Strain softening, grain boundary migration and dynamic recrystallization of Ni during high temperature low cycle fatigue

doi:10.1016/0001-6160(88)90046-6

Acta Metallurgica

Volume 36, Issue 12, December 1988, Pages 3093-3101

https://doi.org/10.1016/0001-6160(88)90046-6 Get rights and content

Abstract

The flow stress behavior and microstructural development of nickel polycrystals during high temperature low cycle fatigue were studied. All specimens exhibited a flow stress decrease at large cycle numbers, even at total strain amplitudes as low as 0.4% but distinctly before crack initiation. The microstructure revealed a tendency toward grain coarsening due to grain boundary migration, but locally grain refinement owing to dynamic recrystallization is the major softening mechanism at large cumulative strains and the cause of the observed flow stress maximum. Observations on Al, which does not recrystallize dynamically support this conclusion. After large cycle numbers the dislocations were arranged in a well recovered cell structure, and the average cell size scaled with the reverse flow stress. The results substantiate that dynamic recrystallization can be set off at drastically lower stress in cyclic deformation than in monotonie deformation. This supports the stipulation that occurrence of dynamic recrystallization is nucleation controlled.

Résumé

Le comportement de la limite élastique et le développement des microstructures de polycristaux de nickel ont été étudiés aucours d'essais de fatigue à haute température et à faible nombre de cycles. Tous les échantillons présentent une décroissance de la limite élastique pour les grands nombres de cycles, même pour des amplitudes de déformation totale aussi faibles que 0,4%, mais nettement avant l'initiation de la fissure. La microstructure révèle une tendance au grossissement du grain due à la migration des joints, mais localement l'affinement du grain d à la recristallisation dynamique est le principal mécanisme d'adoucissement pour les grandes déformations cumulatives et la cause du maximum de limite élastique observé. Des observations dans l'aluminium, qui ne recristallise pas dynamiquement, confirment cette conclusion. Après un grand nombre de cycles, les dislocations s'arrangent en une structure cellulaire bien restaurée et la taille moyenne d'une cellule varie comme l'inverse de la limite élastique. Ces résultats établissent que la recristallisation dynamique peut démarrer pour des contraintes beaucoup plus faibles en déformation cyclique qu'en déformation monotone. Ceci est en faveur de l'hypothèse selon laquelle l'apparition de la recristallisation dynamique est contrôlée par la germination.

Zusammenfassung

Das Verfestigungsverhalten und die Mikrostrukturentwicklung von Nickel Vielkristallen bei der Hochtemperaturermüdung mit groβer Dehnamplitude wurde untersucht. Alle Proben zeigten einen Spannungsabfall bei groβen Zyklenzahlen, sogar bei Dehnamplituden von nur 0.4%, aber deutlich vor Beginn der Ermüdungsriβildung. Die MikroStruktur zeigte eine Neigung zur Kornvergröberung durch Korngrenzenwanderung, aber lokal eine Kornfeinung infolge dynamischer Rekristallisation, die bevorzugt Keimbildung an Korngrenzen erkennen lieβ. Die Mikrostrukturentwicklung läβt sich damit erklären, daβ dynamische Rekristallisation der Hauptentfestigungsmechanismus und der Grund für das beobachtete Flieβspannungsmaximum bei groβen cumulativen Dehnungen ist. Ergebnisse an Aluminium, das nicht dynamisch rekristallisiert, unterstützen diese Interpretation. Nach groβen Zyklenzahlen befanden sich die Versetzungen in guterholten Zellstrukturen, und die mittlere Zellgröβe änderte sich umgekehrt mit der Flieβspannung. Die Ergebnisse belegen, daβ dynamische Rekristallisation während zyklischer Verformung bei drastisch kleineren Flieβspannungen ausgelöst werden kann, als während einsinniger Verformung. Dieses Verhalten stützt die Hypothese, daβ dynamische Rekristallisation durch die Keimbildung kontrolliert wird.

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Strain softening, grain boundary migration and dynamic recrystallization of Ni during high temperature low cycle fatigue

Abstract

Résumé

Zusammenfassung

Treatise on Materials Science and Tech-nology

Acta metall.

Acta metall.

Acta metall.

Acta metall.

Scripta metall.

Acta metall.

Acta metall.

Acta metall.

Acta metall.

Acta metall.

Scripta metall.