We report an ab initio study of the semiconducting Mg(2)X (with X = Si, Ge) compounds and in particular we analyze the formation energies of the different point defects with the aim of understanding the intrinsic doping mechanisms. We find that the formation energy of Mg(2)Ge is 50% larger than that of Mg(2)Si, in agreement with the experimental tendency. From a study of the stability and the electronic properties of the most stable defects, taking into account the growth conditions, we show that the main cause of the n doping in these materials comes from interstitial magnesium defects. Conversely, since other defects acting like acceptors such as Mg vacancies or multivacancies are more stable in Mg(2)Ge than in Mg(2)Si, this explains why Mg(2)Ge can be of n or p type, in contrast to Mg(2)Si. The finding that the most stable defects are different in Mg(2)Si and Mg(2)Ge and depend on the growth conditions is important and must be taken into account in the search for the optimal doping to improve the thermoelectric properties of these materials.

No abstract

The second-order ordering transition of the AuAgZn2 alloy has been studied by coherent x-ray scattering. Within a few degrees above the critical temperature Tc, equilibrium critical fluctuations are observed together with some pre-transitional local ordering connected to sample defects. The speckles observed correspond to heterodyne interference between local ordering and fluctuations and show a mixed static and dynamical behaviour in a narrow domain of a few tenths of degree above Tc. The dynamical behaviour is shown to correspond to the critical slowing down of the fluctuations in the vicinity of the transition (model "A" of Hohenberg and Halperin 1 ). A rough comparison can be carried out with the classical diffusion models. Some improvements of the method are discussed.

A residual stress depth profile up to 1 mm is determined with the Ortner method in a single crystal of a nickel-based superalloy which has been subjected to shotpeening. An optimization procedure is assessed to minimize uncertainties connected to Bragg angle, mosaic spread and numerical stability. The theoretical background is reviewed to highlight the connections between Bragg angle positions and the stress tensor components in different coordinate systems and also to obtain a mathematically consistent formulation. Transformation matrices required to express the strain components with respect to the initial state are provided for the general case. It is shown that, when a stress gradient occurs beneath the sample surface plane, the value of the 33 component of the stress tensor determined from measurements is twice its true value. For a sample surface oriented along a h100i crystallographic direction, the data analysis shows that the compressive stresses which develop in the 150 mm-thick surface layer are compensated for by small tensile stresses developing at long scale rather than a specific layer of finite size featuring high tensile stresses. At least 17 Bragg angles are required to have stable solutions with standard deviations close to 30 MPa. Maximum compressive stresses of 1000 or 1400 MPa depending on the assumption used to describe the initial state occur at a 30 mm depth.

The aim of this paper is to investigate the consequences of finite size effects on the thermodynamics of nanoparticle assemblies and isolated particles. We consider a binary phase separating alloy with a negligible atomic size mismatch and equilibrium states are computed using off-lattice Monte Carlo simulations in several thermodynamic ensembles. First, semi-grand canonical ensemble is used to describe infinite assemblies of particle with the same size. When decreasing the particle size, we obtain a significant decrease of the solid/liquid transition temperatures as well as a growing asymmetry of the solid state miscibility gap related to surface segregation effects. Second, a canonical ensemble is used to analyze the thermodynamic equilibrium of finite monodisperse particle assemblies. Using a general thermodynamic formulation, we show that a particle assembly may split into two sub-assemblies of identical particles. Moreover, if the overall average canonical concentration belongs to a discrete spectrum, the sub-assemblies concentrations are equal to the semi-grand canonical equilibrium ones. We also show that the equilibrium of a particle assembly with a prescribed size distribution combines a size effect and the fact that a given particle size assembly can adopt two configurations. Finally, we have considered the thermodynamics of an isolated particle to analyze whether a phase separation can be defined within a particle. When studying rather large nanoparticles, we found that the region in which a two-phase domain can be identified inside a particle is well below the bulk phase diagram but the concentration of the homogeneous core remains very close to the bulk solubility limit.

The dynamics of the order fluctuations in the AuAgZn 2 close to the critical point (T c ≃ 609 K) was observed by coherent x-ray scattering. With the high beam intensity of the ID10 ESRF beamline and with the new pixel detector, the dynamics was measured with a few tens of millisecond resolution. The intensity connected to the diffuse scattering corresponding to fluctuations was unambiguously distinguished from the surface pretransitional ordering occurring in this system close to T c . The variations of the fluctuation time with temperature and wave vectors were measured in this system belonging to the universality class of Ising second order transition with a non-conserved order parameter. The direct observation of the critical slowing down in the vicinity of the second-order transition led to an estimate of the dynamic exponent z ≃ 1.96(11), in rough agreement with theory (model "A" of Ref. 1).

In nickel-based superalloys, temperatures related to the formation or the dissolution of the different types of γ' precipitates are important parameters for optimizing the mechanical properties of components but also for developing models which can reproduce the kinetics of their phase transformation. We showed that the electrical resistivity variations during heat treatment of the N18 superalloy was sufficient to monitor the kinetics related to secondary and tertiary γ' precipitates. In particular, the effects of the heating rate and the initial microstructure on the dissolution kinetics of the γ' phase were investigated. Experimental results were also compared to outputs of a precipitation model developed for the N18 alloy showing that in situ electrical resistivity measurements can be used for calibration and validation purposes.

No abstract