J. Hjelen

SEM/EBSD based in situ studies of deformation induced phase transformations in supermartensitic stainless steels

Abstract Recent years equipment design has enabled the combination of in situ deformation tests with near real time electron backscatter diffraction (EBSD) mapping of the microstructure evolution in the scanning electron microscope (SEM). The present work involves studies of deformation induced phase transformations in supermartensitic steel containing ∼40 vol.-% retained austenite at room temperature. The martensite formation was initiated already at low strains, and increased gradually with increasing plastic strains up to ∼10%. It was observed that the martensite formed homogeneously within the microstructure, independent of the crystallographic orientations of the retained austenite. But no new martensite variants, besides those already present in the as received condition, did form during deformation. At the same time, the mutual distribution of these variants remained approximately constant throughout the deformation process.

Application of EBSD technique to ultrafine grained and nanostructured materials processed by severe plastic deformation: Sample preparation, parameters optimization and analysis

Abstract With the help of FESEM, high resolution electron backscatter diffraction can investigate the grains/subgrains as small as a few tens of nanometers with a good angular resolution (∼0.5°). Fast development of EBSD speed (up to 1100 patterns per second) contributes that the number of published articles related to EBSD has been increasing sharply year by year. This paper reviews the sample preparation, parameters optimization and analysis of EBSD technique, emphasizing on the investigation of ultrafine grained and nanostructured materials processed by severe plastic deformation (SPD). Detailed and practical parameters of the electropolishing, silica polishing and ion milling have been summarized. It is shown that ion milling is a real universal and promising polishing method for EBSD preparation of almost all materials. There exists a maximum value of indexed points as a function of step size. The optimum step size depends on the magnification and the board resolution/electronic step size. Grains/subgrains and texture, and grain boundary structure are readily obtained by EBSD. Strain and stored energy may be analyzed by EBSD.

Grain refinement of magnesium alloys processed by severe plastic deformation

Abstract Grain refinement of AZ31 Mg alloy during cyclic extrusion compression (CEC) at 225–400 °C was investigated quantitatively by electron backscattering diffraction (EBSD). Results show that an ultrafine grained microstructure of AZ31 alloy is obtained only after 3 passes of CEC at 225 °C. The mean misorientation and the fraction of high angle grain boundaries (HAGBs) increase gradually by lowering extrusion temperature. Only a small fraction of twinning is observed by EBSD in AZ31 Mg alloys after 3 passes of CEC. Schmid factors calculation shows that the most active slip system is pyramidal slip and basal slip {0001} at 225–350 °C and 400 °C, respectively. Direct evidences at subgrain boundaries support the occurrence of continuous dynamic recrystallization (CDRX) mechanism in grain refinement of AZ31 Mg alloy processed by CEC.

Optimization of EBSD parameters for ultra-fast characterization

Ultra‐fast pattern acquisition of electron backscatter diffraction and offline indexing could become a dominant technique over online electron backscatter diffraction to investigate the microstructures of a wide range of materials, especially for in situ experiments or very large scans. However, less attention has been paid to optimize the parameters related to ultra‐fast electron backscatter diffraction. The present results show that contamination on a clean and unmounted specimen is not a problem even at step sizes as small as 1 nm at a vacuum degree of 6.1 × 10−5 Pa. There exists an optimum step size at about 50 data acquisition board units. A new and easy method to calculate the effective spatial resolution is proposed. Effective spatial resolution tends to increase slightly as the probe current increases from 10 to 100 nA. The fraction of indexed points decreases slightly as the frame rate increases from 128 patterns per second (pps) to 835 pps by compensating the probe current at the same ratio. The value 96 × 96 is found to be the optimum pattern resolution to obtain optimum speed and image quality. For a fixed position of electron backscatter diffraction detector, the fraction of indexed points as a function of working distance has a maximum value and drops sharply by shortening the working distance and it decreases slowly with increasing the working distance.

High-Speed Orientation Microscopy with Offline Solving Sequences of EBSD Patterns

A high speed in acquisition of backscatter Kikuchi patterns (BKP) and solving the stored raw patterns offline has many advantages over online EBSD. No compromise is made between speed and reliability. Automated backscatter Kikuchi diffraction in the scanning electron microscope (SEM) is about to become a tool for process and quality control. Mandatory requirements for these applications are measures to enable re-examination of the results at any time and a high speed. Therefore, fast acquisition of pattern sequences and off¬line indexing will soon become standard. Online pattern solving is optional, but at the disadvantage of reduced speed and reliability.

Orientation microscopy with fast EBSD

Abstract Automated backscatter Kikuchi diffraction in the SEM is about to become a tool for process and quality control. Mandatory requirements for these applications are a high speed and provisions to enable re-examination of the results at any time. Means and motivation for high speed electron backscattering diffraction are discussed. Separate acquisition and store of pattern sequences in an unprocessed data format, followed by offline calculation of the grain orientations, has many advantages over online orientation microscopy. With actually >1000 acquired patterns per second it is significantly faster than conventional electron backscattering diffraction. The data can be examined again using the original backscatter Kikuchi sequences, and the presence of a priori unknown phases can be checked for. Online pattern solving, as a dual task, is optional.

Study of microstructure and texture evolution using in-situ EBSD investigations and SE imaging in SEM

The crystallographic slip activity in several grains deformed by simple tension is determined by use of in-situ deformation in combination with Electron Back Scattering Diffraction (EBSD)-investigations and Secondary Electron (SE) imaging. This technique is also used to determine grain lattice rotation paths of grains with different initial orientation, providing information on basic deformation mechanisms of grains present in texture gradients. Both slip activity and grain lattice rotation paths depend on the initial orientation and are influenced by the neighbouring grain orientations. This indicates that predictions of the forming behaviour of extruded profiles with a strong through thickness texture gradient relate to a very complex nature.

Characterization of microstructure and strain response in Ti−6Al−4V plasma welding deposited material by combined EBSD and in-situ tensile test

Abstract Additive layer manufacturing (ALM) of aerospace grade titanium components shows great promise in supplying a cost-effective alternative to the conventional production routes. Complex microstructures comprised of columnar remnants of directionally solidified β -grains, with interior inhabited by colonies of finer α -plate structures, were found in samples produced by layered plasma welding of Ti–6Al–4V alloy. The application of in-situ tensile tests combined with rapid offline electron backscatter diffraction (EBSD) analysis provides a powerful tool for understanding and drawing qualitative correlations between microstructural features and deformation characteristics. Non-uniform deformation occurs due to a strong variation in strain response between colonies and across columnar grain boundaries. Prismatic and basal slip systems are active, with the prismatic systems contributing to the most severe deformation through coarse and widely spaced slip lines. Certain colonies behave as microstructural units, with easy slip transmission across the entire colony. Other regions exhibit significant deformation mismatch, with local build-up of strain gradients and stress concentration. The segmentation occurs due to the growth morphology and variant constraints imposed by the columnar solidification structures through orientation relationships, interface alignment and preferred growth directions. Tensile tests perpendicular to columnar structures reveal deformation localization at columnar grain boundaries. In this work connections are made between the theoretical macro- and microstructural growth mechanisms and the observed microstructure of the Ti–6Al–4V alloy, which in turn is linked to observations during in-situ tensile tests.

Future prospects on EBSD speeds using a 40 nA FESEM

This paper describes a method of estimating the maximum obtainable mapping speed by the use of a new ultra-fast EBSD detector system [1], both with current equipment and with even faster CCDs that will be available in the future.

Characterization of morphology and microstructure of different kinds of materials at NTNU Mater Sci EM Lab

The NTNU Materials Science and Engineering EM Lab is equipped with the electron microscopy facilities assisting NTNU multi-disciplinary materials science research. The laboratory services research activities from various material departments at the university. Here we present several recent characterization examples from different kinds of materials.