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Copper-rich nanoclusters: ferritic steels strengthened

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Abstract

Cu-rich nanocluster strengthening in ferritic steels has attracted considerable attention in recent years and has become the cornerstone for the development of high-strength low-carbon steels. This entry intends to provide a comprehensive review of the recent developments of the Cu-rich nanocluster-strengthened ferritic steels. The unique structural evolution and interfacial segregation of the Cu-rich nanoclusters, as characterized by scanning transmission electron microscope and atom probe tomography, will be presented and discussed. In addition, their tensile properties, impact resistance, welding properties, and radiation effects will all be summarized and analyzed.

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... Recently, the co-precipitation of Cu-rich precipitates and NiAl intermetallics has been proven to be a representative approach for nano-precipitation design. These nanoprecipitates are typically on a sufficiently fine diameter of less than 5 nm, and coherent with the body-centered cubic (bcc) ferrite/martensite matrix [16]. Zhang et al. [17] performed aging at 500 C for 10 h on a Fe-2.5Cu-2.1Al-1.5Mn ...
... These body-centered cubic (bcc) structured Cu-rich precipitates are coherent with the ferrite matrix [22], as also indicated from our synchrotron X-ray results. When more Cu atoms diffuse into these precipitates, Fe atoms are interstitially partitioned to the matrix, and such Cu-rich precipitates grow [16]. As a result, the cores of larger precipitates are enriched more with Cu atoms. ...
... The resistance of these bcc Cu precipitates to the moving dislocations is size-dependent. With the increase of the diameter, the bcc precipitates enriched with more Cu atoms, and became easier to be sheared [16]. Considering the size distribution of these bcc Cu-rich precipitates in Fig. 7b, the moving dislocations cut through these precipitates at different strain levels, leading to the significantly leveraged strain-hardening curve. ...
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... The origin of the Cu-rich nano-precipitation lies behind the limited solubility (only~0.15 wt% at 550°C) of Cu solutes in the BCC Fe solid solution [93,94] and the positive enthalpy of mixing between the Fe and Cu atoms [95]. Once cooled down from the homogenization temperature, a solid solution supersaturated with Cu solutes is created and this drives the nucleation of Cu-rich precipitates. ...
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The variations in the FCGR curve, the J–R curve, the blunting-line slope and the stretch zone width (SZW) for systematic variation of microstructure in Cu-strengthened HSLA-100 steel has been studied. The microstructural variation in the steel has been introduced through ageing at various temperatures after an initial quenching treatment. This has resulted in progressive tempering of the as-quenched martensitic matrix, accompanied by nanoscale precipitation of coherent Cu particles that gradually coarsen and loose coherency with overageing. It was observed that although there was a systematic trend, the FCGR curves were least sensitive to microstructural changes. The variation of fracture toughness, characterized by JQ in most cases and by KQ for the microstructures displaying highest strengths, correlated well with the inverse relationship between fracture toughness and strength. A systematic trend was also observed for the pre-exponent and exponent of the power-law tearing curve (for cases in which brittle fracture was precluded), the blunting-line slope and the SZW. The effect of coherency of precipitates in restricting plastic flow, as implied from the variation of mechanical properties with ageing temperature is thought to be responsible for the effects observed.
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Oxide dispersion strengthened ferritic steels are being considered for a number of advanced nuclear reactor applications because of their high strength and potential for high temperature application. Since these properties are attributed to the presence of a high density of very small (nanometer-sized) oxide clusters, there is interest in examining the radiation stability of such clusters. A novel experiment has been carried out to examine oxide nanocluster stability in a mechanically alloyed, oxide dispersion strengthened ferritic steel designated 12YWT. Pre-polished specimens were ion irradiated and the resulting microstructure was examined by atom probe tomography. After ion irradiation to ∼0.7dpa with 150keV Fe ions at 300°C, a high number density of ∼4nm-diameter nanoclusters was observed in the ferritic matrix. The nanoclusters are enriched in yttrium, titanium and oxygen, depleted in tungsten and chromium, and have a stoichiometry close to (Ti+Y):O. A similar cluster population was observed in the unirradiated materials, indicating that the ultrafine oxide nanoclusters are resistant to coarsening and dissolution under displacement cascade damage for the ion irradiation conditions used.
Article
Samples of the welds from the Midland and Palisades reactors in the unirradiated condition and after neutron irradiation to a high fluence of up to 3.4×1023m−2 (E>1MeV) have been characterized with the Oak Ridge National Laboratory’s local electrode atom probe. High number densities, ∼5 and ∼7×1023m−3, respectively, of ∼2-nm-diameter copper-, nickel-, manganese- and silicon-enriched precipitates were observed after neutron irradiation. These copper-enriched precipitates were observed both in the matrix of the steel and also preferentially located along the dislocations. No appreciable differences were observed in the sizes or the compositions of the precipitates in the matrix and on the dislocations. The average interparticle distance along the dislocations was 11±3nm. Phosphorus segregation was also evident along the dislocations in both welds. No other nanoscale intragranular phases were observed in these neutron irradiated welds.
Article
Displacement cascades have a much greater power to disrupt the crystalline lattice than do single displacement events. The effects of displacement cascades on particle resolution, on disordering, and on amorphization are considered herein. Particle resolution is found to be a relatively slow process, even with the assistance of displacement cascades. Larger displacement cascades which give greater recoil distances are found to give faster resolution. Numerous experimental studies of oxide particles in metallic matrices have revealed at most the first stages of radiation recoil dissolution. Ordered intermetallic compounds are readily disordered by low temperature irradiation, even in the absence of displacement cascades. It was recently shown theoretically that displacement cascades may give disordering when equivalent displacement conditions without cascades leave the alloy ordered. Radiation amorphization of intermetallic compounds has been observed under a variety of irradiation conditions. Some compounds amorphize readily under cascade-inducing heavy ion irradiation but not under electron irradiation. Recent modelling, studies have shown that clusters of defects, such as may be formed in cascades, may be needed for amorphization.
Article
High-resolution electron microscopy experiments have been performed to explore the bcc—9R transformation and the subsequent elastic relaxation of Cu precipitates in an Fe—Cu alloy aged at 550°C. It was found that both electron irradiation (at an electron energy of 400 kV) and thermal annealing caused rotation of the close-packed (009)9R planes in twinned 9R Cu precipitates. For 400 kV electron irradiation, such rotations were observed in precipitates smaller than about 12nm in diameter. For specimens cooled from the ageing temperature of 550°C to a given temperature up to −60°C, and then annealed at 400°C, the rotation of (009)9R planes was found to occur only in precipitates above a size which depended on the temperature to which the specimen had been cooled. This critical size ranged from about 9 nm for specimens cooled to 400°C, to 4 nm for specimens cooled to −60°C. It is argued that these critical sizes are indicative of the sizes at which coherent bcc precipitates transform martensitically at different temperatures. At the ageing temperature of 550°C, the transformation to 9R takes place when precipitates reach a size of about 12nm. The number of twin segments in the transformed 9R precipitates is determined by the transformation, depending on the precipitate size. The annealing-induced plane rotations are shown to be connected with the diffusional relaxation of elastic strains, which are created upon the martensitic transformation. From the precipitate size and annealing time dependence of the rotations, it is concluded that the elastic strains relax by atomic diffusion along the interfaces between the Fe matrix and Cu precipitates. The activation energy for the interfacial diffusion is evaluated to be 1.7eV.
Article
Transmission electron microscopy and high-resolution electron microscopy investigations of the structure of Cu precipitates in the size range 4–30 nm were carried out as part of a wider investigation into Cu precipitation from thermally aged Fe-Cu and Fe-Cu-Ni ferritic model alloys. A twinned 9R structure was found to be present in precipitates slightly larger than 4nm in diameter, following transformation from b.c.c. Two twin-related 9R variants were observed in all the smallest 9R particles studied. The 9R precipitates were observed to grow subsequently as spherical, multiply twinned particles up to approximately 17 nm, indicating that further twinning must occur during growth in the 9R phase. At sizes larger than 17 nm, a second transformation to the more stable 3R structure takes place. Observations on these 3R particles indicate that, following transformation from 9R, the precipitates are untwinned and have a distorted f.c.c. structure. The particle-matrix orientation is close to, but not exactly, the Kurdjumov-Sachs relationship. Larger 3R particles are observed to have the expected f.c.c. structure, aligned according to the Kurdjumov-Sachs relationship, suggesting that lattice relaxation occurs during diffusional growth of the 3R precipitates.
Article
An atom probe tomography microstructural characterization has been performed on an A533B pressure vessel steel (JRQ) after irradiation to a fluence of 5 × 1023 n m-2 (E > 1 MeV) and a subsequent annealing treatment of 168 h at 460 °C and also through two cycles of neutron irradiation (0.85 × 1023 n m-2 (E > 1 MeV)) and annealing (168 h at 460 °C). The alloy that was neutron irradiated to a fluence of 5 × 1023 n m-2 exhibited a high number density of Cu-enriched precipitates and a shift in the ductile-to-brittle transformation temperature of DeltaT41J = 96 °C. Annealing for 168 h at 460 °C coarsened these Cu-enriched precipitates and recovered the embrittlement. The material that was re-irradiated to a total fluence 1.7 × 1023 n m-2 also exhibited a high number density of Cu-enriched precipitates and a DeltaT41J shift of 56 °C. Annealing the re-irradiated material for 168 h at 460 °C coarsened these precipitates and recovered the embrittlement.
Article
The influence of simulated heat-affected zone thermal cycles on the microstructural evolution in a blast-resistant naval steel was investigated by dilatometry, microhardness testing, optical microscopy, electron backscatter diffraction and atom-probe tomography (APT) techniques. Coarsening of Cu precipitates were observed in the subcritical and intercritical heat-affected zones, with partial dissolution in the latter. A small number density of Cu precipitates and high Cu concentration in the matrix of the fine-grained heat-affected zone indicates the onset of Cu precipitate dissolution. Cu clustering in the coarse-grained heat-affected zone indicated the potential initiation of Cu reprecipitation during cooling. Segregation of Cu was also characterized by APT. The hardening and softening observed in the heat-affected zone regions was rationalized using available strengthening models.
Article
The influence of small amounts of copper on the neutron-irradiation-induced embrittlement of reactor pressure vessel steels is of considerable current interest. Previous work has shown that the embrittlement is associated with the formation of copper-rich precipitates, but uncertainties remain regarding their composition and form. This Letter reports the preliminary results from a structural investigation of such precipitates in solution-treated and thermally aged Fe–-Cu and Fe–-Cu–-Ni model alloys using fluorescence extended X-ray absorption spectroscopy. The most significant result is the first direct observation that small clusters in the peak hardness condition have a b.c.c. structure.
Article
Interactions between precipitates and dislocations were investigated using atomistic computer simulations. In particular, the effect of Cu precipitates on the core structures, slipping behavior, and Critical Resolved Shear Stress (CRSS) of an edge dislocation in a b.c.c. Fe single crystal was considered. Three-dimensional (3D) molecular statics were performed on a a0/2(11-0) [111] edge dislocation passing through spherical Cu precipitates under shear deformations. The atomic stresses on the dislocation slip plane are calculated for different deformation stages. The characteristics of stress distributions and dislocation core structures are analyzed. The Peierls stress of a pure edge dislocation and the CRSS in the presence of Cu precipitates are determined. Finally, the mechanisms of the dislocation passing through Cu precipitates were analyzed by examining the evolution of atomic configurations.
Article
The reactor pressure vessel (RPV) surrounding the core of a commercial nuclear power plant is subject to embrittlement due to exposure to high energy neutrons. The effects of irradiation embrittlement can be reduced by thermal annealing at temperatures higher than the normal operating conditions. However, a means of quantitatively assessing the effectiveness of annealing for embrittlement recovery is needed. The objective of this work was to analyze the pertinent data on this issue and develop quantitative models for estimating the recovery in 30 ft-lb (41 J) Charpy transition temperature and Charpy upper shelf energy due to annealing. Data were gathered from the Test Reactor Embrittlement Data Base and from various annealing reports. An analysis data base was developed, reviewed for completeness and accuracy, and documented as part of this work. Independent variables considered in the analysis included material chemistries, annealing time and temperature, irradiation time and temperature, fluence, and flux. To identify important variables and functional forms for predicting embrittlement recovery, advanced statistical techniques, including pattern recognition and transformation analysis, were applied together with current understanding of the mechanisms governing embrittlement and recovery. Models were calibrated using multivariable surface-fitting techniques. Several iterations of model calibration, evaluation with respect to mechanistic and statistical considerations, and comparison with the trends in hardness data produced correlation models for estimating Charpy upper shelf energy and transition temperature after irradiation and annealing. This work provides a clear demonstration that (1) microhardness recovery is generally a very good surrogate for shift recovery, and (2) there is a high level of consistency between the observed annealing trends and fundamental models of embrittlement and recovery processes.
Article
Advanced fission and future fusion energy will require new high-performance structural alloys with outstanding properties that are sustained under long-term service in ultrasevere environments, including neutron damage producing up to 200 atomic displacements per atom and, for fusion, 2000 appm of He. Following a brief description of irradiation damage and damage resistance, we focus on an emerging class of nanostructured ferritic alloys (NFAs) that show promise for meeting these challenges. NFAs contain an ultrahigh density of Y-Ti-O-enriched dispersion-strengthening nanofeatures (NFs) that, along with fine grains and high dislocation densities, provide remarkably high tensile, creep, and fatigue strength. The NFs are stable under irradiation up to 800°C and trap He in fine-scale bubbles, suppressing void swelling and fast fracture embrittlement at lower temperatures and creep rupture embrittlement at high temperatures. The current state of the development and understanding of NFAs is described, along wi...
Article
Precipitation hardening has long been used to increase the strength of commercial alloys, such as quenched and tempered steels and the duralumin type aluminium alloys. The theoretical treatments of precipitation hardening are briefly considered. The equations for strengthening by ‘hard’ indeformable particles and by ‘soft’ deformable particles are presented, and the implications are discussed. These lead to the concept of an optimum particle size for a given system, but the optimum can vary from system to system depending upon the particle characteristics. A broad comparison is made between the increments in strength that occur due to precipitation in commercial alloys and the predictions of the theories; an important contribution to these increments in strength is shown to derive from variations in the volume fraction of precipitated particles that can be employed in the various systems.
Article
High-resolution transmission electron microscopy and conventional transmission electron microscopy have been used to investigate in detail the transformation processes of twinned 9R copper precipitates to final stable structures via a 3R structure in a thermally aged Fe-Co model alloy. In 9R precipitates of size greater than about 13nm, the motion of the twin boundaries and the elimination of the regular stacking faults on every third (009)9R close packed plane were observed to occur nearly simultaneously. Direct evidence was found that 3R is a non-cubic structure obtained when the regular stacking faults on the (009)9R basal planes are removed. In larger precipitates (of size up to about 26 nm), lattice plane rotations and plane spacing changes in 3R variants took place towards stable fcc and fct structures, suggesting the occurrence of lattice relaxation involving the diffusion of atoms. The fct structure had larger lattice constants a=b=0.369 nm and c=0.366 nm than bulk fcc copper (a=0.361 nm). Precipitates of size 26-40 nm consisted of nearly twin-related variants with both fcc and fct segments, and aligned with the iron matrix according to the Kurdjumov-Sachs orientation relationship. Larger precipitates were observed to be fcc and fct single crystals.
Article
We present experimental evidence that deformation can induce the transformation of small Cu-rich precipitates from the coherent bcc phase to the 9R phase in aged binary FeCu alloys. This is in broad agreement with molecular dynamics simulations of the interaction of dislocations with copper precipitates carried out by Bacon and co-workers [Acta Mater. 50 195/209 (2002); Mater. Sci. Eng. A 400/401 353 (2005); J. Nucl. Mater. 329/333 1233 (2004)].
Article
Nanoscale co-precipitation in a novel high-strength low-carbon steel is studied in detail after isothermal aging. Atom-probe tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M2C (M is any combination of Cr, Mo, Fe, or Ti) carbide strengthening precipitates. Coarsening of Cu precipitates is offset by the nucleation and growth of M2C carbide precipitate, resulting in the maintenance of a yield strength of 1047 ± 7 MPa (152 ± 1 ksi) for as long as 320 h of aging time at 450 °C. Impact energies of 153 J (113 ± 6 ft-lb) and 144 J (106 ± 2 ft-lb) are measured at −30 °C and −60 °C, respectively. The co-location of Cu and M2C carbide precipitates results in non-stationary-state coarsening of the Cu precipitates. Synchrotron-source X-ray diffraction studies reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due to dissolution of cementite, Fe3C, which is the source of carbon for the nucleation and growth of M2C carbide precipitates. Less than 1 vol.% austenite is observed for aging treatments at temperatures less than 600 °C, suggesting that transformation-induced plasticity does not play a significant role in the toughness of specimens aged at temperatures less than 600 °C. Aging treatments at temperatures greater than 600 °C produce more austenite, in the range 2–7%, but at the expense of yield strength.
Article
The local composition of small, coherent Cu-rich precipitates with a metastable body-centered cubic structure in a ferritic a-Fe matrix of a high-strength low-carbon steel was studied by conventional atom-probe tomography. The average diameter, AEDae, of the precipitates is 2.5 ± 0.3 nm at a number density of (1.1 ± 0.3) · 10 24 m À3 after direct aging at 490 °C for 100 min to a near-peak hardness condition, yielding a value of 84 Rockwell G. Besides Cu, the precipitates contain 33 ± 1 at.% Fe and are enriched in Al (0.5 ± 0.1 at.%). Nickel and Mn are significantly segregated at the a-Fe matrix/precipitate heterophase interfaces. The Gibbsian interfacial excesses relative to Fe and Cu are 1.5 ± 0.4 atoms nm À2 for Ni and 1.0 ± 0.3 atoms nm À2 for Mn. The reduction of the interfacial free energy, calculated utilizing the Gibbs adsorption isotherm, is 16 mJ m À2 for Ni and 11 mJ m À2 for Mn.
Article
Quenched and tempered 5.5Ni steel was embrittled by hydrogen charging and broken in air at room temperature. The primary fracture mode was transgranular quasicleavage. The quasicleavage facets were studied by scanning electron fractography and by transmission electron microscopy of profile fractographic specimens. The latter were prepared by plating the fracture surface with nickel and thinning so that the fracture surface was contained within the region of the specimen that was transparent to the electron beam. The fracture surface generally followed martensite lath boundaries. In addition, interlath microcracks were frequently found in the material immediately beneath the fracture surface. These results suggest that transgranular hydrogen embrittlement in this steel is primarily an interlath cracking phenomenon. Since the lath boundary planes tend to lie in {110}, the results also explain the prevalence of {110} quasicleavage in the embrittled specimens, which contrasts with the {100} cleavage found in uncharged specimens broken below the ductile-to-brittle transition temperature.
Article
A new high strength, high toughness steel containing Cu for precipitation strengthening was recently developed for naval, blast-resistant structural applications. This steel, known as BlastAlloy160 (BA-160), is of nominal composition Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct). The evident solidification substructure of an autogenous gas tungsten arc (GTA) weld suggested fcc austenite as the primary solidification phase. The heat-affected zone (HAZ) hardness ranged from a minimum of 353HV in the coarse-grained HAZ (CGHAZ) to a maximum of 448HV in the intercritical HAZ (ICHAZ). After postweld heat treatment (PWHT) of the spot weld, hardness increases were observed in the fusion zone (FZ), CGHAZ, and fine-grained HAZ (FGHAZ) regions. Phase transformation and metallographic analyses of simulated single-pass HAZ regions revealed lath martensite to be the only austenitic transformation product in the HAZ. Single-pass HAZ simulations revealed a similar hardness profile for low heat-input (LHI) and high heat-input (HHI) conditions, with higher hardness values being measured for the LHI samples. The measured hardness values were in good agreement with those from the GTA weld. Single-pass HAZ regions exhibited higher Charpy V-notch impact toughness than the BM at both test temperatures of 293K and 223K (20°C and –50°C). Hardness increases were observed for multipass HAZ simulations employing an initial CGHAZ simulation.
Article
Metastable solid solutions of Fe and Cu, which are immiscible in equilibrium, have been formed using high-energy ball milling of elemental powder mixtures. Single-phase face-centered-cubic (fcc) solid solution was obtained for 0≪x≤60, and body-entered-cubic (bcc) solid solution for 75≤x≪100. The transition from fcc to bcc occurred near x=70, where a mixture of fcc and bcc phases was obtained. The enthalpy of transformation to equilibrium was measured using differential scanning calorimetry. The average atomic volume of the phases exhibits a positive deviation from Vegard’s law, in qualitative agreement with the large positive enthalpy of mixing in this system. The magnetic moments and Curie temperatures for the metastable solid solutions have been determined and compared with those reported for Fe-Cu alloys formed by vapor deposition. Calculations of the formation enthalpy (ΔH) and free energy (ΔG) have been performed based on calphad data, with corrections based on our magnetization measurements. The calculated ΔG results are used to explain the observed fcc-bcc transition under polymorphous constraints.
Article
Strengthening due to small coherent BCC precipitates of copper is an important component of in-service irradiation hardening of ferritic pressure-vessel steels. The dislocation effects involved are studied here by atomic-scale computer simulation. Many-body interatomic potentials for the Fe–Cu alloy system are used to investigate stacking-fault-energy surfaces and to simulate the atomic structure of the <111> screw dislocation in both pure α-iron and the metastable BCC phase of copper. In iron, the core has the well-known three-fold form of atomic disregistry. In BCC copper, however, the core becomes delocalised by transformation of the copper as the lattice parameter is reduced to mimic the strain experienced by precipitates. Simulation of the screw dislocation threading through the centre of a BCC copper precipitate in an α-iron matrix shows that the extent of core delocalisation depends on precipitate size. The dislocation energy changes indicate a significant dislocation pinning effect due to this dislocation-induced precipitate transformation process.
Article
The size, number density and compositions of ultrafine copper–manganese precipitates that form in Fe–0.80at.% Cu and Fe–0.78at.% Cu–1.05at.% Mn model alloys that were neutron irradiated to a fluence of ∼1×1023 nm−2 (E>1 MeV) at 288 °C have been estimated by atom probe tomography and small-angle neutron scattering (SANS) experiments. The number density of precipitates was approximately an order of magnitude higher in the Fe–Cu–Mn alloy than in the Fe–Cu alloy. A direct comparison of the microstructural parameters estimated by each technique revealed good agreement between the radii of gyration and the number densities. However, the copper content in precipitates inferred from SANS was significantly higher than estimated from the atom probe results.
Article
Atom probe tomography has played a key role in the understanding of the embrittlement of neutron irradiated reactor pressure vessel steels through the atomic level characterization of the microstructure. Atom probe tomography has been used to demonstrate the importance of the post weld stress relief treatment in reducing the matrix copper content in high copper alloys, the formation of ∼2-nm-diameter copper-, nickel-, manganese- and silicon-enriched precipitates during neutron irradiation in copper containing RPV steels, and the coarsening of these precipitates during post irradiation heat treatments. Atom probe tomography has been used to detect ∼2-nm-diameter nickel-, silicon- and manganese-enriched clusters in neutron irradiated low copper and copper free alloys. Atom probe tomography has also been used to quantify solute segregation to, and precipitation on, dislocations and grain boundaries.