Lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae

Lamellae (materials)

A lamella is a gill-shaped structure: fine sheets of material held adjacent to one another, often with fluid in between though sometimes simply a set of 'welded' plates. They appear in biological and engineering contexts, such as filters and heat exchangers...

. They are often observed in cases where a phase

Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

 transformation front moves quickly, leaving behind two solid products, as in rapid cooling of eutectic (such as solder

Solder

Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

) or eutectoid (such as pearlite

Pearlite

Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...

) systems.

Such conditions force phases of different composition to form but allow little time for diffusion

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 to produce those phases' equilibrium compositions. Fine lamellae solve this problem by shortening the diffusion distance between phases, but their high surface energy

Surface energy

Surface energy quantifies the disruption of intermolecular bonds that occur when a surface is created. In the physics of solids, surfaces must be intrinsically less energetically favorable than the bulk of a material, otherwise there would be a driving force for surfaces to be created, removing...

 makes them unstable and prone to break up when annealing

Annealing (metallurgy)

Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...

 allows diffusion to progress. A deeper eutectic or more rapid cooling will result in finer lamellae; as the size of an individual lamellum approaches zero, the system will instead retain its high-temperature structure. Two common cases of this include cooling a liquid to form an amorphous solid

Amorphous solid

In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....

, and cooling eutectoid austenite

Austenite

Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...

 to form martensite

Martensite

Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...

.