Atom Probe

We report on the microstructure, texture and deformation mechanisms of a novel ductile lean duplex stainless steel (Fe–19.9Cr–0.42Ni–0.16N–4.79Mn–0.11C–0.46Cu–0.35Si, wt.%). The austenite is stabilized by Mn, C, and N (instead of Ni). The microstructure is characterized by electron channeling contrast imaging (ECCI) for dislocation mapping and electron backscattering diffraction (EBSD) for texture and phase mapping. The material has 1 GPa ultimate tensile strength and an elongation to fracture of above 60%. The mechanical behavior is interpreted in terms of the strength of both the starting phases, austenite and ferrite, and the amount, dispersion, and transformation kinetics of the mechanically induced martensite (TRIP effect). Transformation proceeds from austenite to hexagonal martensite to near cubic martensite (c !e ! a0). The e-martensite forms in the austenite with an orientation relationship close to Shoji–Nishiyama. The a0-martensite nucleates at the intersections of deformation bands, especially e-bands, with Kurdjumov–Sachs and Nishiyama–Wassermann relationships. The ferrite deforms by dislocation slip and contains cell substructures.

We investigate the effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3um and 50um were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall–Petch-type relation for the twinning stress using the same Hall–Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid’s law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 111//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid’s law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid’s law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains.

Invited lecture, German-Chinese High-level Workshop on “Microstructure-driven Design and Performance of Advanced Metals” held 12-16. April 2013 in the Institute of Metals Research (IMR) of the Chinese Academy of Science (CAS), Shenyang, China
D. Raabe, Y. Li, D. Ponge, S. Sandlöbes, P. Choi, T. Hickel, R. Kirchheim, J. Neugebauer

The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs.

Carbon partitioning from martensite into austenite in the quenching and partitioning (Q&P) process has been suggested to be controlled by the constrained carbon equilibrium (CCE) criterion. It defines an approach for predicting the carbon concentration in austenite under the condition that competing reactions such as carbide formation and bainite transformation are suppressed. Carbide precipitation in martensite is, however, often observed during the partitioning step, even in low-carbon steels as well as in high-carbon steels, even when containing a high amount of Si. Therefore, carbon partitioning from martensite into austenite is studied here, considering carbide precipitation in martensite. Carbon partitioning was investigated by means of a field-emission electron probe micro analysis (FE-EPMA) and atom probe tomography (APT), using 1.07 wt.% and 0.59 wt.% carbon steels with various martensite volume fractions. Carbon partitioning from martensite to austenite was clearly observed in all specimens, even though a considerable amount of carbide precipitated inside the martensite. The austenite carbon concentration after the partitioning step was not influenced by either the martensite volume fraction or the bulk carbon content. A modified model for predicting the austenite carbon concentration after the partitioning step was proposed to explain the experimental results by assuming carbon equilibria between austenite, ferrite and cementite under a constrained condition.

The transformation-induced plasticity (TRIP) effect is a pathway for obtaining high-strength and tough steels with excellent formability. Conventional TRIP steels containing about 0.15 wt.% C, 1.5 wt.% Mn and 1.5 wt.% Si exploit the transformation of metastable retained austenite into martensitic or bainitic constituents to control the mechanical properties [1–4]. Conventionally heat-treated TRIP steels, processed by a two-step process, namely intercritical annealing (IA) and isothermal bainitic transformation (IBT) annealing (the latter also referred to as austempering), have a complex microstructure consisting of a soft, ductile intercritical ferrite matrix with retained austenite (cR), bainite, martensite and fine carbides [5,6]. The product of ultimate tensile strength (UTS) and total elongation (TE) of these alloys can be further improved by controlling
thermal processing and composition, as both factors determine the stability of the metastable cR. It has been proposed that a high stability and sufficient fraction of cR are crucial for achieving a high tensile strength in conjunction with an acceptable ductility [7]. Carbides are considered detrimental in these steels (either transition carbides or cementite) as they can initiate cleavage fracture and void formation. Moreover, carbides act as sinks for C, thus reducing partitioning of C into austenite. To avoid these effects, the addition of Si to TRIP-aided steels is important [4,7]. Si has very low solubility in cementite (Fe3C) and hence kinetically retards precipitation during the bainitic transformation owing to its low mobility compared to that of carbon. Consequently, Si-containing TRIP steels show an increase in austenite stability via C partitioning which results in improved toughness and UTS [4,7]. However, Si also deteriorates the galvanizability of low-C TRIP steels owing to the formation of a very stable Mn2SiO4 oxide film adherent to the steel substrate [8]. As a result, the steels
have poor coatability [9]. To overcome this problem, the addition of Al instead of Si was suggested [9–11]. Al slows down the formation kinetics of cementite and reduces its thermodynamic stability. Hence, substituting Si by Al leads to steels with better coating behavior without sacrificing mechanical properties [10–12]. Furthermore, Al influences the activity and solubility of C in ferrite which leads to
the acceleration of bainite transformation [13,14]. In order to achieve high strength and toughness, nanostructured bainitic steels, which contain a high concentration of C (0.79 wt.%), but do not form carbides, have been designed [15–17]. These steels consist of nanoscale bainitic ferrite (aB) plates and metastable C-enriched cR. They have a strength of 2.5 GPa and a fracture toughness of 30–40 MPa m1/2. Following this concept, recent efforts
addressed modifications of composition and processing with the aim of reducing the C content and the heat-treatment times [18,19]. Bainitic steels containing a low C content of 0.20 wt.% have great potential for lightweight sheet forming and automotive crash applications [18]. To elucidate the redistribution behavior of solute C and the formation of nanosized cementite, atom probe tomography
(APT) was applied [17,19]. It was pointed out that nano-scale carbide precipitation can take place despite a high Al (1.01 wt.%) and Si (1.50 wt.%) concentration. Irrespective of the observation of nanoscaled carbides in these materials, an atomistic understanding of the alloying effects such as the Si:Al ratio, of partitioning and of the thermal treatment on the bainitic transformation in high-carbon bainitic–austenitic TRIP steels is still lacking. APT is well suited for studying the phase transformation as it provides information on the topology and chemistry in the vicinity of aB/cR interfaces and on the partitioning behavior of elements at near-atomic resolution [17,19–26]. Previous APT studies on TRIP steels suggest that there is no redistribution of substitutional elements, such as Cr, Mn, Al and Si, at the aB/cR interface during bainitic transformation [15–19,25–27]. This observation suggested a displacive mechanism of incomplete bainitic transformation occurring under para-equilibrium (PE) [28,29] or no parti-
tion equilibrium conditions [30]. In general, PE represents a constrained equilibrium, in which C is in local equilibrium and substitutional alloying elements are considered unaffected at the phase boundaries between proeutectoid ferrite
(a) and prior austenite (c) or at the aB/cR interfaces. On the other hand, alternative models have predicted the redistri bution of substitutional elements during proeutectoid ferrite transformation under local-equilibrium, negligible-
partitioning (LENP) conditions [31–36]. Previous publications on quaternary Fe–C–Mn–Si alloys [37,38] have studied the transition from the PE to the LENP regime and the formation of Mn spikes during the growth of proeutectoid ferrite. This phenomenon was interpreted in terms of a reconstructive mechanism using a stationary-interface approximation and also analyzed by scanning
transmission electron microscopy (STEM). Recently, a rough estimate employing area density values of a Mn concentration spike at a a/c interface in a Fe–Mn–C steel was made via STEM analysis [38]. Moreover, theoretical DICTRA calculations and APT analyses have been carried out to better elucidate which type of local equilibrium boundary condition is prevalent at the a/c interface in a low carbon Fe–MnC alloy [39]. However, no atomic-scale understanding has been achieved so far regarding compositional spikes and/or partitioning behavior of alloying elements (especially C, Si and Mn) across the aB/cR interface in high-carbon TRIP steels [13,39–41]. Therefore, a specific challenge for advanced APT analysis lies in determining the type of equilibrium conditions prevailing at the aB/cR interface and identifying the occurrence of possible solute drag effects during bainite formation. More specifically, it is the aim of this study to elucidate
the relationship between the mechanical properties and the nanostructure of steels containing 0.71 wt.% C as a function of the Si:Al ratio, austempering temperature and partitioning effects from an atomic perspective. The steels
studied in this work have a bainitic–austenitic microstructure, and are referred to as super-bainitic TRIP (SB-TRIP) steels. We subject these materials to an experimental multiscale analysis from the micrometer down to the atomic
regime, paying particular attention to the stability of the cR phase and on the redistribution of substitutional elements in the vicinity of aB/cR interfaces during the bainite transformation. We expect that an insight into the atomic nature of the occurring phase transformations and partitioning effects involved gives us a better understanding of the relevant nanostructure–property relationships in these materials.
Therefore, understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic–austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation.

Carbon partitioning between ferritic and austenitic phases is essential for austenite stabilization in the most advanced steels such as those produced by the quenching and partitioning (Q&P) process. The atomistic analysis of the carbon partitioning in Q&P alloys is, however, difficult owing to the simultaneous occurrence of bainite transformation, which can also contribute to carbon enrichment into remaining austenite and hence overlap with the carbon partitioning from martensite into austenite. Therefore, we provide here a direct atomic-scale evidence of carbon partitioning from martensite into austenite without the presence of bainite transformation. Carbon partitioning is investigated by means of atom probe tomography and correlative transmission electron microscopy. A model steel (Fe–0.59 wt.% C (2.7 at.% C)–2.0 wt.% Si–2.9 wt.% Mn) with martensite finish temperature below room temperature was designed and used in order to clearly separate the carbon partitioning between martensite and austenite from the bainite transformation. The steel was austenitized at 900 C, then water-quenched and tempered at 400 C. Approximately 8 vol.% retained austenite existed in the as-quenched state. We confirmed by X-ray diffraction and dilatometry that austenite decomposition via bainite transformation did not
occur during tempering. No carbon enrichment in austenite was observed in the as-quenched specimen. On the other hand, clear carbon enrichment in austenite was observed in the 400 C tempered specimens with a carbon concentration inside the austenite of 5–8 at.%. The results hence quantitatively revealed carbon partitioning from martensite to austenite, excluding bainite transformation during the Q&P heat treatment.

This article focuses on four topics that demonstrate the importance of atom probe tomography for obtaining nanostructural information that provides deep insights into the structures of metallic alloys, leading to a better understanding of their properties. First, we discuss the microstructure–coercivity relationship of Nd-Fe-B permanent magnets, essential for developing a higher coercivity magnet. Second, we address equilibrium segregation at grain boundaries with the aim of manipulating their interfacial structure, energies, compositions, and properties, thereby enabling benefi cial material behavior. Third, recent progress in the search to extend the performance and practicality of the next generation of advanced high-strength steels is discussed. Finally, a study of the temporal evolution of a Ni-Al-Cr alloy through the stages of nucleation, growth, and coarsening (Ostwald ripening) and its relationship with the predictions of a model for quasi-stationary coarsening is described. This information is critical for understanding high-temperature mechanical properties of the
material.

In the present work, we report our recent progress in the development, optimization, and application of a technique for the three-dimensional (3-D) high-resolution characterization of crystalline microstructures. The technique is based on automated serial sectioning using a focused ion beam (FIB) and characterization of the sections by orientation microscopy based on electron backscatter diffraction (EBSD) in a combined FIB-scanning electron microscope (SEM). On our system, consisting of a Zeiss-Crossbeam FIB-SEM and an EDAX-TSL EBSD system, the technique currently reaches a spatial resolution of 100 × 100 × 100 nm3 as a standard, but a resolution of 50 × 50 × 50 nm3 seems to be a realistic optimum. The maximum observable volume is on the order of 50 × 50 × 50 mum3. The technique extends all the powerful features of two-dimensional (2-D) EBSD-based orientation microscopy into the third dimension of space. This allows new parameters of the microstructure to be obtained---for example, the full crystallographic characterization of all kinds of interfaces, including the morphology and the crystallographic indices of the interface planes. The technique is illustrated by four examples, including the characterization of pearlite colonies in a carbon steel, of twins in pseudonanocrystalline NiCo thin films, the description of deformation patterns formed under nanoindents in copper single crystals, and the characterization of fatigue cracks in an aluminum alloy. In view of these examples, we discuss the possibilities and limits of the technique. Furthermore, we give an extensive overview of parallel developments of 3-D orientation microscopy (with a focus on the EBSD-based techniques) in other groups.

For 5000 years, metals have been mankind’s most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe–9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces.The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.

tructural defects such as interfaces or dislocations in crystalline solid solutions are disturbed regions and attract solute segregation when diffusion is enabled (1–5). According to the Gibbs isotherm, the driving force is the reduction of the system’s energy. Extending this concept also to nonisostructural
cases suggests that local structural transformations can occur if the chemical composition and stress at a defect reach a level sufficient for stabilizing a state different from that of the matrix
(6, 7). The concept of interface complexions (8–17) extends the classical isothermto interface-stabilized states that have a structure and composition different from that of the matrix and
remain confined in the region where they form. We observed such a phenomenon also at linear defects—edge dislocations—in a binary Fe–9 atomic % Mn model alloy in which a stable face-centered cubic (fcc; austenitic) confined structure forms in an otherwise body-centered cubic (bcc; martensitic) crystal. This is a phenomenological one-dimensional (1D) analog of the previously observed complexions that were observed
at planar defects (8). We homogenized the Fe–9 atomic%Mn alloy at 1100°C and then quenched and cold-rolled it to 50% reduction for increasing the dislocation density. Subsequent annealing at (i) 400°C for 336 hours (2weeks); (ii) 450°C for 6 hours, 18 hours, and 336 hours; and (iii) 540°C for 6 hours enabled Mn diffusion (18). To characterize structure and composition at the same positions, we conducted correlative scanning transmission electron microscopy–atom probe tomography (STEM-APT) analysis (19–23).We identified  structural defects using STEM and cross-correlated it with solute decoration observed with APT (Fig. 1). Two grain boundaries and a single dislocation line are highlighted in Fig. 1 by blue arrows in the STEMmicrograph and in the 3D atommap,
in which they are visible asMn-enriched regions. The correlative STEM experiments clearly identify the linear Mn-enriched features in the APT volumes as dislocations. As evident from the STEM micrograph, not all dislocations attract solute segregation high enough to be detectable with APT (Fig. 1A, red arrow “1”).
We obtained 1D compositional profiles along cylindrical regions with 1 nm diameter at individual locations (Fig. 1E). Profile 1 shows a concentration of 25 ± 2 atomic % Mn at the dislocation
core, which corresponds to an enrichment factor of 2.7 compared with the matrix concentration of Mn (9.1 atomic %). The average thickness of the Mn-enriched zone is ~1 nm.
Besides the high Mn content at the dislocation cores, the composition along the dislocation line (profile 2) reveals periodic Mn changes— enriched and depleted zones alternating with a spacing of ~5 nm, resembling a nano-sized pearl
necklace.

The influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)100xAgx melts was
investigated (x = 0–40 at.%). Fully glassy alloys were obtained for 0 6 x 6 20 at.% Ag, which are characterized
by a homogeneous microstructure without any indication of phase separation. For 30 6 x 6 40 at.%
Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous
matrix phase Cu40Zr40Ag20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes
increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for
30 6 x 6 40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which
were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid
phase of the ternary Ag–Cu–Zr system may occur only for x > 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al10Ag10 alloy a homogeneous element distribution is observed in accordance with the microstructure of ternary (Cu0.5Zr0.5)100xAgx glasses (x = 10, 20 at.%).

The influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)100ÿxAgx melts was investigated (x = 0–40 at.%). Fully glassy alloys were obtained for 0 6 x 6 20 at.% Ag, which are characterized by a homogeneous microstructure without any indication of phase separation. For 30 6 x 6 40 at.%
Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous matrix phase Cu40Zr40Ag20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for
30 6 x 6 40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid phase of the ternary Ag–Cu–Zr system may occur only for x > 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al10Ag10 alloy a homogeneous element distribution is observed in accordance with the
microstructure of ternary (Cu0.5Zr0.5)100ÿxAgx glasses (x = 10, 20 at.%).

Rapid annealing (4–10 s) induced primary crystallization of soft magnetic Fe–Si nanocrystals in a Fe73.5Si15.5Cu1Nb3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30–60 min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50 C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe–Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of 30% smaller Fe–Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu.

The interaction of dislocations with precipitates is an essential strengthening mechanism in metals, as exemplified by the superior high-temperature strength of Ni-base superalloys. Here we use atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a gamma' precipitate in molecular dynamics simulations. It is shown that the precipitate morphology, in particular its local curvature, and the local chemical composition significantly alter both, the misfit dislocation network which forms at the precipitate interface, and the core structure of the misfit dislocations. Simulated tensile tests reveal the atomic scale details of many experimentally observed dislocation–precipitate interaction mechanisms, which cannot be reproduced by idealized simulation setups with planar interfaces. We thus demonstrate the need to include interface curvature in the study of semicoherent precipitates and introduce as an enabling method atom probe tomography-informed atomistic simulations.

In an Fe–9 at.% Mn maraging alloy annealed at 450 C reversed allotriomorphic austenite nanolayers appear on former Mn decorated
lath martensite boundaries. The austenite films are 5–15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron
microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation)
in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics
of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite
stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation
at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.

Observing solute hydrogen (H) in matter is a formidable challenge, yet, enabling quantitative imaging of H at the atomic-scale is critical to understand its deleterious influence on the mechanical strength of many metallic alloys that has resulted in many catastrophic failures of engineering parts and structures. Here, we report on the APT analysis of hydrogen (H) and deuterium (D) within the nanostructure of an ultra-high strength steel with high resistance to hydrogen embrittlement. Cold drawn, severely deformed pearlitic steel wires (FeÀ0.98CÀ0.31MnÀ0.20SiÀ0.20CrÀ0.01CuÀ0.006PÀ0.007S wt%, e ¼ 3:1) contains cementite decomposed during the pre-deformation of the alloy and ferrite. We find H and D within the decomposed cementite, and at some interfaces with the surrounding ferrite. To ascertain the origin of the H/D signal obtained in APT, we explored a series of experimental workflows including cryogenic specimen preparation and cryogenic-vacuum transfer from the preparation into a state-of-the-art atom probe. Our study points to the critical role of the preparation, i.e. the possible saturation of H-trapping sites during electrochemical polishing, how these can be alleviated by the use of an outgassing treatment, cryogenic preparation and transfer prior to charging. Accommodation of large amounts of H in the under-stoichiometric carbide likely explains the resistance of pearlite against hydrogen embrittlement.

Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 lm) but with the same martensite content (30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 lm) to aging at 170 C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite.

In the present work, we report our recent progress in the development, optimization, and application of a technique for the three-dimensional (3-D) high-resolution characterization of crystalline microstructures. The technique is based on automated serial sectioning using a focused ion beam (FIB) and characterization of the sections by orientation microscopy based on electron backscatter diffraction (EBSD) in a combined FIB–scanning electron microscope (SEM). On our system, consisting of a Zeiss–Crossbeam FIB-SEM and an EDAX-TSL EBSD system, the technique currently reaches a spatial resolution of 100 × 100 × 100 nm3 as a standard, but a resolution of 50 × 50 × 50 nm3 seems to be a realistic optimum. The maximum observable volume is on the order of 50 × 50 × 50 μm3. The technique extends all the powerful features of two-dimensional (2-D) EBSD-based orientation microscopy into the third dimension of space. This allows new parameters of the microstructure to be obtained—for example, the full crystallographic characterization of all kinds of interfaces, including the morphology and the crystallographic indices of the interface planes. The technique is illustrated by four examples, including the characterization of pearlite colonies in a carbon steel, of twins in pseudonanocrystalline NiCo thin films, the description of deformation patterns formed under nanoindents in copper single crystals, and the characterization of fatigue cracks in an aluminum alloy. In view of these examples, we discuss the possibilities and limits of the technique. Furthermore, we give an extensive overview of parallel developments of 3-D orientation microscopy (with a focus on the EBSD-based techniques) in other groups.

ABSTRACT Elemental processes underlying the temperature-initiated decomposition of low-conductive Ti-Al-N thin films is investigated on an atomic-scale by laser-assisted three-dimensional atom probe tomography. This allows three-dimensional observation of spinodal decomposition in Ti-Al-N. Experimental evidence is presented for the formation of an interconnected network of Ti- and Al-rich domains and the subsequent phase transformation under increased thermal load to a dual phase structure of TiN and AlN areas.

The difference in quantitative analysis performance between the voltage-mode and laser-mode of a local electrode atom probe (LEAP30 0 0X HR) was investigated using a Fe-Cu binary model alloy. Solute copper atoms in ferritic iron preferentially field evaporate because of their significantly lower evaporation field than the matrix iron, and thus, the apparent concentration of solute copper tends to be lower than the actual concentration. However, in voltage-mode, the apparent concentration was higher than the actual concentration at 40 K or less due to a detection loss of matrix iron, and the concentration decreased with increasing specimen temperature due to the preferential evaporation of solute copper. On the other hand, in laser-mode, the apparent concentration never exceeded the actual concentration, even at lower temperatures (20 K), and this mode showed better quantitative performance over a wide range of specimen temperatures. These results indicate that the pulsed laser atom probe prevents both detection loss and preferential evaporation under a wide range of measurement conditions.

"In order to investigate the thermodynamic driving force for the experimentally observed accumulation of C in ferritic layers of
severely plastically deformed pearlitic wires, the stabilities of C interstitials in ferrite and of C vacancies in cementite are investigated as a function of uniaxial stain, using density-functional theory. In the presence of an applied strain along [110] or [111], the C interstitial in ferrite is significantly stabilized, while the C vacancy in cementite is moderately destabilized by the corresponding strain states in cementite [100] and ([010]). The enhanced stabilization of the C interstitial gives rise to an increase in the C concentration within the ferritic layers by up to two orders of magnitude. Our results thus suggest that in addition to the generally assumed non-equilibrium, dislocation-based mechanism, there is also a strain-induced thermodynamic driving force for the experimentally observed accumulation of C in ferrite."