Formation of deformation twins in f.c.c. crystalsFormation des macles de deformation dans les cristaux c.f.c.Die bildung von verformungszwillingen in f.c.c. kristallen
Abstract
It is proposed that the dislocation reaction may govern the nucleation of three-layer twins in f.c.c. crystals. Conceivably a macroscopic twin may evolve when these three-layer embryonic twins, distributed at different levels within a localized slipped region, grow into each other.
Using transmission electron microscopy, the circumstantial evidence in support of the model has been developed by correlating the crystallographic features of twins, faults and associated slip in cobalt-9.5 wt. % iron alloy deformed at 77°K. The observed correlations are totally consistent with the suggested reaction. They cannot, however, be explained in terms of the dislocation reaction considered by Venables and Cohen and Weertman.
Our “glide” sources for twins are compared with Venables' “glide” and “prismatic” sources. On the basis of both models, it is possible to satisfactorily account for the orientation dependence of the twinning behavior observed by Blewitt, Coltman and Redman, and Suzuki and Barren. However, the crystallography of twins observed by Chin, Hosford and Mendorf during constrained deformation of the [11̄0] (110) oriented cobalt-8 wt. % iron crystal cannot be rationalized on the basis of Venables' and Cohen-Weertman's approach, although it can be explained in a satisfactory manner in terms of our model.
Résumé
Les auteurs proposent un modèle suivant lequel la réaction de dislocations peut régir la formation de macles à trois couches atomiques dans les cristaux c.f.c. On peut imaginer qu'une macle macroscopique puisse se former quand ces maeles embryonnaires à trois couches, distribuées à différents niveaux dans une région de glissement localisé, grossissent en s'interpénétrant.
Ce modèle a pu être confirmé à partir d'observations au microscope électronique en transmission effectuées sur l'alliage Co-9,5%Fe en poids déformé à 77K, en reliant les caractéristiques cristallographiques des maeles, des fautes et du glissement associé. Les relations observées sont tout à fait compatibles avec la réaction proposée. On ne peut pas par contre les expliquer à l'aide de la réaction de dislocations utilisée par Venables, et par Cohen et Weertman.
Les auteurs comparent leurs “sources de glissement” pour les maeles aux sources prismatiques et de glissement de Venables. Les deux modèles permettent d'expliquer de façon satisfaisante l'influence de l'orientation sur le maclage, observée par Blewitt, Coltman et Redman ainsi que par Suzuki et Barrett. Cependant, la cristallographie des maeles observées par Chin, Hosford et Mendorf au cours de la déformation sous contrainte du cristal [11̄0] (110) de Co-8%Fe en poids ne peut pas être interprétée à l'aide du modèle de Venables et de Cohen et Weertman, alors que le modèle proposé ici fournit une interprétation satisfaisante.
Zusammenfassung
Es wird vorgeschlagen, daβ die Keimbildung von Dreischichtzwillingen in f.c.c. Kristallen durch die Versetzungsreaktion bestimmt ist. Es ist denkbar, daβ sich aus diesen Dreischicht-Zwillingskeimen makroskopische Zwillinge entwickeln, wenn die innerhalb eies lokalisierten Abgleitbereichs über mehrere Ebenen verteilten Keime ineinander wachsen.
Mit Hilfe der Durchstrahlungs-Elektronenmikroskopie wurden experimentelle Hinweise gefunden, die dieses Modell unterstützen. Zu diesem Zweck wurden die kristallographischen Eigenschaften von Zwillingen, Stapelfehlern und der in den bei 77°K verformten Kobalt-9,5 Gew. % Eisen-Legierungen damit verbundenen Gleitung untersucht. Die gefundenen Zusammenhänge sind vollständig konsistent mit der vorgeschlagenen Reaktion. Sie können jedoch nicht mit der von Venables und Cohen und Weertman betrachteten Versetzungsreaktion erklärt werden.
Unsere “Gleit”-quellen för Zwillinge werden mit Venables “Gleit”- und prismatischen Quellen verglichen. Beide Modelle erlauben eine befriedigende Erklärung der Orientierungsabhängigkeit des von Blewitt, Coltman und Redman und von Suzuki und Barrett beobachteten Zwillingsverhaltens. Die Kristallographie der von Chin, Hosford und Mendorf während der Druckverformung von [11̄0](110)-orientierten Kobalt-8 Gew. % Eisen-Kristallen kann jedoch nicht anhand der Modelle von Venables und Cohen und Weertman verstanden werden; dagegen erlaubt unser Modell eine befriedgende Erklärung.
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