Josef Pešička

The evolution of dislocation density during heat treatment and creep of tempered martensite ferritic steels

Abstract The evolution of dislocation density in two tempered martensite ferritic steels (a 12% and a 9% chromium steel, “X20” and “P91”) during heat treatment and creep is analyzed using transmission electron microscopy (TEM) and X-ray diffraction (XRD); both methods yield results which are in good agreement when rationalizing the XRD-data based on densities of free dislocations. It is shown that due to the intermediate formation of martensite, standard heat treatments produce very high dislocation densities in tempered martensite ferritic steels (TMFSs). Long term tempering and creep are characterized by a decrease of dislocation density; but dislocation densities are still high as compared to alloys where the formation of microstructure does not involve a martensitic transformation. A heterogeneous microstructure after long term tempering and creep is a characteristic feature of TMFSs. Micro grains with high dislocation densities co-exist next to micro grains without dislocations. The XRD method yields average data and cannot account for this microstructural heterogeneity; but it supports the TEM results which in isolation suffer from providing insufficient statistics. The results of the present study are discussed in the light of earlier work published in the literature and contribute to a better understanding of the role of free dislocations in TMFSs during tempering, creep and high temperature low cycle fatigue.

Mechanisms of serrated flow in aluminium alloys with precipitates investigated by acoustic emission

By measuring the acoustic emission, it is possible to distinguish between deformation which is controlled by normal dislocation motion through a crystal lattice, and deformation controlled by the shearing of coherent particles. These results from alloys with different precipitate structures can be correlated with the appearance of serrated flow: when shearing is pronounced, serrated flow is minute. The claim that shearing is directly responsible for serrated flow is not supported by these findings.

Deposition of composite metal/C:H films-the basic properties of Ag/C:H

Abstract Composite Ag C:H films were deposited using an electromagnetron with Ag target operated in the dc unbalanced mode in a working gas mixture of n-hexane/Ar. The C:H component of this composite was investigated and its polymeric nature revealed. Structure, basic optical and dc electrical properties are described with the main emphasis on the ageing processes. A qualitative explanation is proposed.

The relation between the shape of the stress anomaly and the structure of Fe3Al alloys

Abstract The stress anomaly for Fe 3 Al intermetallics exhibits various shapes in the D0 3 phase region. Two in situ TEM investigations at high temperatures were performed to explain this behaviour—the growth of D0 3 domains and the measurement of the long range order (LRO) in the D0 3 phase during aging. It is shown, that LRO depends on the depth under the surface of the specimen and changes during aging. In the light of this fact is possible to explain the various shape of stress anomaly.

Composite metal/C:H films prepared by unbalanced magnetron sputtering: Ni/C:H

Abstract Composite Ni/C:H films were prepared in two ways: using a working gas mixture butane/Ar in a closed field unbalanced twin magnetron system powered by 40 kHz voltage and using n-hexane/Ar mixture also in an unbalanced planar magnetron system with RF powered substrate support that could provide additional dc negative bias. General electrical and optical properties, morphology observed by TEM and composition by XPS of Ni/C:H films are described. Because the films showed ageing in the morphology and in dc electrical resistance a more detailed study of this phenomenon in case of Ni/C:H films deposited by dc unbalanced magnetron using n-hexane/Ar mixture was performed. These films were compared to those prepared when additional dc negative bias up to − 300 V was imposed during deposition. Qualitative explanation of the ageing is proposed.

Temperature induced structural rearrangements of Ag/a-C:H composite films and their dc electrical conduction

Abstract Silver doped hydrogenated amorphous carbon (a-C:H) films were prepared in an rf (13.56 MHz) glow discharge onto negatively biased substrates using a butane-argon mixture and a resistively heated boat for silver co-evaporation. Temperature induced structural rearrangements were studied by TEM in connection with the changes of the dc electrical conduction and optical transmission. Qualitative explanation was based on TEM micrographs. It was found that electrical and optical properties showed irreversible change for temperatures above 280°C. The dc electrical conduction of Ag/a-C:H films has been found to be rather complex for high temperatures but for low temperatures it can be described in terms of a theory for granular metals.

Superellipsoids: A unified analytical description of the geometry of nanoscale second-phase particles of any shape

An equation for the analytical description of the geometry of nanoscale second-phase particles of any shape (spheres, cubes, plates, cylinders, and fibers) is presented: they are elegantly described as superellipsoids. The respective equation involves four parameters, which can be experimentally derived from just one micrograph.

Composite germanium/C:H films prepared by DC unbalanced magnetron sputtering

Abstract Composite Ge/C:H films (germanium doped hard plasma polymer (C:H)) have been deposited using unbalanced planar magnetron equipped with germanium/graphite target and operated in argon/ n -hexane gas mixture. The composition of the deposited films was determined by Rutherford back scattering (RBS), elastic recoil detection (ERD) and X-ray photoelectron spectroscopy (XPS) analytical methods. Contents of germanium from 0 up to 30 at.% was confirmed with rather homogenous distribution of germanium through the cross-section of the composite films. Transmission electron microscopy (TEM) investigation of the samples revealed that germanium forms clusters with a maximum diameter of 2 nm embedded in C:H and GeC alloy matrix. An optical gap ranging from 1.9 to 1.0 eV with corresponding refractive index ranging from 2 to 3 were determined. DC electrical properties were measured in the planar electrodes -composite film-configuration. The electrical conduction is strongly dependent on the germanium content and on the substrate temperature. Current-voltage characteristics are linear at low electrical field and become superlinear at higher field.

Long-term aging of Ag/a-C:H:O nanocomposite coatings in air and in aqueous environment

Abstract Nanocomposite coatings of silver particles embedded in a plasma polymer matrix possess interesting properties depending on their microstructure. The film microstructure is affected among others also by the RF power supplied during the deposition, as shown by transmission electron microscopy. The optical properties are characterized by UV–vis–NIR spectroscopy. An anomalous optical absorption peak from the Ag nanoparticles is observed and related to the microstructure of the nanocomposite films. Furthermore, a long-term aging of the coatings is studied in-depth in ambient air and in aqueous environments. It is shown that the studied films are not entirely stable. The deposition conditions and the microstructure of the films affect the processes taking place during their aging in both environments.

TEM investigations on the structure of antiphase boundaries in D03 ordered Fe3Al

Abstract Antiphase boundaries (APBs) in D0 3 long-range ordered Fe 3 Al were studied by transmission electron microscopy. It was found that the APBs gave rise to a diffraction contrast not only in superlattice reflections but also in fundamental ones. This indicates that the total fault displacement R across the APBs is not exactly equal to the basic displacement R 0 but involves an additional one r so that R = R 0 + r. The basic displacement R 0 is either R 0 = 1/4(111) for the nearest-neighbor APBs or R 0 = 1/2(111) for the next nearest-neighbor APBs. R 0 was responsible for the normal, strong APB contrast when imaged with superlattice reflections. Anomalous APB contrast imaged with fundamental reflections can be attributed to the additional non-crystallographic displacement r. The APB contrast caused by r was generally weak and asymmetric in the dark field image. While the nearest-neighbor APBs were clearly visible when imaged with fundamental reflections, the next nearest-neighbor APBs gave no observable anomalous contrast. The additional displacement r is therefore dependent on R 0 . Its magnitude is smaller for R 0 = 1 /2(111) than for R 0 = 1 /4(111). Furthermore, the direction of the additional displacement r across the nearest-neighbor APB varies with the orientation of the domain boundaries. The main component of the fault vector r was determined to be perpendicular to the fault plane. The nearest-neighbor APB is of extrinsic character. The lattice near the APB is dilated.