The intergranular segregation of boron in Ni3Al: Equilibrium segregation and segregation kinetics
Abstract
The grain boundary B content of high-purity Ni-24 at.% Al alloys containing 0.048, 0.144, 0.240 and 0.480 at.% B (100, 300, 500, 1000 ppm mass) has been determined for samples aged from 1323 to 873 K for sufficient times to attain equilibrium. The B content was derived from Auger electron spectra of the intergranular fracture facets. Many facets were exposed during fracture at ≈ 300 K, and additional facets were formed upon fracturing following hydrogen charging after heat treatment. For each alloy sample, about 25 facets were analyzed. The grain boundary B contents were in the range of 0.5–2.5 at.%. The grain boundary B content increased with decreasing temperature and with increasing bulk B content in the alloys. The energy of binding of a B atom to the grain boundary was calculated using McLean's segregation theory and assuming a unique binding energy for each alloy. The values were in the range of 0.15–0.45 eV/atom, and increased with increasing temperature and with decreasing bulk B content. These results have been rationalized in terms of a spectrum of binding energies for a given alloy. However, when the entropy of adsorption was taken into account, an enthalpy of adsorption of B to the grain boundary of 0.13 eV/atom was obtained, independent of temperatire and bulk B content. This is interpreted to mean that the spectrum of binding energies is quite restricted. The grain boundary B content of these alloys has also been measured as a function of annealing time at 773, 873, 973 and 1173 K. The diffusion coefficient of B in Ni3Al at 773 K is about 5 × 10−21 m2/s, and the equilibrium grain boundary B content is attained at about 3000 s. The diffusion coefficient at 973 K is between 10−16 and 10−17 m2/s. The activation energy for diffusion of B in Ni3Al is between 200,000 and 300,000 J/mol.
Résumé
On détermine la teneur en bore des joints de grains d'alliages de Ni de haute pureté-24 at.% Al contenant 0,048, 0,144, 0,240 et 0,480 at.% de bore (100, 200, 300, 500, 1000 ppm en masse) pour des échantillons vieillis entre 1323 et 873 K pendant un temps suffisant pour atteindre l'équilibre. La teneur en bore est tirée des spectres Auger des facettes de rupture intergranulaire. Plusieurs facettes sont exposées pendant la rupture à , et des facettes additionalles se forment pendant la rupture suivant la charge en hydrogéne aprés le traitement thermique. Pour chaque échantillon d'alliage, environ 25 facettes sont analysées. Les teneur en bore des joints de grains varient entre 0,5 et 2,5% en atomes. Cette teneur augmente lorsque la température décroît et lorsque la teneur globale en bore des alliages augmente. L'énergie de liaison d'un atome de bore au jount de grains est calculée en utilisant la théorie de la ségrégation de McLean et en supposant une énergie de liaison unique pour chaque alliage. Les valeurs varient entre 0,15 et 0,45 eV/atome et augmentent lorsque la température croît et lorsque la teneur globale en bore décroît. Ces résultats sont rationalisés en fonction du spectre des énergies de liaison pour un alliage donné. Cependant, quand l'entropie de l'adsorption est prise en compte, on obtient une enthalpie d'absorption du bore sur le joint de grains de 0,13 eV/atome, indépendante de la température et de la teneur globale en bore. Ceci montre que le spectre des énergies de liaison est trés restreint. La teneur en bore des joints de grains de ces alliages est aussi mesurée en fonction du temps de recuit à 773, 873, 973 et 1173 K. Le coefficient de diffusion de B dans Ni3Al à 773 K est environ 5.10−21 m2/s et la teneur en bore des joints de grains à l'équilibre est atteinte au bout d'environ 3000 s. Le coefficient de diffusion à 973 K est situé entre 10−16 et 10−17 m2/s. L'énergie d'activation de la diffusion du B dans Ni3Al est située entre 200.000 et 300.000 J/mol.
Zusammenfassung
Der Borgehalt der Korngrenzen in hochreinem Ni-24 At.-% Al-Legierungen, die 0,048, 0144, 0,240 und 0,480 At.-% B (100, 300, 500 und 1000 Massen-ppm) enthalten, wird ermittelt an Proben, die ausreichend lange bis zum Gleichgewicht bei Temperaturen von 1323 und 873 K ausgelagert worden sind. Der Borgehalt wird aus Augerelektronenspektren der intergranularen Bruchfacetten abgeleitet. Viele Facetten lagen während des Bruches bei ≈300 K frei, zusätzliche Facetten wurden gebildet beim Bruch nach Wasserstoffbeladung nach einer Wärmebehandlung. Für jede Lgierung wurden etwa 25 Facetten untersucht. Der Borgehalt der Korngrenzen liegt bei 0,5 bis 2,5 Atom-%; er nimmt mit fallender Temperatur und steigendem Volum-Borgehalt zu. Die Bindungsenergie eines Boratons in der Korngrenze wird mit der Segregationstheorie von McLean unter der Annahme, daß eine einheitliche Energie für jede Legierung besteht, berechnet. Sie liegt zwischcen 0,15 und 0,45 eV/Atom und nimmt mit steigender Temperatur und fallendem Volum-Borgehalt zu. Diese Ergebnisse werden anhand eines Spektrums der Bindungsenergien erklärt. Wird allerdings die Adsorptionsentropie berücksichtigt, dann ergibt sich eine Adsorptionsenthalpie an die Korngrenze von 0,13 eV/Atom unbhängig von der Temperature und dem Volum-Borgehalt. Dieses Ergebnis wird dahingegehend interpretiert, daß das Spektrum der Bindungsenergien sehr eng ist. Der Borgehalt der Korngrenzen dieser Legierungen wird auch in Abhängigkeit von der Auslagerungszeit bei 773, 873, 973 und 1173 K gemessen. Der Diffusionskoeffizient des Bors in Ni3Al bei 773 liliegt bei 5 × 10−21m2/s, die Gleichggewichtskonzentration des Bors stellt sich an der Korngrenze nach etwa 3000 sec ein. Der diffusionskoeffizient bei 973 K liegt zwischen 10−16 und 10−17 m2/s. Die Aktivierungsenergie der Diffusion des Bors in Ni3Al liegt zwischen 200.000 und 300.000 J/Mol.
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