E. Jezierska

Application of the shock compaction technique for consolidation of hard magnetic powders

A shock compaction technique has been successfully applied for the consolidation of hard magnetic powders without appreciable changes in the phase structure and grain size. Pr–Fe–B and Sm–Fe–N materials, prepared with the application of melt spinning and mechanical alloying, respectively, were shock compacted with a shock wave speed of 1200 m/s and a pressure of 4.4 GPa. We found that the coercivity of high RE containing material, for which the magnetic hardness is controlled by good magnetic separation of magnetic grains by a RE-rich paramagnetic phase, decreases slightly, by ∼5 %, due to the closer proximity of the crystallites in a dense material. For low RE, exchange coupled materials, the densification does not change the coercivity.

Hydrogen surface effects in ferritic stainless steels

Abstract The surface microstructure of ferritic chromium stainless steels subjected to hydrogen charging was investigated. Mono- and polycrystalline samples after severe cathodic hydrogen charging were examined using optical, scanning and transmission electron microscopy. The H-induced multiple twinning effect was observed, which manifested itself in the formation of grain oriented needles with a pronounced surface relief, similar to those appearing in the quasi-martensitic transformation. This was accompanied by a heavy increase in the dislocation density and microhardness, intensification of microcrack formation, a strong refinement of the ferrite grains and a radiation-like damage to the structure of the surface layer several microns thick. These results indicate that the surface zone is much more saturated with hydrogen, which is “implanted” into the steel during cathodic charging, than the bulk.

Studies of the relationship between the microstructure and anisotropy of the plastic properties of industrial aluminum-lithium alloys

Al-Li alloys are one of the groups of materials used in the modern aircraft industry. Their mechanical properties make them very attractive for this application. A considerable plastic deformation during the technological operation results in the development of a significant crystallographic texture in the Al-Li alloys. In this paper we show the results of studies on the anisotropy of the properties of the 8090 alloy with attention paid to precipitates and the anisotropy of all the pertinent elements in the materials microstructure.

Influence of Milling Media on Mechanically Exfoliated MoS2

Wet ball milling was used to exfoliate MoS2. The aim of the study was to examine how the surface energy of all the individual materials affected the formation of the nanosheets. Two types of milling balls were selected: one made of steel and one made of Al2O3. The liquids used were water (H2O), isopropyl alcohol (C3H8O) and kerosene. The obtained nanosheets were identified and characterized using transmission/scanning electron microscopy and electron diffraction. Following sedimentation, scattered light intensity was measured. Our experiments showed that the material of the milling balls played a significant role in the experiment and had an influence on the number of the nanosheets obtained. Irrespective of the liquid employed, the number of nanosheets in the suspension obtained by milling with Al2O3 balls was greater by 100% than those obtained via milling with steel balls.

Improvement of the Properties of Hard Magnetic NdFeb/Fe Nanocomposites by Minor Addition of Refractory Metals

The effect of the addition of selected refractory metals on structure and magnetic properties was studied for the nanocomposite Nd9Fe77−x B14M x (M=Ti, Mo, Nb) system, consisting of hard magnetic Nd2Fe14B phase, and soft magnetic phases (Fe, Fe3B). It was found that 2–5 at % of Ti, Mo or Nb addition leads to a substantial increase of the coercivity and maximum energy product of the nanocomposite magnets maintaining the remanence unchanged. The highest properties were obtained for the alloys containing 4–5 at% of the refractory metals. The hysteresis loops for the alloys containing additional elements are smooth and characteristic of single phase alloys. The initial magnetization curve indicates the change of the coercivity mechanisms giving rise for pinning of domain walls, which is caused by the reduction of the crystallite size.

CBED and LACBED: characterization of antiphase boundaries

Convergent-beam electron diffraction (CBED) and large-angle convergent-beam electron diffraction (LACBED) techniques are well adapted to the characterization of several types of crystal defects. In fact, dislocations, grain boundaries and stacking faults have already been successfully characterized with these methods. In the present paper, we describe the CBED and LACBED characterization of another type of crystal defect showing a special interest in materials science: antiphase boundaries (APBs). The first part of the paper is devoted to the determination of the effects of antiphase boundaries on CBED and LACBED patterns that could be expected from a theoretical point of view. It indicates that the superlattice excess lines present on these patterns are split into two lines with equal intensity when the incident beam is located on an APB. In the second part, we experimentally test these theoretical predictions on a specimen showing two different known types of antiphase boundaries. In a third part we indicate how these methods could be used to identify unknown APBs in a specimen. Finally, the advantages and disadvantages of both methods for the characterization of antiphase boundaries are discussed.

Identification of Phases in Alloy Steels after Quenching and after Isothermal Quenching

The purpose of the work is the analysis and the identification of phases in alloy steels after various heat treatments by means of transmission electron microscopy. We have investigated the phases formed during austenitising followed by quenching with various rates or by isothermal quenching, as martensite, upper and lower bainite. In order to identify the phases, we used their morphological characteristics and the kind of orientation relationship between the given phase and the austenite. We concluded that the identification of phases occurring in steels after various heat treatments may be performed in a clear manner according to their morphological features combined with the diffraction patterns analysis as observed by transmission electron microscopy.

Antiphase boundaries in Ni3Al ordered intermetallic—application of the CBED method

Abstract The antiphase domain structure in Ni 3 Al ordered intermetallics with perfect L1 2 superstructure has been examined. Transmission electron microscopy (TEM) investigations were performed using Jeol JEM 3010 and Philips CM30 microscopes. Conventional TEM studies, convergent-beam electron diffraction (CBED) and high-resolution electron microscopy (HRTEM) were used to elucidate the structure of the alloys. For advanced studies of the nature of antiphase boundaries, the large-angle convergent-beam electron diffraction (LACBED) method was employed. The recognition between symmetry-related and displacive domains can be performed with high accuracy using LACBED and dark-field images, even for nanosized domains.

TEM study of inhomogeneities in Nd-Fe-B sintered magnets

Nd-Fe-B sintered magnets with a nominal composition Nd/sub 18.8/Fe/sub 73.6/B/sub 6.9/Al/sub 0.7/ and Nd/sub 19/Fe/sub 69.8/B/sub 6.4/Al/sub 4.5/ were prepared by conventional powder metallurgy processes and post-sintering heat treatment. Microstructure studies were performed by transmission electron microscopy (TEM) using Philips EM300 and Hitachi (200 kV). Specimens for TEM were prepared by electropolishing and ion thinning. The structure of these magnets contains following inhomogeneities nanocrystalline phase on grain boundaries and interfaces, grain boundaries without isolation of the Nd-rich phase, interface irregularities or anomalous strain contrast, low angle boundaries and coincidence boundaries between Nd/sub 2/Fe/sub 14/B phase and coherent interfaces. It was observed that higher Al concentration improves magnetic properties by allowing grain boundary phase to form equal thickness layer without stress concentration on the interface. >

HRTEM and LACBED of Zigzag Boundaries in GaN Epilayers

Defects recognition in GaN epilayers was performed using HRTEM and LACBED images. Edge type dislocations, basal plane and prismatic stacking faults were determined from HRTEM analysis. Stacking mismatch boundaries on zigzag steps were found and examined using LACBED patterns in bright and dark field. For stacking faults Bragg lines split into a main and a subsidiary line. The fault plane and displacement vector can be identified from trace analysis performed on LACBED patterns.