Formation and Application of Grain Boundary Serrations

Abstract

Serrations develop in the grain boundaries (GB) of many metals and alloys as a result of local migrations to absorb dislocation walls. The serrations form only under deformation conditions where the subgrains reach about 2-10 J1m in diameter; the conditions of temperature and strain rate depend on the metal and the effects of alloying on dynamic recovery. The small serrations evidently diminish GB sliding, making lattice deformation rate controlling. If the conditions are changed to produce lower strain rates, then initial serrations increase the creep life. In alloys with limited dynamic recovery, the serrations are also related to initiation of dynamic recrystallization, which is very important in preventing progression of GB cracking, and thus increasing the ductility in hot-forming operations. The formation of serrations or of a necklace of fine recrystallized grains along the GB of large grains (with good creep resistance) are noted for improving fatigue resistance.Resume-Des dentelures se developpent dans les joints de grains (GB) de plusieurs metaux et alliages, resultat de migrations locales pour absorber les murs de dislocation. Les dentelures se forment seulement sous les conditions de deformations OU les sous-grains atteignent a peu pres 2-10 J1m de diametre; les conditions de temperature et la vitesse de deformation dependent du metal et les effets d'alliages de l'adoucissage dynamique. Les petites dentelures diminuent de maniere evidente Ie glissement des GB, ce qui fait que la vitesse de deformation est controlee par Ie reseau. Si on change les conditions de fayon a produire des vitesses de deformation moins grandes, les dentelures initiales augmentent la duree de vie en fluage. Dans les alliages a faible adoucissage dynamique, les dentelures sont aussi reliees a l'initiation de la recristallisation, ce qui est tres important pour prevenir la progression de fissurations de GB et done accroitre 1amalleabilite dans 1esoperations de formage a chaud. La formation de dentelures ou de colliers de grains fins recristallises Ie long des GB de gros grains (avec une bonne resistance au fluage) sont remarquables pour leur resistance accrue a 1afatigue.

Serration of Grain Boundary in Ni-300Fe Alloy through High Temperature Deformation.

“…Many studies have shown that serrations develop at the grain boundaries during hot deformation for various materials like Al, Al alloys, [1][2][3] Ni alloys, 4) and stainless steels. 4) Serrated boundary plays an important role in the microstructural evolution during the hot deformation.…”

“…6) Furthermore, the serration prevents grain boundary sliding, resulting in the enhancement of the creep resistance and the fatigue resistance. 4) The ledge formation by the transgranular slip 7,8) or the grain boundary migration that interacted with the subboundary formation 9) has been thought to cause the serrated boundary. Recently, Hasegawa et al 9) showed that the interaction between the grain boundary and the sub-boundary is mainly responsible for the development of the grain boundary serration during high temperature deformation for Al alloys by direct observations with TEM.…”

“…4) Serrated boundary plays an important role in the microstructural evolution during the hot deformation. The bulged area that is surrounded by the serrated boundaries can act as a nucleus for recrystallization.…”

Dynamic Restoration Process of Ni-30Fe Alloy during Hot Deformation.

“…Then the serrated boundaries may impinge on each other (marked by arrows) to form the equiaxed grains. For Al and Al-Mg alloys, McQueen et al 17) and Drury et al 18) showed that the interpenetration of the serrated grain boundaries could result in microstructures consisting of equiaxed grains. The phenomenon of equiaxed grains evolving without the discontinuous recrystallization process is called 'geometric recrystallization' 15) or 'grain refining dynamic recovery'.…”

“…The phenomenon of equiaxed grains evolving without the discontinuous recrystallization process is called 'geometric recrystallization' 15) or 'grain refining dynamic recovery'. 17) The following relationship is usually adopted for determining the thickness of the pancaked austenite grain (TH g ) corresponding to the compressive strain (e): (4) where K is a constant, D o is the initial grain size and D is the subgrain size or dislocation cell size during deformation.…”