EBSD coupled to SEM in situ annealing for assessing recrystallization and grain growth mechanisms in pure tantalum

An in situ annealing stage has been developed in‐house and integrated in the chamber of a Scanning Electron Microscope equipped with an Electron BackScattered Diffraction system. Based on the Joule effect, this device can reach the temperature of 1200°C at heating rates up to 100°C/s, avoiding microstructural evolutions during heating. A high‐purity tantalum deformed sample has been annealed at variable temperature in the range 750°C–1030°C, and classical mechanisms of microstructural evolutions such as recrystallization and grain coarsening phenomena have been observed. Quantitative measurements of grain growth rates provide an estimate of the mean grain boundary mobility, which is consistent with the value estimated from physical parameters reported for that material. In situ annealing therefore appears to be suited for complementing bulk measurements at relatively high temperatures, in the context of recrystallization and grain growth in such a single‐phase material.     

A study of recrystallization in single-phase aluminium using in-situ annealing in the scanning electron microscope

In‐situ annealing experiments were performed in the scanning electron microscope on a single‐phase Al?0.13Mg alloy cold rolled to different strain levels. Once the validity of the technique had been verified by comparison of the recrystallization kinetics and final grain size with bulk annealed samples, the method was used in combination with electron back‐scattered diffraction (EBSD) to study the potential mechanisms for recrystallization in this alloy. During annealing of material rolled to moderate strains (?_t < 0.7), the primary mechanism was strain‐induced boundary migration (SIBM). In material rolled to higher true strains (?_t > 1.4), recrystallization occurred extensively along pre‐existing cube bands and EBSD measurements showed that the mean size of cells within the cube bands was larger than for all other orientations measured, suggesting a size advantage was responsible for the strengthening of cube texture during recrystallization. SIBM was shown to occur concurrently with the nucleation along cube bands but this contributed a lower proportion of nucleation sites during recrystallization.     

A MEMS‐based heating holder for the direct imaging of simultaneous in‐situ heating and biasing experiments in scanning/transmission electron microscopes

The introduction of scanning/transmission electron microscopes (S/TEM) with sub‐Angstrom resolution as well as fast and sensitive detection solutions support direct observation of dynamic phenomena in‐situ at the atomic scale. Thereby, in‐situ specimen holders play a crucial role: accurate control of the applied in‐situ stimulus on the nanostructure combined with the overall system stability to assure atomic resolution are paramount for a successful in‐situ S/TEM experiment. For those reasons, MEMS‐based TEM sample holders are becoming one of the preferred choices, also enabling a high precision in measurements of the in‐situ parameter for more reproducible data. A newly developed MEMS‐based microheater is presented in combination with the new NanoEx?‐i/v TEM sample holder. The concept is built on a four‐point probe temperature measurement approach allowing active, accurate local temperature control as well as calorimetry. In this paper, it is shown that it provides high temperature stability up to 1,300°C with a peak temperature of 1,500°C (also working accurately in gaseous environments), high temperature measurement accuracy (<4%) and uniform temperature distribution over the heated specimen area (<1%), enabling not only in‐situ S/TEM imaging experiments, but also elemental mapping at elevated temperatures using energy‐dispersive X‐ray spectroscopy (EDS). Moreover, it has the unique capability to enable simultaneous heating and biasing experiments. Microsc. Res. Tech. 79:239–250, 2016. © 2016 Wiley Periodicals, Inc.     

Recrystallization phenomena in an IF steel observed by in situ EBSD experiments

In situ electron backscatter diffraction microstructural analysis of recrystallizing interstitial free steels deformed to strains of 0.75 and 1.6 has been carried out in a FEG‐SEM. The experimental procedures are discussed, and it is shown that there is no degradation of the electron backscatter diffraction patterns at temperatures up to 800°C. Analysis of the surface and interior microstructures of annealed samples shows only minor difference, which suggests that in situ annealing experiments are of value. In addition, it is shown that in situ measurements allow a detailed comparison between the same areas before and after annealing, thereby providing information about the recrystallization mechanisms. Sequential recrystallization phenomena, such as initiation and growth of new grains, are observed at temperatures over 740°C, and depending on the deformation histories, different recrystallization behaviour is observed. It is found that {111}〈123〉 recrystallized grains are preferentially formed in the highly deformed material, whereas no strong recrystallization texture is formed in the lower strained material.     

Scanning electron microscopy and transmission electron microscopy in situ studies of grain boundary migration in cold-deformed aluminium bicrystals

The crystallography of recrystallization has been investigated in channel‐die deformed pure aluminium bicrystals with {100}<011>/{110}<001> orientations. The microstructural and microtextural changes during the early stages of recrystallization were followed by systematic local orientation measurements using scanning and transmission electron microscopes. In particular, orientation mapping combined with in situ sample heating was used to investigate the formation and growth of new grains at very early stages of recrystallization. Grain boundary migration and ‘consumption’ of the as‐deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation.     

Recrystallization behaviour of the nickel-based alloy 80 A during hot forming

The dynamic recrystallization as well as meta‐dynamic and static recrystallization of the nickel‐based alloy 80A was investigated by means of electron backscatter diffraction (EBSD). Specimens were hot compressed at a temperature of 1120°C and a strain rate of 0.1/s at varying strain and soak times to describe the recrystallization behaviour. Various approaches were tested in order to differentiate between recrystallized and deformed grains based on EBSD data. The grain orientation spread was clearly found to be the most reliable procedure. A high twinning of the recrystallized grains was observed, and as a consequence the measured grain size was strongly dependent on whether the coherent and incoherent twin boundaries were regarded as genuine boundaries or removed.     

The application of a hot deformation SEM stage, backscattered electron imaging and EBSD to the study of thermomechanical processing

The technique of combining in situ hot‐deformation and high resolution electron backscattered diffraction (EBSD) has been applied to study the mechanisms operating during the thermomechanical processing of metals. A simple hot tensile‐straining stage is installed in a field emission gun scanning electron microscope equipped with an EBSD system and has been used successfully for a number of preliminary investigations. These investigations include substructure formation, dynamic subgrain and grain growth, superplastic deformation in aluminium alloys, and dynamic recrystallization in copper. Despite the surface topography, which inevitably increases during plastic deformation, channelling contrast backscattered electron micrographs have been successfully obtained after strains of up to ～50%. Good quality EBSD maps have been obtained after strains of up to 100%. Most observations and measurements from the in situ experiments are consistent with what is known about the mechanisms occurring in the bulk. The microstructures revealed in the centre of the in situ samples after later repolishing are generally similar to those at the surface.     

The Annealing Behavior of the Subsurface Zone Induced by Friction in Bismuth Detected by Positron Lifetime Technique

The annealing behavior of the subsurface zone (SZ) in pure bismuth induced by dry sliding was studied using the positron lifetime measurement. This measurement allows us to detect the SZ and its recovery, and recrystallization processes. The comparative measurements of the sample exposed to compression revealed the thermal stability of the SZ. The compressed sample rebuilt its structure due to the recovery and recrystallization processes at the temperature of 60 °C, whereas the sample exposed to dry sliding does it at higher temperature of 260 °C, which is close to the melting point. The isothermal annealing at the temperature of 100 °C confirmed these results. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that the concentration of defects at the worn surface gradually decreases, but at the depth between 50 and 170 μm, the generation of new defects takes place at the temperature of 75, 100 and even at 175 °C. At the temperature of 175 °C, the defects still are extended up to the depth of about 60 μm from the worn surface. The results were qualitatively confirmed by the measurements of the Vickers microhardness depth profile. Similar annealing behavior of the SZ was observed in pure magnesium.     

Development of high‐temperature strain instrumentation for in situ SEM evaluation of ductility dip cracking

Nowadays, the implementation of sophisticated in situ electron microscopy tests is providing new insights in several areas. In this work, an in situ high‐temperature strain test into a scanning electron microscope was developed. This setup was used to study the grain boundary sliding mechanism and its effect on the ductility dip cracking. This methodology was applied to study the mechanical behaviour of Ni‐base filler metal alloys ERNiCrFe‐7 and ERNiCr‐3, which were evaluated between 700°C and 1000°C. The ductility dip cracking susceptibility (threshold strain; ε_min) for both alloys was quantified. The ε_min of ERNiCrFe‐7 and ERNiCr‐3 alloys were 7.5% and 16.5%, respectively, confirming a better resistance of ERNiCr‐3 to ductility dip cracking. Furthermore, two separate components of grain boundary sliding, pure sliding (S_p) and deformation sliding (S_d), were identified and quantified. A direct and quantitative link between grain boundary tortuosity, grain boundary sliding and ductility dip cracking resistance has been established for the ERNiCrFe‐7 and ERNiCr‐3 alloys.     

The role of misorientation and phosphorus content on grain growth and intergranular fracture in iron–carbon–phosphorus alloys

The relationship between the crystallography of intergranular fracture and phosphorus segregation has been investigated in a Fe?0.06wt%P?0.002wt%C alloy aged for 1 h at temperatures between 600 °C and 1000 °C. Two novel techniques were devised for the investigation: first, electron back‐scatter diffraction (EBSD) across the reconstructed fracture surface and, second, a combination of Auger electron spectroscopy, stereophotogrammetry and microscopy to measure phosphorus and carbon on fracture facets combined with EBSD measurements direct from the fracture surface. In total, 700 misorientations were measured from across the reconstructed fracture surface and in ‘control’ areas away from the fracture. It was found that Σ 3s were in general more resistant to brittle fracture than were random boundaries, and it was suggested that alloys of this type could be grain boundary engineered to improve fracture resistance by a short anneal in the austenite region to increase the final proportion of Σ 3s. Sixteen fracture facets yielded combined Auger/EBSD data. The combined Auger/EBSD methodology to acquire joint crystallographic and segregation information from facets was shown to be feasible, although laborious. There were significantly more {110} planes than any other type in the sample population of facets from which combined segregation/crystallography data had been collected. The data suggested that there was on average lower phosphorus segregation on fracture facets that were near {110} than on other intergranular fracture facets.     

Oscillating sliding wear behaviour of SiC,TiC, TiB2, 59SiC–41TiB2 and 52SiC–24TiC–24TiB2 materials up to 750°C in air

The tribological behaviour of different monolithic and composite ceramics was evaluated in the temperature range between room temperature and 750°C. The test method was oscillating sliding with a ball‐on‐disk arrangement in an SRV machine. Alumina balls were used as counter body. The friction behaviour was determined on‐line, and the wear behaviour was determined from calculations on the basis of wear scar dimensions and profilometric measurements. The friction depends on temperature and shows an increase for most materials for increasing temperature; the smallest friction at all temperatures is found for monolithic TiC. The wear behaviour shows different trends for the different materials. In tests against SiC a maximum of wear is found at 500°C, for TiC at 200°C and for TiB₂ at 750°C. The composite ceramics suffer the smallest wear of all materials in the range from 200°C to 500°C. Copyright © 2006 John Wiley & Sons, Ltd.     

Ion beam polishing for three‐dimensional electron backscattered diffraction

Serial sectioning by focused ion beam milling for three‐dimensional electron backscatter diffraction (3D‐EBSD) can create surface damage and amorphization in certain materials and consequently reduce the EBSD signal quality. Poor EBSD signal causes longer data acquisition time due to signal averaging and/or poor 3D‐EBSD data quality. In this work a low kV focused ion beam was successfully implemented to automatically polish surfaces during 3D‐EBSD of La‐ and Nb‐doped strontium titanate of volume 12.6 × 12.6 × 3.0 μm. The key to achieving this technique is the combination of a defocused low kV high current ion beam and line scan milling. The line scan was used to restrict polishing to the sample surface and the ion beam was defocused to ensure the beam contacted the complete sample surface. In this study 1 min polishing time per slice increases total acquisition time by approximately 3.3% of normal 3D‐EBSD mapping compared to a significant increase of indexing percentage and pattern quality. The polishing performance in this investigation is discussed, and two potential methods for further improvement are presented.     

Electron backscatter diffraction applied to lithium sheets prepared by broad ion beam milling

Due to its very low hardness and atomic number, pure lithium cannot be prepared by conventional methods prior to scanning electron microscopy analysis. Here, we report on the characterization of pure lithium metallic sheets used as base electrodes in the lithium‐ion battery technology using electron backscatter diffraction (EBSD) and X‐ray microanalysis using energy dispersive spectroscopy (EDS) after the sheet surface was polished by broad argon ion milling (IM). No grinding and polishing were necessary to achieve the sufficiently damage free necessary for surface analysis. Based on EDS results the impurities could be characterized and EBSD revealed the microsctructure and microtexture of this material with accuracy. The beam damage and oxidation/hydration resulting from the intensive use of IM and the transfer of the sample into the microscope chamber was estimated to be <50 nm. Despite the fact that the IM process generates an increase of temperature at the specimen surface, it was assumed that the milling parameters were sufficient to minimize the heating effect on the surface temperature. However, a cryo‐stage should be used if available during milling to guaranty a heating artefact free surface after the milling process. Microsc. Res. Tech., 78:30?39, 2015. © 2014 Wiley Periodicals, Inc.     

Chromosome morphology after long-term storage investigated by scanning near-field optical microscopy

Fluorescence in situ hybridization coupled with far‐field fluorescence microscopy is a commonly used technique to visualize chromosomal aberrations in diseased cells. To obtain the best possible results, chromatin integrity must be preserved to ensure optimal hybridization of fluorescence in situ hybridization probes. However, biological samples are known to degrade and storage conditions can be critical. This study concentrates its investigation on chromatin stability as a function of time following fluorescence in situ hybridization type denaturing protocols. This issue is extremely important because chromatin integrity affects the fluorescence response of the chromosome. To investigate this, metaphase chromosome spreads of human lymphocytes were stored at both ?20 and ?80 °C, and were then imaged using scanning near‐field optical microscopy over a nine month period. Using the scanning near‐field optical microscope's topography mode, chromosome morphology was analysed before and after the application of fluorescence in situ hybridization type protocols, and then as a function of storage time. The findings revealed that human chromosome samples can be stored at ?20 °C for short periods of time (～ several weeks), but storage over 3 months compromises chromatin stability. Topography measurements clearly show the collapse of the stored chromatin, with variations as large as 60 nm across a chromosome. However, storage at ?80 °C considerably preserved the integrity with variations in topography significantly reduced. We report studies of the fluorescent response of stored chromosomes using scanning near‐field optical microscopy and their importance for gaining further understanding of chromosomal aberrations.     

The integration of experimental in-situ EBSD observations and numerical simulations: a novel technique of microstructural process analysis

The combination of subgrain‐ and grain‐scale microstructural data collected during in‐situ heating experiments and numerical simulations of equivalent microstructural development offers an innovative and powerful tool in the advancement of the understanding of microstructural processes. We present a system that fully integrates subgrain‐ to grain‐scale crystallographic data obtained during in‐situ observations during heating experiments in a scanning electron microscope and the two‐dimensional hybrid numerical modelling system Elle. Such a system offers the unique opportunity to test and verify theories for microstructural development, as predictions made by numerical simulations can be directly coupled to appropriate physical experiments and, conversely, theoretical explanations of experimental observations should be testable with numerical simulations. Discrepancies between data obtained with both techniques suggest the need for an in‐depth investigation and thus open up new avenues of theory development, modification and verification. In addition, because in numerical models it is possible to select the processes modelled, the effect of individual processes on the microstructural development of a specific material can be quantified. To illustrate the potential and methodology of the so‐called EBSD2Elle system, two in‐situ experiments and their equivalent numerical experiments are presented. These are static heating experiments of (a) an annealed Ni‐foil coupled with a front tracking model for grain growth and (b) a cold deformed rock salt with kinetic Monte Carlo simulations for subgrain growth.     

Effect of different temperatures on the metallographic structure and tensile property of 2024‐T4 alloy in integral heating single point incremental forming

To study the tensile property and metallographic structure evolution of 2024‐T4 high‐strength aluminum alloy in integral heating single point incremental forming (IHSPIF), the warm tensile tests were carried out at 120–240°C with the strain rates of 0.1–0.001 s^?1. Its results could provide a certain theoretical reference to the IHSPIF. The integral heating was different from the local heating, which was to heat the overall sheet to be deformed. It was found in the tensile tests that at the strain rate of 0.01 s^?1, the optimum forming temperature was determined to be 210°C at which the ductility was the best. The material dynamically recovered at 240°C. The following IHSPIF tests were conducted at different temperatures. By observing the organization of the sidewall of the square tapered parts, the alloy dynamically recovered appeared at 210°C and its grains coarsened at 240°C. Considering the temperature interval of 30 and below the recrystallizing temperature of aluminum alloy, it was concluded that the optimal temperature for the integral heating IHSPIF was about 150°C.     

A preliminary electron backscattered diffraction study of sintered NdFeB-type magnets

This paper reports, for the first time, the use of electron backscattered diffraction (EBSD) to study orientation in sintered NdFeB type magnets. The magnetic properties of NdFeB magnets are greatly improved if a strong crystallographic texture is firstly achieved, namely, the direction of the c‐axis is along the direction of magnetization. A systematic survey of sample preparation techniques showed that samples that were mechanically polished and then etched gave the most reliable EBSD data. Analyses were made using both fully automated EBSD scans and by EBSD measurements taken after manual movement of the beam. The EBSD results are presented as secondary electron SEM micrographs, orientation images and 001 pole figures. For the selection of grains investigated, the deviation of the c‐axis was shown to be between 10° and 30° from the ideal [001]//magnetization direction. It is demonstrated that EBSD is a valuable tool for characterizing the microstructure and texture relationships and for assessing the performance of the processing routes of NdFeB magnets.     

EBSD investigation of the microstructure and texture characteristics of hot deformed duplex stainless steel

The microstructure and crystallographic texture characteristics were studied in a 22Cr‐6Ni‐3Mo duplex stainless steel subjected to plastic deformation in torsion at a temperature of 1000 °C using a strain rate of 1 s^?1. High‐resolution EBSD was successfully used for precise phase and substructural characterization of this steel. The austenite/ferrite ratio and phase morphology as well as the crystallographic texture, subgrain size, misorientation angles and misorientation gradients corresponding to each phase were determined over large sample areas. The deformation mechanisms in each phase and the interrelationship between the two are discussed.     

Analysis of Lubricating Oil for Cu,Fe, Mn,Ni, and Zn Using Electrothermal Atomic Absorption Spectrometry

Abstract

Electrothermal atomic absorption spectrometry (ETA‐AAS) was utilised for the estimation of Cu, Fe, Mn, Ni, and Zn, which are additives for desired properties in different lubricating oils, directly and when subjected to 500°C with and without the addition of xylene. The results show that, for the samples subjected to 500°C and treated with xylene before analysis, the coefficient of variation and, hence, precision improved for the elements. The result obtained also shows that there are no significant variations, indicating that, with the ETA‐AAS technique, the samples require no chemical pre‐treatment. This is so because the chemical matrix is removed by ashing prior to atomisation.     

Static and dynamic experiments in cryo-electron microscopy: comparative observations using high-vacuum, low-voltage and low-vacuum SEM

We carried out a unique comparative study between three modes of cryo‐scanning electron imaging: high‐vacuum, low‐voltage and low‐vacuum, using ice cream as a model system. Specimens were investigated both with and without a conductive coating (Au/Pd) and at temperatures for which ice either remains fully frozen (< ?110 °C) or undergoes sublimation (?110 to ?90 °C). At high magnification, high‐vacuum imaging of coated specimens gave the best results for ‘static’ specimens (i.e. containing fully frozen ice). Low voltages, such as 1 kV, could be used for imaging uncoated specimens at high vacuum, although slight ‘classical’ charging artefacts remained an issue, and the reduced electron beam penetration tended to decrease the definition between different microstructural features. However, this mode was useful for observing in situ sublimation from uncoated specimens. Low‐vacuum mode, involving small partial pressures of nitrogen gas, was particularly suited to in situ sublimation work: when sublimation was carried out in low vacuum in the absence of an anti‐contaminator plate, sublimation rates were significantly reduced. This is attributed to a small partial pressure of sublimated water vapour remaining near the specimen surface, enhancing thermodynamic stability.