Martensitic transformation, the sudden, dramatic change in a material’s crystal structure that gives hardened steel its strength, has been implicitly or explicitly exploited for centuries. While the growth of martensite was reasonably well-described, the mechanism of its initial birth, or nucleation, remained elusive.
The key to unlocking this mystery may have been sitting on a library shelf since 1974. Gregory Bruce Olson proposed a new way of thinking about this age-old problem.
The work proposes that a martensitic transformation is not a random phase fluctuation that can begin anywhere in a uniform material. Instead, it is always triggered by a specific variety of pre-existing flaw.
The very first step is a faulting on planes of closest packing. Imagine the material as an orderly stack of atomic playing cards; the transformation begins when a few of these cards slip slightly out of alignment, a motion initiated and guided by the defect.
This heterogeneous nucleation starts at special sites.
The process requires just 4 or 5 lattice dislocations arranged on top of each other with the proper spacing. This works for transformation to both hexagonal close-packed (hcp) structure and body-centered cubic (bcc) transformation.
The specific number is critical. A group of 4 or 5 dislocations is just probable enough to exist in real-world materials, but also just rare enough to explain why the transformation only starts in specific, sparse locations. The thesis explains this delicate balance perfectly:
Thus we see that the probability of a group of dislocations being arranged with a particular spacing decreases rapidly with the number of dislocations required for the nucleation event. The idea that 4 or 5 dislocations could happen to be spaced by two planes is sufficiently probable to be realistic, and at the same time sufficiently improbable (in terms of the expected number of such defects per unit volume of parent phase) to account for the known sparseness of the initial nucleation sites which trigger the martensitic transformation.
This surprisingly small cluster of defects raises a deeper question: what is it, physically, that this handful of misaligned atoms actually creates?
The stacking fault created by the dissociation of this array of dislocations on parallel planes is the embryo. An intrinsic stacking fault is actually an hcp embryo, two planes in thickness. A bcc embryo is more sophisticated but conceptually robust.
For the embryo to grow, it must be energetically favorable to do so. The available driving force thus determines whether the dissociation leads to a perceptible martensite plate. Under the right thermodynamic conditions (such as cooling the material to a specific temperature), the fault energy can become zero or even negative. When this tipping point is reached, the model predicts that there is no barrier to nucleation.
This concept of barrier-less nucleation is essential for explaining one of the most famous characteristics of martensitic transformations: their incredible speed. Once the conditions are right, the defect that houses the embryo becomes unstable. The restraining forces vanish, and the transformation proceeds spontaneously and rapidly, propagating through the material.
The thesis concludes by presenting an analogy: martensitic nucleation can be viewed as a type of spontaneous plastic deformation.
Plastic deformation is a change in shape that occurs through the rearrangement of atoms under an external force. The thesis argues that nucleation is a similar process of atomic rearrangement, but the driving force isn't an external push or pull. Instead, it is an "internal chemical stress"—the built-in energetic desire of the material to change from one crystal structure to another.
This provides a satisfying framework for understanding how a chemical driving force can manifest as a physical, structural rearrangement, kicked off by a tiny flaw and proceeding with speed.
Olson’s thesis argued that martensite nucleates from simple, pre-existing defects through a process of atomic faulting. This successfully explained a wide variety of experimental observations that had previously been unaccounted for.