Determination of stacking fault probability in fcc Fe–Mn–Si–Al alloy by electron diffraction

Abstract

The stacking disorder in the fcc structure of Fe–23Mn–2Si–2Al alloy after tensile testing at −75°C was investigated by electron diffraction, using a relationship between the stacking fault probability and the shift of diffraction spots deduced in the present work (and based on Kakinoki’s theory [Acta Crystallogr. 23, 875 (1967)] and Kajiwara’s previous work [Jpn. J. Appl. Phys. 9, 385 (1970); J. Phys. Soc. Jpn. 22, 795 (1967)]). Shifts of diffraction spots along the [111] direction in the fcc structure were observed, and the stacking fault probabilities in two selected areas with different densities of stacking faults were determined as α=0.15 and α=0.35. The stacking fault probabilities measured by electron diffraction are much larger than average values determined by x-ray diffraction, indicating that the distribution of stacking faults is localized. A mechanism for the γfcc→εhcp transformation is suggested, whereby during deformation the following sequences take place: dissociation of perfect dislocation→localization of stacking faults→evolution from stacking disorder to stacking order→the formation of the perfect hcp martensite (α=1) or the hcp martensite with stacking faults (α→1). This proposed mechanism for strain-induced hcp martensite formation should be also applicable to the thermally induced hcp martensite.