Nucleation of cavities at second phase particles in grain boundaries
Abstract
A kinetic approach is used, to explain the nucleation of cavities in grain boundaries at elevated temperature. Under the influence of a tensile stress, vacancies cluster together and form cavities. However a free energy barrier exists for the nucleation of a cluster which is stable. Heterogeneous nucleation, especially at the junctions of grain boundaries and second phase particles, is favored. Also an incubation time is required to form a vacancy cluster of critical size. Its duration depends upon the volume of the cluster and the diffusivity of vacancies. In this manner a stress and time condition for the nucleation of cavities in grain boundaries is obtained.
Résumé
On utilise une approche cinétique pour expliquer la germination des cavités dans les joints de grains, aux températures élevées. Sous l'influence d'une contrainte de traction, les lacunes se rassemblent pour former des cavités. Il existe cependant une barrière d'énergie libre à franchir pour la germination d'un amas stable. La germination hétérogène, particulièrement aux jonctions des joints de grains et sur les particules d'une seconde phase, est favorisée. Il existe également un temps d'incubation pour la formation d'un amas de taille critique. Ce temps d'incubation dépend du volume de l'amas et de la diffusivité des lacunes. On obtient ainsi une condition de contrainte et de temps pour la germination de cavités aux joints de grains.
Zusammenfassung
Die Keimbildung von Hohlräumen an Korngrenzen bie erhöhten Temperaturen wird mit einer kinematischen Näherung erklärt. Unter dem Einfluβ einer Zugspannung lagern sich Leerstellen zusammen und bilden Hohlräume. Jedoch besteht eine Barriere in der freien Energie für die Bildung einer stabilen Anhäufung. Die heterogene Keimbildung ist begünstigt, besonders an Korngrenzgabelungen und an Teilchen einer zweiten Phase. Auβerdem gibt es eine Inkubationszeit für die Bildung einer Leerstellenanhäufung mit kritischer Gröβe; deren Länge hängt von dem Volumen der Anhäufung und dem Leerstellendiffusionskoeffizienten ab. Auf diese Weise wird eine Spannungsund Zeitbedingung für die Keimbildung von Hohlräumen in Korngrenzen erhalten.
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