The morphology of martensite in iron alloys

Light and electron microscopy have been used to determine the main structural differences between the two major types of martensite in ferrous alloys. In the martensite that forms in dilute alloys of iron, the basic transformation unit takes the shape of a lath, and hence the term lath martensite is appropriate for identifying this morphology. Each lath is the result of a homogeneous shear, and successive shears produce a packet of parallel laths containing a high density of tangled dislocations. The other type, plate martensite, differs in the shape taken by a transformation unit and its transformation sequence is characterized by nonparallel plate formation. Investigation of a large number of binary ferrous systems shows that alloy composition and the transformation temperature influence the transition from lath to plate martensite. These two factors are discussed in terms of their possible effects on the plastic deformation mechanisms which must occur in the parent austenite and product martensite during transformation.