Fe 10Ni Research Articles

Selective oxidation of FeNiCrAl-based alloys under low oxygen pressure and their coking resistance

Selective oxidation of Fe–25Ni–20Cr–xAl-based alloys under low oxygen pressure was studied to improve oxidation and coking resistance. The morphology and oxidation kinetics of the oxide layer were analyzed using multi-scale techniques such as scanning electron microscopy, electron probe micro-analysis, and X-ray photoelectron spectroscopy. The results indicated that when the Al content exceeded 2.5 wt%, a duplex-structure oxide film, mainly consisting of M2O3 (M: Al and Cr) outer and Al2O3 inner layer, was formed on the surface of the alloy after pre-oxidation. When the Al content exceeded 8 wt%, a complete and dense Al2O3 external oxide film was formed through a two-step oxidation process. The decrease in oxygen partial pressure and surface enrichment of aluminum during the pre-oxidation process accelerated the formation of an external Al2O3 oxide film. Therefore, it isolated the catalytic coking reaction of Fe and Ni transition metals in the alloy with the pyrolysis atmosphere, significantly improving the anti-coking performance of the alloy.

Creep deformation and damage behavior of an alumina-forming austenitic alloy strengthened by intermetallic compounds

Based on Fe–30Ni–15Cr, a new alumina-forming austenitic (AFA) alloy, free of carbon, was designed for potential use in ultra-supercritical generator set. The creep behavior of alloy has been investigated by analyzing the creep constitutive equation, creep damage tolerance factor (λ), microstructure images, and oxide layer structure under different stresses at 750 °C. The creep microstructure and oxide morphology were obtained by electron microscopy. The minimum creep rate (ε˙m) is positively correlated with the stress, and when the stress increases from 120 MPa to 250 MPa, ε˙m increases from 6.26E-3 1/h to 6.34E-4 1/h, almost ten times of the latter. The creep constitutive equation at 750 °C was determined as: ε˙m=9.29×10−11σ3.3. B2–NiAl and L12-Ni3Al were precipitated during creep. The relationship between the misfit of nanoscale L12-Ni3Al phase and γ matrix are perfectly coherent. The L12-Ni3Al phase grows more slowly and remains at the nanoscale size after creep at 750 °C for 869h, which contribute to increase creep strength and reduce the creep rate of the material. However, the coarsened σ-FeCr phase reduced the creep productivity of the alloy. The microstructural degradation caused by precipitate coarsening was found to be responsible for creep failure, as indicated by the creep damage tolerance factor (λ) and creep fracture morphology. The elemental distribution of the oxide layer indicates the absence of a dense alumina protective scale at 750 °C. The extensive presence of oxidation cracks in the matrix suggests that the effect of oxidation cannot be ignored in the analysis of creep life. The fact that σ-FeCr is detrimental to creep damage indicates that the alloy composition needs further optimization.

Effect of Radiation Dose on the Deformation Behavior of the Single-Crystal Fe–10Ni–20Cr Alloy

Effect of Radiation Dose on the Deformation Behavior of the Single-Crystal Fe–10Ni–20Cr Alloy

Optimizing magnetic performance of Fe–50Ni alloy for electric motor cores through LPBF: A study of as-built and heat-treated scenarios

This study aims to identify the optimal combination of process variables for laser powder bed fusion (LPBF) of electric motor (EM) cores using Fe–50Ni alloy. A thorough analysis of mechanical and magnetic properties, with a focus on its dynamic magnetic performance within 50–500 Hz frequency range, is presented. Optimized process parameters yielded relative densities above 99%. In the as-built condition, high hardness (twice that of conventionally processed alloy) and high ductility (>30% at rupture) were achieved. The as-built samples demonstrated magnetic properties below the requirements, but significant improvement was observed in the semi-static magnetic properties after heat treatment, with acceptable coercivity (44 A/m) and maximum permeability (∼104) attributed to a notable reduction in geometrically necessary dislocations (GNDs) density. Heat treatment did not significantly reduce the total loss at high flux densities or elevated testing frequencies because the energy loss in the as-built microstructure is lower than what is expected due to the activation of more domain walls resulting in a homogeneous distribution of eddy currents. The superior semi-static performance of the optimum sample is related to its texture, which was more oriented toward the easy axis of magnetization in this alloy (<111> direction). This research demonstrates the LPBF process's potential for manufacturing electric motor soft cores, providing acceptable surface integrity, roughness levels, and desired coercivity and permeability. However, the high total loss, specifically at elevated frequencies, highlights the need for additional capabilities of LPBF, such as fabricating multi-materials, to mitigate energy losses without resorting solely to heat treatment.

Improvement of shape recovery by controlling the microstructure and mechanical properties in a thermomechanically-treated Fe-based alloy

In this research, the effects of thermomechanical treatment including heavy cold rolling and post-deformation annealing on the improvement of shape recovery in an Fe–9Ni–7Mn (at.%) martensitic alloy were investigated. Experimental results showed that the 80 % cold rolling produced a little fraction of strain-induced austenite phase in the martensitic matrix due to the strain concentration in the microstructure and the temperature rise under deformation. During post-deformation annealing at 590 °C in a dual phase (α+γ) region, a greater fraction of the austenite was introduced in the martensitic matrix such that its amount first increased by increasing the annealing time to 1.8 ks and then decreased at a more time of reversion. The microstructural evolution revealed that the austenite reversion under post-deformation annealing initially occurred by a shear nature at the early steps of reversion and then continued by diffusion. Tensile testing represented a good mixture of a relatively high ultimate tensile strength of ∼780 MPa and proper ductility of ∼8.5 % in the post-deformation annealed (PDA) sample for 0.9 ks. In addition, the shape memory properties were examined under the conditions of cold rolling and post-deformation annealing. The results showed a maximum shape recovery rate of ∼23 % for the cold rolled specimen which increased to ∼50 % after post-deformation annealing for 0.9 ks. The presence of a relatively high amount of retained austenite with proper stability and strength in the microstructure of the PDA sample for 0.9 ks was the reason for achieving the highest value of shape recovery.

Fe‐Based Filler Material Forming Austenite‐Covered Packet Lath Martensite for Gas Tungsten Arc Welding of 9% Ni Steel

Fe‐based filler material with a composition of Fe–20Ni–5Co–2.5Mn–0.2C is developed for welding 9% Ni steel. To ensure high weldability through compositional similarity, the composition of the Fe‐based filler material is designed based on that of 9% Ni steel. The filler material has elements such as Ni, Mn, Co, and C to enhance its compatibility with the 9% Ni steel. The Fe‐based filler material is designed to control the martensite formation temperature, resulting in the appearance of the martensite–austenite dual phase during rapid cooling. The weld joint shows a distinct microstructure characterized by the presence of austenite‐covered packet lath martensite. The mechanical properties of the weld joint are investigated via Vickers harness and tensile tests at 25 and −163 °C. In comparison to 9% Ni steel, the weld joint prepared using the Fe‐based filler material exhibits yield and tensile strengths that are ≈5.5% higher. Additionally, the tensile fracture of the specimen occurs exclusively within the base metal, without any involvement of the weld metal or heat‐affected zone. Further, the fracture surface is observed. In the findings of this study, the applicability of the Fe‐based filler material for welding 9% Ni steel is demonstrated.

Investigation of chemical short range order strengthening in a model Fe–12Ni–18Cr (at. %) stainless steel alloy: A modeling and experimental study

Solid solution strengthening remains the basis for many industrial alloys, yet chemical short-range order (CSRO) can also play a significant role in the strengthening of alloys with appreciable alloying additions, such as in Austenitic stainless steels. In this work, we study the evolution of CSRO under various annealing temperatures and its role on the mechanical strength of a ternary Fe–12Ni–18Cr (at.%) alloy using molecular dynamics (MD) and Monte Carlo atomistic simulations and experimental measurements from mechanical microindentation and atom-probe tomography (APT). A general model that incorporates the role of CSRO into a solute–solution strengthening model is proposed, based on a family of analytical models due to Varvenne et al. (2016) which takes into account dislocation-solute misfit interaction, and a more recent extension due to Nag and Curtin (2020) which incorporates the role of solute–solute interactions during dislocation slip. Predictions of the modified model are validated against MD shearing simulations using two commonly used embedded atom model (EAM) interatomic potentials for Austenitic stainless steel alloys, and mechanical microindentation experiments on cast alloys of the same composition. Our results suggest that, while significant CSRO is predicted in the simulations, there is little experimental evidence in cast and annealed alloys analyzed by APT.

Effects of dislocation arrangement and character on the work hardening of lath martensitic steels

Work-hardening behavior of a lath martensitic Fe–18Ni alloy during tensile deformation is discussed based on the Taylor's equation. The dislocation characteristics are monitored using in situ neutron diffraction. In the specimens of as-quenched (AQ) and tempered at 573 K (T573), the dislocations are extremely dense and randomly arranged. The dislocations in AQ and T573 form dislocation cells as deformation progresses. Consequently, a composite condition comprising cell walls and cell interiors is formed, and the coefficient α in the Taylor's equation increases. Cells are already present in the specimen tempered at 773 K (T773), which has a low dislocation density and a large fraction of edge-type dislocations. As deformation continues, the dislocation density of T773 increases, its cell size decreases, and its composite condition become stronger. Simultaneously, the edge-type dislocation fraction decreases, keeping α unchanged. Thus, both the dislocation arrangement and character affected α, thereby affecting the work-hardening behavior.

Dynamic strain aging behavior of an advanced alumina-forming austenite stainless steel based on Fe–30Ni–15Cr–3.5Al

Alumina-forming austenite (AFA) steels have emerged as promising candidate materials for ultra-supercritical units because of their excellent high-temperature performance. Dynamic strain aging (DSA) can increase the strength, but at the expense of plasticity, which will inevitably reduce the service life of materials. The aim of this study was to determine the serration behavior of a new AFA steel from room temperature (RT) to 700 °C at different strain rates from 1.33 × 10−5 s−1 to 1.33 × 10−2 s−1. Serrations were found at 300–650 °C under the tested conditions. Regardless of the strain rate, three types of serration arose: type A, A + B, and C. Plateaus of yield strength, ductility minima, and sensitivity to negative strain rates were observed. Ludwik's relationship was more accurate than Holloman's relationship for expressing the relationship between the true stress and the true strain during plastic deformation. Below 600 °C, the critical plastic strain showed a positive DSA, but it showed an inverse DSA above 600 °C. The value of (m+β) at the normal DSA was 2.27, and the activation energy was 81.624 kJ/mol. Depending on the value of the activation energy, the DSA was attributed to pinning dislocations by substitutional aluminum atoms. SEM examination of the fracture surfaces revealed mixed mode with dimples and facets at tested temperature, but facts fracture became more intense with increasing temperature and ductility minima arose at 650 °C. The results of transmission electron microscopy showed increase interaction of dislocations with solute atoms with increase in the temperature up to 700 °C.

Corrosion of the Fe–25Ni–15Cr superalloy under the combined effects of marine atmospheric exposure and high-temperature oxidation

Corrosion of the Fe–25Ni–15Cr superalloy under the combined effects of marine atmospheric exposure and high-temperature oxidation

Lattice parameters of austenite and martensite during transformation for Fe–18Ni alloy investigated through in-situ neutron diffraction

Martensitic transformation is accompanied by the generation of microscale and macroscale internal stresses during cooling below the martensitic transformation start temperature. These internal stresses have been determined through X-ray or neutron diffraction, but the reported results are not consistent, probably because the measured lattice parameter is influenced not only by the internal stress but also by several factors, including solute elements and crystal defects. Therefore, in-situ neutron diffraction combined with dilatometry measurements during martensitic transformation and subsequent cyclic tempering were performed for an Fe–18Ni alloy. The phase strains calculated by lattice parameter variations show that a hydrostatic compressive strain in austenite and a tensile strain in martensite arose as the martensitic transformation progressed during continuous cooling or isothermal holding. However, the phase stresses of austenite and martensite estimated from these strains failed to hold stress balance law when dense crystal defects involved in the processes. After these crystal defects were removed by appropriate tempering, the stress balance law held well. Meanwhile, the phase stresses of austenite and martensite were changed to opposite, revealing their true identity. Furthermore, the lattice parameter of quenched full martensite and the specimen diameter were both decreased by tempering, demonstrating that there is a direct correlation among lattice parameter, specimen volume and dislocation density. As a result, various crystal defects in austenite and martensite, introduced by plastic accommodation, were suggested to affect their lattice parameters and then their phase stresses.

The effect of quenching on the hinderance of the NbC growth process in a model microalloyed steel

ABSTRACT The effect of vacancies on the evolution of strain-induced NbC precipitates (SIP) was studied in a model austenitic Fe–30Ni–0.1Nb-0.1C steel deformed at 925 °C. Precipitation growth and coarsening characteristics have been investigated in detail by employing high resolution transmission electron microscopy. In general, the SIP preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls and grew over the varying holding time following the conventional precipitation growth and coarsening theory. Nevertheless, the current results demonstrated unexpected sluggishness in the process of growth and coarsening of the precipitates on the introduction of intermediary population of vacancies by a quenching step added in the thermomechanical processing schedules. It has been suggested that the formation of stable vacancy-solute complexes may reduce the effective availability of solutes for the precipitate growth.

Significance of Mn concentration on aging behavior, microstructure evolution and mechanical properties of Fe–Ni–Mn alloys

Fe–Ni–Mn alloys, are ultra-high-strength steels, with suitable ductility and age-hardenability in the solution annealed (SA) condition. However, they suffer from grain boundary embrittlement after aging at ∼480OC, when Mn content goes beyond a specific value. This study was conducted to investigate systematically the effects of the Mn concentration on the microstructural evolution, aging behavior, and the resulting mechanical properties of Fe–10Ni–xMn alloys containing 3.5, 6 and 9Mn wt.%. Samples containing 3.5 and 6 wt.% Mn showed age-hardenability during aging and higher Mn content resulted in a higher hardness value which in turn showed higher strength. The Mn content of about 9 wt.% led to the stability of the austenite phase at room temperature (RT). The results showed that although age-hardening did not occur in this microstructure, a good combination of strength and ductility was achieved. Studying the effects of Mn content in a wide range, resulted in different properties by introducing the proper chemical composition and the preferred heat treatments in this type of alloys, which could be age-hardenable, easy to fabricate, and low-cost.

Corrosion behavior of (Fe,Ni)–Gd intermetallic compounds in FeNi-based neutron-absorbing steels

In this paper, microstructures of Fe–12Ni–1.4Mn–(0–4 wt%)Gd steels were quantitatively analyzed, and corrosion behavior of the alloys in solutions containing H3BO3 and Cl– was investigated through potentiodynamic polarization tests and corrosion morphology observation. The addition of Gd to the Fe–12Ni–1.4Mn alloy formed secondary phases, mainly (Fe,Ni)17Gd2 and a small amount of (Ni,Fe)4Gd. As the Gd content increased, the total amount of the secondary phase linearly increased. In a NaCl solution, the pitting corrosion resistance of the alloys was reduced by the addition of Gd. In a HCl solution, both uniform and pitting corrosion occurred, and the resistance to both types of corrosion decreased as the Gd content increased. In the two solutions, the corrosion susceptibility increased in the order of matrix < (Fe,Ni)17Gd2 < (Ni,Fe)4Gd. The high susceptibility to corrosion of the Gd-rich phase is due to the fact that Gd is electrochemically active compared to the other constituent elements of the matrix and also has a low ability to form passive films. Also, in a H3BO3 solution, both uniform and pitting corrosion were observed in the alloys. Although Gd is electrochemically active compared to Fe–Ni–Mn matrix, uniform corrosion resistance was improved by addition of Gd and the Gd-rich phase behaved as a noble phase only in the H3BO3 solution. This is considered to be because H3BO3 selectively adsorbs to the Gd-rich phase and protects it from dissolution. However, because the protective ability of the H3BO3 adsorbed layer was weaker than that of the passive film of the matrix, pitting occurred at the Gd-rich phase.

Strengthening and fracture mechanisms of Fe–20Ni–14Cr–2Cu alumina-forming austenitic steel during creeping

Strengthening and fracture mechanisms of Fe–20Ni–14Cr–2Cu alumina-forming austenitic steel during creeping

Role of Nb in promoting protective scale formation on Al2O3-forming austenitic alloys

The high-temperature oxidation behavior of Fe–25Ni–17Cr–7.7Al–0.03Zr alloys with and without 0.5 or 1 at% Nb addition was investigated at 800 °C in air. Nb addition was found to promote the formation of an external Al2O3 scale. The Nb-free alloy formed a scale consisting of a thick Fe/Ni-rich oxide layer and a Cr2O3 layer, beneath which Al was internally oxidized. However, a continuous protective Al2O3 scale was developed below a thin Cr2O3 layer on the 1 Nb alloy, which significantly decreased the oxidation rate. Examination of the initial oxidation process during heating to 800 °C revealed that all of the alloys formed a Cr2O3 layer and an amorphous Al2O3 layer upon heating to 650 °C, but these layers became discontinuous upon further heating to 800 °C for the Nb-free alloy. The beneficial role of Nb was ascribed to its ability to maintain the initially formed thin Cr2O3 layer by the formation of a Nb-containing Cr-rich oxide layer at the interface between the Cr2O3 and amorphous Al2O3 layers. This Nb-containing Cr-rich layer prevented the breakdown of the transient amorphous Al2O3 layer until its transition to a protective crystalline Al2O3 scale.

Creep Behavior of Fe–25Ni–15Cr‐Based Alumina‐Forming Austenitic Steel

The creep behavior of Fe–25Ni–15Cr‐based aluminum‐forming austenitic stainless steel is investigated at 700 °C, under the selected stress of 100, 140, 170, and 180 MPa, respectively. Creep properties are analyzed in terms of stress exponent (n) and creep damage tolerance factor (λ). Within the tested stress range, there is an order of magnitude difference between the minimum creep rate and steady creep rate. The evolution of the microstructures after creep test with the stresses of 140, 170, and 180 MPa is characterized by scanning electron microscopy and electron backscatter diffraction detectors, respectively. With the decreased creep stress, the creep void size shows an increasing trend. Higher stress would accelerate the process of recrystallization during the creep process at 700 °C. The proportion of high‐angle grain boundaries increases to a certain extent under high‐stress conditions. A large amount of Laves and NiAl phases are observed after creep test at 140, 170, and 180 MPa, respectively.

Evaluation of texture weakening and microstructural evolution in an Fe–10Ni–7Mn martensitic steel severely deformed by six turns of high-pressure torsion

The effect of high-pressure torsion (HPT) on textural components and microstructural evolution of an Fe–10Ni–7Mn steel was investigated. Six turns of HPT, with a rotation rate of 1 rpm and with a pressure of 6.0 GPa, were applied to a solution annealed steel. Microstructural and textural evolution, including dislocation characteristics and grain size/misorientation measurements, were accomplished using electron backscatter diffraction (EBSD). The results showed that a fully martensitic structure, containing a high density of dislocations, was developed during the solution annealing stage and subsequently the application of six turns of HPT process converted the low-angle grain boundaries (LAGBs) of the solution annealed condition into high-angle grain boundaries (HAGBs). Thus, the sub-grain boundaries established in the solution annealed condition were altered into very sharp HAGBs. The grain size was also decreased from ∼ 25 μm in the solution annealed situation to ∼210 nm in the severely deformed condition of six-turn high-pressure torsion state. Yield strength, and tensile strength were increased from 765 MPa, and 840 MPa in the solution annealed case to 1890 MPa, and 2230 MPa in the six-turn HPT-processed sample, respectively. Under such a condition, ductility was decreased from 15.2% to 6.1% in the severely deformed state. Applying six turns of HPT developed a predominantly {111}//ND texture. However, the overall texture intensity of the HPT-processed sample was weakened to a value of about one-quarter of the as-quenched solution treated sample. This was related to dynamic recrystallization and grain subdivision as well as a dislocation density reduction in the HPT-processed sample. The texture components of strong Copper ({112}<111>), S ({123}<634>) and some average intensity Cube {100}<0001> were observed in the {100} pole figure in the solution annealed condition. As well, γ-fibre (<111>//ND) and β-fibre were seen in this condition. However, {110}<uvw> fibres were mostly developed in the six-turn sample with the texture components of E1{011‾}<111>, E2{01‾1}<111>, J1{01‾1}<2‾11>, J2+{11‾0}<1‾1‾2>, D2{1‾1‾2}<111> , D1{112‾}<111> and F {110}<001>. In addition, the Cu{112}⟨111⟩ component was significantly diminished. This weakened Cu{112}⟨111⟩ component would have caused enhancement of the mechanical properties, particularly the formability and toughness, as previously reported by the current authors. Finally, it was found that applying severe plastic deformation (SPD) in the form of HPT reduces the effect of martensitic texture in the solution treated steel.

Strengthening via Grain Refinement in Lath Martensite on Low Carbon Fe–18Ni Alloys

The lath martensite structure in steel offers high strength with a complex substructure, and its strength increases with carbon content. However, the mechanism of carbon strengthening is yet to be elucidated. In this study, we evaluate the tensile properties of as-quenched lath martensite without retained austenite in Fe–18Ni alloys containing 4–570 ppm carbon. In the 4 mass ppm carbon alloy, whose carbon is almost trapped by Ti(CN) particles, the work hardening behavior during uniform elongation is constant regardless of the size of the effective grain surrounded by a high angle boundary. In contrast, the yield point (YP), 0.2% proof stress (σ0.2%), 0.6% proof stress (σ0.6%), and maximum tensile strength (TS) in 7, 110, and 570 mass ppm carbon alloys increase with the refinement of their effective grain, consistent with the Hall–Petch relationship. The Hall–Petch intercepts for the YP, σ0.2%, σ0.6%, and TS are constant and unaffected by the carbon content. This suggests that the non-occurrence of solution hardening by solute carbon atoms in the lath martensite. The Hall–Petch coefficients for the YP, σ0.2%, σ0.6%, and TS increase with carbon content and are proportional to the square root of the carbon content. This indicates that the increase in carbon content increases the strength of the lath martensite via the refinement of effective grains and the increase in the effectiveness of grain refinement.

Molecular dynamics study of interactions between edge dislocation and irradiation-induced defects in Fe–10Ni–20Cr alloy

Irradiation-induced defects frequently impede the slip of dislocations, resulting in a sharp decline in the performance of nuclear reactor structural materials, particularly core structural materials. In the present work, molecular dynamics method is used to investigate the interactions between edge dislocations and three typical irradiation-induced defects (void, Frank loop, and stacking fault tetrahedron) with the sizes of 3 nm, 5 nm, and 7 nm at different temperatures in Fe–10Ni–20Cr alloy. The critical resolved shear stresses (CRSSs) are compared among different defect types after interacting with edge dislocations. The results show that the CRSS decreases with temperature increasing and defect size decreasing for each defect type during the interaction with edge dislocations, except for the case of 3-nm Frank loops at 900 K. According to a comparison, the CRSS in Frank loop is significantly higher than that of others of the same size, which is due to the occurrence of unfaulting and formation of superjog or stacking-fault complex during the interaction. The atomic evolution of irradiation-induced defects after interacting with dislocations can provide a novel insight into the design of new structural materials.