the habit plane

When a martensite plate or a twin or a precipitate forms, it does not grow as a blob — it grows as a flat plate or lath lying on one particular crystal plane of the parent. That preferred plane, the one the new feature sits on, is called the habit plane. If you sliced the parent crystal open you would see the plates all lying parallel to the same set of planes, the way roof slates all lie in one plane.

For a martensitic transformation the habit plane has a deep meaning: it is the invariant plane — the one plane that the transformation shear leaves undistorted and unrotated, so parent and product match along it without stretching or tearing. Its indices are usually irrational and depend on the alloy: roughly {111}gamma in some low-carbon steels, near {225}gamma or {259}gamma in higher-carbon steels. That the habit plane is left invariant is the geometric heart of the phenomenological theory of martensite: the shape change is an invariant-plane strain.

Because the habit plane is the well-matched, low-energy interface, it sets the shape, orientation and arrangement of the transformation product — the plates, laths and needles you see in a micrograph all trace their habit planes. Reading habit planes therefore lets a metallurgist deduce the shear system that produced them.