Deformation induced martensite formation in metastable austenitic steel during in situ fatigue loading in a scanning electron microscope

Abstract
Aim of the study is to identify quantitatively the influence of deformation-induced phase transformation on the fatigue damage of a metastable austenitic steel during loading in the high cycle fatigue regime. Cyclic deformation tests were carried out in situ in a scanning electron microscope (SEM) in combination with automated electron backscatter diffraction (EBSD) used for phase analysis and crystallographic orientation mapping. The in situ experiments were supported by ex situ cycling in a servohydraulic testing machine. The examined metastable austenitic steel (AISI 304L) transforms diffusion less from the fcc austenite lattice to the bcc α´ martensite lattice either spontaneously at very low temperatures or at room temperature when a critical value of monotonic or accumulated cyclic plastic strain is exceeded. The experiments showed that already after some initial 10,000 cycles of fatigue loading at stress amplitudes close to the fatigue limit a nucleation of martensite occurs as needles near activated slip systems as a consequence of localized plastic deformation. Once first microstructurally short cracks have nucleated, strong martensitic transformation occurs within the plastic zone ahead of the crack tip. Due to the higher specific volume the martensite is considered to shield the crack tip, i.e., transformation-induced crack closure takes place. The role of deformation-induced phase transformation on (i) crack initiation and (ii) the mechanism of fatigue microcrack propagation is discussed in detail in the present paper.