Morphological and Crystallographic Characteristics of alpha Structure in a Low-Carbon Iron-Nickel Alloy

The features of a (body-centered cubic) structures were investigated in a low-carbon multicomponent alloy from morphological and crystallographic perspectives. In addition to apparent features of granular bainite and lamellar martensite, a morphological similarity can be found between lath martensite and lath bainite. Therefore, it is of interest to explore possible discrepancies between lath martensite and lath bainite from a crystallographic perspective. These microstructures were obtained by various cooling rates (i.e., water quenching, 5 degrees C/s, and 0.05 degrees C/s) and then were characterized by a combination of scanning electron microscopy and electron backscattered diffraction techniques. It is shown that: (1) Lath martensite (LM) formed in the samples that were water-quenched, and a mixture of LM and lath bainite (LB) and granular bainite (GB) formed in the samples cooled at rates of 5 degrees C/s and 0.05 degrees C/s, respectively; (2) A Kurdjumov-Sachs relationship was mostly found in as-quenched martensite, while a Greninger-Troiano relationship represented the orientation relationship of LB and GB; (3) As the cooling rate decreased, the dislocation densities in corresponding microstructures were reduced, while the tendency of variant grouping was enhanced.

Effect of cooling conditions on microstructures and mechanical behaviors of reheated low-carbon weld metals

Rui Cao, Cyril Cayron, Roland Logé, Gaojun Mao

An investigation was conducted to systematically explore the relationships among microstructure evolution, continuous cooling transformation kinetics, and mechanical properties development in a low-carbon reheated weld metal (0.052C-1.5Mn-0.3Si-0.7Cr-0.8Mo-0.014Ti-0.004V-4Ni, wt%). Microstructure was characterized by optical microscopy, scanning electron microscopy and electron backscattered diffraction techniques. The effect of different cooling rates on morphologic features and crystallographic characteristics (i.e., orientation relationship, variant selection) of various microstructures was researched. Transformation kinetics at various cooling rates were studied on the basis of the dilatometric data using a Gleeble simulator. Impact energy at 223 K, ultimate tensile strength at room temperature, and microhardness were tested. Present results were compared with previously published literature. It is shown that the primary microstructure varies from martensite, to a mixture of martensite and lath bainite, to a mixture of lath bainite and acicular ferrite, and to complete granular bainite with decreasing cooling rates from water quenching, to 30 K/s, to 10 K/s, to 3 K/s, and to 0.3 K/s, respectively, which was also accompanied by a decrease in Vickers microhardness. The best combination of toughness and strength in specimens with microstructures was made up of lath bainite and acicular ferrite obtained with the cooling rates of 10 K/s and 3 K/s.

ELSEVIER SCIENCE SA2019

Characterization at the Atomistic Level of Defective Structures in Complex Materials

Piyush Agrawal

Defects are key to enhance or deploy particular materials properties. In this thesis I present analyses of the impact of defects on the electronic structure of materials using combined experimental and theoretical Electron energy loss spectroscopy (EELS) in (Scanning) Transmission Electron Microscopy. The energy loss near-edge structure (ELNES) in EELS reflects the element-specific electronic structure providing insights into bonding characteristics of individual atomic species. New electron optical devices have boosted the analytical capabilities by which materials can be investigated with atomic resolution and single atom sensitivity using (scanning) transmission electron spectroscopy (STEM/TEM).With the help of aberration correctors for forming small electron probes, high intensity electron beams can nowadays be focused to clearly less than 100 pm which has enhanced the resolution and sensitivity in analytical scanning transmission electron microscopy (STEM). Various kinds of defects in different complex oxides were studied: point defects like oxygen vacancies in BiVO4 and SrMnO3, edge-dislocations in BiFeO3, and planar defects in GaAs. By comparison with experimental data, structures for these systems were proposed based on all-electron density functional theory (DFT) code Wien2k. By comparing theoretical calculations and experimental data, a pronounced surface reduction in the oxidation state of vanadium in BiVO4 from +5 to +4 was unveiled, which is due to a high density of oxygen vacancies, and its importance in potential application of BiVO4 in photoelectrochemical energy conversion. A similar study was performed on a series of SrMnO3 thin films with different epitaxial strain where theoretical investigations revealed the impact of oxygen non-stoichiometry and strain on the O-K ELNES. In the next study, molecular dynamics simulations were combined with FEFF-based EELS calculations and its comparison with experiments was helpful for the correct prediction of the edge dislocation core structure in BiFeO3. This study also confirmed the presence of Fe atoms in the core of the edge dislocation which possibly makes these defects ferromagnetic whereas the bulk structure is known to be antiferromagnetic. This thesis has established methodologies for utilizing different codes, illustrating how links between experimental and theoretical ELNES can be used in revealing structural information around defects and how defects affect materials properties. This tandem methodology of theory and experiments is applicable to various future materials where the reliable interpretation of EELS data is pivotal in unfolding mysteries of such technologically important materials.

EPFL2017

Study on Bainite/Martensite Transformation in Reheated Weld Metals

Rui Cao, Cyril Cayron, Roland Logé, Gaojun Mao

A new combination of laser scanning confocal microscope (LSCM) and electron backscattering diffraction (EBSD) with a field-emission scanning electron microscope (SEM) is utilized to study the mechanism of bainite transformation in reheated low carbon bainitic weld metal. The LSCM observations show that laths grow on the surface at various rates, from 30 mu m/s to 240 mu m/s, which is greatly larger than those referred in literature for bainite. In order to confirm that the laths are bainite and not surface martensite, additional experiments were performed. The crystallographic characteristics of surface bainite were compared with those of bulk bainite obtained during isothermal treatments and those of bulk martensite obtained by water quenching. By means of a dedicated EBSD data-treatment software, orientation relationship, variant selection and packet groups were identified; it was shown that both the surface laths and bulk bainite share the same misorientation, habit plane, and have similar variant distribution. Experiments are running to compare these features with those of bulk martensite. If the distinction between martensite and bainite is successful, the very high growth rates of the surface laths could be used to discuss the displacive/diffusive nature of bainitic transformations.

TRANS TECH PUBLICATIONS LTD2018

Characterization at the Atomistic Level of Defective Structures in Complex Materials

Piyush Agrawal

EPFL2017