Temperature dependent Grain Boundary Migration Microstructures:
Examples from Abnormal Grain Growth in Rock Salt
Sandra Piazolo1, Michel Bestmann1, Chris Spiers2, Dave Prior1, Colin Peach2
1Department of Earth Sciences, 4 Brownlow Street, Liverpool, L69 3GP
2
Utrecht University, Budapestlaan 4, 3584 TA Utrecht, the NetherlandsStepwise in-situ heating experiments show that different types of microstructures are formed by grain boundary migration (GBM) at different temperatures. This feature was observed during heating experiments of dry deformed rock salt samples (axially compressed at 175° C to 30% strain) performed in a scanning electron microscope. GBM occurs between new, substructure and lattice distortion free grains and old, substructured grains. New grains appear from the 3rd dimension and become eventually one magnitude bigger than the grains of the original microstructure. The preexisting substructure of old grains does not change significantly throughout experiments (Bestmann et al., this volume).
The large difference in internally stored strain energy between new and old grains results in a high driving force (F) for GBM causing abnormally fast grain growth. We assume that the magnitude of F does not vary significantly neither with temperature nor throughout experiments. Three main temperature dependent types of microstructures are associated with abnormally grain growth. With increasing temperature they are:
Type I: irregular, serrated grain boundaries and substructure-free, growing grains with a large amount (up to 15 area%) of 5-50 mm inclusions of old grains which results in a "mosaic" microstructure.
Type II: bulging, smooth grain boundaries, often elongate fingering features, and substructure-free grains with some inclusions of old grains.
Type III: evenly curved to straight grain boundaries and "inclusion-free", substructure-free grains.
Type I microstructures develop from GBM where grain boundaries move relatively slow. A moving boundary circumvents an 5-50 mm "inclusion" of old grain. During further migration the boundary leaves often irregular shaped inclusions behind. In addition, developing grain boundaries move in a stop-and-go manner. Type II microstructures develop from grain boundaries which move faster and more continuously than in Type I. Inclusions are rare and initially large (up to 200 mm), shrink with time and often exhibit a near circular shape. Elongate fingers of the new grains develop by preferential GBM along preexisting subgrain boundaries. Type III microstructures are formed by continuous and fast moving grain boundaries. Inclusions of old grains closely resemble those observed in Type II microstructures.
According to these observations and observations outlined by Bestmann et al. (this volume), the mechanism of GBM may be significantly affected by migration rate. Models to be reviewed include a mechanism of cooperative transfer of groups of atoms during GBM (Merkle & Thompson, 2002) and vacancy and defect drag. Dislocations drive GBM; however, at the same time vacancies are generated which decrease grain boundary migration rates. This interplay may influence the mechanisms and rates of GBM.
Merkle, K.I. & L.J. Thompson, 2002. Phy. Rev. Letters, 88, 225501