Under the right conditions, the very atomic structure of a material can rapidly reconfigure itself in the blink of an eye. This process is known as a "martensitic transformation," a fascinating, split-second structural change that occurs in strong steels. One of the most explosive versions of this change, the "burst" transformation, was studied in meticulous detail in a 1977 MIT doctoral thesis by Gerald Albert Knorovsky.
The core mechanism driving the rapid spread of a martensitic transformation is a process called autocatalysis—in simple terms, the transformation itself creates the conditions for more transformation to occur.
The transformation spreads in a self-sustaining chain reaction, like a series of cascading dominoes, once the first one is tipped.
The "burst" variant of the martensitic transformation is fast and highly coordinated event. Sometimes, as much as 60-75% of the material can transform in a few milliseconds.
"Martensitic phase transformations have been described in the past as 'military' in character. This choice of word emphasizes the coordinated nature of the atomic motions required for the parent phase to form a single region of martensite."
The burst of transformation is so abrupt that it generates audible acoustic emissions. Some of the earliest evidence for this phenomenon was the German word "umklapp" (meaning "to flip over") being applied to the distinct clicking sounds made by the metal during its transformation.
Most metals are polycrystalline, composed of countless microscopic crystal "grains," each with a slightly different atomic orientation. The interfaces where these grains meet are known as grain boundaries.
Knorovsky’s research found that these boundaries act as firebreaks. The stress-induced burst of autocatalysis is significantly less efficient in polycrystals than in single crystals. Grain boundaries disrupt the propagation of stress, effectively slowing or stopping the chain reaction. This means the grain size of an alloy can be engineered to control the extent of the burst of transformation.
It was not clear prior to Knorovsky's work what controlled the trigger temperature (known as the $M_b$ temperature) for the burst transformation.
The thesis concluded that the burst temperature does not depend on grain size, provided that "spurious chemical changes are avoided." In simpler terms, previous contrary results were likely caused by unintended chemical reactions—such as the loss of carbon (decarburisation) from the sample surface—rather than the grain size itself.
Knorovsky's thesis reveals the burst of transformation is a complex chain reaction. This "military" precision, while extremely fast, is governable, with internal crystal boundaries acting as firebreaks.
The thesis contains unique experiments in which the progress of martensite was studied in both temperature gradients and chemical composition gradients, to identify the chemical driving force at which plate growth stops.
Thesis reproduced with the permission of Gerald Albert Knorovsky. There is an effect of external stress on bursts of martensitic transformation.