Metastable Transformations in Crystals
Crystals are a unique form of matter in which the atoms maintain a regular and periodic arrangement. This can give extendable physical properties that are hugely varied in various crystalline structures. Structural transformations, when atoms reorganize, are a fundamental aspect of their behavior. Many of these transformations are very quick, taking place in less than a millionth of a second, before rearrangement is complete, with the change of crystal structure signified by the release and absorption of thermal energy, giving rise to the changes in the physical properties of the crystal. This is largely known as a phase change, between two different phases in the crystal.
Marginal stability is when a crystal assumes a metastable structure in which the atoms have been shifted around, however, due to the discrepancy in energies of the various states, a return to the primary structure is inhibited. This can be thought of as being, energetically speaking, in an unstable equilibrium state. In such cases, the crystal will, eventually, return to its original form, or it may even transform to another form.
One of the most studied metastable transformations in crystals is the martensitic transformations. It is also one of the oldest, with the first such transformation being observed in 1681. It occurs in steels and alloys, when mechanical stress is applied to the material. This can create a phase change from the simple cubic structure of Fe to the less symmetrical body centered cubic, leading to an increased strength in the material.
The atomic mechanisms of a martensitic transformation have complicated origins but, essentially, come down to a process know as lattice mismatch, which occurs when a crystal lattice is strained beyond its natural limits between two states. This is known as making a ‘crystallographic dislocation’, which then proceeds to distort the crystal lattice into the more metastable form, releasing the energy initially needed to make the transformation.
Another transformation closely related to the martensitic is the Bain transformation. This occurs when the material is cooled at a certain rate and involves ductile-to-brittle transition. Bain transformations also depend on external conditions such as temperature, with brittle crystalline structures also known as cold worked or stress-annealed.
There are also a range of other transformations related to nonlinear constituative equations, and structures formed from them. These transformations, although very complicated in nature, are governed by two distinct parameters: one that concerns the directionality of the grain and the other concerning the orientation of the grain within the crystal structure.
These two parameters can lead to complex geometrical forms within the crystal structure, such as helical twists and grain boundaries, which are extremely important in determining the range of physical properties that the crystal will display.
The study of crystal transformations has been an important part of physical chemistry for centuries, but has often been shrouded in confusion due to the complexity of the process. However, recent developments have meant that these transformations can now be accurately calculated and simulated, enabling more reliable predictions of the physical properties of the crystal under certain conditions. As the understanding of crystal transformations increases, the application of such processes to various materials will only increase, allowing the development of materials with new and unique properties.
