In the present study, the influence of the deformation-induced non-diffusional phase transformation from austenite into martensite of a metastable stainless steel on the short crack growth behaviour in the high cycle fatigue regime was investigated. For this purpose in situ as well as ex situ cyclic deformation experiments in combination with scanning electron microscopy and supplementing imaging and analytical techniques (i.e., electron backscatter diffraction for orientation and phase analysis) were employed. Cyclic loading of the AISI304L stainless steel investigated at stress amplitudes slightly above the classical fatigue limit was found to lead to a local transformation from the face centred cubic (fcc) austenite into the body centred cubic (bcc) α´-martensite. The nucleation takes place in selected areas of the microstructure in form of lamellae even already after few initial cycles near active slip systems of high Schmid factors of almost 0.5. In the majority of cases cracks initiate at twin boundaries in the absence of any martensite formation. After a first stage of shear-stress-controlled propagation along the respective twin boundary, the cracks change growth mode and direction and continue to grow in a transcrystalline manner perpendicular to the loading axis. In this stage, an alternating growth on two adjacent {111} slip systems results in low-indexed crack planes such as {100} or {110}. As a consequence of the plastic deformation at the tip of the propagating short crack a transformation into α´-martensite occurs. This martensite formation results in an increase of the specific volume and gives rise to compressive stresses leading to premature contact of the crack surfaces and hence to a retardation of the short crack growth.
Fatigue life of structures made of metastable austenitic steels, e.g., in civil engineering or chemical industry, depends strongly on the phenomenon of strain-induced phase transformation from g austenite to a 0 martensite. Due to the higher strength and the higher specific volume of the martensite phase, pronounced cyclic hardening throughout the fatigue life [1] and transformation-induced crack closure during fatigue crack propagation [2,3] can be observed.Contrary to stable austenitic steels, the Ni concentration in the metastable materials is rather low (approximately 8 wt%), and hence, cooling should result in g austenite, d ferrite, and a 0 martensite, according to the Schaeffler diagram, [4] as shown in Figure 1.From a thermodynamic point of view, cooling to room temperature is not sufficient to cause spontaneous transformation. Only when mechanical strain contributes with DG mech to the energy level of the parent austenite phase, the critical Gibbs energy difference DG as driving force for martensite formation is reached. This is schematically shown in Figure 2. By means of the following phenomenologic equations (cf. [5] , the concentration of alloying elements to be inserted in wt%) the martensite start temperature M s and the temperature M d30 where 30% plastic strain results in 50% martensite formation can be calculated:M d30 ¼ 413 À 462ð%C þ %NÞ À 9; 2ð%SiÞ À 8; 1ð%MnÞ À 13; 7ð%CrÞ À 20ð%NiÞ À 18; 5ð%MoÞAccording to earlier work, [6,7] a 0 martensite nucleates at hcp e marteniste band intersections. The e martensite bands are caused by an accumulation of stacking faults in the {111} slip planes of the fcc austenite phase, i.e., plasticity occurring in the g austenite leads to the formation of shear bands which transforms to e martensite with the {0001} basal planes being parallel to the {111} slip planes of the parent austenite phase. [7] During monotonic tension the transformation into a 0 martensite follows a sigmoidal relationship, as it is described COMMUNICATION [**] Acknowledgements, The financial support by Deutsche Forschungsgemeinschaft and the material supply by Deutsche Edelstahlwerke Südwestfalen are gratefully acknowledged.High cycle fatigue (HCF) life of metastable austenitic steels is governed by the ability of the parent austenite phase to transform into a 0 martensite via metastable e martensite. The mechanism of this strain-induced transformation is closely related to the grain size, the crystallographic orientation distribution, as well as to amplitude, and cyclic accumulation of plastic strain. Aim of the present study is to identify and to quantitatively describe the basic principles of strain-induced martensite formation by means of in situ cyclic deformation experiments in a scanning electron microscope (SEM) in combination with electron back-scattered diffraction (EBSD) and numerical modeling using a boundary element approach. It was shown that during HCF loading martensite formation is inhomogeneous and not directly linked with crack initiation. Only when the fatigue crack pro...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.