Diffusional transformations occur when the temperature of a uniform
mixture of materials is lowered into a regime where the uniform
mixture is unstable. The system responds by nucleating second phase
precipitates (e.g. crystals) that then evolve diffusionally until the
process either reaches equilibrium or is quenched by further reducing
the temperature. The diffusional evolution consists of two
phasesgrowth and coarsening. Growth occurs in response to a local
supersaturation in the primary (matrix) phase and a local mass balance
relation is satisfied at each precipitate interface. Coarsening occurs
when a global mass balance is achieved and involves a dynamic
rearrangement of the fixed total mass in the system so as to minimize
a global energy. Typically, the global energy consists of the surface
energy. If the transformation occurs between components in the solid
state, there is also an elastic energy that arises due to the presence
of a misfit stress between the precipitates and the matrix as their
crystal structures are often slightly different.
Diffusional phase transformations are responsible for producing the
material microstructure, i.e. the detailed arrangement of distinct
constituents at the microscopic level. The details of the
microstructure greatly influence the material properties of the alloy
(i.e., stiffness, strength, and toughness). In many alloys, an in situ
coarsening process can occur at high temperatures in which a
dispersion of very small precipitates evolves to a system consisting
of a few very large precipitates in order to decrease the surface
energy of the system. This coarsening severely degrades the properties
of the alloy and can lead to in service failures. The details of this
coarsening process depend strongly on the elastic properties and
crystal structure of the alloy components. Thus, one of the goals of
this line of research is to use elastic stress to control the
evolution process so as to achieve desirable microstructures.
More details, images and results coming soon...