The crystallographic structure of most materials is dependend on external conditions as temperature and pressure. When the external conditions change the crystals may undergo a phase transition from the inital phase, often called parent phase, into the child phase. While both phases still have the same chemical composition their crystallographic structure might be quite different. A typical example are the alpha and beta phase of titanium. While the parent beta phase is cubic
the child alpha phase is hexagonal
be the orientation of the atomic lattice befor phase transition and
oriChild the orientation of the atomic lattice after the phase transition. Since during a phase transition the atoms reorder with respect to a minimal energy constraint, both orientations
oriChild are in a specific orientation relationship with respect to each other. In the case of alpha and beta Titanium the dominant orientation relationship is described by the Burger orientation relationship
A corresponding child orientation would then be
This orientation relationship is characterised by alignment of hexagonal (0001) plane with the cubic (110) plane and alignment of the hexagonal [2-1-10] direction with the cubic [-11-1] direction.
We could also use these alignment rules to define the orientation relationship as
The advantage of the above definition by the alignment of different crystal directions is that it is independent of the convention used for the hexagonal crystal coordinate system.
Due to crystal symmetry each orientation of a parent beta grain has 24 different may transform into up to 24 child orientations.
beta2alpha phase relationship to these 24 different represenations we obtain 24 child orientations.
Some of these child orientations are symmetrically equivalent with respect to the hexagonal child symmetry. In fact there are 12 pairs of symmetrically equivalent child orientations as depicted in the following inverse pole figure.
These 12 pairs are called the variants of the parent orientation
oriParent with respect to the orientation relation ship
beta2alpha. They can be computed more directly using the command
As we can see each variant can be associated by a
variantId. You can pick specific variants by their
variantId using the syntax
It is important to understand that the reduction to 12 variants from 24 symmetrically equivalent parent orientations comes from the specific Burger orientation relationship. For a general orientation relationship, e.g., if we disturb the OR a little bit
we will always have exactly 24 variants. For the above example we observe how the 12 pairs of orientations which diverge slightly.
Sometimes one faces the inverse question, i.e., determine the
variantId from a parent and a child orientation or a pair of child orientations. This can be done using the command
calcVariants which is discussed in detail in the section parent grain reconstruction.