In this chapter, we discuss one important means of altering the mechanical response of metals and ceramics: martensitic transformation. Martensitic transformation is a highly effective means of increasing the strength of steel. An annealed medium-carbon steel (such as AISI 1040) has a strength of approximately 100 MPa. By quenching (and producing martensite), the strength may be made to reach about 1 GPa, a tenfold increase. The ductility of the steel is, alas, decreased.
A quite different effect is observed in ceramics. Martensitic transformation can be exploited to enhance the toughness of some ceramics. If a ceramic undergoes a martensitic transformation during the application of a mechanical load, the propagation of cracks is inhibited. For example, partially stabilized zirconia has a fracture toughness of approximately 7 MPa m1/2. An equivalent ceramic not undergoing martensitic transformation would have a toughness less than or equal to 3 MPa m1/2.
An additional, and very important, effect associated with martensitic transformations is the “shape-memory effect.” Alloys undergoing this effect “remember” their shape prior to deformation. The three effects just described have important technological applications.
Structures and Morphologies of Martensite
Quenching has been known for over 3,000 years and is, up to this day, the single most effective mechanism known for strengthening steel. However, it is only fairly recently that the underlying mechanism has been studied in a scientific manner and understood.