Microstructure and Grain Boundary Engineering

Nanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nano-sized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials.

Standard test methods such as the Electrochemical Potentiokinetic Reactivation Test (EPR–ASTM G108) and the Double-Loop EPR test (DL-EPR–ISO12732) are commonly used to characterise sensitisation behaviour in austenitic stainless steels. These tests provide a quantitative assessment of microstructure susceptibility. Factors such as different grain size may be accounted for, but additional information on the network of sensitised boundaries is neglected. This paper reports a new approach to characterise the development of sensitisation, applied to a Type 304 austenitic stainless steel subjected to thermo-mechanical processing. DL-EPR testing is augmented by large area Image Analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. Comparison is made with the standard assessment methods, and a new method is proposed, based on normalisation by a cluster parameter to describe the network of susceptible grain boundaries....

The development of an intergranular stress corrosion crack initiation site in thermally sensitized type 304 austenitic stainless steel has been observed in situ in high temperature oxygenated water using digital image correlation of time-resolved optical observations.  The grain boundary normal stresses were calculated using the Schmid-Modified Grain Boundary Stress (SMGBS) model of Was et al, applying three-dimensional data for the grain boundary planes and grain orientations.  The initiation site coincided with the most highly stressed sensitised boundary, demonstrating the importance of the combined contributions to crack initiation of grain boundary structure and plastic strain incompatibility.

Hydrogen embrittlement in 304L austenitic stainless steel fabricated by laser powder-bed-fusion (LPBF) was investigated and compared to conventionally produced 304L samples with two different processing histories; casting plus annealing (CA) and CA plus thermomechanical treatment (CA-TMT). Interestingly, no significant difference in the amount of deformation-induced α ′ martensite between the LPBF and CA-TMT samples was observed, suggesting that the solidification substructure in the LPBF sample enhanced the strength without promoting the harmful hydrogen embrittlement effect. These results are discussed in terms of the chemical in-homogeneity, hydrogen-assisted cracking behavior, and hydrogen diffusion and trapping in the present 304L samples.

Cellular automata are algorithms that describe the discrete spatial and temporal evolution of complex systems by applying local (or sometimes longrange) deterministic or probabilistic transformation rules to the cells of a regular (or non-regular) lattice. The space variable in cellular automata usually stands for real space, but orientation space, momentum space, or wave vector space can be used as well. Cellular automata can have arbitrary dimensions.

Multi-axial incremental forging and shearing (MAIFS), as a new severe plastic deformation technique, was successfully applied up to eight passes on the workpieces of commercially pure Al (AA1100). The microstructure evolutions and mechanisms of the grain refinement in the billets deformed through various passes of process were studied using the electron backscatter diffraction (EBSD) analysis. Microhardness measurements and tensile tests were carried out to evaluate the mechanical properties and deformation behavior of the material after successive passes of the MAIFS process. Measured microhardness evolution indicated that while the distribution of hardness was non-uniform after odd-numbered passes up to four passes, but thereafter outstanding deformation homogeneity was achieved when the consecutive MAIFS passes were applied. Tensile tests indicated that yield stress and ultimate tensile strength increased rapidly during the primary pass of process but thereafter there was only a minor increase up to four passes. After that, a little drop could be observed in strength and then it reached to a saturated magnitude. Measured microhardness distribution values exhibited the same trend, viz. it increased through successive passes to a limiting value beyond which it showed a minor decline by disappearance of points having maximum hardness. Some coarsening was taken place and the dislocation walls between the boundaries were reduced significantly in going from four to six passes. It was suggested that the absorption of the dislocations into grain boundaries as an effective recovery process under large deformations and short-range migration of grain boundaries might be significant mechanisms responsible for the softening observed after four passes of process.

Tool Steel Pacification and Heat-treatment of Wear resistance Steel and D3 Steel
ISBN: 978-977-90-5579-4
Study on tool steels pacification, Heat-treatment and its effect in Microstructure and Mechanical Property (Hardness) for all type of Wear Resistance Steel and specially D3 steel.
This is e-book media (not paper print format), The book format is "pdf". English, 152 pages, size A4, file size 9.21 Mb
This is study on tool steel pacification and heattreatment
for all type of wear resistance steel and specially our D3 steel. Supported with value review.
This research “theoretical and experimental” in Effect of
Heat-treatment “hardening temperature, tempering
temperature, and cooling rate“ in the Mechanical property “Hardness”, and Microstructure transformation on Tool Steel, all type of wear resistance steel, and specially D3 steel.
The research study heat-treatment for two pieces of D3 die steel "Working at cone pulleys for nail manufacture" to modify there mechanical properties.
The classification its type, properties and set on condition effect on it was discussed.
Two set of experiments were done one to determine the effect of austenitization temperature and tempering temperature on the mechanical properties "hardness" and another "the basic one" to discuses improving its mechanical properties from view of microstructure transformations "type, amount, and properties" for each
heat-treatment stage.
This study was APPROVED as my BS graduate project from Metallurgy Engineering Collage, Cairo University 2001, and published after addition in experimental review at 2018.

Metallography is one of the strongest tools to observe the microstructure of a material. This study has two main goals. First goal is to observe the microstructure and to determine the mean grain size of Mg AZ31 alloy examining it under light microscopy and investigate the relationship between microstructure and physical properties of a material. Second is to learn specimen preparatory techniques for metallographic microscopy. The results which are obtained for the mean grain size and the yield stress are in the expected interval. This experiment proves that there is a strong relation between physical properties and the microstructure.

Developing strong, damage-tolerant, and functional steels shapes the backbone for industrial innovations in manufacturing, energy, transportation, and safety. Examples are Fe-Cr steels for emission-reduced turbines; weight reduced and ultra-high strength Fe-Mn steels for light-weight and safe mobility; magnetic Fe-Si steels for low-loss electrical motors and generators; or stainless steels for power plants. These examples document the necessity of developing improved high strength and yet ductile steels. Most traditional hardening mechanisms, however, such as enabled by solutes, dislocations, or precipitates, albeit leading to high strength, often reduce ductility rendering the material brittle and susceptible for failure. This phenomenon is sometimes referred to as the inverse strength-ductility problem.
The reduction of the grain size offers a pathway for increasing both, strength and toughness of steels. Here we develop this concept further in that we combine this strategy with the manipulation of individual interfaces by grain boundary segregation and local phase transformation. More specific, we enable grain boundaries in steels not only as barriers against dislocation motion but also as regions where segregation and nanoscale phase transformation occur. Such locally transformed regions can act as compliance layers impeding for instance crack penetration among lath martensite lamellae.

In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strainhardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength.

Assume that the physics on the microscale (interactions between atoms,
molecules, defects in crystals . . . ) is understood.What is the appropriate mesoscale
continuum theory for the problem? What are the assumptions involved and how do
they define the limitations of the continuum model? To answer these questions, we
begin with the definition of mesoscale continuum kinematics from the microscale
kinematics. The geometry of micro-structure (e.g., order vs. disorder) has a decisive
role in defining the continuum kinematics. We thus arrive at three kinematic
formulations: mass continuum, lattice continuum and granular continuum. Then,
upon formulating the power balance, we use the principle of virtual power to
arrive at a variety of mathematical formulations: simple continuum with moving
boundaries, phase field formulation, and, a higher order, size-dependent continuum.
The problems considered include: mixing of fluids and capillary flows, granular
flow/deformation, and, polycrystalline diffusional/dislocation creep accompanied
by dislocation plasticity.

To achieve safety and reliability in pipelines installed in seismic and permafrost regions, it is necessary to use linepipe materials with high strength and ductility. The introduction of dual-phase steels, e.g., with a bainite and dispersed martensite–austenite (MA) constituent, would provide the necessary ingredients for the improvement of the strain capacity (as required by a new strain-based linepipe design approach) and toughness. To fine-tune the alloy design and ensure these dual-phase steels have the required mechanical properties, an understanding of the governing deformation micromechanisms is essential. For this purpose, a recently developed joint numerical–experimental approach that involves the integrated use of microscopic digital image correlation analysis, electron backscatter diffraction, and multiphysics crystal plasticity simulations with a spectral solver was employed in this study. The local strain and stress evolution and microstructure maps of representative microstructural patches were captured with a high spatial resolution using this approach. A comparison of these maps provides new insights into the deformation mechanism in dual-phase microstructures, especially regarding the influence of the bainite and MA grain size and the MA distribution on the strain localization behavior.

Grain boundaries are intrinsic and omnipresent microstructural imperfections in polycrystalline and nanocrystalline materials. They are short-circuit diffusion paths and preferential locations for alloying elements, dopants, and impurities segregation. They also facilitate heterogeneous nucleation and the growth of secondary phases. Therefore, grain boundaries strongly influence many materials' properties and their stabilities during application. Here, we propose an approach to measure diffusion, segregation, and segregation-induced precipitation at grain boundaries at a sub-nanometer scale by combining atom probe tomography and scanning transmission electron microscopy. Nanocrystalline multilayer thin films with columnar grain structure were used as a model system as they offer a large area of random highangle grain boundaries and inherent short diffusion distance. Our results show that the fast diffusion flux proceeds primarily through the core region of the grain boundary, which is around 1 nm. While the spatial range that the segregated solute atoms occupied is larger: below the saturation level, it is 1,2 nm; as the segregation saturates, it is 2-3.4 nm in most grain boundary areas. Above 3.4 nm, secondary phase nuclei seem to form. The observed distributions of the solutes at the matrix grain boundaries evidence that even at a single grain boundary, different regions accommodate different amounts of solute atoms and promote secondary phase nuclei with different compositions, which is caused by its complex threedimensional topology.

Grain boundaries (GBs) are planar lattice defects that govern the properties of many types of polycrystalline materials. Hence, their structures have been investigated in great detail. However, much less is known about their chemical features, owing to the experimental difficulties to probe these features at the atomic length scale inside bulk material specimens. Atom probe tomography (APT) is a tool capable of accomplishing this task, with an ability to quantify chemical characteristics at near-atomic scale. Using APT data sets, we present here a machine-learning-based approach for the automated quantification of chemical features of GBs. We trained a convolutional neural network (CNN) using twenty thousand synthesized images of grain interiors, GBs, or triple junctions. Such a trained CNN automatically detects the locations of GBs from APT data. Those GBs are then subjected to compositional mapping and analysis, including revealing their in-plane chemical decoration patterns. We applied this approach to experimentally obtained APT data sets pertaining to three case studies, namely, NiP , Pt-Au, and Al-Zn-Mg-Cu alloys. In the first case, we extracted GB specific segregation features as a function of misorientation and coincidence site lattice character. Secondly, we revealed interfacial excesses and in-plane chemical features that could not have been found by standard compositional analyses. Lastly, we tracked the temporal evolution of chemical decoration from early-stage solute GB segregation in the dilute limit to interfacial phase separation, characterized by the evolution of complex composition patterns. This machine-learningbased approach provides quantitative, unbiased, and automated access to GB chemical analyses, serving as an enabling tool for new discoveries related to interface thermodynamics, kinetics, and the associated chemistry-structure-property relations.

The formation of submicron structural defects within austenite (γ), ε-and α′-martensite during cold rolling was followed in a 17.6 wt.% Mn steel. Several probes, including XRD, EBSD, and ECCI-imaging, were used to reveal the complex superposition of the strain hardening mechanisms of these phases. The maximum amount of ε-martensite is observed at a strain of ε = 0.11. At larger strains, the amount of ε decreases suggesting that it precedes the α′-formation (γ → ε → α′). Stacking faults and twins are the main planar defects noticed in ε-martensite. The remaining γ is finely subdivided by stacking faults and twins up to ε = 0.22. From ε = 0.51 on, twinning and multiplication of dislocations are the principal strain hardening mechanisms in austenite. Deformation is accommodated in α′ by the rearrangement of dislocation tangles into dislocation cells plus shear banding at ε = 1.56. During cold rolling, austenite develops a Brass-type texture component, which can be associated to mechanical twinning. ε-martensite presents its basal planes tilted ∼24° from the normal direction towards the rolling direction. The α′-martensite develops and strengthens both, the bcc α-and γ-texture fibers during cold rolling.

Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.

This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.

A texture component crystal plasticity finite element method (TCCP-FEM) is used for the simulation of cup drawing of a ferritic stainless steel sheet (X6Cr17, AISI 430, EN 1.4016). The simulation includes the through-thickness texture gradient of the starting hot band. It predicts the development of the orientation distribution and the earing profile during cup forming considering 48 slip systems (12{1 1¯ 0}〈1 1 1〉, 12{1 1 2¯}〈1 1 1〉, 24{123¯}〈1 1 1〉). The earing profiles are compared to FE results obtained by use of a Hill 48 yield surface and to experimental data.

The mechanical response of engineering materials evaluated through continuum fracture mechanics typically assumes that a crack or void initially exists, but it does not provide information about the nucleation of such flaws in an otherwise flawless microstructure. How such flaws originate, particularly at grain (or phase) boundaries is less clear. Experimentally, “good” vs. “bad” grain boundaries are often invoked as the reasons for critical damage nucleation, but without any quantification. The state of knowledge about deformation at or near grain boundaries, including slip transfer and heterogeneous deformation, is reviewed to show that little work has been done to examine how slip interactions can lead to damage nucleation. A fracture initiation parameter developed recently for a low ductility model material with limited slip systems provides a new definition of grain boundary character based upon operating slip and twin systems (rather than an interfacial energy based definition). This provides a way to predict damage nucleation density on a physical and local (rather than a statistical) basis. The parameter assesses the way that highly activated twin systems are aligned with principal stresses and slip system Burgers vectors. A crystal plasticity-finite element method (CP-FEM) based model of an extensively characterized microstructural region has been used to determine if the stress–strain history provides any additional insights about the relationship between shear and damage nucleation. This analysis shows that a combination of a CP-FEM model augmented with the fracture initiation parameter shows promise for becoming a predictive tool for identifying damage-prone boundaries.

Austenitic stainless steels with their good weldability, superior corrosion resistance and excellent performances in higher temperatures are an important material for engineering applications in industrial plants. Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels is a critical failure mechanism where cracking can result from sensitisation of certain grain boundaries after heat treatment (e.g. post-weld stress relief) or fast neutron irradiation in nuclear plant. Sensitisation is a decrease in the local resistance to stress corrosion, to a degree that depends on the grain boundary structure. Developments of predictive models for stress corrosion crack nucleation require more information about the effect of several external parameters (e.g. stress and time) on the likely extent of crack growth. Understanding is also required about how the grain boundary crystallography and the orientations of grain boundary plane and its surrounding grains affect crack propagation.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.

We study grain boundary embrittlement in a quenched and tempered Fe–Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper-embrittlement at 450 °C. Tempering at 600 °C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth.

The effect of cooling methods after heat treatment on the microstructure and mechanical properties of reinforced high strength low alloy steel were studied. The microstructure characteristics were observed under OM (optical microscope), SEM (Scanning Electron Microscope). Charpy V Notch impact tests at different temperatures (-30°C, -10°C, 25°C) along with Tensile tests were performed and the mechanical properties were evaluated. The evaluated results indicated that cooling with water led to a mix of ferritic matrix and tempered martensite, whereas after air cooling the microstructure was a mix of ferritic matrix and tempered bainite. This is basically due to the intensity of cooling rate in both the cooling techniques. Ultimate tensile strength, yield strength, % elongation were compared for the different cooling techniques. In air cooling the combination of strength and toughness was better than in comparison with water cooling.

The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization.

The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislo-cation density (~10 14 m −2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE mi-cromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between fer-rite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels.

In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30-Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural
investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (~300 mm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (~4 mm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys.

The global demand for lightweight design is increasing to provide sustainable solutions to counteract climate change. We developed a novel Ti-bearing lightweight steel (8% lower mass density than general steels), which exhibits an excellent combination of strength (491 MPa ultimate tensile strength) and tensile ductility (31%) at elevated temperature (600 • C). The developed steel is suitable for parts subjected to high temperature at reduced dynamical load. The composition of the developed steel (Fe-20Mn-6Ti-3Al-0.06C-NbNi (wt%)) lends the alloy a multiphase structure with austenite matrix, partially ordered ferrite, Fe 2 Ti Laves phase, and fine MC carbides. At elevated temperature (600 • C), the ductility of the new material is at least 2.5 times higher than that of conventional lightweight steels based on the Fe-Mn-Al system, which become brittle at elevated temperatures due to the inter/intragranular precipitation of κ-carbides. This is achieved by the high thermal stability of its microstructure and the avoidance of brittle κ-carbides in this temperature range.

Ultrafine ferrite grains in a plain C–Mn steel (0.3 mass%
C) were produced by large-strain warm compression and
subsequent annealing treatment in a temperature range between 773 K and 1003 K. The samples were investigated
by means of high-resolution electron back-scatter diffraction.
The resulting microstructures showed very fine ferrite
grains and homogeneously distributed cementite particles.
The majority of the grain boundaries (55 – 70%) were classified
as high-angle ones (³ 15° misorientation). When considering
only these high-angle grain boundaries, the average
grain size changed from 0.9 lm at a deformation
temperature of 773 K to 2.2 lm at a deformation temperature
of 1003 K. For the same range the average subgrain
sizes increased from 0.6 lm to 1.5 lm. The basic result of
this study is that the grain size characterization of polycrystalline microstructures with ultrafine grains requires the use of the high-resolution electron back-scatter diffraction
method in conjunction with a careful analysis of the grain
boundary character.

Superplastic alloys exhibit extremely high ductility (>300%) without cracks when tensile-strained at temperatures above half of their melting point. Superplasticity, which resembles the flow behavior of honey, is caused by grain boundary sliding in metals. Although several non-ferrous and ferrous superplastic alloys are reported, their practical applications are limited due to high material cost, low strength after forming, high deformation temperature , and complicated fabrication process. Here we introduce a new compositionally lean (Fe-6.6Mn-2.3Al, wt.%) superplastic medium Mn steel that resolves these limitations. The medium Mn steel is characterized by ultrafine grains, low material costs, simple fabrication, i.e., conventional hot and cold rolling, low deformation temperature (ca. 650 °C) and superior ductility above 1300% at 850 °C. We suggest that this ultrafine-grained medium Mn steel may accelerate the commercialization of superplastic ferrous alloys.

Here 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 (3D EBSD; EBSD tomography). 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 50x50x50 nm^3 seems to be a realistic optimum. The maximum observable volume is on the order of 50x50x50um^3. The technique extends all the powerfulfeatures 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 pseudo nano-crystalline 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.

This report aims at providing an improved fundamental under-
standing on the micro-mechanical response of lath martensitic
microstructures. Lath martensite is of immense importance for
structural alloys, since it is among the major strength-providing
microstructural constituents in martensitic or multi-phase steels (e.g. dual phase steel, transformation-induced plasticity steel, complex phase steels, quench-partition steels, etc.). Despite its long history and use, efforts to better understand the microstructure development and the mechanical behavior of lath martensite are still ongoing. Here, we are specifically interested in martensitic constituent size variation effects which have been rarely investigated so far [1–3], but drastically influence e.g. the autotempering behavior [4] and toughness properties [5]. Effectively any analysis associated with lath martensitic microstructures is hindered due to the complexities arising from (i) crystallographic and (ii) compositional aspects of the underlying microstructure. In order to motivate the novel analysis strategy developed here, we first discuss these two challenges in the following two paragraphs.
Regarding martensite crystallography most pioneering works
were based on transmission electron microscopy analyses [6,7].
TEM provides sufficient spatial resolution to resolve fine martensitic features (e.g. laths [6]), however, it provides only limited statistics of larger martensitic constituents (e.g. prior austenite grains) due to its limited field of view arising from the specimen and beam geometries. It is the development of the electron backscatter diffraction (EBSD) technique that enabled the systematic characterization of the hierarchical martensitic microstructure spanning multiple scales, i.e. ranging from prior austenite grains of hundreds of microns down to laths of tens of nanometers [8–11].
Yet, it is also clear that the standard 2D EBSD-based analysis provides a rather simplified representation of the lath martensite crystallography. For example, 3D EBSD and 3D FIB [12–15] analyses, as well as TEM observations [1,16,17] reveal significant heterogeneities in the size and morphology of martensite sub-units even within a single alloy, which cannot be fully captured by stand-alone 2D investigations. Also, even in optimized conditions, EBSD cannot resolve the fine details of the martensitic sub-structure.
To improve the fundamental understanding of the multi-scale characteristics of martensitic microstruc-
tures and their micro-mechanical properties, a multi-probe methodology is developed and applied to
low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron
channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels.

The recently developed dual-phase high-entropy alloys are characterized by pronounced strain hardening and high ductility under monotonic loading owing to the associated transformation induced plasticity effect. Fatigue properties of high-entropy alloys have not been studied in depth so far. The current study focuses on the low-cycle fatigue regime. Cyclic tests were conducted and the microstructure evolution was studied post-mortem. Despite deformation-induced martensitic transformation during cycling at given plastic strain amplitudes, intense strain hardening in the cyclic stress-strain response is not observed. This behavior is attributed to the planar nature of slip and partial reversibility of deformation.

The grinding process commonly used in manufacturing requires a specific surface finish, critical size, and required ground surface. The grinding process is used in order to improve final products 'quality by reducing surface roughness and increasing tolerance integrity. To improve final products quality, it is essential to know which grinding wheel should be used and how to avoid surface burn by getting a better analysis of forces acting on the material. This project is the investigation of the grinding process with respect to the theoretical and experimental methods. Theoretical methods provide the background information about the grinding process, where one can use it, the aim of the grinding process, elements of grinding process and previous results of grinding process done with precise parameters. Theoretical knowledge obtained via journals, library and literature database. Experimental methods will do by simulation on Matlab software and surface grinding operations. Simulation on Matlab software is the methodology to simulate and provide results, which are verisimilitudinous and necessary to eliminate costs and time in the manufacturing process. and provides respectively concurrent results to the grinding process with various types of inputs such as different grinding stones with different grain numbers, grinding wheel size by height and width, thickness, and material. In addition, experimental results were obtained using a surface grinding machine to investigate surface roughness and grinding forces by using various types of grinding stones. Furthermore, abrasive grains' characteristics are being studied, their effect on the grinding process, and the measurement of abrasion.

Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at
973 K (60 K below Ae1, the equilibrium austenite–pearlite transformation temperature). Scanning electron microscopy, electron
backscatter diffraction and atom probe tomography results reveal the mechanism of austenitic transformation below Ae1. The initial
fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations.

The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution during the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed.

Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (l phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations,

Two approaches in materials physics have proven immensely successful in alloy design: First, thermodynamic and kinetic descriptions for tailoring and processing alloys to achieve a desired microstructure. Second, crystal defect manipulation to control strength, formability and corrosion resistance. However, to date, the two concepts remain essentially decoupled. A bridge is needed between these powerful approaches to achieve a single conceptual framework. Considering defects and their thermodynamic state holistically as 'defect phases', provides a future materials design strategy by jointly treating the thermodynamic stability of both, the local crystalline structure and the distribution of elements at defects. Here, we suggest that these concepts are naturally linked by defect phase diagrams describing the coexistence and transitions of defect phases. Construction of these defect phase diagrams will require new quantitative descriptors. We believe such a framework will enable a paradigm shift in the description and design of future engineering materials.

Niobium pentoxide (Nb2O5) was added to cobalt spinel ferrite (CoFe2O4) powders for the first time, at varying amounts of 0, 5, 10 and 15 wt%. The purpose was to evaluate the effect of niobia on the crystalline phases, microstructure and complex electromagnetic behaviour (complex permittivity and permeability between 300 MHz and 10 GHz) of CoFe2O4. The samples were prepared by conventional ceramic methods and sintered at 1475 °C, as potential applications are as aerospace materials (radomes) which have to survive at such temperatures upon re-entry. The only crystalline phase observed in all samples was CoFe2O4 , but microstructural evaluation showed that a non-crystalline, niobium-rich intergranular region was formed between the grains with niobium addition, and this apparent liquid/glassy phase aided sintering as considerable grain growth was also observed. It was shown by Raman spectroscopy that this niobium-rich amorphous intergranular phase was FeNbO4. The electromagnetic measurements of complex permittivity (ε*) and permeability (μ*) measurements indicated a steady decrease in both permittivity and permeability with increasing niobium oxide addition, although the values for each sample were relatively stable between 300 MHz and 10 GHz. The real permittivity, ε′, decreased from ~ 12 in the pure CoFe2O4 to ~ 5.5 with increasing addition, while its imaginary part assumed values very close to zero. At the same time, the real permeability, μ′, decreased from ~ 1.4 to ~ 1.1, and a similar effect can be observed in the permeability curves. The results of the complex measurements also allowed us to obtain reflectivity graphs representing the energy loss of the incident electromagnetic wave when crossing the layer of the evaluated composition. The graphs are presented in the frequency domain, and indicate that the reflection loss increases with the addition of niobia.

Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (l-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized l-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels.

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.