Anna Zielińska-Lipiec

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.

Influence of Cold Plastic Deformation on the Development of the Texture in High-Manganese Austenitic Steel

The research was carried out on 26Mn-3Si-3Al steel having a low value of SFE. After casting, steel was forged and hot-rolled at the temperatures within the range of from 1200°C to 900. Next, the steel was cold rolled until the deformation of 66% was obtained. Upon the basis of X-ray examination and the observations of microstructures, it was found out that during cold rolling mechanical twinning and the martensitic transformation of austenite in hcp-ε martensite occurred. The crystallographic texture of the γ phase (austenite) and ε phase (martensite) was also examined.

Cracking of High-Strength Steel Welded Joints

ABSTRACT Fracture evaluation of welded joints in high-strength steels, with bainitic and martensitic structures, is presented and cracking mechanisms discussed. Hot cracks or microcracks formed during welding are further expanded as cold cracks on cooling. The cause of cracking is shown to be low temperature of weld solidification and deformation-induced contraction. Hydrogen can also be an important factor in this cracking.

Microstructure and Properties of a Repair Weld in a Nickel Based Superalloy Gas Turbine Component

Abstract The aim of the present study was to characterize the repair weld of serviced (aged) solid-solution Ni-Cr-Fe-Mo alloy: Hastelloy X. The repair welding of a gas turbine part was carried out using Gas Tungsten Arc Welding (GTAW), the same process as for new parts. Light microscopy, scanning electron microscopy, transmission electron microscopy, microhardness measurements were the techniques used to determine the post repair condition of the alloy. Compared to the solution state, an increased amount of M6C carbide was detected, but M23C6 carbides, sigma and mu phases were not. The aged condition corresponds to higher hardness, but without brittle regions that could initiate cracking.

The microstructure of melt-spun alloys with liquid miscibility gap

Effect of the melt ejection temperature on the microstructures of (FexCu1-x)62Si13B9Al8Ni6Y2 (x=0.48, 0.60 and 0.71) alloys was investigated. Rapid cooling of the examined alloys from the temperature range conventional to single-phase amorphous alloys brought about a non-uniform microstructure due to a liquid/liquid phase separation prior to cooling. The systems with a miscibility gap need to be heated over a critical temperature where homogeneous liquid exists. Microstructures of the ribbons melt spun from the homogeneous melt region are characterized by presence of the spherical precipitates distributed in homogeneous matrix. Both, precipitates and matrix, can constitute either the Fe-rich or the Cu-rich phases depending on the alloy composition. The studies confirmed amorphous nature of the Fe-rich phases, both matrix and precipitates whereas the Cu-rich liquid crystallized due to lower glass forming ability.

Microstructural evolution in deformed duplex stainless steels

Abstract The present research is part of a project concerning texture and microstructure evolution upon deformation and annealing in a series of various duplex stainless steels. This article shows a comparison of deformation behavior in two grades of duplex steels subjected to cold-rolling over a wide range of reductions. Both steels under examination showed clear differences in chemical composition and their preliminary treatment included different processes of hot plastic working. Optical microscopy was used to examine changes in morphology and formation of a ferrite–austenite band-like structure upon cold-rolling. Transmission electron microscopy was applied to analyze microstructural evolution and deformation behavior within the ferrite and austenite bands. Additional research included texture measurements for both component phases. Special attention was paid to the ferrite–austenite co-deformation and the structural effects resulting from strain localization at high rolling reductions.

Structural Aspects of Ferrite and Austenite Co-Deformation in Duplex Stainless Steel

The present research is a part of project dealing with structural aspects of ferrite and austenite co-deformation in duplex stainless steels. The examination concerned a development of ferrite and austenite microstructures, major deformation mechanisms operating in both phases and texture formation upon cold-rolling of a model duplex type steel. The investigations showed that the band-like morphology of two-phase structure formed upon processing together with specific starting textures obtained after a preliminary thermo-mechanical treatment exerted significant influence on texture and microstructure development in both constituent phases. Microstructure and texture evolution in examined duplex steel significantly differed from those in one-phase steels. These differences resulted first of all from the role of the phase boundaries acting as the barriers for dislocation movement and affecting the processes of strain localization at higher deformations.

TEM Investigations of the Microstructure of 7CrMoVTiB10-10 Steel Weld Metal

7CrMoVTiB10-10 (T24) steel is recommended for fabrication of boiler components such as water walls, water panels or headers without post weld heat treatment (PWHT). The chemical composition of this steel and cooling conditions after welding influence the susceptibility to cracking in weld metal during and after welding. TEM investigations showed that in the as-welded microstructure mixtures of bainite and martensite were dominating. This has an important implication for technological properties.

VM12 Steel for Advanced Power Generation Plants – Metrology of the Precipitates by Electron Microscopy

A new high chromium martensitic steel, VM12, was recently developed for advanced coal-fired ultra-supercritical steam power plants. A very important factor in maintaining high creep resistance during service exposure is its stable microstructure, in particular high dispersion particle strengthening, which slow down the recovery and softening processes of the matrix. Quantitative characterisation of the precipitates in the VM12 steel as received condition and after creep tests up to about 30000 h at 625 °C using TEM was carried out.

Morphological Evolutions in Steels during Continuous Rapid Heating

The influence of high energy density sources on morphological changes in steels was studied by a physical simulation. Strips of carbon steels were subjected to heat cycles including continuous rapid heating to temperatures between 400 and 1200°C and immediate water quenching. The heat cycles were carried out by passing a high intensity electrical current through trapezoidal specimens in a special device allowing to obtain heating rates up to 10000°/s with an excellent temperature control. Real-time temperature recordings were drawn so as to define some characteristic temperatures of continuous austenitization. The evolution of the morphology, initially composed of ferrite and martensite, was examined by means of light and electron (SEM and TEM) microscopy using standard techniques. The results of the examination were related to microhardness measurements. Three distinct stages of the morphological evolution were finally analyzed: - T > AC1 - Beginning of austenite formation in zones of as-tempered martensite, mostly at former austenite grain boundaries. - AC1 << T < AC3 - Development of irregular lath-shaped interface between the growing (austenite) and shrinking (ferrite) phases. - T > AC3 - Final massive transformation of supersaturated ferrite areas.