Uncovering Microstructure-Property Relationship in Ni-Alloyed Fe-Mn-Al-C Low-Density Steel Treated by Hot-Rolling and Air-Cooling Process

This paper focuses on the relationship between the microstructure and tensile properties of Fe-Mn-Al-C low-density high-strength steel processes by hot-rolling and air-cooling process. The microstructure analysis reveals that the combination of hot-rolling and air-cooling results in the formation of heterogeneous structures comprising different-sized γ and B_(2) phases in the low-density steel with the addition of nickel (Ni). The addition of Ni promotes the formation of the B_(2) phase and induces the pinning of B_(2) phase particles at the γ grain boundaries. This pinning effect effectively hinders the growth of the γ grains, leading to grain refinement. The tensile test results demonstrate that LDS-5Ni (low-density steel, LDS) exhibits excellent high strength and ductility combination, e.g., a tensile strength of 1535 MPa, yield strength of 1482 MPa, and elongation of 23.3%. These remarkable mechanical properties are primarily attributed to the combined strengthening contributions of grain refinement and duplex nano-sized second-phase precipitation hardening.

Tempering Behavior of Ductile 1700 MPa Mn-Si-Cr-C Steel Treated by Quenching and Partitioning Process Incorporating Bainite Formation

We obtained a good combination of strength and ductility in a 0.4C-2.0Mn-1.7Si-0.4Cr（wt%） steel,namely,;.7 GPa of ultimate tensile strength and;6% of elongation,by conducting a Q-P-T（quenching-partitioning- tempering） process incorporating the formation of carbide-free bainite. The tempering behavior of this steel was discussed by using experimental finding(scanning electron microscopy,X-ray diffraction（XRD）,transmission electron microscopy and dilatometer) and CCE（constrained carbon equilibrium） modeling. The XRD results combined with CCE calculation prove that carbon partitioning from martensite to austenite occurs during tempering. Consequently,the thermodynamic stability of retained austenite is enhanced. This idea can be utilized to design novel Q-P-T processes in future.

Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite,retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue(HCF/VHCF)regimes.The subsurface microstructural fatigue crack initiation,referred to as“non-inclusion induced crack initiation,NIICI”,is a leading mode of failure of bainitic steels within the HCF/VHCF regimes.In this regard,there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF.To address this aspect,we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure.This was accomplished by uniquely combining electron microscopy,three-dimensional confocal microscopy,focused ion beam,and transmission Kikuchi diffraction.Interestingly,the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels,including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI,which resulted in the different distribution of fine grains in the crack initiation area.The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries.The formation of fine grains assisted the advancement of small cracks.Another important aspect discussed is the role of retained austenite(RA)during cyclic loading,on crack initiation and propagation in terms of the morphology,distribution and stability of RA,which determined the development of localized cyclic plastic deformation in multiphase structure.