Deformation induced martensitic transformation of metastable stainless steel AISI 301

Abstract: Metastable stainless steels are promising engineering materials demonstrating good corrosion resistance and mechanical properties. Their mechanical properties are however significantly affected by the deformation induced martensitic transformation. Hence, in order to use these steels to their full potential it is vital to have profound knowledge on this martensitic phase transformation. The aim of this thesis was therefore to investigate the evolution of phase fractions, texture, microstrains and microstructure to improve the current understanding of the deformation induced martensitic transformation in AISI 301. To investigate the deformation behavior of AISI 301, in-situ high-energy x- ray diffraction during tensile loading has been performed on samples suffering different cold rolling reduction. Ex-situ transmission electron microscopy, electron back-scattered diffraction and optical microscopy were also used to characterize the microstructure at different deformation levels. The results show that parts of the austenite transforms to both ά- martensite and ε-martensite during deformation of AISI 301. The transformation behavior of ά-martensite is however completely different from the transformation behavior of ε-martensite. ε-martensite forms in a parabolic behavior, while the ά-martensite transformation can be divided in three characteristic stages. The third transformation stage of ά-martensite has previously not been reported and it is characterized by a series of rapid transformations, each of which is followed by a period of yielding without any transformation. Moreover, the lattice strain evolution in the austenite at high plastic strains was found to be oscillatory, which is correlated with the stepwise transformation of ά-martensite as well as changes in x-ray peak broadening. This behavior was also coupled with the evolution of microstructure, where a distinct banded structure consisting of slip bands and Ü-martensite was observed at low plastic strains. This banded structure was however broken at high plastic strains when the ά-martensite grew larger and formed a block- shaped morphology. These findings lead to the conclusion that the three stages of ά- martensite transformation is due to different stages of nucleation and growth. The ά-martensite will first form as small nucleus, mainly at dislocation pile-ups along slip bands. The nucleuses will grow moderately in size and the structure will become saturated with nucleuses. Hence, the only way more ά-martensite can form is by growth of the existing nucleuses. This growth is very localized and seen as bursts in the transformation curve. The oscillatory behavior observed for the lattice strains during martensite formation possibly originate when semicoherent boundaries between austenite and ά-martensite become incoherent as the ά-martensite grow large.