Si-modified Fe-Cr-Ni austenitic steels exhibit superior corrosion resistance in liquid lead-bismuth eutectic coolant; however, their application is limited by Si-induced austenite decomposition at elevated temperatures. This decomposition arises from pronounced Si rejection during M23C6 precipitation, thereby creating a local chemical environment favorable for α-ferrite formation. Herein, Nb addition is introduced to regulate Si redistribution and modify the decomposition pathway during aging at 550 °C for up to 3000 h. Nb addition suppresses M23C6 precipitation through preferential NbC formation and promotes the formation of (Nb, Si)-rich clusters via strong Nb-Si interactions. The Nb-modified austenite decomposition proceeds via two distinct pathways: an M6C-associated γ → α-ferrite transformation at intermediate aging times (∼500 h) and a G-phase-mediated γ → G-phase + α-ferrite eutectoid transformation during prolonged aging. The (Nb, Si)-rich clusters act as chemically favorable precursors for M6C carbide and G-phase nucleation, while the partitioning of Nb and Si mainly governs the transformation kinetics. A stabilization treatment involving the formation of dispersed nanosized NbC further suppress decomposition by reducing the availability of C and Nb in the austenite matrix. These findings demonstrate that Nb addition effectively mitigates Si-induced austenite decomposition by altering precipitation sequences and local solute redistribution, providing a strategy for improving the long-term microstructural stability of Si-modified austenitic steels.
NiAl-(Cr,Mo) in-situ composites such as NiAl-28Cr-6Mo (at.%) exhibit a high fracture toughness due to their directionally solidified two-phase microstructure. The reinforcing (Cr,Mo)-phase hinders the crack propagation through the brittle B2-NiAl-matrix. In this study, two different morphologies with a range of achievable phase thicknesses in a NiAl-28Cr-6Mo alloy are produced by additive manufacturing. This resulted in either an isotropic, interconnected rod network or a lamellar microstructure. Due to the small size of the phases, the fracture properties of the so formed nanostructured composites are analysed locally by focused ion beam (FIB) milled microcantilevers. It was shown for the first time that an isotropic, interconnected network exhibits a maximum fracture toughness of 9 MPa√m at a (Cr,Mo)-phase thickness of 50–70 nm. In contrast, the fracture toughness of the plate morphology (6–8 MPa√m) is largely independent of the (Cr,Mo)-phase thickness. However, (Cr,Mo)-plates with a favorable crack arrester orientation demonstrate significantly higher work of fracture due to active toughening mechanisms during extended stable crack growth, compared to the rod-network morphology or plates with unfavorable orientation. It could be shown that fundamental toughening mechanisms are also active in additive manufactured NiAl-(Cr,Mo) composites even down to the size of nano-structured plates.
Revealing the interaction between dopants and grain boundaries (GBs) is crucial for tailoring the material properties by elemental doping. It is generally believed that the segregation behavior of dopants depends on the initial structure of pristine GBs and the effect of GB segregation is localized since the dopants only affect the arrangement of neighboring atoms. In this study, aberration-corrected transmission electron microscopy observations reveal that pristine α-Fe2O3 possesses three types of (0001) coherent twin boundaries (TBs) with different displacement vectors, which evolve into the same TB after co-doping of Sr and Si. The dopant co-segregation at the TBs causes the localized TB structural transition of Fe2O3 grains, which consequently fundamentally alters the local atomic coordination and the relative alignment of the adjacent grains. First principles calculations suggest that the spontaneous segregation of large-sized Sr atoms induces a local interfacial modification and promotes the segregation of Si atoms. These findings suggest that dopants can not only completely modify the structure of GBs but also induce collective atomic rearrangements across the interface, thereby providing insights for atomic-scale interfacial engineering.
Understanding the kinetics of solid-vapor phase transitions at interfaces is essential for describing high-temperature material behavior under thermodynamic non-equilibrium conditions. While sublimation and evaporation rates have traditionally been characterized by the Hertz-Knudsen equation for decades, this classical framework cannot account for atomic-scale interface migration, where local non-equilibrium interface configurations lead to spatially non-uniform desorption kinetics. In this study, we establish an atomic-scale interface kinetic theory of non-equilibrium solid-vapor phase transition in crystalline metals. Atomic-resolution observations of non-uniform sublimation kinetics in the ultrathin Ag nanosheet using transmission electron microscopy reveal that transitions between different sublimation pathways are energy-dependent. The tendency toward surface energy minimization generates a net vapor flux, accompanied by variations in local energy barriers at atomic sites undergoing sublimation. By introducing surface step density as a structural state variable, we derive a rate equation for non-uniform sublimation that quantitatively predicts local atomic desorption rates along the interface. Extending this framework to nanoparticles and nanowires, we further develop a corresponding numerical model to track the resulting interface evolution, and the agreement between simulation and experiments across these nanostructures demonstrates the generality of the kinetic description and establishes a predictive framework for non-equilibrium interface migration in crystalline metals.
The structural and chemical properties of austenite/ferrite (γ/α) interfaces were investigated in a model duplex stainless steel (Fe–Cr–Ni–Mo–N) using a correlative approach combining scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDS), and atom probe tomography (APT). Two interface types were selected based on their orientation relationship (OR): a near-Kurdjumov-Sachs (K-S) interface and a non-K-S interface. The near K-S interface presents a planar terrace-ledge morphology with atomically sharp structural transitions at the terraces, whereas the non-K-S interface is rough with protrusions of 3–5 nm amplitude. In both cases, the chemical transition width measured by EELS and APT exceeds the crystallographic interface width determined from STEM imaging, indicating that chemical potential perturbations extend beyond the structural discontinuity. The OR is found to have a clear effect on the solute segregation behavior: Mo and N segregate exclusively to the non-K-S interface, while Cr segregates to both interface types. Ni shows no detectable interfacial excess at either interface. The 2D APT composition maps reveal a spatially heterogeneous distribution of Cr at the near K-S interface, with a strong spatial correlation between Cr and N enrichment attributed to their attractive thermodynamic interaction. Gibbsian interfacial excess values and segregation energies are reported for all solutes, distinguishing between effective and intrinsic contributions. Thermodynamic self-consistency of the derived segregation energies is only achieved for an interface width of 1–2 nm, consistent with the chemical width from EELS. This provides an experimentally grounded estimate of the interface width directly applicable to solute drag models of γ/α interface migration.
