ON NUCLEATION OF MARTENSITE IN IRON PRECIPITATES

Copper-rich copper-iron alloys containing small defect-free iron precipitates, which in the austenitic condition have practically the same lattice parameter as the surrounding copper matrix, transform to martensite only when the alloy is cold worked. The transformation occurs by the formation of laths with a Kurdjumov-Sachs orientation relationship. Strain energy calculations indicate that the driving force for the transformation in these precipitates, once having introduced the necessary defect, is large enough to accommodate a lath-shaped martensite in agreement with observation. Energy calculations show that the spontaneous transition of an austenite sphere to a martensite ellipsoid is unlikely; furthermore dislocations moving close to precipitates have no chance of triggering off the transformation. The critical step to bring about the transformation is for the precipitate to be cut by the moving matrix dislocation. However, precipitates smaller than about 200 Å in diameter do not transform even although they are cut by dislocations and on this basis it is shown that there is a critical size of martensite nucleus which is only a few atoms in thickness. © 1969.