A. Pisch

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.

Development of Mg-Sc-Mn alloys

Abstract Scandium is a potential alloying element for improving the high temperature properties (above 300°C) of magnesium alloys. Scandium ( T m =1541°C) increases the melting point of the magnesium solid solution, diffuses slowly in magnesium and has a lower density than other potential alloying additions (3 g cm −3 ). Based on thermodynamic equilibrium calculations the alloys MgSc6Mn1 and MgSc15Mn1 were prepared using the squeeze casting technique. The alloys showed a strong annealing response due to the formation of Mn 2 Sc precipitates. The low diffusivity of the alloying elements retards the overageing process. The existence of the Mn 2 Sc precipitates was confirmed by X-ray diffraction and energy dispersive X-ray microanalysis. The ultimate tensile strength of the new alloys was somewhat lower than the corresponding values for the high creep resistant conventional alloy WE43 (Mg–4.0wt.% Y–1.0wt.% Heavy Rare Earths–2.25wt.% Nd–0.5wt.% Zr) whereas the tensile yield strength was comparable to WE43 at higher Sc content. The alloys showed a low elongation to fracture due to a strongly localised plastic deformation. Creep tests of the material in the as cast condition resulted in secondary creep rates which were comparable to WE43 at higher stresses but significantly lower at lower stresses. A T5 heat treatment of the new alloys led to creep rates which were up to two orders of magnitude lower than for WE43. Hence it is possible to increase the operating temperature by 50°C for the new alloys.

State of the art in the modelling of SiC sublimation growth

Different computational tools have helped to provide additional information on the sublimation growth of SiC single crystals by the modified-Lely method. The modelling work was motivated by the need of a better control of the local temperature field inside the crucible. Because there is an environment of strong thermal radiation in which the SiC boule growth process occurs, heat transfer must therefore be coupled with gaseous species transport and reactivity. This highly coupled model must take into account all geometric modifications in crucibles which strongly influences the crystal growth process. Local thermochemical equilibrium linked to heat and mass transfer is the model proposed in this paper to give the magnitude of the growth rate and the shape of the crystal. This modelling field is still too young to propose a software package including all modelling aspects and a reliable material database. However, some parts of the modelling work have reached maturity like electromagnetic heating and thermal modelling coupled with simplified chemical models. We show in this paper selected examples in order to demonstrate the types of information which can be routinely available by simulation and how to approach defect formation from a macroscopic point of view. Minor geometric modifications of the holes for pyrometric measurements drastically change the magnitude of thermal gradients in the crucible. Geometric modifications of the crucible change the computed crystal shapes. The calculated results complete the experimental knowledge by a quantification of the local macroscopic fields.

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.

First-principles structural stability in the strontium–titanium–oxygen system

First principles calculations have been made to theoretically interpret the structural stability of SrTiO3 as a function of pressure or strain. We show that the orthorhombic Pbnm structure is more stable than the ideal perovskite one due to rotations of TiO6 octahedrons at high pressures or deformations of TiO6 octahedrons at low pressures. Such a study is extended to the structural stability of the Ruddlesden–Popper Sr n +1Ti n O3 n +1 phases for n = 1, 2 and 3, in which the lattice constraints are induced by the intercalation of SrO layers in the perovskite structure.

Standard enthalpies of formation of some Mn–Y and Mn–Sc intermetallic compounds

Abstract The standard enthalpies of formation of Mn2Y, Mn23Y6 and Mn2Sc have been determined by aluminium drop solution calorimetry in an isothermal double cell Tian–Calvet high temperature microcalorimeter at 1123 K. The heats of dissolution of the elements and the intermetallic compounds have been measured at infinite dilution in liquid aluminium. Mn, Y and the three intermetallic compounds dissolved rapidly. It was not possible to determine the value for Sc directly due to the low solubility of this element in Al, which led to a precipitation of Al3Sc without further dissolution. The value has therefore been determined indirectly by the dissolution of two intermetallic compounds — Sc5Ge3 and Al2Sc — for which the enthalpies of formation are known from the literature.

Thermodynamic optimization of the systems Mn–Gd and Mn–Y using new experimental results

Abstract The literature concerning binary systems of manganese and rare earth metals is very scarce. Thermodynamic optimizations using the Calphad method are only available for the Mn–Y system [Flandorfer et al., Z. Metallkd. 88 (1997) 529–538]. In this work, thermodynamic optimizations for the binary systems Gd–Mn and Mn–Y are investigated. New experimental data for the enthalpies of formation of the binary Mn–Y phases were considered. The resulting enthalpies fit in the trend of stabilities within the series of RE–Mn systems described by Saccone et al. [Z. Metallkd. 84 (1993) 563–568].

HTS thin films by innovative MOCVD processes

Abstract The successful achievement of high temperature superconductor (HTS) YBa 2 Cu 3 O 7 films by MOCVD is not straightforward and depends critically on the volatility and stability of the precursor materials which must be transformed into the product film. In the present study, new deposition processes based on mixed liquid sources are described, where the unstable precursor is preserved from thermal decomposition and where the film growth rates can be considerably improved. For the synthesis of YBa 2 Cu 3 O 7 , these processes, named aerosol-assisted MOCVD, injection MOCVD and band-injection MOCVD, use a single solution source of Y, Ba and Cu β-diketonates dissolved in suitable organic solvents. Experimental details of these new processes are described and the effects of different process parameters are studied in relation with the quality of the deposited layers. Thin and thick films of YBa 2 Cu 3 O 7 and buffer layers (CeO 2 , SrTiO 3 ,……) have been grown using these processes. The superconducting properties of the high quality films obtained ( T c = 90 K, J c > 10 6 A cm −2 at 77 K) will be discussed in view of their use for large scale applications.

Application of computational thermochemistry to Al and Mg alloy processing with Sc additions

Abstract Thermochemical calculations have been used to discuss various aspects of AlMgSc alloy processing: alloying of scandium, alloy melting, solidification and heat treatment. The feasibility of replacing expensive scandium by cheap scandium oxide for alloying into liquid Al is supported by these quantitative data, whereas the alloying into liquid Mg is found to be unfeasible. Calculations of ternary AlMgSc phase equilibria are based on an experimentally supported thermodynamic data set. Liquidus surfaces of both the Al and Mg rich corners depict the narrow ranges for complete melting of alloys subject to reasonable temperature limits. The joint solid solubilty limits in both the (Al) and (Mg) matrix are given as a basis for heat treatments. The solidification of one typical Al and one Mg alloy are discussed based on the equilibrium calculations and also using the Scheil model.

Thermodynamics of the Al–C–O Ternary System II. High-Temperature Mass Spectrometric Study of the Vaporization of the Alumina-Graphite System

Thermodynamic calculations in the Al-C-O system were performed to understand the vaporization behavior of the aluminacarbon section in open conditions. The gaseous phase in equilibrium with the alumina-graphite system was analyzed by high-temperature mass spectrometry using an effusion cell: Al(g), Al 2 O(g), and CO(g) molecules were the only vaporizing species in the temperature range 1200-1700 K. The measured partial pressures were lower than those attended at equilibrium and second and third calculations for different vaporization reactions never agree, this means a hindered vaporization. As complementary chemical analysis of the samples after experiment by XRD and XPS show that alumina and graphite do not produce any oxycarbide and carbide, the deduced main vaporization reaction is Al 2 O 3 (s) + C(s) + 2Al(g) ↔ CO(g) + 2Al 2 O(g). This confirms that the Al 2 O 3 -C section vaporizes congruently with a kinetic barrier. The analysis of this behavior leads to correlated limited composition ranges for the gaseous and condensed phases which are evaluated.