Carbon is the most important alloying element in steels and Fe-C alloys have been studied intensively. Carbon has various functions and has a strong influence on the transformed structures and mechanical properties. There are plenty of experimental data of Fe-C alloys but we still have many unsolved problems. The first-principles calculation and molecular dynamics (MD) method can evaluate effects of the slight change of the position and distribution of carbon atoms on energy and physical properties. So they are very useful for clarifying the nature and state of carbon in steels and for solving the problems which cannot be made clear by experiments. The electronic structure of carbon in iron, diffusion of carbon, site occupation of carbon atom in martensite, tetragonality, C-C interaction, carbon cluster and spinodal decomposition of martensite are the topics of this review paper. The studies using first-principles calculation and MD method are mainly reviewed. How much of the unsolved issues are clarified and what kind of problems remain are shown.
The solubility of boron in α- and γ-iron solid solutions was investigated by diffusion-couple experiments in the Fe-B, Fe-Ni-B, and Fe-Si-B systems. Diffusion couples composed of iron foils and Fe2B bulk specimens were annealed at various temperature between 750ºC and 1100ºC, and the concentration profile of boron from the end-surface of the iron part were measured by the radio-frequency glow-discharge optical emission spectroscopy (rf-GD-OES). It was found that rf-GD-OES was effective in analyzing boron of low concentrations in iron. The solubility limit of boron in α- and γ-iron determined by analyzing the concentration profiles in the diffusion couples was found to agree with that reported by Cameron and Morral for α-iron, and that by Brown et al. for γ-iron. It was also suggested that the invariant reaction between α-iron, γ-iron and Fe2B was a eutectoid reaction, as had been indicated by Cameron and Morral.
We report an attempt to determine the diffusion coefficient of B in α Iron by measuring the penetration profile by means of secondary-ion mass spectrometry (SIMS). Pure iron plates of grain-size of 1 to 3 mm were prepared, and thin films of Fe-B alloy (200 nm) and alumina (50 nm) were deposited on the surface as a B source and a capping layer, respectively. The samples were subjected to diffusion annealing at 700ºC, 800ºC, and 900ºC for certain periods of time, and the intensity of secondary ions of B was measured as a function of depth by SIMS. The mesa method was employed, in which a groove is prepared first around the target area by sputtering, and then the depth profile of B through the inner pillar was obtained. The concentration profiles thus obtained were analysed with the thin-film solution, the error-function solution, and also using Hall’s method, depending on the form of the profile. The diffusion coefficient was of the order of 10–18 m2 s–1 in all the cases, which is seven to eight orders of magnitude smaller than those evaluated from deboronising experiments in the 1950 s, but is close to recent theoretical prediction for substitutional diffusion.
Thermodynamic analysis of the Fe-Mo-B ternary system was performed using the CALPAHD approach coupled with the first-principles calculation, and effect of Mo addition in steels on the grain boundary segregation tendency of B was discussed by means of the grain boundary phase model. The calculated phase diagrams and thermodynamic properties well reproduced the experimental data as well as the results of the first-principles computation, and thus the parameter set with high accuracy for this ternary system was evaluated. Grain boundary segregation behavior of B and Mo was analyzed by means of the parallel tangent scheme. The Gibbs energy of the liquid phase obtained in the present work was adopted for that of the grain boundary phase. According to the model, the amount of segregated B in grain boundaries decreased with descending temperature. This phenomenon is due to the formation of boride phases such as Fe2B and τ2 phases in the iron matrix.
To understand the effect of Fe23(C,B)6 precipitation on hardenability of boron (B) added low carbon steels, we investigated the distribution of B at prior austenite grain boundaries by the advanced analysis using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The Mo-B combined steel which has no precipitation of Fe23(C,B)6 showed flat intensity profiles of BO2– ions at prior γ grain boundaries, while the B added steel showed the decrease in signal intensities of BO2– ions at the prior γ grain boundary between two Fe23(C,B)6 precipitates. The results suggested that “solute B depleted zone” is formed near the precipitates and the Fe23(C,B)6 precipitation promotes ferrite transformation not only by assisting itself but also by forming the solute B depleted zone.
In this study, the free energy of iron-carbon binary BCT martensite was calculated using the first-principles calculation and the cluster expansion and variational method. The free-energy curves of BCT martensite show the possibility of promoting the clustering of carbon atoms in the tempering process, because there is two-phase separation associated with the formation of metastable BCT-Fe2C ordered structure. This BCT-Fe2C structure was found to have many crystallographic similarities to η-carbide (Fe2C). Then, the energy barrier required for the transition from the BCT-Fe2C ordered structure to η-carbide was calculated by means of the G-SSNEB method. The obtained activation energy was sufficiently small, suggesting that η-carbide may be formed through the BCT-Fe2C ordered structure. According to these findings, it was suggested that η-carbide in the low-temperature annealing process of BCT martensite may precipitate through a two-step process in which BCT-Fe2C ordered structure is formed by the two-phase separating tendency in BCT martensite, and the ordered structure transitions to η-carbide over the energy barrier.
The precipitation of iron carbides is a crucial factor that determines the properties of tempered martensite. However, the effect of alloying elements on the carbon concentration of ε carbide has not yet been clarified. In this work, we studied the effect of alloying elements on the carbon concentration of ε carbide using first-principles calculations and a three-dimensional atom probe. The first-principles calculations showed that ε carbide with a lower carbon concentration tends to form by the inclusion of Si. The carbon concentration in ε carbide measured by the three-dimensional atom probe was consistent with the first-principles calculations.
The resistance to temper softening in low carbon martensite with its underlying origin, by microalloying of strong carbide-forming alloying elements (V, Nb and Ti) to an Fe-0.1C-1.5Mn-0.05Si (mass%) alloy, was investigated in this study. With similar hardness in as-quenched condition in all the alloys used, the hardness of tempered martensite is increased by V, Nb and Ti additions, particularly after treatment at higher temperature with longer time. The increment in hardness becomes larger by more amount of V addition, while with almost the same amount of microalloying additions, Nb and Ti provide larger strengthening than that of V. Atom probe measurements have revealed that a high density of nano-sized alloy carbides are formed in those alloys with V, Nb and Ti additions at 923 K, where large secondary hardening was observed. At 723 K, where there is some resistance to temper softening, however, almost no precipitation of V, Nb and Ti can be detected. The X-ray line profile analysis of the tempered alloys implies that the reduction in dislocation density during tempering is strongly retarded by V, Nb and Ti additions. This should be the major reason for their resistance to temper softening at relatively lower temperature, even without nano-precipitation of alloy carbides.
The pop-in phenomenon as a plasticity initiation during nanoindentation was analyzed to investigate the effect of solute carbon on the plastic deformation for Fe-C binary alloys and interstitial free (IF) steel. The maximum shear stress τc calculated from the critical pop-in load increases as solute carbon concentration increases. Based on a model in which the elementary step of pop-in is the nucleation of a dislocation loop, it is considered that solute carbon increases the resistance to the growth of a dislocation loop, thus higher stress is necessary for the nucleation. Frequency distributions of the pop-in load shows two peaks when the sample includes solute carbon. One peak on the lower load corresponds to the nucleation at a region of solute carbon free. The other peak on the higher load is attributed to the nucleation at a region with solute carbon. Displacement burst during pop-in Δh decreases with increase in solute carbon concentration. Considering that the Δh is proportional to the number of dislocations, it is concluded that solute carbon decreases the mean free path of dislocation movement.
The upper and lower yield points of ferritic steel containing a small amount of carbon were discussed in terms of the critical stress for dislocation emission from a grain boundary, namely, “critical grain boundary shear stress”, on the assumption of the pile-up model. Considering some experimental results such as tensile testing, relaxation testing and nanoindentation testing on grain boundaries, we concluded that both upper and lower yield points could be similarly understood as a phenomenon of dislocation emission from dislocation sources existing at grain boundaries. The difference in stress between upper and lower yield points was explained in terms of the density of mobile dislocations, which determines the extent of stress concentration at grain boundary caused by pile-up of the dislocations. Slow cooling after annealing or aging at low temperature, by which Cottrell atmosphere is formed, leads to a significant decrement of the mobile dislocation density, and this results in an occurrence of the sharp upper yield point because of a reduced number of piled-up dislocations and insufficient stress concentration at grain boundaries.
The mechanisms of the static strain aging phenomenon of polycrystalline ferritic steels were investigated focusing on segregation of solute C atoms to dislocations and grain boundaries, precipitation of carbides and structural changes of dislocations and grain boundaries, using partially C-stabilized Nb-bearing ULC steel sheets and IF steel sheets with solute C content of 0–4 mass ppm and ferrite grain sizes from 9.5 μm to 183 μm with aging temperatures from 70ºC to 600ºC. The partially C-stabilized steel sheets exhibited two definite hardening stages. The first hardening appeared in the every partially C-stabilized steel sheet accompanied by an increase in the grain-interior strength, σ0, which saturated at around 30 MPa. During the first hardening stage, linear and strong segregation of solute C atoms was developed, which was observed by 3DAP, and their concentration was in good agreement with Cottrell’s formula. The second hardening significantly appeared in fine-grained steels, reaching 90 MPa and accompanied by a large increase in the critical grain boundary strength, τc. The apparent activation energy of the second hardening was 135 kJ/mol. Neither carbide precipitation nor obvious segregation of carbon atoms to grain boundaries was detected during the aging period. Increase in τc was exhibited in a quite wide range of aging temperatures from 170ºC to 600ºC and τc reached the value close to that of the recrystallized condition. Local recovery or rearrangement of Fe atoms in deformed grain boundaries is proposed as a possible mechanism of the second hardening.
The influence of Mn addition on fatigue properties of ferritic steel containing solute carbon was examined using rotating bending fatigue tests on water-quenched Fe-0.016C-1.9Mn ferritic-pearlitic steel containing 0.0035 mass% solute carbon in comparison with water-quenched Fe-C ferritic steels containing 0.0063-0.017 mass% solute carbon. The fatigue tests were carried out at ambient temperature around 300 K and a frequency of 50 Hz with a stress ratio of –1. The Fe-C-Mn steel exhibited a comparable hardness and fatigue limit to the water-quenched Fe-0.017C ferritic steel which contains about three times the amount of solute carbon than the Fe-C-Mn steel. In addition, the Fe-C-Mn steel exhibited a comparable coaxing effect to the Fe-0.017C ferritic steel when started from a stress amplitude near the fatigue limit. Crack initiation sites were changed by stress amplitude unlike in the Fe-C ferritic steels. Specifically, intergranular cracks were observed at high stress amplitudes and transgranular cracks were observed at low stress amplitudes near fatigue limit. It was concluded that the Mn addition suppresses intergranular cracking at the low stress amplitudes.
Fatigue crack initiation and propagation behavior of a water-quenched fully ferritic nitrogen steel were investigated by means of tension-compression fatigue tests. The Fe-0.011 mass% N steel showed no serrated flow associated with dynamic strain aging, and showed a fatigue limit of 150 MPa alongside a non-propagating fatigue crack. The major mode of crack initiation was at the grain boundaries, and the cracks propagated along the grain boundaries and interiors at and above the fatigue limit. The Fe-N steel did not exhibit a significant level of coaxing effect. The results were compared with our previous findings in Fe-0.006 and 0.017C steels, and the similarity and difference were discussed.
We investigated the effect of solute nitrogen on threshold stress intensity factor range, ΔKth, of the growth of small cracks using a water-quenched Fe-0.011N (wt.%) binary alloy, in terms of strain-age hardening. Fatigue tests were carried out for micro-notched specimens at 20°C and 160°C at a frequency of 30 Hz with a stress ratio of –1. The nitrogen effect on ΔKth at room temperature was significant, but smaller than the carbon effect. On the other hand, the thermal stability of the strain aging effect on ΔKth was higher in the Fe-0.011N steel than in Fe-C steels containing supersaturated carbon, because the nitrogen solubility above room temperature is higher than the carbon solubility in ferritic steels.