Martensite is a type of microstructure of iron-based alloys that is formed at relatively low temperatures, approximately between 200 and 900°C (392 and 1652°F). It is a metastable-phase, which is a supersaturated solid solution as compared to the equilibrium phase austenite. Martensite is a very hard and brittle crystalline structure, which can be viewed in the microstructure of a metallographic sample.
When a ferrous alloy is heated to a temperature at which austenite is formed, it shifts the equilibrium ratio of the alloy’s constituents. However, if the alloy is cooled rapidly the ratio of the constituents can shift to where the equilibrium ratio is no longer applicable and the alloy will experience a transformation to the metastable phase. This transformation is known as diffusionless transformation because the atoms remain in the same lattice structure they were in as austenite.
The diffusionless transformation of austenite to martensite takes place as the austenite spontaneously reorganizes on a microscopic level and the lattice structure begins to distort. The atoms of ferrite (Fe) and cementite (Fe3C) will become separated in the process, with the partially dissolved ferrite atoms occupying larger interstitial site. Simultaneously, the cementite will form smaller precipitates where the lattice will be distorted, causing a phase change from austenite to martensite. It is during this stage in which the distortion of the lattice leads to considerable strength and hardness increases in the material.
Martensitic microstructures can take on a variety of shapes and sizes, with the most common being platelets also known as laths. The laths come in three sers, lath martensite, plate martensite and two variants of stack martensite all of which have misorientation of their axes. The different misorientation of their axes is related to the different crystallography of the phases, which occurs during the transformation.
The most common shape, however, is an elongated ferrite shape or lath, which is created by the sideways distortion of the lattice. Martensite can also take on variously shaped particles and clusters, with the particles possessing a tetragonally distorted structure. When these particles escape and form separate phases, they are called Widmanstatten ferrite, or cementite insterlite.
These diverse shapes and sizes are a result of a wide range of factors such as the cooling rates, alloying elements and carbon concentrations. Depending on the shape, size, and crystal structure of the martensite, it can possess a variety of properties such as a high strength-to-weight ratio and excellent wear resistance.
Martensite is a preferred microstructure in many applications due to its combination of high strength with modest toughness. Additionally, martensite is a very hard and strong material, because the transformation is accompanied by an increase in martensite volume of up to 10%.
Martensite can be further strengthened by heat treating and tempering, which result from reheating the material at a point below its full transformation temperature. The tempering process enriches the material with carbon, likewise allowing for more effective hardening by quenching processes.
Martensite is a useful for applications ranging from surgical instruments and cutting tools to car body parts and springs, due to its diverse shapes, sizes and properties. The properties of martensite also make it difficult to machining, requiring finer tools and machines than when working with other microstructures. The metastable nature of Martensite also means it is not easily softened by heating or reheating to recovery the austenite.
