The ever-increasing demand for designing high-specific strength metallic materials with enhanced ductility suitable for automotive applications has led to the development of low-density steels (LDS) which are primarily constituted by Fe, Mn, Al and C. These steels mainly derive their strength from nano-scale precipitates (intermetallics) formed in a disordered solid solution matrix. The mechanical response of these LDS depends on a number of factors including the size, morphology, distribution and volume fraction of these precipitates which necessitates their near atomic-scale analysis. Hence, an overall microstructure-property correlation is highly crucial for service-life prediction of these materials. To this end, this seminar presents the temporal evolution of different phases and the associated mechanical response of 5Ni-alloyed Fe-16Mn-9Al-0.9C (wt.%) LDS susceptible to nano-scale precipitation. After subjecting the as-cast steel to a series of thermomechanical processing treatments including hot-rolling, the LDS was subjected to thermal (annealing) treatments in the temperature range of 600-1200 ᵒC with 30 min holding time. Bulk phase analysis using X-ray diffraction (XRD) showed the presence of three phases, namely FCC structured γ, L12’ structured κ and BCC phases in as-rolled condition which is in good agreement with CALPHAD-based thermodynamic phase stability estimates. Combined microscopic analysis involving three-dimensional atom probe tomography (3D-APT) coupled with small-angle X-ray scattering (SAXS) based structural analysis revealed the localisation of nano-scale κ and B2 precipitates within majority γ and banded BCC structured regions respectively. Following this, bulk mechanical response was studied under uniaxial tensile and compressive testing with varied thermal treatments. To understand the contribution of different phases to the overall strength of the steel, site-specific nanomechanical responses from different phases using nanoindentation was performed which indicated the relatively higher strength and reduced modulus of the banded BCC structured phase. Further, the role of different interfaces towards influencing the overall strengthening response was studied using novel correlative transmission Kikuchi diffraction (TKD)-APT analysis which also indicated that the interfaces were devoid of any preferential solute segregation. Therefore, the Gibbsian interfacial excess calculation was performed by the intentional addition of tracer element. Finally, deformation micro-mechanisms were investigated from the deformed microstructures of samples after tensile and compressive tests. The novel experimental approaches employed in this work, revealed the localization of nano-scale ordered precipitates in a predominantly disordered solid solution matrix which strengthened the steel. Therefore, a systematic microstructure-processing-property correlation was performed which further provides a pathway for the design of next-generation automotive grade Fe-Mn based LDS with enhanced mechanical properties.
Reference
Saha, M., Ponnuchamy, M.B., Sadhasivam, M. et al. Revealing the Localization of NiAl-Type Nano-Scale B2 Precipitates Within the BCC Phase of Ni Alloyed Low-Density FeMnAlC Steel. JOM 74, 3181–3190 (2022). https://doi.org/10.1007/s11837-022-05349-2.