A popular approach of obtaining improved combinations of properties involves the inclusion of two or more phases within a microstructure, each with substantially different properties. For instance, recent publications have shown that dual-phase steels use a hard martensite second phase within a soft ferrite matrix to create an alloy with a good combination of strength and ductility. Ideally, dual phase steels typically have the soft ferrite as the matrix phase to enable good ductility and work hardening rates, with the harder phase being added to improve strength. The major shortfall of the aforementioned microstructure design methodology is the partitioning of strain from the hard phase onto the soft phase, due to the mismatch in strength between the ferrite matrix and the harder martensite phase. Noteworthy literature recently reported on the development of laboratory-based series of steels that show excellent combinations of strength and ductility in axial loading compared to conventional dual phase steels. However, it is not yet clear whether these new bainite steels will have the same characteristics as the martensite steels.
Further, some bainites have been reported to retain soft films of austenite between the bainite laths. It remains largely unstudied on what happens to this softer phase during cyclic loading, and how much strain can actually be accommodated under these conditions. On this account, Professor Nikki Stanford from the University of South Australia together with Dr. J. Wang and Dr. T. Hilditch at the Deakin University in Australia examined the new steels that contain hard bainite matrix phases. To be specific, they focused on studying three multi-phase steels, all comprising 80% bainite and 20% ferrite under low cycle fatigue. Their work is currently published in International Journal of Fatigue.
In their work, the researchers used micro-grid based local strain measurements and finite element modelling to assess the impact of the strain partitioning on the low cycle fatigue behavior of the new steels. Generally, various microstructure characterization techniques were used, such as; Supra Scanning Electron Microscope and FEI Quanta dual beam focused ion beam SEM.
The authors reported that under monotonic loading, the assessed steels exhibited high strengths and reasonable ductility of 10–20% elongation. Further, the researchers reported that under cyclic loading the steels exhibited an initial cyclic hardening followed by a cyclic softening response. In fact, when compared to traditional dual-phase steels, the bainitic alloys showed an inferior fatigue lifetime, attributed to strain partitioning to the ferrite.
In summary, the study by Professor Nikki Stanford and colleagues presented in-depth assessment of a set of three multi-phase steels, all comprising 80% bainite and 20% ferrite under low cycle fatigue. In the study, the local distribution of strain between the hard bainite and soft ferrite phases were quantified using a micro-grid on the sample surface. In a statement to Advances in Engineering, Professor Nikki Stanford explained that their finite element study revealed that the strength of the hard phase must be less than 3 times the strength of the soft phase so as to minimize stain partitioning.
N. Stanford, J. Wang, T. Hilditch. Quantification of strain partitioning during low cycle fatigue of multi-phase steels containing a bainite matrix. International Journal of Fatigue, volume 129 (2019) 105218.