The microstructure of dislocated martensitic steel: Theory

Recent experimental studies of the microstructure of dislocated martensite in low-carbon steel have shown that its superficially complex microstructure is, in fact, a simple hierarchical pattern whose basic element is a composite block containing two Kurdjumov-Sachs variants of the α′ phase that share the same Bain axis. The blocks are plates with interfaces generally parallel to {0 1 1}α planes that can be stacked into four crystallographically distinguishable packets. When the prior austenite grain contains all four packets its transformation can be a simple dilatation that involves no net shear. In the present paper we use the crystallographic theory of dislocated martensite to show that the stackable composite martensite plate is the simplest transformed plate that can be constructed that preserves a close-packed plane ({0 1 1}α||{1 1 1}γ) and has an invariant {1 1 1}γ habit plane, as the experimental results require. The 12 distinct variants of this plate can be configured into a microstructure that duplicates the patterns and pole figures found experimentally. The results suggest that there is no single-variant martensite plate that can be stacked in this way without substantial strain, though the theory does predict the single-variant plates with {5 5 7}α habit planes and Greninger-Troiano orientation relations that are found in alloys with large fractions of retained austenite.