Ermüdungsverhalten nanostrukturierter Bainite

• Antragstellerin: Dr. Lucia Morales-Rivas
• Fachliche Zuordnung: Mechanische Eigenschaften von metallischen Werkstoffen und ihre mikrostrukturellen Ursachen ; Metallurgische, thermische und thermomechanische Behandlung von Werkstoffen
• Förderung: Förderung von 2018 bis 2022
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 411091845

• Erstellungsjahr: 2022

Zusammenfassung der Projektergebnisse

Nanostructured bainitic steels owe their good mechanical properties to their microstructure, basically consisting of two phases, bainitic ferrite and retained austenite. The absence of coarse carbides is related to the C enrichment of austenite during the transformation, stabilizing this phase and making possible its presence at room temperature. Retained austenite features present typically very heterogeneous morphologies: from films with thicknesses down to the nanoscale, to sub-micrometric or coarser blocks. Knowing the influence that the microstructural features and the different crystallographic boundaries have over the fatigue response has been long wished, especially with the purpose to develop and fully exploit novel carbide-free bainitic microstructures obtained by efficient steel processing routes. Results from a previous project indicated the capability of high-angle boundaries to deflect the fatigue crack path at the earlier stage of growth. However, many questions remained: the effect of crystallographic boundaries on crack arrestment, the role of retained austenite, and the performance variations at different fatigue stages and conditions, including the presence of notches and defects. New results and interpretations derived from the present DFG project, based on fatigue testing of both blunt-notched and artificial defect-containing specimens, suggest that, as opposed to conventional bainitic steels, there is a strong coupling between crack nucleation and crack growth events. It means that nucleation of a propagating crack requires not only to exceed certain stress values, but to have a large volume of material subjected to such nucleation-critical conditions. It would explain the unexpected high divergence between Kf and Kt in notched specimens. Likewise, that theory is consistent with the observation of multiple crack nucleation and coalescence, a mechanism that allows the crack to propagate, "avoiding” the more restrictive stress intensity factor threshold for long cracks. Nucleated micrometric cracks have been observed to get confined within bainite blocks, i.e. high-angle grains of bainitic ferrite which also contain thin films of austenite (the latter normally not resolved under EBSD techniques). Crack arrestment at boundaries between bainitic ferrite and thick features of austenite has been observed at least at fatigue stage Ia, while crack deflection at such boundaries has also been observed, not only at stage Ia, but also at stage Ib. In consequence, minimizing microstructural texture (or optimizing it if loading conditions are well defined), the bainite block size, and finding a compromise between the size of retained austenite features and their stability against mechanically-induced martensitic transformation are strategies to follow in future developments of advanced bainitic steels.

Projektbezogene Publikationen (Auswahl)

• Explaining the dilatometric behavior during bainite transformation under the effect of variant selection. Journal of Alloys and Compounds, 864, 158130.
  Eres-Castellanos, Adriana; Morales-Rivas, Lucia; Caballero, Francisca G. & Garcia-Mateo, Carlos
  
• Fine granular area linked to very high cycle fatigue in martensitic and bainitic steels: Characterization by means of EBSD-dictionary indexing. Scripta Materialia, 194, 113644.
  Morales-Rivas, Lucia; Ram, Farangis; Spriestersbach, Daniel; Sippel, Jan; De Graef, Marc & Kerscher, Eberhard
  
• Parent austenite reconstruction tolerant to 180° ambiguity: Application to a very-high-cycle fatigue tested bearing-steel. Materials Characterization, 178, 111253.
  Morales-Rivas, Lucia & Sundaram, Ganesh
  
• Fatigue behavior of nanostructured bainite: A morphological study of crack path. Procedia Structural Integrity, 39(2022), 515-527.
  Morales-Rivas, Lucia; Azadi, Ava & Kerscher, Eberhard
  
• Viewpoints on Technological Aspects of Advanced High-Strength Bainitic Steels. Metals, 12(2), 195.
  Morales-Rivas, Lucia
  
• Short fatigue crack growth in an artificial-defect-containing nanostructured bainitic steel. International Journal of Fatigue, 166, 107219.
  Morales-Rivas, Lucia; Azadi, Ava & Kerscher, Eberhard