On the Mechanism of Martensite Formation during Short Fatigue Crack Propagation in Austenitic Stainless Steel:Experimental Identification and Modelling Concept

摘要

During fatigue of metastable austenitic steels,local plasticity is linked with the formation of martensite.It was shown that cubic a' martensite nucleates at slip band and 8 martensite intersection sites ahead of growing fatigue cracks.Since such intersections require the operation of alternate slip systems,martensite formation was found only for shear-controlled single-slip crack propagation (mode iD prevailing during the early propagation phase of microstructurally short fatigue cracks.Surprisingly,cracks were found not to initiate within martensitically transformed grains but by a fraction of 70％ along twin boundaries.The extent of martensite formation ahead of a propagating crack increases with increasing crack length and eventually,gives rise to transformation-induced crack-closure effects.The interaction between the crack tip plasticity and the local microstructure was quantitatively analysed by automated electron back-scatter diffraction (EBSD) in the scanning electron microscope (SEM) applied to the electro-polished surfaces of fatigue specimens.Following a geometrical model for martensitic transformation,further analysis of the relevant fatigue mechanism became possible aiming to both mechanism-based life prediction and tailoring fatigue resistant microstructures.