Beata Dubiel

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.

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.

Influence of polyetheretherketone coatings on the Ti–13Nb–13Zr titanium alloy's bio-tribological properties and corrosion resistance

Polyetheretherketone (PEEK) coatings of 70-90μm thick were electrophoretically deposited from a suspension of PEEK powder in ethanol on near-β Ti-13Nb-13Zr titanium alloy. In order to produce good quality coatings, the composition of the suspension (pH) and optimized deposition parameters (applied voltage and time) were experimentally selected. The as-deposited coatings exhibited the uniform distribution of PEEK powders on the substrate. The subsequent annealing at a temperature above the PEEK melting point enabled homogeneous, semi-crystalline coatings with spherulitic morphology to be produced. A micro-scratch test showed that the coatings exhibited very good adhesion to the titanium alloy substrate. Coating delamination was not observed even up to a maximal load of 30N. The PEEK coatings significantly improved the tribological properties of the Ti-13Nb-13Zr alloy. The coefficient of friction was reduced from 0.55 for an uncoated alloy to 0.40 and 0.12 for a coated alloy in a dry sliding and sliding in Ringers solution, respectively. The PEEK coatings exhibited excellent wear resistance in both contact conditions. Their wear rate was more than 200 times smaller compared with the wear rate of the uncoated Ti-13Nb-13Zr alloy. The obtained results indicate that electrophoretically deposited PEEK coatings on the near-β titanium alloy exhibit very useful properties for their prospective tribological applications in medicine.

TEM, HRTEM, electron holography and electron tomography studies of γ′ and γ″ nanoparticles in Inconel 718 superalloy

The aim of the study was the identification of γ′ and γ″ strengthening precipitates in a commercial nickel‐base superalloy Inconel 718 (Ni‐19Fe‐18Cr‐5Nb‐3Mo‐1Ti‐0.5Al‐0.04C, wt %) using TEM dark‐field, HRTEM, electron holography and electron tomography imaging. To identify γ′ and γ″ nanoparticles unambiguously, a systematic analysis of experimental and theoretical diffraction patterns were performed. Using HRTEM method it was possible to analyse small areas of precipitates appearance. Electron holography and electron tomography techniques show new possibilities of visualization of γ′ and γ″ nanoparticles. The analysis by means of different complementary TEM methods showed that γ″ particles exhibit a shape of thin plates, while γ′ phase precipitates are almost spherical.

Real-Time Atomic Scale Imaging of Nanostructural Evolution in Aluminum Alloys

We present a new approach to study the three-dimensional compositional and structural evolution of metal alloys during heat treatments such as commonly used for improving overall material properties. It relies on in situ heating in a high-resolution scanning transmission electron microscope (STEM). The approach is demonstrated using a commercial Al alloy AA2024 at 100-240 °C, showing in unparalleled detail where and how precipitates nucleate, grow, or dissolve. The observed size evolution of individual precipitates enables a separation between nucleation and growth phenomena, necessary for the development of refined growth models. We conclude that the in situ heating STEM approach opens a route to a much faster determination of the interplay between local compositions, heat treatments, microstructure, and mechanical properties of new alloys.

The 3D Imaging and Metrology of CMSX-4 Superalloy Microstructure Using FIB-SEM Tomography Method

STEM-EDX and FIB-SEM tomography studies have been carried out to visualize three-dimensional morphology of the γ’ precipitates in different zones of ex-service turbine blade made of CMSX-4 single crystal superalloy. The results allowed the three dimensional analysis of the changes in microstructure of blade as resulting of operating conditions. Tomographic reconstructions provided quantitative data about γ and γ’ phase shape, size and volume fraction. It was shown that FIB-SEM tomography technique is suitable for 3D reconstruction of the objects of 100 nm in size or even smaller and thus enables the accurate quantitative microstructural analysis of this superalloy.

Analytical TEM and SEM characterisation of aluminide coatings on nickel based superalloy CMSX-4

Abstract Aluminide coatings are applied on gas turbine components to improve oxidation and corrosion resistance or as bond coats for thermal barrier coatings (TBC). In the present study, coatings produced on a CMSX-4 superalloy by pack gas phase aluminising and subsequently annealed at 1050°C for 2 h were investigated. Microstructure, morphology and phase composition of the coating were characterised using light microscopy, X-ray diffraction, scanning electron microscopy and analytical transmission electron microscopy (TEM). Scanning electron microscopy (SEM) investigations of the 39 µm thick coating revealed a complex multilayered microstructure produced by diffusion at 1050°C at the surface of CMSX-4 alloy. Detailed TEM phase analysis was performed on cross-section thin foils by electron diffraction and energy dispersive X-ray spectrometry (EDS). The outer zone (1) consisted of β-NiAl phase. The interdifusion zone (2) contained two sublayers consisting of a Ni5Al3 matrix with precipitates of topologically close packed phases, most probably σ and µ. Close to the surface of the CMSX-4 superalloy, a further zone (3) consisting of γ+γ′ with large precipitates (possibly µ and P phases) was observed.

Characterization of the μ and P phase precipitates in the CMSX‐4 single crystal superalloy

A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning‐transmission electron microscopy (STEM) using high‐angle annular‐dark‐field (HAADF) imaging, focussed ion beam‐ scanning electron microscopy (FIB‐SEM) tomography, selected area electron diffraction with beam precession (PED), as well as spatially resolved energy‐dispersive X‐ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), was used to investigate topologically close‐packed (TCP) phases, occurring in the CMSX‐4 superalloy subjected to high temperature annealing and creep deformation. Structural and chemical analyses were performed to identify the TCP phases and provide information concerning the compositional partitioning of elements between them. The results of SEM and FIB‐SEM tomography revealed the presence of merged TCP particles, which were identified by TEM and PED analysis as coprecipitates of the μ and P phases. Inside the TCP particles that were several micrometres in size, platelets of alternating μ and P phases of nanometric width were found. The combination of STEM‐HAADF imaging with spatially resolved EDS and EELS microanalysis allowed determination of the significant partitioning of the constituent elements between the μ and P phases.

Microstructural changes during creep of CMSX‐4 single crystal Ni base superalloy at 750°C

TEM studies of creep tested CMSX‐4 nickel‐base single crystal superalloy were performed to analyse a microstructure evolution during creep at temperature 750°C, and uniaxial tensile stress of 675 MPa. Microstructural analyses were focused mainly on examination of dislocation configurations during primary and secondary creep stages of high temperature deformation. At such low temperature and high stress creep deformation proceed by cutting of γ′ particles by dislocations. It was found that primary creep is initiated by movement of dislocations with Burgers vector a/2 <110> in the γ phase. The second type of dislocations active at primary creep stage are extended dislocation ribbons with overall a<112> Burgers vector, separated by superlattice stacking faults, cutting both the γ and γ′ phases. The movement of the dislocation ribbons is inhibited at secondary creep stage by dislocation networks formed at γ−γ′ interfaces.