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Copper-rich nanoclusters: ferritic steels strengthened
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.
... 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. ...
Breaking the strength-ductility paradox of very-low (≤0.05 wt.%)-C medium (3–12 wt.%)-Mn steels (MMnS) has been a hard-wired topic, since in these steels multi-step heat treatments are usually required to obtain austenite for improved ductility and precipitates for higher strength. In this study, a compact two-step heat treatment comprising short (2 min) annealing and tempering was developed to investigate the synergetic effect of austenite reversion and nano-precipitation on the tensile behavior of a very-low-C MMnS containing 1.5 wt.% Cu and 1.5 wt.% Ni. The annealing step promoted considerable amount of reverted austenite (33 vol.%), and the annealing was short to prevent Cu and Ni from partitioning into austenite, since they were supposed to maintain in the ferrite phase and then promote the nano-precipitation in the subsequent tempering stage. During the subsequent tempering step, the nano-precipitates with Cu concentration of 20–50 at.% in the precipitation core and enriched with Cu, Ni, Al and Mn were observed in the ferrite phase. The volume fraction of reverted austenite reached 38.5 vol.% after tempering, which led to the ultimate tensile strength of 1222 MPa and total elongation of 29 % by the transformation induced plasticity during plastic deformation. The current study demonstrates the beneficial influence of the compact two-step heat treatment on the austenite reversion and nano-precipitation behavior of very-low-C MMnS with the addition of Cu and Ni, which subsequently enables an enhanced strain hardening behavior, thereby improving the mechanical property profile of the MMnS.
... 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. ...
Low-carbon advanced nanostructured steels have been developed for various structural engineering applications, including bridges, automobiles, and other strength-critical applications such as the reactor pressure vessels in nuclear power stations. The mechanical performances and applications of these steels are strongly dependent on their microstructural features. By controlling the size, number density, distribution, and types of precipitates, it is possible to produce nanostructured steels with a tensile strength reaching as high as 2 GPa while keeping a decent tensile elongation above 10% and a reduction of area as high as 40%. Besides, through a careful control of strength contributions from multiple strengthening mechanisms, the nanostructured steels with superior strengths and low-temperature impact toughness can be obtained by avoiding the temper embrittlement regime. With appropriate Mn additions, these nanostructured steels can achieve a triple enhancement in ductility (total tensile elongation, TE of ∼30%) at no expense of strengths (yield strength, YS of ∼1100 to 1300 MPa, ultimate tensile strength, UTS of ∼1300 to 1400 MPa). More importantly, these steels demonstrate good fabricability and weldability. In this paper, the microstructure-property relationships of these advanced nanostructured steels are comprehensively reviewed. In addition, the current limitations and future development of these nanostructured steels are carefully discussed and outlined.
A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without adding any strong carbide-forming element was designed to investigate the co-precipitation behaviors of Cu-rich (CRPs) and β-NiAl precipitates by using electron back-scatter diffraction (EBSD) and atom probe tomography (APT). In the light of phase transformation and hardness-temperature curves, it is estimated that the nanoscale clusters and precipitates formed at 450 and 500°C tempering respectively are responsible for the strong precipitation strengthening effect. The number density of the co-precipitated couples is significantly increased by an order of magnitude while the size almost keeps constant. The limited growth of the dispersed small precipitates is in association with the co-precipitation reactions itself and the addition of Co that possibly hinders fast coarsening of the co-precipitate couples. The strength contributions from dislocated martensite, solid solution and precipitation were roughly calculated and compared with the experimental values. It is found the precipitation hardening is dominant in strengthening and the solid solution hardening exhibits a non-negligible effect as well.
High-strength low-carbon ferritic steels attaining a maximum yield strength of 1600 MPa by combined Cu and NiAl precipitation-strengthening were developed. The yield strength of the alloys increases monotonically with the total concentration of the principal alloying elements i.e. Mn, Cu, Ni and Al. At 12.40 at.%, a 1600 MPa yield strength is achieved after solution treatment at 950 °C followed by aging at 550 °C for 2 h. For all three alloys investigated, the hardness reached a maximum after 1–2 h aging at 500–550 °C. At peak hardness, the combined precipitation of the body-centered cubic (bcc) Cu-alloy and B2-ordered NiAl-type intermetallic precipitates is observed by atom probe tomography (APT). The morphology, composition and structure of the Cu-alloy and NiAl-type precipitates were characterized using APT and transmission electron microscopy, as a function of aging time at 550 °C. In peak hardness conditions, the equiaxed bcc Cu-alloyed precipitates contain substantial amounts of Fe and are enriched in Ni, Al and Mn. Ni and Mn segregate at the Cu-alloy precipitate/ferrite matrix interface. In addition to the segregation, B2 NiAl-type precipitates nucleate at the Cu-alloy precipitates. After aging for 2 h, most Cu-alloy precipitates have a NiAl-type precipitate attached to their side. On subsequent further aging, the Cu-alloyed precipitates enrich progressively with Cu and elongate, indicating a transformation to the 9R or face centered cubic structure. The Cu-alloyed precipitates coarsen slower than the NiAl-type precipitates due to interfacial energy differences between the two types of precipitates, slower diffusion kinetics of Cu through the NiAl precipitates, different matrix equilibrium solubility and solute transfer from Cu-alloyed precipitates to NiAl-type precipitates. The relatively slow growth and coarsening of Cu-alloyed precipitates are consistent with the observation of an only modest decrease of hardness with extended aging.
Bulk metallic glasses (BMGs) have been classified according to the atomic size difference. heat of mixing (Delta H-mix) and period of the constituent elements in the periodic table. The BMGs discovered to date are classified into seven groups on the basis of a previous result by Inoue. The seven groups are as follows: (G-I) ETM/Ln-LTM/BM-Al/Ga, (G-II) ETM/Ln-LTM/BM-Metalloid (G-III) Al/Ga-LTM/BM-Metalloid, (G-IV) IIA-ETM/Ln-LTM/BM, (G-V) LTM/BM-Metalloid, (G-VI) ETM/Ln-LTM/BM and (G-VII) IIA-LTM/BM. where ETM. Ln, LTM, BM and IIA refer to early transition, lantlianide, late transition, group IIIB-IVB and group IIA-group metals, respectively. The main alloying element of ternary G-I, G-V and G-VII, ternary G-II and G-IV, and tertiary G-VI BMGs is the largest, intermediate and smallest atomic radius compared to the other alloying elements, respectively. The main alloying element of tertiary BMGs belonging to G-I. G-V. G-VI and G-VII is an element in the atomic pair with the largest and negative value of Delta H-mix (<(Delta H)under bar >(mix)(LN)). while the main element of ternary BMGs belonging to G-II and G-IV is independent of the atomic pair with <(Delta H)under bar >(mix)(LN). The characteristics of the main element derived for the ternary BMGs are directly applicable to multicomponent BMGs belonging to G-I. G-II, G-IV (Mg-based BMGs), G-V and G-VII. The main element can be the larger-sized element in the atomic pair with <(Delta H)under bar >(mix)(LN) or in the same group as the other elements for multicomponent BMGs belonging to G-III. G-IV (Be-containing Zr-based BMG) and G-VI. The main element of BMGs belonging to G-VI tends to change from the element with the smallest atomic radius in a ternary system to an element with a relatively large atomic size in a in multicomponent system. The change is due to an increase in glass-forming ability through multicomponent alloying of BMGs belonging to G-VI. The results of the classification of BMGs obtained in the present study are important for further development of BMGs, with the results providing a road map for the development of new BMG compositions.
Structural materials represent the key for containment of nuclear fuel and fission products as well as reliable and thermodynamically efficient production of electrical energy from nuclear reactors. Similarly, high-performance structural materials will be critical for the future success of proposed fusion energy reactors, which will subject the structures to unprecedented fluxes of high-energy neutrons along with intense thermomechanical stresses. Advanced materials can enable improved reactor performance via increased safety margins and design flexibility, in particular by providing increased strength, thermal creep resistance and superior corrosion and neutron radiation damage resistance. In many cases, a key strategy for designing highperformance radiation-resistant materials is based on the introduction of a high, uniform density of nanoscale particles that simultaneously provide good high temperature strength and neutron radiation damage resistance.
The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77,
Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 C to 700 C for 1
hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel
revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of
retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed
on tempering at 450 C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate
tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 C and −85
C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 C, resulting in a phenomenal
rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 C and −85 C) at the expense of YS and
UTS. The best combination of strength and toughness has been obtained on tempering at 600 C for 1 hour (YS-1015 MPa and UTS-1068
MPa, with 88 J at −85 C).
The phase decomposition in alpha(bcc) phase and the subsequent structural phase transformation from alpha to gamma(fcc) phase during isothermal aging of an Fe-Cu-Mn-Ni quaternary alloy, which is a base alloy of the light-water reactor pressure vessel, have been simulated by the phase-field method. At the early stage of spinodal decomposition, Cu-rich alpha phase is formed, and the Mn and Ni, which are minor components, are partitioned to the Cu-rich phase. As the Cu composition in the precipitate is increased, the Ni atoms inside the precipitates move to the interface region between the precipitate and matrix, but Mn atoms remain inside the Cu particles. When the Cu-rich particles eventually transform to the fee structure, the Mn atoms also move to the interface region, which results in the shell structure of the fee Cu precipitates, where each particle is surrounded by a thin layer enriched in Mn and Ni. This microstructural change can be reasonably explained by considering the local equilibrium at the surface region of the Cu-rich particles.
Systematic ab initio calculations of lattice constants, elastic constants and magnetic mo- ments of Fe, Co and Ni in bcc, fcc and hcp structures have been carried out, in part to understand the phase stability and magnetic properties of artificial Fe, Co and Ni structures. The calculations are based on the local spin-density functional theory with generalized gra- dient corrections (GGA) which are found to describe the properties of these materials rather well. In particular, the calculated lattice constants and bulk modulus of the observed struc- tures are in good agreement with experiments. Also, the theoretical elastic constants agree with the measurements within the numerical uncertainties. Interestingly, several energetically metastable structures are found to be elastically unstable. It is predicted that magnetism has a pronounced influence on both the size and sign of some elastic constants, and hence on the structural stabilities. For example, while both nonmagnetic Fe fcc and hcp structures are metastable phases, ferromagnetic Fe fcc and hcp structures are not. These results would help to gain insight into the formation of certain artificial thin films and superlattices.
Precipitate size and number density are two key factors for tailoring the mechanical behavior of nanoscale precipitate-hardened alloys. However, during thermal aging, the precipitate size and number density change, leading to either poor strength or high strength but significantly reduced ductility. Here we demonstrate, by producing nanoscale co-precipitates in composition-optimized multicomponent precipitation-hardened alloys, a unique approach to improve the stability of the alloy against thermal aging and hence the mechanical properties. Our study provides compelling experimental evidence that these nanoscale co-precipitates consist of a Cu-enriched bcc core partially encased by a B2-ordered Ni(Mn, Al) phase. This co-precipitate provides a more complex obstacle for dislocation movement due to atomic ordering together with interphases, resulting in a high yield strength alloy without sacrificing alloy ductility.
The nucleation and growth of nanoscale precipitates in a new class of high-strength, multicomponent, ferritic steels has been studied with complementary state-of-the-art microstructural characterization techniques of atom probe tomography for individual embryos and precipitates and small-angle neutron scattering for their statistical averages. Both techniques revealed a bimodal size distribution, with subnanometer embryos, and nanoscale precipitates. The embryos, which have a radius of ∼0.4 nm, are enriched in Cu and served as preferential sites for nucleation. The critical radius for nucleation was determined to be ∼0.7 nm. Subsequent growth of the precipitates is dictated by volumetric diffusion, as predicted by the Lifshitz–Slyozov–Wagner theory.
A phase with the stoichiometry Ni0.5(Al0.5−xMnx) is observed at heterophase interfaces of Cu-rich precipitates in an α-Fe matrix, utilizing atom-probe tomography. First-principles calculations are utilized to determine the substitutional energies, yielding EMn→Ni = 0.916 eV atom−1 and EMn→Al = −0.016 eV atom−1 indicating that the manganese atoms prefer substituting at Al sublattice sites instead of Ni sites. A synchrotron radiation experiment demonstrates that the identified phase possesses the B2 structure.
In recent years the development of atomistic models dealing with microstructure evolution and subsequent mechanical property change in reactor pressure vessel steels has been recognised as an important complement to experiments. In this framework, a literature study has shown the necessity of many-body interatomic potentials for multi-component alloys. In this paper we develop a ternary many-body Fe-Cu-Ni potential for this purpose. As a first validation, we used it to perform a simulated thermal annealing study of the Fe-Cu and Fe-Cu-Ni alloys. Good qualitative agreement with experiments is found, although fully quantitative comparison proved impossible, due to limitations in the used simulation techniques. These limitations are also briefly discussed here.
Application of a systems approach to computational materials design led to the theoretical design of a transformation toughened ultratough high-strength plate steel for blast-resistant naval hull applications. A first prototype alloy has achieved property goals motivated by projected naval hull applications requiring extreme fracture toughness (C
v
> 85 ft-lbs or 115 J corresponding to K
Id≥ 200 ksi.in1/2 or 220 MPa.m1/2) at strength levels of 150–180 ksi (1,030–1,240 MPa) yield strength in weldable, formable plate steels. A continuous casting process was simulated by slab casting the prototype alloy as a 1.75′′ (4.45 cm) plate. Consistent with predictions, compositional banding in the plate was limited to an amplitude of 6–7.5 wt% Ni and 3.5–5 wt% Cu. Examination of the oxide scale showed no evidence of hot shortness in the alloy during hot working. Isothermal transformation kinetics measurements demonstrated achievement of 50% bainite in 4 min at 360 °C. Hardness and tensile tests confirmed predicted precipitation strengthening behavior in quench and tempered material. Multi-step tempering conditions were employed to achieve the optimal austenite stability resulting in significant increase of impact toughness to 130 ft-lb (176 J) at a strength level of 160 ksi (1,100 MPa). Comparison with the baseline toughness–strength combination determined by isochronal tempering studies indicates a transformation toughening increment of 65% in Charpy energy. Predicted Cu particle number densities and the heterogeneous nucleation of optimal stability high Ni 5 nm austenite on nanometer-scale copper precipitates in the multi-step tempered samples was confirmed using three-dimensional atom probe microanalysis. Charpy impact tests and fractography demonstrate ductile fracture with C
v
> 80 ft-lbs (108 J) down to −40 °C, with a substantial toughness peak at 25 °C consistent with designed transformation toughening behavior. The properties demonstrated in this first prototype represent a substantial advance over existing naval hull steels. Achieving these improvements in a single design and prototyping iteration is a significant advance in computational materials design capability.
The topic of precipitation hardening is critically reviewed, emphasizing the influence of precipitates on the CRSS or yield
strength of aged alloys. Recent progress in understanding the statistics of dislocation-precipitate interactions is highlighted.
It is shown that Pythagorean superposition for strengthening by random mixtures of localized obstacles of different strengths
is rigorously obeyed in the limit of very weak obstacles; this had been known previously as a result of computer simulation
experiments. Some experimental data are discussed in light of this prediction. All of the currently viable mechanisms of precipitation
hardening are reviewed. It is demonstrated that all versions of the theory of coherency hardening are woefully inadequate,
while the theory of order hardening is capable of accurately predicting the contribution of γ′ precipitates to the CRSS of
aged Ni-Al alloys. It is also convincingly shown that a new theory based on computer simulation experiments of the motion
of dislocations through arrays of obstacles having a finite range of interaction cannot explain these same data, and is of
doubtful validity in other instances for which its success has been proclaimed. A new theory of hardening by spinodal decomposition
is proposed. It is based on the statistics of interaction between dislocations and diffuse attractive obstacles, and is shown
to be in very good quantitative agreement with much of the limited data available. Some of the problems that remain to be
addressed and solved are discussed.
The steels used for structural and other applications ideally should have both high strength and high toughness. Most high-strength
steels contain substantial carbon content that gives poor weldability and toughness. A theoretical study is presented that
was inspired by the early work of Weertman on the effect that single or clusters of solute atoms with slightly different atom
sizes have on dislocation configurations in metals. This is of particular interest for metals with high Peierls stress. Misfit
centers that are coherent and coplanar in body-centered cubic (bcc) metals can provide sufficient twisting of nearby screw
dislocations to reduce the Peierls stress locally and to give improved dislocation mobility and hence better toughness at
low temperatures. Therefore, the theory predicts that such nanoscale misfit centers in low-carbon steels can give both precipitation
hardening and improved ductility and fracture toughness. To explore the validity of this theory, we measured the Charpy impact
fracture energy as a function of temperature for a series of low-carbon Cu-precipitation-strengthened steels. Results show
that an addition of 0.94 to 1.49wtpct Cu and other accompanying elements results in steels with high Charpy impact energies
down to cryogenic temperatures (198K [–75°C]) with no distinct ductile-to-brittle transition. The addition of 0.1wtpct
Ti results in an additional increase in impact toughness, with Charpy impact fracture energies ranging from 358J (machine
limit) at 248K (–25°C) to almost 200J at 198K (–75°C). Extending this concept of using coherent and coplanar misfit centers
to decrease the Peierls stress locally to other than bcc iron-based systems suggests an intriguing possibility of developing
ductile hexagonal close-packed alloys and intermetallics.
An investigation of a low-carbon, Fe-Cu–based steel, for Naval ship hull applications, with a yield strength of 965MPa, Charpy
V-notch absorbed impact-energy values as high as 74J at –40°C, and an elongation-to-failure greater than 15 pct, is presented.
The increase in strength is derived from a large number density (approximately 1023 to 1024m−3) of copper-iron-nickel-aluminum-manganese precipitates. The effect on the mechanical properties of varying the thermal treatment
was studied. The nanostructure of the precipitates found within the steel was characterized by atom-probe tomography. Additionally,
initial welding studies show that a brittle heat-affected zone is not formed adjacent to the welds.
BlastAlloy 160 (BA160) is a low-carbon martensitic steel strengthened by copper and M2C precipitates. Heat-affected zone (HAZ) microstructure evaluation of BA160 exhibited softening in samples subjected to the
coarse-grained HAZ thermal simulations of this steel. This softening is partially attributed to dissolution of copper precipitates
and metal carbides. After subjecting these coarse-grained HAZs to a second weld thermal cycle below the A
c1 temperature (at which austenite begins to form on heating), recovery of strength was observed. Atom-probe tomography and
microhardness analyses correlated this strength recovery to re-precipitation of copper precipitates and metal carbides. A
continuum model is proposed to rationalize strengthening and softening in the HAZ regions of BlastAlloy 160.
The structural steels used in critical construction applications have traditionally been heat-treated low-alloy steels. These
normalized and/or quenched and tempered steels derive strength from their carbon contents. Carbon is a very efficient and
cost-effective element for increasing strength in ferrite-pearlite or tempered martensitic structures, but it is associated
with poor notch toughness. Furthermore, it is well known that both the overall weldability and weldment toughness are inversely
related to the carbon equivalent values, especially at high carbon contents. The stringent control needed for the welding
of these traditional steels is one of the major causes of high fabrication costs.
In order to reduce fabrication cost while simultaneously improving the quality of structural steels, a new family of high-strength
low-alloy steels with copper additions (HSLA-100) has been developed. The alloy design philosophy of the new steels includes
a reduction in the carbon content, which improves toughness and weldability.
A slight modification of the nucleation theory enables reliable calculations of cluster free energies of precipitate nucleation. This method divides the free energy into the cluster energy and entropy terms. The former, consisting of the enthalpy of driving force and the interface energy, is precisely calculated by first-principles calculations. The latter, which is entropy loss from scattered atoms condensing into a cluster, is estimated by the ideal solution model. Model calculations have been performed for an Fe–Cu alloy, Ni added Fe–Cu alloy, and vacancy behavior around Cu clusters.
The nucleation (to a limited extent), growth and coarsening behavior of Cu-rich precipitates in a concentrated multicomponent Fe–Cu-based steel aged at 500 °C from 0.25 to 1024 h is investigated. The temporal evolution of the precipitates, heterophase interfaces, matrix compositions and precipitate morphologies are presented. With increasing time, Cu partitions to the precipitates, Ni, Al and Mn partition to the interfacial region, whereas Fe and Si partition to the matrix. Coarsening time exponents are determined for the mean radius, 〈R(t)〉, number density, NV(t), and supersaturations, which are compared to the Lifshitz–Slyzov–Wagner (LSW) model for coarsening, modified for concentrated multicomponent alloys by Umantsev and Olson (UO). The experimental results indicate that the alloy does not strictly follow UO model behavior. Additionally, we delineate the formation of a Ni–Al–Mn shell with a stoichiometric ratio of 0.51:0.41:0.08 at 1024 h, which reduces the interfacial free energy between the precipitates and the matrix.
Comparison of the yield stress and Young's modulus behavior on aging at 475 and 500 C confirms E. Hornbogen's suggestion that a two- st%age aging %process occurs in solution treated Fe-Cu alloys. That is, formation of bcc copper clusters precedes precipitation of bcc copper phase, epsilon.
Maraging steels are high-strength steels combined with good toughness. They are used particularly in aerospace and tooling applications. Maraging refers to the ageing of martensite, a hard microstructure commonly found in steels. Maraging steels: modelling of microstructure, properties and applications covers the following topics: Introduction to maraging steels; Microstructure of maraging steels; Mechanical properties of maraging steels; Thermodynamic calculations for quantifying the phase fraction and element partition in maraging systems and precipitation hardening steels; Quantification of phase transformation kinetics in maraging steels; Quantification of age hardening in maraging steels; Maraging steels and overageing; Precipitation hardening stainless steel; Applications of artificial neural network on modelling maraging steel properties. With its distinguished authors, Maraging steels: modelling of microstructure, properties and applications is a standard reference for industry and researchers concerned with maraging steels and modelling as well as users of maraging steels in the aerospace and tooling sectors. The book includes both conventional maraging steels and precipitation hardened (PH) stainless steels.
There is an increasing demand for ultrahigh-strength ferritic steels strengthened by nanoprecipitates. Improvement of the precipitation strengthening response requires an understanding of the nanoscale precipitation mechanisms. In this study, the synergistic effects of Cu and Ni on nanoscale precipitation and mechanical properties of ferritic steels were thoroughly investigated, and new steels with ultrahigh strength and high ductility have been developed. Our results indicate that Ni effectively increases the number density of Cu-rich nanoprecipitates by more than an order of magnitude, leading to a substantial increase in yield strength. It appears that Ni decreases both the strain energy for nucleation and the interfacial energy between the nucleus and the matrix, thereby decreasing the critical energy for nucleation of Cu-rich nanoprecipitates. Cu and Ni are also found to be beneficial to grain-size refinement, resulting from lowering the austenite-to-ferrite transformation temperature, as determined from thermodynamic calculations. In addition, the strengthening mechanisms of Cu and Ni were quantitatively evaluated in terms of precipitation strengthening, grain refinement strengthening and solid-solution strengthening. The current findings shed light on the composition–microstructure–property relationships in nanoprecipitate-strengthened ferritic steels.
The evolution of precipitates in a Fe–2.5 Cu–1.5 Mn–4.0 Ni–1.0 Al multicomponent ferritic alloy during annealing at 500 °C was systematically investigated by aberration-corrected scanning transmission electron microscopy. The atomic-scale structure and chemistry characterization reveal that primary precipitates with enriched Cu, Ni, Mn and Al originate from continuous growth of B2 ordered domains in the as-quenched alloy. The formation of a Cu-rich body-centered cubic (bcc) phase takes place by the decomposition of the B2 ordered primary phase, which forms a Cu-rich bcc core and ordered B2-Ni(Al,Mn) shell. The B2 shells serve as a buffer layer to moderate the coherent strain and to prohibit the inter-diffusion between the Cu-rich precipitates and bcc-Fe matrix, giving rise to a low coarsening rate of the precipitates. The Cu-rich precipitates experience a structural transformation from bcc to 9R at a critical size of ∼6 nm during long time annealing, corresponding to obvious coarsening of the precipitates and dramatic loss in hardness of the alloy.
Phase transition of Cu precipitates during aging of an Fe–3Si–2Cu alloy was studied by transmission electron microscopy. The precipitation of 3–5-nm-sized body-centered cubic (bcc) Cu in ferrite matrix was confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The bcc Cu precipitates transformed to 9R Cu as they grew. Many 9R Cu precipitates were twinned, but untwinned 9R Cu particles were also observed. The 9R Cu transformed to twinned face-centered cubic (fcc) Cu by the glide of ±a/3 [1 0 0]9R Shockley-type partial dislocations. Formation of the 3R structure previously reported could not be confirmed in this study. Finally, twins in fcc Cu precipitates disappeared to form stable fcc Cu particles. The importance of electron beam-orientation-dependent moiré fringes in the correct identification of Cu structure is discussed in detail.
Newly-developed precipitate-strengthened ferritic steels with and without pre-existing nanoscale precipitates were irradiated with 4 MeV protons to a dose of ~5 mdpa at 50 C and subsequently examined by nanoindentation and atom probe tomography (APT). Irradiation-enhanced precipitation and coarsening of pre-existing nanoscale precipitates were observed. Copper partitions to the precipitate core along with a segregation of Ni, Al and Mn to the precipitate/matrix interface after both thermal aging and proton irradiation. Proton irradiation induces the precipitation reaction and coarsening of pre-existing nanoscale precipitates, and these results are similar to a thermal aging process. The precipitation and coarsening of nanoscale precipitates are responsible for the changes in hardness. The observation of the radiation-induced softening is essentially due to the coarsening of the pre-existing Cu-rich nanoscale precipitates. The implication of the precipitation on the embrittlement of reactor-pressure-vessel steels after irradiation is discussed.
The scavenging effect of precipitated austenite in a low carbon, commercial Fe-5.5Ni cryogenic alloy was investigated through
observation of the dissolution of cementite precipitates during intercritical tempering and study of the associated change
in Charpy impact toughness. Cementite precipitates initially located along prior austenite grain boundaries were gradually
dissolved into reverted austenite as the intercritical tempering proceeded. The austenite tends to form at or around the carbide
particles and may be catalyzed by their presence. The Charpy impact energy is changed through both a decrease in the ductile-brittle
transition temperature and an increase in the upper shelf energy. The latter effect is specifically associated with the dissolution
of the carbides which act as preferential void nucleation sites in the untempered alloy.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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...
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.
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.
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)].
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.
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.
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.
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.
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.
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.
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.
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.