HOW WOULD WE RECOGNIZE A MARTENSITIC-TRANSFORMATION IF IT BUMPED INTOUS ON A DARK AND AUSTY NIGHT

The history of the meaning of ''martensitic transformation'' over the past forty years is briefly reviewed, and an area of common agreement is identified centered on the concepts of ''cooperative atomic motion' ' and ''shape change''. A search is made for the minimum elements requ ired in a definition that will be simple, compact, precise and easily tested. It is argued that cooperative atomic motion implies acoustic e mission, and that this could provide an important operational element in the definition. A distillation of past definitions, with this new i ngredient added yields, as a proposed irreducible definition: A marten sitic transformation involves a cooperative motion of a set of atoms a cross all interface causing a shape change and sound.