P. Donnadieu

Hardening precipitation in a Mg–4Y–3RE alloy

Abstract Early stages of precipitation in a Mg–Y–Nd based alloy aged at 150 °C have been studied using TEM and small angle X-ray scattering (SAXS). The former brings information concerning nature, morphology and size of precipitates, and the latter adds qualitative and quantitative information concerning populations of precipitates in terms of size and volume fraction. Precipitation at 150 °C involves formation of DO19 monoplanar precipitates, which further develop into the β″ and β′ phases having platelet and globular morphologies, respectively. TEM observations on samples aged at 150 °C reveal the formation mechanism of the bco-β′ structure by the ordering of monoplanar DO19-β″ precipitates. Additional examinations at 250 °C revealed the DO19-β″→bco-β′ transformation, as well as β1 precipitates. Estimation of the volume fraction deduced from SAXS is discussed on the basis of the TEM results.

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.

Experimental investigation of the MgAl phase diagram from 47 to 63 at.% Al

The purpose of this work was to investigate the MgAl phase diagram in the composition range from 47 to 63 at.% Al which has not previously been consistently determined. The rhombohedral e phase n ear 56 at.% Al is found to form at 410°C following a peritectoid reaction (γ + β → e) and not a peritectic reaction as reported by Schumann and Voss (Giessereiforschung. 33 (1981) 43). The respective homogeneity ranges of the γ phase in the Al-rich part and of the β phase have been determined. Besides the γ, β and e phases, the existence of a high temperature phase (named λ) between 435 and 445°C is suggested. The existence of the ξ phase and of the relevant invariant reactions reported by Schumann and Voss are not confirmed by the present results.

A small‐angle neutron scattering study of fine‐scale NbC precipitation kinetics in the α‐Fe–Nb–C system

The fine-scale precipitation of NbC in ferrite has been quantitatively characterized in the temperature range 873-1073 K for two alloy compositions, containing respectively 800 p.p.m. Nb and 400 p.p.m. Nb (by weight). Transmission electron microscopy (TEM) has revealed that the precipitates are located on dislocations, and have a plate-like morphology with an average aspect ratio between 2 and 3. Small-angle neutron scattering (SANS) has been systematically used to determine the precipitation kinetics. The validity of the quantitative SANS measurements of size and volume fraction has been assessed by TEM image analysis and chemical dissolution experiments. The precipitation kinetics is observed to depend strongly on temperature but to be similar for the two alloy compositions. From the measurements, it is inferred that precipitate nucleation is extremely rapid, in relation to the nature of the nucleation sites. A time-temperature transformation diagram is built from the kinetic data, showing a maximum reaction rate between 973 and 1073 K.

Early stages of precipitation and microstructure control in Mg–rare earth alloys

Hardening precipitation frequently occurs in Mg–rare earth (RE) alloys after heat treatment in the 150–200°C range. Early stages of precipitation have been studied in detail by transmission electron microscopy in two Mg–RE alloys (Mg–Y–Gd and Mg–Y–Nd). Two types of structures may be involved in the precipitation sequence: a DO19 phase and the so-called orthorhombic β′ phase. The structural relationship between DO19 and β′ phases has been established in underaged and overaged states from the observations at peak ageing. We show that the earliest precipitates play a key role in the selection of phases developing in overaged states. Depending on the habit plane of the precipitates present in the early states, either the DO19 or the β′ phase will grow in further ageing. The Mg–Y–Gd and Mg–Y–Nd alloys illustrate the different microstructures resulting from such selection. Due to the selective growth of the β′ phase, the Mg–Y–Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.

TEM study of NbC heterogeneous precipitation in ferrite

The heterogeneous precipitation of NbC in ferrite has been quantitatively characterized by transmission electron microscopy in a Fe–C–Nb model alloy for different isothermal heat treatments. The elongation and size distribution of precipitates were derived using dark field imaging. For each precipitation state, the precipitation of NbC occurs on dislocations due to the as-quenched state. This precipitation mechanism leads to characteristic arrays of precipitates in which precipitates grow in a self-similar manner. A detailed study of these arrays has shown that most dislocations decorated by these arrays are edge dislocations with ⟨112⟩ type line vectors. There is only one variant on a given dislocation. This selection can be interpreted by the interaction between dislocation and precipitate strain fields.

Morphology, crystallography and defects of the intermetallic χ-phase precipitated in a duplex (δ + γ) stainless steel

AbstractThe ferritic matrix in the Fe-22Cr-5Ni-3Mo-0.03C ferritic-austenic duplex stainless steel undergoes a variety of decomposition processes when aged in the temperature range 650–750°C. These processes involve the precipitation of the austenite and of the σ and χ Frank-Kasper phases. The intermetallic χ-phase is found at both the grains boundaries (homo and heterophase interfaces) and inside the ferritic grains where it adopts an unexpected hexagonal shape. At the early stage of its precipitation, it nucleates at the δ/γ and δ/σheterophase interfaces and then grows by expanding exclusively in the ferritic matrix. This study is basically focused on this intermetallic χ-phase. The crystal structure and the chemical composition are respectively studied by electron diffraction and energy dispersive X-ray spectroscopy. The χ-phase exhibits rational orientation relationships with the austenite and the σ-phase with which it is in contact and an invariably cube-on-cube orientation relationship with the ferritic matrix into which it grows. Based on the orientation relationship, the morphology and the number of variants of this χ-phase are understood in terms of the group theory. The planar defects present in a large density in the χ-phase, are roughly parallel to {0 1 1}χ//{0 1 1}δ. The fault vectors are determined as:

Microstructural and analytical study of heavily faulted Frank-Kasper R-phase precipitates in the ferrite of a duplex stainless steel

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