Low cycle fatigue behavior of high strength gun steels

The low cycle fatigue (LCF) behavior of two high strength steels, withnominal chemical compositions (mass fraction, %) of 0.40C-1.5Cr-3Ni-0.4Mo-0.2V (PCrNi3MoV) and0.25C-3Cr-3Mo-0.8Ni-0.1Nb (25Cr3Mo3NiNb), was investigated by using the smooth bar specimenssubjected to strained-controlled push-pull loading. It is found that both steels show cyclicsoftening, but 25Cr3Mo3NiNb steel has a lower tendency to cyclic softening. 25Cr3Mo3NiNb steel hashigher fatigue ductility, and its transition fatigue life is almost three times that of PCrNi3MoV.25Cr3Mo3NiNb steel also shows higher LCF life either at a given total strain amplitude above 0.5% orat any given plastic strain amplitude, despite its lower monotonic tensile strength than that ofPCrNi3MoV. It also means that 25Cr3Mo3NiNb steel can endure higher total strain amplitude andplastic strain amplitude at a given number of reversals to failure within 10~4. 25Cr3Mo3NiNb steelis expected to be a good gun steel with high LCF properties because only several thousand firingsare required for gun barrel in most cases.

Ab Initio Investigations for the Role of Compositional Complexities in Affecting Hydrogen Trapping and Hydrogen Embrittlement:A Review

ydrogen embrittlement(HE)seriously restricts the service safety of structural metallic materials applicate in aerospace,ocean,and transportation.Recent studies aiming at increasing the HE-resistance have been focusing on trapping diffusible H atoms by inherent microstructural features in materials.Alloying-induced compositional complexities,including different types of solute atoms,lattice chemical heterogeneities,and carbide precipitates,have attracted research efforts regarding the H trapping capabilities and potential to reduce the susceptibility to HE.In this paper,we review recent progress in exploiting compositional complexities to regulate the hydrogen trapping characteristics and mechanical properties in H-containing environments.The focus is placed on results and insights from ab initio calculations based on density functional theory(DFT).Quantitative predictions of trapping parameters and atomic scale details that are hardly to be gained through traditional experimental characterizations are provided.Additionally,we overview the electronic/atomistic mechanisms of H trapping energetics in metallic materials.Finally,we propose some key challenges and prospects in simulation of defect interactions,interpretation of experimental characterizations,and developing microstructure-based H diffusion prediction models.For the applications of first principle calculations,we illustrate how the DFT data can complement experimental characterizations to guide composition and microstructure design for better HE-resistant materials.