Transmission Kikuchi diffraction mapping induces structural damage in atom probe specimens

The examination of the local chemistry of a specific interface by atom probe tomography (APT) is increasingly facilitated by using transmission Kikuchi diffraction (TKD) to help position specific crystallographic features sufficiently close to the apex of the needle shaped specimen. However, possible structural damage associated by the energetic electrons used to perform TKD is only rarely considered and is hence not well-understood. Here, in two case studies, we demonstrate that APT specimens are subject to electron beam damage during TKD mapping. First, we analyze a solid solution, metastable \b{eta}-Ti-12Mo alloy, in which the Mo is expected to be homogenously distributed, yet APT reveals a planar segregation of Mo amongst other elements. Second, specimens were prepared near {\Sigma}3 twin boundaries in a high manganese twinning-induced plasticity steel, and subsequently charged with deuterium gas. Beyond a similar planar segregation, voids containing a high concentration of deuterium are detected. Both examples showcase damage from TKD mapping leading to artefacts in the compositional distribution of solutes. We propose that the structural damage is created by surface species, including H and C, subjected to recoil from incoming energetic electrons during mapping, thereby getting implanted and causing cascades of structural damage in the sample

Microstructure, grain boundary evolution and anisotropic Fe segregation in (0001) textured Ti thin films

The structure and chemistry of grain boundaries (GBs) are crucial in determining polycrystalline materials' properties. Faceting and solute segregation to minimize the GB energy is a commonly observed phenomenon. In this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin films is presented. By increasing the power density, a transition from polycrystalline film growth to a maze bicrystalline Ti film on SrTiO3 (001) substrate is triggered. All the GBs in the bicrystalline thin film are characterized to be Sigma 13 [00 01] coincident site lattice (CSL) boundaries. The GB planes are seen to distinctly facet into symmetric {(7) over bar 520} and {13 (4) over bar0} and asymmetric {10 (1) over bar0} // {11 (2) over bar0} segments of 20-50 nm length. Additionally, EDS reveals preferential segregation of iron (Fe) in every symmetric {(7) over bar 520} segment. Both the faceting and the segregation are explained by a difference in the CSL density between the facet planes. Furthermore, in the GB plane containing Fe segregation, atom probe tomography is used to experimentally determine the GB excess solute to be 1.25 atoms/nm(2). In summary, the study reveals for the first time a methodology to obtain bicrystalline Ti thin films with strong faceting and an anisotropy in Fe segregation behaviour within the neighbouring GB facets. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc

Grain boundary segregation of boron and carbon and their local chemical effects on the phase transformations in steels

Boron (B) is the most effective alloying element for increasing the hardenability of steel. It achieves its remarkable effect on hardenability through the grain boundary (GB) segregation at austenite GBs. This suppresses the heterogeneous nucleation of ferrite. It is still fundamentally unclear how B achieves its remarkable effect on the ferrite nucleation and their mechanisms remain not fully understood. Carbon (C), on the other hand, is an important alloying element concerning the mechanical properties of steel. Though the carbon effects on various phase transformations are rather understood well, its GB segregation and local decoration effects at GBs remain not understood completely. The thesis aims to uncover the fine details about the GB segregation of B and C and then to relate its effect on the bulk phase transformation. In the present thesis, two systems of alloys are designed to study the GB segregation of B and C. One is binary Fe-B alloys with increasing B content in it. Another system is a typical low C steel with and without B. These alloys are subjected to various heat treatments in the dilatometer, in a controlled manner. Electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and atom probe tomography (APT) are employed on the heat-treated specimens for the site-specific correlative investigation to probe GBs. Through systematic study and careful investigation, the austenitization temperature and the cooling rate dependence of B segregation are unraveled. The corresponding segregation mechanisms are discussed in detail. It has been found that the precipitation of carbo-borides is more pronounced with a slower cooling rate compared with quenching. Additionally, B segregation and borides/carbo-borides precipitation are revealed through the APT analysis. Based on the observed segregation, GB energy reduction is estimated to be about 286 mJ/m2 in B-added low C steel due to B segregation. Thermodynamic and nucleation kinetics analyses reveal that this substantially affects the most favorable heterogeneous nucleation sites like grain corner and grain edge heterogeneous nucleation sites. However, it has been pointed that the effects of borides/carbo-borides precipitation on retarding the ferrite nucleation cannot be excluded. Detailed investigation of the corresponding mechanisms of B effects on ferrite nucleation is given based on the experimental and thermodynamic investigation. On the other hand, the difficulties in studying the GB segregation of C are discussed and possible routes to overcome these difficulties are highlighted. C-rich regions near the PAGBs, the packet, and the lath boundaries are commonly observed in various heat-treated specimens of low-C steels with and without B. The effect of these regions on the ferrite nucleation is excluded as these regions mostly appear due to the auto-tempering effects or due to the C redistribution in the austenite. Enhanced segregation of C leading to GB cementite formation is observed in steels that contain B. Through this, the importance of consideration of GB solute interactions is highlighted. It is pointed out that these solute interactions at GB can lead to GB (2D) phase formation/transition. Such low-dimensional phase formation/transition can promote the bulk phase transformations such as cementite, boride, or boro-carbides

Modeling and simulation of dynamic recrystallization in super austenitic stainless steel employing combined cellular automaton, artificial neural network and finite element method

A cellular automaton (CA) model for dynamic recrystallization (DRX) is established by employing Moore's neighboring rule to predict flow stress, DRX grain size (DDRX) and DRX fraction (XDRX). The CA model has been optimized for super austenitic stainless steel at different strain rates (0.001–10 s−1) and temperatures (1173–1423 K) under isothermal deformation conditions. The output of the CA simulation has been used for establishing ANN-based constitutive models. The trained ANN-based constitutive models have been further implemented in FEM software (ABAQUS 6.14) to evaluate flow behavior and microstructure response of the alloy under various non-isothermal deformation conditions. The conventional CA (CAC) model has failed to provide a good depiction of the microstructure evolution, as it revealed a very low correlation coefficient (R) for XDRX (R ~ 0.75) and DDRX (R ~ 0.8). This inaccuracy of the model could be related to its inherent inability to consider the effect of solute drag on grain growth and DRX kinetics. Therefore, a modified cellular automata (CAM) model has been developed by introducing a new temperature-strain rate-dependent mobility parameter for numerically considering the solute drag effect. Employing non-isothermal simulations, the CAM model has revealed a higher correlation coefficient than the CAC model for predicting XDRX (R ~ 0.95) and DDRX (R ~ 0.98). Moreover, the developed CAM model has also predicted the flow behavior of the alloy in the entire domain investigated, revealing a higher correlation coefficient (R ~ 0.987) and a low average absolute relative error (8.6). © 2021 Elsevier B.V

Thermodynamic Assessment of Steelmaking Practices for the Production of Re-sulfur Steels

FactSage has become one of the most important modeling tools in simulating the high-temperature metallurgical processes. The usefulness of the FactSage has been demonstrated in this work using several examples of steelmaking processes. Primary steelmaking (basic oxygen furnace) simulation was done with the available process data, and process charts similar to the standard ones were obtained. Ladle refining furnace process for free-cutting steels was simulated and it was observed that absolute non-equilibrium condition exists in steel during casting due to S injection. It was found that non-metallic inclusion formation is thermodynamically possible at final processing stages during Ca and S injection with variable recoveries. A significant change in the nature of non-metallic inclusions formed in re-sulfur steel causes clogging during continuous casting of liquid steel, and its influence on the process has been discussed, for mere 2 ppm of Ca difference in the liquid steel composition

Cementite decomposition in 100Cr6 bearing steel during high-pressure torsion: Influence of precipitate composition, size, morphology and matrix hardness

Premature failure of rail and bearing steels by White-Etching-Cracks leads to severe economic losses. This failure mechanism is associated with microstructure decomposition via local severe plastic deformation. The decomposition of cementite plays a key role. Due to the high hardness of this phase, it is the most difficult obstacle to overcome in the decaying microstructure. Understanding the mechanisms of carbide decomposition is essential for designing damage-resistant steels for industrial applications. We investigate cementite decomposition in the bearing steel 100Cr6 (AISI 52100) upon exposure to high-pressure torsion (maximum shear strain, Ƴmax = 50.2). Following-up on our earlier work on cementite decomposition in hardened 100Cr6 steel (Qin et al., Act. Mater. 2020 [1]), we now apply a modified heat treatment to generate a soft-annealed microstructure where spherical and lamellar cementite precipitates are embedded in a ferritic matrix. These two precipitate types differ in morphology (spherical vs. lamellar), size (spherical: 100–1000 nm diameter, lamellar: 40–100 nm thickness) and composition (Cr and Mn partitioning). We unravel the correlation between cementite type and its resistance to decomposition using multi-scale chemical and structural characterization techniques. Upon high-pressure torsion, the spherical cementite precipitates did not decompose, but the larger spherical precipitates (≥ 1 μm) deformed. In contrast, the lamellar cementite precipitates underwent thinning followed by decomposition and dissolution. Moreover, the decomposition behavior of cementite precipitates is affected by the type of matrix microstructure. We conclude that the cementite size and morphology, as well as the matrix mechanical properties are the predominating factors influencing the decomposition behavior of cementite. The compositional effects of Cr and Mn on cementite stability calculated by complementary density functional theory (DFT) calculations are minor in the current scenario. © 2021 Elsevier B.V