Duplex stainless steels (DSS) are defined by their equal phase composition of ferrite and austenite. However, the in-situ formation of this duplex microstructure in laser-based additive manufacturing ...(AM) is still a challenging topic. Nanoparticle addition is a promising approach to tailor the microstructure of steels in AM. Therefore, DSS doped with 0.5 wt.-% NiO nanoparticles was fabricated by laser-based powder bed fusion (PBF-LB) and directed energy deposition (DED-LB). While having no impact on the phase composition in PBF-LB, the addition of NiO nanoparticles showed a significant increase in austenite content of 9% compared to the unmodified powder in DED-LB.
The yield-point phenomena in ferrite–pearlite duplex steels were investigated using multiscale computational simulations. In these multiscale simulations, the stress–strain relationship of the ...ferrite phase was characterized by an elastoplastic constitutive model considering the stress-drop behavior, and its material constants were determined by minimizing the residual error between a computational simulation and a tensile test experiment, where the yield-point phenomenon in a tensile test of ferrite steel was reproduced. Using the determined material response of the ferrite phase, finite element analyses of the ferrite–pearlite duplex microstructure were executed to scrutinize both the macroscopic material response and microscopic deformation mechanisms. Subsequently, finite element analyses of tensile tests, based on numerical results from microstructural analyses, were carried out to replicate the yield-point phenomena in ferrite–pearlite duplex steels. Consequently, the study characterized the strengthening effect of the pearlite constituent while considering microscopic heterogeneity and yield-point phenomena in the ferrite phase. The findings from the multiscale simulations underscored the necessity for a more accurate estimation of local mechanical properties in both the ferrite phase and pearlite constituent for quantitative simulations.
The yield-point phenomena in ferrite–pearlite duplex steels were investigated using multiscale computational simulations. In these multiscale simulations, the stress–strain relationship of the ...ferrite phase was characterized by an elastoplastic constitutive model considering the stress-drop behavior, and its material constants were determined by minimizing the residual error between a computational simulation and a tensile test experiment, where the yield-point phenomenon in a tensile test of ferrite steel was reproduced. Using the determined material response of the ferrite phase, finite element analyses of the ferrite–pearlite duplex microstructure were executed to scrutinize both the macroscopic material response and microscopic deformation mechanisms. Subsequently, finite element analyses of tensile tests, based on numerical results from microstructural analyses, were carried out to replicate the yield-point phenomena in ferrite–pearlite duplex steels. Consequently, the study characterized the strengthening effect of the pearlite constituent while considering microscopic heterogeneity and yield-point phenomena in the ferrite phase. The findings from the multiscale simulations underscored the necessity for a more accurate estimation of local mechanical properties in both the ferrite phase and pearlite constituent for quantitative simulations.
Density functional theory (DFT) calculations were used to model G-phase precipitates of formula X6M16Si7 where X is Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W and Zr and M is either Fe or Ni. It was found that ...the occupancy of the d-orbital is correlated to the formation enthalpies of each structure. Past thermal expansion coefficient data was used to predict the lattice misfit between each G-phase and body centred cubic (BCC) Fe. All except Hf and Zr containing G-phases were predicted to have zero misfit between 581−843 K. Of the Ni containing G-phases, Mn6Ni16Si7 was predicted to have the most similar elastic properties to BCC Fe. DFT calculations of the substitution energies of Al, Cr Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies onto the Mn6Ni16Si7 G-phase from BCC Fe were performed. It was predicted that Cu, P and vacancies favour G-phase substitution. Suppression of the G-phase is predicted when Si content is reduced by half, at which point the BCC phase is favoured. It is hypothesised that including Zr to form a (Mn,Zr)6Ni16Si7 precipitate will allow for higher ageing temperature and expediate nucleation in an Fe alloy. Thermocalc was used to predict that a mixture of FebalCr9Ni4Si2(Mn0.6Zr0.4)1.2 (at.%) will produce a G-phase strengthened Fe alloy with potential for a good balance of strength, ductility and oxidation/corrosion resistance at room temperature. This alloy composition was experimentally determined to precipitate the G-phase in ≤24 h with cube-on-cube orientation to the BCC Fe matrix.
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Yield-point phenomena in Ferrite–Pearlite duplex steels were investigated using multi-scale computational simulations. In this multi-scale simulations, stress–strain relationship of Ferrite phase was ...characterized by an elastoplastic constitutive model considering yield-drop behavior and its material constants were determined by minimizing residual error between a computational simulation and experiment of tensile test, where yield-point phenomenon in a tensile test of Ferrite steel was reproduced.Using the determined material response of Ferrite phase, finite element analyses of Ferrite–Pearlite duplex microstructure were performed to examine its macroscopic material response and its microscopic deformation mechanism. Besides, finite element analyses of tensile test based on the numerical results of microscopic finite element analysis were conducted to reproduce yield-point phenomena in Ferrite–Pearlite duplex steels.
Yield-point phenomena in Ferrite–Pearlite duplex steels were investigated using multi-scale computational simulations. In this multi-scale simulations, stress–strain relationship of Ferrite phase was ...characterized by an elastoplastic constitutive model considering yield-drop behavior and its material constants were determined by minimizing residual error between a computational simulation and experiment of tensile test, where yield-point phenomenon in a tensile test of Ferrite steel was reproduced.Using the determined material response of Ferrite phase, finite element analyses of Ferrite–Pearlite duplex microstructure were performed to examine its macroscopic material response and its microscopic deformation mechanism. Besides, finite element analyses of tensile test based on the numerical results of microscopic finite element analysis were conducted to reproduce yield-point phenomena in Ferrite–Pearlite duplex steels.
•A new microstructure-informed three-scale homogenization scheme for elasto-viscoplastic heterogeneous materials is developed.•The model is applied to predict the local and overall mechanical ...behaviors of cast duplex austenitic-ferritic (AF) steels.•The model is inferred from EBSD analyses of the microstructural data.•he stress/strain responses in ferrite and austenite are correlated with the different variants and habit planes considering the Kurdjumov-Sachs orientation relationship.•The effects of primary ferritic grain orientation and aging on the internal mechanical states and on backstress are studied in the light the three-scale model.
A new microstructure-informed three-scale homogenization scheme for elasto-viscoplastic heterogeneous materials is developed. It is applied to predict the mechanical behavior of cast duplex austenitic-ferritic (AF) steels, which are widely used in primary loop of pressurized water reactors (PWR). At the first scale (microscale), the elasto-viscoplastic behavior of single crystals for both phases is modeled using linear elasticity and a viscoplastic crystal plasticity model. An “affine” type formulation based on the first moments of stresses is applied to the inelastic non-linear part of the deformation. Then, at the second scale (mesoscale), an EBSD-informed two-phase austenite/ferrite laminate structure (LS) model is developed with {110}-type habit planes (HP) in ferrite and a Kurdjumov-Sachs orientation relationship (KS-OR) between both phases. At the third scale (macroscale), the model considers a single ferritic primary grain as an aggregate of spherical two-phase laminate structure domains corresponding to the 24 KS-OR variants. The elasto-viscoplastic self-consistent scheme (EVPSC) is used through the Translated Fields (TF) method to obtain the effective behavior at this scale. The TF-EVPSC scheme is also used for an ensemble of primary ferritic grains to identify materials parameters from experimental tensile curves corresponding to as received and aged specimen. From EBSD measurements, the crystallographic data are thoroughly analyzed to physically feed the three-scale model. The results are discussed in terms of stress/strain responses in ferrite and austenite, which are correlated to the different KS-OR variants and HP orientations. The effect of primary ferritic grain crystallographic orientation and aging is also studied regarding monotonic and cyclic tests to study the possible origins of backstress in this material.
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Duplex and super-duplex stainless steels are increasingly used in applications where good fatigue strength is demanded in addition to corrosion resistance. In this research work, the fatigue strength ...of duplex and super-duplex steels was investigated experimentally, using standard fatigue strength assessment methods, and theoretically, using conventional design methods and a novel effective notch stress-based procedure, the 4R method. The experimental tests included testing of welded joints with and without post-weld treatment. The experimental results were compared with the 4R method. The test results indicated good fatigue strength properties for both materials in the as-welded (ASW) condition, and post-weld treatment by high frequency impact (HFMI)-treatment improved the fatigue resistance at low stress ratios. No improvement, however, was found in the case of high mean stress of the applied load. The results obtained by the theoretical investigation agreed quite well with the experimental results.
Yield-point phenomena in Ferrite–Pearlite duplex steels were investigated using multi-scale computational simulations. In this multi-scale simulations, stress–strain relationship of Ferrite phase was ...characterized by an elastoplastic constitutive model considering yield-drop behavior and its material constants were determined by minimizing residual error between a computational simulation and experiment of tensile test, where yield-point phenomenon in a tensile test of Ferrite steel was reproduced. Using the determined material response of Ferrite phase, finite element analyses of Ferrite–Pearlite duplex microstructure were performed to examine its macroscopic material response and its microscopic deformation mechanism. Besides, finite element analyses of tensile test based on the numerical results of microscopic finite element analysis were conducted to reproduce yield-point phenomena in Ferrite–Pearlite duplex steels.
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