Jean Bernardini

Self-diffusion in α-Al2O3 I. Aluminium diffusion in single crystals

Abstract Aluminium self-diffusion has been measured in single crystals of α-Al2O3 in the temperature range 1540–1697°C using 26Al as a radioactive tracer. Two diffusion mechanisms are involved: a lattice diffusion mechanism and a subboundary mechanism. Lattice and subboundary diffusion coefficients obey equations and respectively. The results show that lattice diffusion coefficients of aluminium are much lower than those determined earlier. The comparison with oxygen diffusion determined by other workers for the same specimens shows that oxygen and aluminium diffusion coefficients are of the same order of magnitude in the lattice but that aluminium diffusion coefficients are greater than oxygen diffusion coefficients in subboundaries.

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation : effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (18O) and aluminium (26Al) self-diffusion coefficients in Al2O3 were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped α-alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr2O3.

Characterization, thermomechanical behaviour and micromechanical-based constitutive model of shape-memory CuZnAl single crystals

Abstract CuZnAl shape memory alloys are grown as single crystals by the Bridgman technique with a final shape directly suitable for thermomechanical tests (cylinders with tapered heads: 25 mm gauge length, 4 mm in diameter). The four classical transformation temperatures are checked by Differential Scanning Calorimetry and resistivity. The orientation of crystal structure is investigated by X-ray diffraction. Isothermal pseudoelastic tensile tests show that the width of the hysteresis loops and the slope of the stress-strain curves during phase transformation increase as the applied stress rate increases. A micromechanical-based constitutive model allows us to describe this single crystal behaviour.

Equilibrium intergranular segregation and diffusion in AgSn (0–6%) alloys

Silver and tin self-diffusion measurements have been made in polycrystalline single phase solid solutions (2 × 10−2 ⩽ CSnv ⩽6 at%) AgSn, over the temperature range 281–450°C. Contrary to what is observed with bulk diffusion, the variations of the parameters PAg and PSn, characteristic of the grain boundary diffusion are not a simple function of the volume tin content. The results are discussed in relation to an equilibrium grain-boundary segregation of tin; the segregation coefficient is almost constant for the “low” tin bulk concentrations and it decreases for “high” tin contents; this last fact is in agreement with a saturation of the grain boundary with tin.

Role of the solid/liquid interface faceting in rapid penetration of a liquid phase along grain boundaries

A model is developed for describing rapid penetration of a liquid phase along a grain boundary. It is based on the assumption of a highly faceted solid/liquid interface. Experiments showing the faceting of the solid/liquid interface in grain boundary penetration experiments are presented. The basic hypothesis of the model are an undersaturated solid and a positive spreading coefficient of the liquid phase along the grain boundary. The model explains the apparent concave shape of the tip of the groove and the reason why penetration also occurs if the liquid phase is pre-saturated with the material of the solid. Moreover it predicts a power law with an exponent close to unity for the time dependence of the depth of penetration of the liquid layer along the grain boundary.

Volume diffusion of iron in Fe3Al: Influence of ordering

Abstract The lattice self diffusion coefficient of 59 Fe in Fe 3 Al has been determined in the temperature range 847–1503 K using the classical radio-tracer method. Below 1014 K the Arrhenius plot has a downwards curvature resulting from the B2 type of atomic ordering. The temperature dependence of the self-diffusion coefficient can be described by: D v = ( 3.2 1.5 + 2.8 × 10 − 5 ) × exp ( − ( 204 ± 6.1 kJ / mol ) × ( 1 + ( 0.1 ± 0.03 ) × S 2 ) R T ) ( m 2 / s ) where S is the long-range order parameter.

Grain-boundary diffusion: Analysis of the c kinetic regime

Abstract The possibility of measuring the ‘true’ grain boundary (gb) diffusion coefficient in the so-called C kinetic regime is demonstrated for self-diffusion of silver. Although the criteria for the C-type diffusion have sometimes been experimentally established, we point out the existence of a transitional region in which the classical analysis of gb diffusion profiles (assuming either B- or C-type conditions) is wrong. In this region an alternative method, initially proposed by one of us, is tested. It allows the determination of both the true gb double products and the true gb diffusion coefficients from the experimental profiles. The ratio of these values gives the thickness δ of the gb, and the values obtained justify the usual assumption δ=0·5 nm.

Ni diffusion in near-equiatomic Ni-Ti and Ni-Ti(-Cu) alloys

Tracer volume diffusion of 63 Ni in Ni-50.07 at.% Ti binary and Ni-48.83 at.% Ti-9.29 at.% Cu ternary alloys have been measured between 783 and 1288 K. The temperature dependence of the diffusion coefficients can be well described by a straight Arrhenius function This illustrates that a diffusional anomaly (typical in some bcc metallic alloys and related to the well-known phonon softening), if there is any in this system, is negligible. On the other hand, the small activation energy (about half the value expected from simulations for the commonly accepted mechanism with thermally activated vacancies) shows that the mechanism of diffusion is probably mediated by structural vacancies.

Cobalt lattice diffusion in bulk cobalt disilicide

Abstract Lattice diffusion of cobalt disilicide has been studied using 60 Co, conventional sectioning techniques and bulk samples of different compositions (stoichiometric, with small excess of silicon or cobalt). In the temperature range 760–1100°C, the cobalt lattice diffusion coefficients vary between 3 x 10 -15 and 9 x 10 -12 cm 2 /s with an activation energy of 2.83 eV. They are not influenced by silicon or cobalt excess. These values and the “relatively” high activation energy indicate a slow cobalt diffusion.

Effect of segregation on the shape of grain boundary diffusion profiles Experimental study of the Cu[sbnd]Ag system

Abstract Grain boundary diffusion of Ag in Cu and in a Cu-0.091 at.% Ag alloy has been investigated. An experimental method to separate the effect of different grain boundaries present in the samples and the effect of segregation on grain boundary diffusion profiles is described. It is shown that in the case of silver diffusion in pure copper the curved penetration plots are due to segregation effects. For pure Cu the obtained activation energy and pre-exponential factor of the triple product P are (90 ± 6.2)kJ mol−1 and (3.7+7.3 −2.5) × 10−14 m3s−1, respectively. For Cu(Ag) alloy these values are (119.4 ± 6.6)kJ mol−1 and (2.1+4.8 −1.5) × 10−12m3s−1, respectively. The activation energy of grain boundary segregation is estimated as 37± 13kJmol−1.