Influence of Temperature on Fatigue-Induced Martensitic Phase Transformation in a Metastable CrMnNi-Steel
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UCTION
THE deformation behavior of high-alloy metastable austenitic steels is controlled primarily by the austenite stability and the stacking fault energy csf,[1–5] which are influenced primarily by the chemical composition and temperature.[6–8] Thus, depending on these parameters, different deformation mechanisms in terms of (i) deformation-induced transformation of austenite into e- and a-martensite,[9] (ii) deformation-induced twinning[10,11] and (iii) conventional dislocation glide are distinguished. In this study, e-martensite is regarded as a high density of stacking faults on each second lattice plane leading to a hexagonal structure (ABAB stacking sequence of lattice planes).[12] The transformation into emartensite occurs most readily at csf < 15 to 20 mJ/m2, whereas at 15 to 20 mJ/m2 < csf < 40 to 50 mJ/m2, deformation-induced twinning is the dominant deformation mechanism. Finally, a value of csf > 40 to 50 mJ/m2 supports conventional dislocation glide.[13,14]
HORST BIERMANN, Professor, and ALEXANDER GLAGE and MATTHIAS DROSTE, Research Assistants, are with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg, 09599 Freiberg, Germany. Contact e-mail: biermann@ ww.tu-freiberg.de Manuscript submitted September 22, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
It is important to note that the transitions between the different deformation mechanisms are fluent and the respective values of the stacking fault energy depend on the alloying system.[15] The austenite stability can be estimated using the parameters of the martensite start temperature Ms for athermally induced a-martensite and the Md/Md30 temperatures for deformation-induced martensite. The Md temperature represents the highest temperature at which deformation-induced a-martensite occurs, whereas at a temperature of Md30 50 pct a-martensite are formed due to a true strain of 30 pct. Various research works have observed the martensitic phase transformation under cyclic loading in several metastable austenitic steels of different chemical composition—see for instance.[16–30] The deformation-induced martensitic phase transformation under cyclic loading has an important impact on the cyclic deformation behavior of metastable austenitic steels. It is generally agreed that there is a significant difference in the fatigue lifetime between strain-controlled and stresscontrolled tests.[16,31] In the latter case the martensitic transformation results in a fatigue-life improvement as a result of the increasing strength, and thus reduced plastic deformation. Strain-controlled tests at high strain amplitudes in the low cycle fatigue (LCF) regime reveal a fatigue life reduction which is attributed to the remarkable cyclic hardening caused by the martensitic
phase transformation. Moreover, some authors state that a-martensite is a preferred site for crack nucleation.[17,32] Conversely, at low strain amplitudes in the high cycle fatigue (HCF) regime, a fatigue-life improvement was observed which correlated with the strengthening effec
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