions in a face-centered cubic high entropy alloy was conducted using the state-of-the-art spherical aberration corrected transmission electron microscopy. We experimentally measured the stacking fault energy of the high entropy alloy from the atomic images and diffraction contrast of dislocation cores. The low stacking fault energy results in widely dissociated dislocations and extensive dislocation reactions, which leads to the formation of immobile Lomer and Lomer-Cottrell dislocation locks. These dislocation locks act as both dislocation barriers and sources and are responsible for the significant work hardening with a large hardening rate in the alloy. Based on the atomic-scale characterization and classical dislocation theory, a simple equation was derived to describe the work hardening behavior of the high entropy alloy in the early-stage of plastic deformation.
Type 316LN stainless steel (SS) weld joints are widely used in several high temperature applications. The variations of creep properties across the microstructurally heterogeneous regions of 316LN SS weld joint such as base metal, weld metal and heat affected zone (HAZ) have been evaluated using Small Punch Creep (SPC) technique at 923 K. Both the SPC rupture and deformation behaviors were investigated. The creep strength gradient across 316LN SS weld joint showed a smooth increasing trend from base metal to weld metal. However, the trend of the rupture life for weld metal shifted with change in load level. The SPC deformations of various regions of weld joint have been analyzed according to the equation,δ=δ0+δT(1−e−κt)+δ̇st+δ3e[ϕ(t−tr)]. The variations of transient, secondary and tertiary creep parameters were determined across the weld joint. The master curve for transient creep deflection was obtained for various regions of weld joint.
Austenitic stainless steels are widely used in current commercial BWR and PWR systems as in-core and surrounding structural materials. These versatile steels have also been the main materials considered and applied for many advanced reactor technologies, including fast-breeder reactors and magnetic fusion reactors. For advanced reactors for higher temperatures or high lifetime damage levels, modified versions of the standard commercial stainless steel grades have been developed to provide improved performance properties and radiation resistance in specific reactor environments. An overview of the unirradiated properties and a summary of some important radiation-induced changes in properties are presented. The development of several alloys for more radiation resistance or better performance in specific reactor environments is highlighted.
Many common metals such as copper, silver, gold, aluminum, nickel, and their alloys, have a face-centered cubic crystal structure. The deformation behavior is closely related to the atomic structure of the core of dislocations. In face-centered cubic crystal structure, the atomic structure of the core of dislocations is more complex than other crystals. The shortest lattice vectors, and therefore the most likely Burgers vectors for dislocations in the face-centered cubic structure, are of the type 1/2〈110〉 and 〈001〉. Since the energy of a dislocation is proportional to the square of the magnitude of its Burgers vector b2, the energy of 1/2〈110〉 dislocations in an isotropic solid will be only half that of 〈001〉, i.e., 2a2/4 compared with a2. Thus, 〈001〉 dislocations are much less favored energetically and, in fact, are only rarely observed. Since 11/2〈110〉 is a translation vector for the lattice, glide of a dislocation with this Burgers vector leaves behind a perfect crystal and the dislocation is a perfect dislocation. Another dislocation arrangement has been observed in metals and alloys of low stacking-fault energy following treatment that produces a supersaturation of vacancies.
Two different methodologies for analysing the deterioration of mechanical properties due to hydrogen embrittlement by means of the small punch test (SPT) have been studied. In the first, specimens were electrochemically pre-charged before testing, while in the second, they were charged at the same time as testing. A novel, simple, easy-to-manage SPT device was developed for the latter purpose. Two different CrMoV steel grades, a base and a weld metal, tempered at different temperatures, were tested. Tensile tests of hydrogen pre-charged specimens as well as hydrogen content measurements were also performed. Greater hydrogen absorption was observed in the higher strength CrMoV weld metal due to its microstructure composed of low tempered bainite. This steel was fully embrittled in both tensile and small punch tests in the presence of hydrogen, and no significant difference between the two SPT methodologies were found in this case. The CrMoV base metal was only embrittled, however, when hydrogen charging was performed at the same time as testing, showing the greater suitability of this small punch test methodology. The fracture pattern of SPT specimens changed completely from ductile to brittle when testing in hydrogen. Typical SPT parameters also exhibited a marked decrease in ductility and fracture toughness, the CrMoV weld metal being more susceptible to hydrogen embrittlement. Finally, the feasibility of the small punch test for ranking the hydrogen embrittlement susceptibility in steels was demonstrated, and the most suitable SPT parameters for analysing the reduction in mechanical properties were defined.
This article reviews recent basic research on two classes of twins: growth twins and deformation twins. We focus primarily on studies that aim to understand, via experiments, modeling, or both, the causes and effects of twinning at a fundamental level. We anticipate that, by providing a broad perspective on the latest advances in twinning, this review will help set the stage for designing new metallic materials with unprecedented combinations of mechanical and physical properties.
The cast austenite stainless steels were investigate in order to understand the microstructural evolution and mechanical properties in the long-term thermal aging at 400. C for up to 20,000. h. Spinodal decomposition and G-phase precipitation in ferrite after long-term thermal aging lead to the degradation of mechanical properties. Ferrite hardness increases with aging time, but the austenite hardness does not change. Tensile strength is not strongly affected by aging time, but the plasticity has a significant decrease after long-term aging. Under impact with high strain rate, the ferrite phases deform by the way of deformation twinning. High stress concentration on the phase boundaries cause the phase boundary separating and the austenite's tearing off. 2013 Elsevier Ltd.
Dislocation cross-slip is introduced as an essential detail mechanism in plastic deformation with implications for strain rate, recovery, hardening, and structural evolution. Different cross-slip scenarios are classified according to different possibilities of re-dissociation of the glide dislocation which has cross-slipped following recombination of the stacking fault. The most important cross-slip models found in literature are reviewed. They fall into one of three main categories: line-tension approximation, linear-elastic treatment with long-distance interaction, and atomistic simulation. Special emphasis is laid on the problem of linear-elastic cutoff radii and critical approach distances and the author's methods of calculating the energy of cross-slip configurations. A critical comparison of different models is made including constriction energies and stress dependence, and the advantages and inherent weaknesses of the different approaches are discussed.
Influence of thermal aging at non-sensitized temperature on the SCC susceptibility of wrought 316LN at high temperature water was investigated. The SCC susceptibility increases with increasing of aging time, especially over 1000 h. The fracture surface for aged samples shows a mixed mode. In particular, the intergranular attack (IGA) has been found in brittle surface of the long-term aged material (5000 h). Kinks and dislocations at the grain boundary (GB) have been found as well as aging-induced hardening. Carbon enrichment on the GB is believed to be an important cause of IGA and higher SCC susceptibility in aged materials.
It is sometimes very convenient to use miniature tests for the mechanical characterization of materials, making use of very small specimens which may be extracted from the components during their normal service life. One of these tests is the small punch test (SPT). Nevertheless,
different expressions for estimating the tensile and fracture properties of metallic alloys by means of the small punch test (SPT) were proposed and their applicability was assessed in this paper after experimental testing a wide range of metallic materials and the application of a numerical model developed to study the effect of specimen thickness on these proposals.
The best estimation of the yield strength was obtained employing the SPT yield load, assessed as the crossing point between the SPT curve and a straight line parallel to the initial slope of the graph, with an offset displacement of t/10. The most suitable relationship for estimating the ultimate tensile strength was obtained by dividing the maximum SPT load by the product of the thickness and the displacement at maximum load (dm). However, a suitable relationship between the SPT displacement at maximum load and the tensile elongation was not obtained in the investigated materials, but it was demonstrated that the fracture toughness of non-brittle steels can be estimated from biaxial fracture strain (εqf), when εqf > 0.8.
In this study, the small punch test (SPT) was conducted to evaluate the stress corrosion cracking (SCC) susceptibility of stainless steel (SS) 304L with surface nanocrystallization (SNC) in 1 mol/L NaCl+0.5 mol/L HCl aq. The surface mechanical attrition treatment (SMAT) was applied to realize the SNC. The mechanical property and micro-structural evolutions of SS 304L induced by SMAT were investigated through optical microscope (OM), X-ray diffraction (XRD), micro-Vickers hardness and transmission electron microscopy (TEM). The grain size on the surface of the material was reduced to 30–100 nm. The SPT was conducted in both ambient air and corrosive solution. The results were investigated by OM and scanning electron microscopy (SEM), showing that in ambient air, the specimen with 30 min SMAT performed a higher yield strength and lower ductility than the solution annealed (SA) counterpart. The SS 304L without SMAT presented a transgranular SCC (TGSCC) mode in chloride solution. In contrast, the SNC 304L SS showed a higher SCC susceptibility with a typical intergranular SCC (IGSCC).
The effect of thermal aging on microstructural changes was investigated in stainless steel weld-overlay cladding composed of 90% austenite and 10% δ-ferrite phases using atom probe tomography (APT). In as-received materials subjected to cooling process after post-welding heat treatments (PWHT), a slight fluctuation of the Cr concentration was already observed due to spinodal decomposition in the ferrite phase but not in the austenitic phase. Thermal aging at 400°C for 10,000h caused not only an increase in the amplitude of spinodal decomposition but also the precipitation of G phases with composition ratios of Ni:Si:Mn=16:7:6 in the ferrite phase. The chemical compositions of M23C6 type carbides seemed to be formed at the austenite/ferrite interface were analyzed. The analyses of the magnitude of the spinodal decomposition and the hardness implied that the spinodal decomposition was the main cause of the hardening.
Ordering and domain grolth kinetics of Ni3Fe have been studied over the temperature range 434-497°C using electron microscopl and X-ral diffraction. Ordering, in general, takes place more luickll at higher temperatures, except for the highest temperatures studied lhere nucleation difficulties appear to become important. Domain grolth takes place in a tlo stage manner, corresponding to (i) a nucleation and grolth stage and (ii) a coalescence stage. The nucleation and grolth stage cannot be detected at lol temperatures because of the fast nucleation kinetics. The parameter n, defined at δ = Ktn. is reduced from the vaque of 1.0 expected during nucleation and grolth, and that of 0.5 expected during coalescence, lith the reduction being more marked at high temperatures. These features mal be interpreted either bl taking into account domain nuclei siles or bl considering the effect of dissolved impuritl.
The ordering transformation based on the Ni2Cr superlattice was studied by monitoring the variations in lattice parameter, electrical resistivity and microhardness in a series of ternary NiCrFe alloys (30–67 at.% NI, 17–32 at.% Cr and 1–51 at.% Fe) after long-term exposures at temperatures between 450 and 600 °C. The microstructural evolution of the alloys at interrupted annealing times was observed and then correlated to the variations in the physical properties. The degree of order and ordering kinetics depend on alloy composition, time and temperature of aging. Short-range order develops in all alloys during the first hours of aging. The degree of short-range order increases with decreasing temperature, increasing Ni and decreasing Fe concentration. In alloys approaching the composition Ni2Cr with additions of up to 5 at.% Fe this structure transforms to long-range order upon aging below the critical temperature for aging durations of the order of thousands of hours. The transformation kinetics depend on temperature and are markedly delayed by Fe addition. The degree of long-range order falls with increasing Cr concentration. In an alloy approaching the Ni2Cr atomic ratio with a 10 at.% Fe addition, long-range order forms after 32 000 h aging at 475 °C.
Past research leads to the conclusion that dislocation cell formation in work-hardened crystalline materials occurs when dislocations assemble into low energy configurations. On the assumptions that the dislocation cells formed in f.c.c. metals in early stage II also approach the lowest energy for a given dislocation content in the material and that the initial dislocation arrangement before cell formation consists of linear dipolar mats, i.e. sets of similar edge dislocations in coplanar arrays but alternating sign from one mat to the next, the structure of the resulting cells is investigated. It is found that the initial pile-up-like arrays should transform into tilt walls with or without some twist component, such that the axis of relative misorientation is roughly parallel to the original edge dislocations. By a simple consideration of energies it is found that cell formation should begin at or below about 1.2τ0 where τ0 is the initial critical flow stress. Again if the minimum energy is considered, it is found that for low stacking fault energy materials Lomer-Cottrell locks should form prominently, such that the primary dislocations become rotated roughly normal to the Lomer-Cottrell locks. These results are in good agreement with available experimental evidence.
Besides the macro-mechanical properties for thermal aging effect published in “Thermal aging effect on Z3CN20.09M Cast Duplex Stainless Steel” (Nuclear Engineering and Design 239(2009) 2217-2223), the thermal aging damage mechanism is investigated in this paper through nano-indentation tests and micro-structures evolution examination. Numerical simulations were carried out with GTN continuum damage model to investigate the different crack propagation process for aging. The nano-indentation hardness values increase with aging time for both phases while the hardness values of the ferrite phase are much higher and increase much more. The nano-indentation energy indicating the toughness decreases for both phases with aging time. TEM results show that the Cr-enriched α′ phase precipitates in the ferrite phase which is considered as the critical reason making the dislocation slip difficult and causing the increase of the strength and reduction of the toughness. The crack initiates from the ferrite phase instead of the austenite phase from the SEM observation and FEA simulation results, which reflects the change of the fracture mechanism for thermal aging.
In this study, double loop electrochemical potentiokinetic reactivation (DLEPR) test was applied to determine the degree of sensitization in 316L type stainless steel, where obtained results were correlated with revealed microstructures after oxalic acid test and weight loss measurements of Streicher and Huey acid tests. Best agreement was provided with test parameters which are 1M H2SO4 and 0.005M KSCN at 0.833mV/s scan rate at 30°C. Specimens were classified structurally as absence of chromium carbides – step, no single grain completely surrounded by carbides – dual and one or more grain completely surrounded by carbides – ditch, in the as-etched structure, if the Ir:Ia (×100) ratios were obtained to be between 0 and 0.2, 0.2 and 5.0 and 5.0 and higher, respectively. It was also found that at high KSCN concentrations, reactivation current profile skewed to higher potentials where this was attributed the formation of metastable pits, during the anodic scan of the test procedure.
The microstructural changes, precipitation behaviour, and mechanical properties of typical austenitic stainless steels (304 H, 316 H, 321 H, 347 H, and Tempaloy A–1) have been examined after long-term aging. The steels were aged statically in the temperature range 600–800°C for up to 50000 h. The microstructural changes were observed by optical and transmission electron microscopy, and the extracted residue was identified using X-ray analysis. Time–temperature precipitation diagrams were made for each steel. The amount of σ-phase was measured in samples aged at 700°C. The hardness and impact-value changes, and the tensile properties of aged samples were measured.MST/358
Small punch test was performed on CF8 duplex stainless steel aged at 370 and 400°C for up to 5000 h to characterize thermal aging embrittlement. At room temperature, the small punch (SP) load–displacement curve was similar in shape to those of ferritic steels and exhibited a good reproducibility in spite of ferrite–austenite structure. As the test temperature was lowered to a certain temperature depending on the degree of aging, the SP load showed a sudden drop followed by curve serration before the SP specimen fractured, resulting from the cracking of ferrite phase. While the aging heat treatment led to a slight increase of the yield strength, the transition appearing in the SP energy versus temperature curves shifted to higher temperature due to the hardening of ferrite phase. Additionally, phase boundary separation was an important factor in the degradation of the steel aged at 400°C.
The origin of planar slip in single-phase and precipitation-hardened f.c.c. alloys is discussed in detail. It is shown that pronounced short range order (SRO) or short range clustering (SRC) in solid solutions are the main reasons causing planar slip. Since the leading dislocations destroy SRO (SRC), glide plane softening occurs; therefore, a yield point or a point of inflection is observed on the stress-strain curve. In precipitation-hardened alloys finely dispersed particles with an atomic order also give rise to planar slip. Distinct planar slip occurs when cross slip is planar too. Other parameters, like a low value of the stacking fault energy or a high value of the yield stress, seem to be only of minor importance for the formation of pronounced planar slip.
This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.
Although Type 316 austenitic stainless steel is widely used in steam generating plants and nuclear reactors the knowledge
about aging reactions, nature of precipitates, and precipitation kinetics during high temperature exposure is limited. Time-temperature-precipitation
(TTP) diagrams were determined between 400° and 900°C for up to 3000 hr as a function of carbon content, solution treatment
temperature, and cold work. The nucleation and growth phenomena, morphology, and composition of the various carbide (M23C6, M6C) and intermetallic phases (σ, χ, η were determined. The complex sequence of phase instabilities can be explained on the
basis of the carbon content, effect of molybdenum and chromium on the carbon solubility, thermodynamic stability of the phases,
and the kinetics of the various precipitation reactions.
The Westinghouse AP1000 Program is aimed at making available a nuclear power plant that is economical in the US deregulated electrical power industry in the near-term. The AP1000 is a two-loop 1000 MWe pressurizer water reactor (PWR). It is an uprated version of the AP600. Passive safety systems are used to provide significant and measurable improvements in plant simplification, safety, reliability, investment protection and plant costs. The AP1000 uses proven technology, which builds on over 35 years of operating PWR experience. The AP1000 received Final Design Approval from the United States Nuclear Regulatory Commission in September 2004; the AP1000 has also received Design Certification by the USNRC in December 2005. The AP1000 and its predecessor AP600 are the only nuclear reactor designs using passive safety technology licensed anywhere in the world. The safety performance of AP1000 has been verified by extensive testing, safety analysis and probabilistic safety assessment. AP1000 safety margins are large and the potential for accident scenarios that could jeopardize public safety is extremely low.Simplicity is a key technical concept behind the AP1000. It makes the AP1000 easier and less expensive to build, operate, and maintain. Simplification also provides a hedge against regulatory driven operations and maintenance costs by eliminating equipment subject to regulation. The AP1000's greatly simplified design complies with NRC regulatory and safety requirements and the EPRI advanced light water reactor (ALWR) utility requirements document.Plans are being developed for implementation of the AP1000 plant. Key factors in this planning are the economics of AP1000 in the de-regulated US electricity market, and the associated business model for licensing, constructing and operating these new plants.
The nucleation of extended dislocations from the grain boundaries in nanocrystalline aluminum is studied by molecular-dynamics simulation. The length of the stacking fault connecting the two Shockley partials that form the extended dislocation, i.e., the dislocation splitting distance, rsplit, depends not only on the stacking-fault energy but also on the resolved nucleation stress. Our simulations for columnar grain microstructures with a grain diameter, d, of up to 70 nm reveal that the magnitude of rsplit relative to d represents a critical length scale controlling the low-temperature mechanical behavior of nanocrystalline materials. For rsplit>d, the first partials nucleated from the boundaries glide across the grains and become incorporated into the boundaries on the opposite side, leaving behind a grain transected by a stacking fault. By contrast, for rsplit<d two Shockley partials connected by a stacking fault are emitted consecutively from the boundary, leading to a deformation microstructure similar to that of coarse-grained aluminum. The mechanical properties of nanocrystalline materials, such as the yield stress, therefore depend critically on the grain size.
The concepts of twinning shears and twinning modes are introduced. The early attempts to predict these features are presented. This is followed by a detailed discussion of the formal theories of Bilby and Crocker and Bevis and Crocker for predicting these elements. Their formalisms are applied to predict twinning modes in single lattice structures, superlattices, hexagonal close packed structure and other double lattice structures. Wherever possible the predicted modes are compared with those observed.The description of fully coherent, rational twin interfaces is presented, and the concepts of elementary, zonal, complementary and partial twinning dislocations are discussed. It is suggested that the irrational K1 twin interfaces may be faceted on the microscopic scale, and these facets may be coherent.Homogeneous and heterogeneous nucleation of twins are discussed. The growth of twins by the nucleation of twinning dislocations on planes parallel and contiguous to the coherent twin boundary is considered. Various dislocation models proposed for the formation of twins in b.c.c., f.c.c., diamond cubic, zinc-blende and h.c.p. structures are critically reviewed. In some cases the supporting experimental evidence is presented. Additionally, the effects of deformation temperature, imposed strain-rate, alloying and doping, prestrain, precipitates and second phase disperions on deformation twinning are discussed.Mechanistic details regarding the accommodation processes occurring at twins terminating within a crystal, slip-twin, twin-slip and twin-twin intersections are reviewed and are compared with the experimental results. The role of twins in the nucleation of fracture in materials is also considered.
Stainless steel components used in nuclear power plant must be capable of maintaining reasonable mechanical properties after thermal ageing and irradiation damage has accumulated over the lifetime of the system. This study examines the fracture toughness behaviour of wrought and welded Type 316 material in long-term thermally aged and irradiated conditions. The results indicate that whilst some potentially detrimental microstructural changes have occurred during ageing, the degradation in mechanical properties is not large. In wrought material some comparisons are made between the toughness of Type 316 grade and recent results obtained on modified Type 316LN grade materials. The effects of welding processes on oxygen and inclusion content have been quantified in MMA and TIG welds, and the results have been used to explain the higher toughness of TIG-welded material. Comparisons of fracture toughness after irradiation have also been made between arc welds in Type 316 and other grades.