Aleksandra Czyrska-Filemonowicz

The Samson phase, β-Mg2Al3, revisited

Co-Authors: Michael Feuerbacher, Carsten Thomas, Julien P. A. Makongo, Stefan Hoffmann, Wilder Carrillo-Cabrera, Raul Cardoso, Yuri Grin, Guido Kreiner, Jean-Marc Joubert, Thomas Schenk, Joseph Gastaldi, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Bernard Billia, Patricia Donnadieu, Aleksandra Czyrska-Filemonowicz, Anna Zielinska-Lipiec, Beata Dubiel, Thomas Weber, Philippe Schaub, Günter Krauss, Volker Gramlich, Jeppe Christensen, Sven Lidin, Daniel Fredrickson, Marek Mihalkovic, Wieslawa Sikora, Janusz Malinowski, Stefan Brühne, Thomas Proffen, Wolf Assmus, Marc de Boissieu, Francoise Bley, Jean-Luis Chemin, Jürgen Schreuer Abstract. The Al−Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg 38.5 Al 61.5, this cubic phase, space group Fd-3m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214 °C a first-order phase transition to rhombohedral β′-Mg2Al3(hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd-3m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400 °C. It essentially agrees with Samsons model, even in most of the many partially occupied and split positions. The structure of β′-Mg2Al3is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.

Dislocation dynamics in the oxide dispersion strengthened alloy INCOLOY MA956

In situ straining experiments in a high-voltage electron microscope have been performed on the oxide dispersion strengthened alloy INCOLOY MA 956 at room temperature and between 640 and 1010°C to study the dynamic behaviour of dislocations. Macroscopic compression experiments including stress relaxation tests have been carried out between room temperature and 900°C. The dynamic behaviour of dislocations, the flow stress and its strain rate sensitivity are discussed in the different temperature ranges in terms of long-range dislocation interactions, the Orowan mechanism, the model of the thermally activated detachment of dislocations from oxide particles, solution hardening and a diffusional point defect drag. It is concluded that the latter controls the thermally activated dislocation processes in a wide temperature range around about 700°C. The detachment model may be active only above 900°C.

Stereological estimation of microstructural parameters of nickel-based superalloy Waspaloy using TEM methods

Abstract The nickel-based superalloy Waspaloy, which is precipitation strengthened by coherent γ′ particles, was investigated using different transmission electron microscopy (TEM) imaging conditions in the as-received condition and after long time exposure at 650°C and 750°C. Scanning–transmission electron microscopy (STEM) bright-field images, conventional TEM dark-field images and convergent-beam electron diffraction (CBED) patterns have been taken for a stereological evaluation of the particle size distributions, particle density and volume fraction of primary and secondary coherent γ′ precipitates.

Effect of rhenium addition on the strengthening of chromium–alumina composite materials

Abstract Chromium–alumina composites are well known for their good mechanical properties in comparison to pure ceramics or metals. These composites are characterized by high hardness and high mechanical strength. The aim of the present work was to improve the properties of chromium–alumina composites even more and expand the range of their possible applications by addition of rhenium. To achieve this goal, chromium–alumina composites containing 2 and 5 vol.% of rhenium were produced via powder metallurgy. The microstructural characterization of the processed material was performed using light microscopy, scanning and transmission electron microscopy as well as X-ray diffraction analysis. Measurement of selected properties such as Youngs modulus, bend strength and hardness revealed an advantageous influence of rhenium additions. The results are discussed in terms of the influence of rhenium volume content on the microstructure and on the physical and mechanical properties of the chromium–alumina composites. The solid solution is only partially formed. The properties strongly depend on the amount and distribution of both aluminium oxide and rhenium content.

Microstructural characterization of nitrided Timetal 834.

The microstructure of Timetal 834, in as‐received condition and after nitriding under glow discharge has been examined by light microscopy and analytical transmission electorn microscopy (TEM) methods (SAED, EDS, EELS and EFTEM). The microstructure of the as‐received alloy consists of the α phase and a small amount of the β phase. Silicide precipitates (Zr5Si4) are present both inside the grains and at the grain boundaries. TEM investigations of cross‐sectional thin foils allow for detailed analysis of the nitrided layer microstructure. It was found that the nitrided layer exhibits a graded character with continuously varying nitrogen content. The outermost sublayer consists of nanocrystals of δ‐TiN. The following sublayers consist mainly of δ′‐Ti2N and ɛ‐Ti2N grains. The last sublayer, closest to the substrate, is identified as a nitrogen‐rich α(N) solid solution containing up to 14 at% of nitrogen.

Electrophoretic deposition and characterization of HA/chitosan nanocomposite coatings on Ti6Al7Nb alloy

Nano-hydroxyapatite/chitosan (nc-HA/chitosan) composite coatings were produced on two phase (α+β) Ti6Al7Nb titanium alloy substrates by electrophoretic deposition (EPD). The microstructure of the coatings was examined by scanning- and transmission electron microscopy methods as well as by X-ray diffractometry. The coatings, 770 nm–800 nm thick, were uniform, without any cracks or presence of large voids and they exhibited good adhesion to the titanium alloy substrate. The microstructure of the coatings consisted of nc-HA needle-like particles homogeneously embedded in a chitosan matrix. The deposited coatings exhibited good adhesion to the substrate. The best adhesion to the titanium alloy was determined for the coating deposited from suspensions containing 4 g/L of HA at 10 V during 240 s. The results confirm EPD as a convenient method to develop uniform and crack-free nanoscale organic-inorganic composite coatings on two phase titanium alloy substrates with potential application in orthopedic and dental implants.

Microstructure characterization of erosion resistant coatings on carbon‐bonded carbon fibre composites

The microstructure of as received and surface treated carbon‐bonded carbon fibre composites has been examined by light microscopy, scanning and transmission electron microscopy. The microstructure of the as received material consists of a bonded together layered carbon fiber network, identified as graphitic carbon (hexagonal close packed). To improve the erosion resistance of the carbon‐bonded carbon fibre composites composite, the SiC and silicate glass–ceramic coatings from the system SiO2–Al2O3–Y2O3 were produced on carbon‐bonded carbon fibre composites composites by a low‐cost slurry technique. Transmission electron microscopy investigations of cross‐section thin foils allowed for detailed analysis of the coatings microstructure. It was found that the SiC coating was consisting mainly of a nanocrystalline SiC (fcc). The multilayered glass–ceramic coating showed a complex microstructure consisting of an external SiO2–Al2O3–Y2O3 layer and an intermediate nanocrystalline SiC layer. The SiO2–Al2O3–Y2O3 layer was composed of SiO2 (fcc), Y2Si2O7 (op) and Al4.644Si1.357O9.68 (op).

Electrophoretic deposition of organic/inorganic composite coatings containing ZnO nanoparticles exhibiting antibacterial properties

To address one of the serious problems associated with permanent implants, namely bacterial infections, novel organic/inorganic coatings containing zinc oxide nanoparticles (nZnO) are proposed. Coatings were obtained by electrophoretic deposition (EPD) on stainless steel 316L. Different deposition conditions namely: deposition times in the range 60-300s and applied voltage in the range 5-30V as well as developing a layered coating approach were studied. Antibacterial tests against gram-positive Staphylococcus aureus and gram-negative Salmonella enteric bacteria confirmed the activity of nZnO to prevent bacterial growth. Coatings composition and morphology were analyzed by thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Moreover, the corrosion resistance was analyzed by evaluation of the polarization curves in DMEM at 37°C, and it was found that coatings containing nZnO increased the corrosion resistance compared to the bare substrate. Considering all results, the newly developed coatings represent a suitable alternative for the surface modification of metallic implants.

Alloying Behavior of Self-Assembled Noble Metal Nanoparticles

The atomic redistribution processes occurring in multiparticle nanostructures are hardly understood. To obtain a more detailed insight, we applied high-resolution microscopic, diffraction and spectroscopic characterization techniques to investigate the fine structure and elemental distribution of various bimetallic aerogels with 1:1 compositions, prepared by self-assembly of single monometallic nanoparticles. The system Au-Ag exhibited a complete alloy formation, whereas Pt-Pd aerogels formed a Pd-based network with embedded Pt particles. The assembly of Au and Pd nanoparticles resulted in a Pd-shell formation around the Au particles. This work confirms that bimetallic aerogels are subject to reorganization processes during their gel formation.

Microstructure and tribological properties of low-friction composite MoS2(Ti,W) coating on the oxygen hardened Ti-6Al-4V alloy

Duplex surface treatment, which combines the oxygen diffusion hardening with a deposition of low friction MoS2(Ti,W) coating, was applied to improve the Ti-6Al-4V alloy load bearing capacity and tribological properties. The coating (3.1 μm thick) was deposited on the oxygen hardened alloy by magnetron sputtering. Microstructure characterisation was performed by scanning- and transmission electron microscopy methods, as well as X-ray diffractometry. The results of micro/nanostructural analyses performed by high-resolution transmission electron microscopy showed that the coatings are composed of MoS2 nanoclusters embedded in an amorphous matrix. Some Ti α, W, and Ti2S nanocrystals were also found in the coating microstructure. The wear resistance and friction coefficient of the hardened oxygen, as well as the coated alloy, was investigated at room temperature (RT), 300 °C, and 350 °C. The presence of the MoS2(Ti,W) coating decreases the friction coefficient from 0.85 for the oxygen hardened alloy to 0.15 (at RT) and 0.09 (at 300 °C and 350 °C) for the coated one. The coating essentially increases the wear resistance of the alloy at RT and 300 °C. It was found that the wear resistance of the coated alloy decreased significantly during the wear test performed at 350 °C.