Effects of B segregation on Mo-rich phase precipitation in S31254 super-austenitic stainless steels: Experimental and first-principles study

Precipitation in super-austenitic stainless steels will significantly affect their corrosion resistance and hot workability. The effects of Cr and Mo on precipitation behaviors were mainly achieved by affecting the driving force for precipitation, especially Mo has a more substantial promotion effect on the formation of the σ phase than Cr. In the present study, B addition to the S31254 super-austenitic stainless steels shows an excellent ability to inhibit precipitation. The effect of B on the precipitation behaviors was investigated by microstructure characterization and theoretical calculations. The experimental observation shows that the small addition of B inhibits the formation of the σ phase along grain boundaries and changes from continuous to intermittent distribution. Moreover, the inhibitory effect increased obviously with the increase of B content. The influence of B addition was theoretically analyzed from the atomic level, and the calculation results demonstrate that B can inhibit the formation of σ phase precipitates by suppressing Mo migration to grain boundaries. It is found that B and Mo are inclined to segregate at Σ 5 and Σ 9 grain boundaries, with B showing the most severe grain boundary segregation tendency. While B distribution at the grain boundary before precipitation begins, the segregation of Mo and Cr will be restrained. Additionally, B's occupation will induce a high potential barrier, making it difficult for Mo to diffuse towards grain boundaries.     

Al，Cr共偏析Fe晶界对其结合强度影响的第一性原理研究

在密度泛函理论的框架下,采用广义梯度近似(GGA)研究了合金化元素Al,Cr在Fe (210)晶界共偏析的作用。结果表明Cr提高了Fe晶界结合,为韧性杂质;而Al减弱了晶界的结合,是脆性杂质。Cr不能够彻底地消除Al的脆化作用,反而使其脆性增强。基于偏聚能分析表明Cr能有效地阻止Al偏析到晶界,改变Fe的力学性能。     

Proton-induced grain boundary segregation in stainless steel

Abstract

A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from 5.3 to 8.7 at %, over 80 times the bulk value. No radiation-induced grain boundary segregation of S was measured in the UHP + S alloy. Results also indicate that the presence of P or S may enhance radiation-induced segregation of major alloying elements at the boundary. Comparison of irradiated and unirradiated regions of the UHP + P alloy indicate that while a prior thermal treatment segregates P to the grain boundary to 5.3 at %, the major element concentrations at the grain boundary are completely different from those under irradiation.     

Service Life Effect on Structure and Phase Composition of DIN 14MoV63 Steel

Russian Physics Journal - The paper studies the structure and phase composition of steel 0.12C–1Cr–1Mo–1V–Fe (grade DIN 14MoV63) after a long-term operation using the...     

Interface adsorption,embrittlement and fracture in metallurgy: A review

Interface adsorption or segregation in metallurgy is reviewed with particular reference to the phenomenon of grain boundary adsorption and its effect on important mechanical properties such as temper brittleness in steels. No attempt is made to provide a complete bibliography but rather to put in perspective the significant and properly documented observations relating to segregation. General theories are then developed which first relate segregation to readily available basic parameters and then, in turn, with a new theory, the degree of embrittlement to segregation. In this way predictions may be made and remedial measures proposed for metallurgical problems involving grain boundary or interfacial segregation.     

Solute segregation to grain boundaries and free surfaces in an Fe---Si multicomponent alloy

Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si_xN_y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmann's theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmann's theory.     

Characterization of the ξ-carbide in ex-service 1Cr-0.5 Mo steels

The ξ-carbide, also described as Fe₂MoC and M_aC_b, has been identified at ferrite grain boundaries in 1Cr-0.5Mo steels exposed to elevated temperatures (500–530°C) for prolonged periods (65,000–170,000 hr). The structure and composition of the phase have been characterized using electron microdiffraction and energy-dispersive X-ray spectroscopy (EDXS) respectively. Analysis of microdiffraction patterns supports the proposal that the ξ-carbide has a monoclinic unit cell, rather than the orthorhombic unit cell first proposed. The ranges of metallic element concentrations in the ξ-carbide (i.e. 48–59 at.% Fe, 22–29 at.% Mo, 7–16 at.% Cr, 4–7 at.% Mn and 3–6 at.% Si) are unique compared to those of other carbide precipitates identified in the 1Cr-0.5Mo steels, which means that the ξ-carbide may be identified rapidly on extraction replicas using qualitative EDXS.     

Brittle fracture in 50Mo–50Re alloys during slow strain rate tensile testing

Tensile tests were conducted on 50 wt% Mo–50 wt% Re alloys in both fully recrystallized and recovery heat-treated conditions at a low strain rate of 10^?6 s^?1 and room temperature in air. It was found that both material conditions exhibited predominantly cleavage fracture with significant intergranular secondary cracking, compared to the predominantly ductile fracture found in the alloys at a higher strain rate. Cracks were often initiated at grain boundary triple junctions at the low strain rate. Electron backscatter diffraction (EBSD) measurements revealed significantly high misorientation gradients (i.e. highly localized change in orientation) at grain boundaries, especially in the vicinity of some grain boundary triple junctions in the deformed alloys. Transmission electron microscopy (TEM) results verified the existence of significant misorientations near grain boundaries in these alloys. Stress-assisted dynamic embrittlement, possibly due to trace interstitials, was the possible cause of brittle fracture in the 50Mo–50Re alloys at the low strain rate.     

Study of ageing effects of a low-interstitial 18Cr-2Mo ferritic steel

The phase transformations which occur in the 18Cr-2Mo ferritic steel by annealing in the temperature range 603–1093 K have been studied using Moessbauer spectroscopy and X-ray diffraction. It is found that the 748 embrittlement is to be ascribed to Cr redistribution and/or to the formation of an intermetallic R-type phase. Precipitation of an h.c.p. Laves phase Fe (Mo,Nb) of MgZn₂-type takes up at about 850 K and Nb to Mo substitution goes on by increasing the annealing temperature.     

Effects of excess point defects flow and interface orientation on grain boundary migration under irradiation

Fe-Cr-Ni, Ni-Al and Ni-Si alloys, and 316L stainless steels as reference were electron-irradiated using a high voltage electron microscope (1MV), and in-situ observations of structural evolution and micro-chemical analysis were carried out. From the compositional analysis it was found that nickel was enriched and chromium depleted near grain boundary in Fe-Cr-Ni alloys including 316L stainless steels, and that simultaneously grain boundary migration was caused during irradiation, even if no grain boundary migration occurred in the un-irradiated area at the same irradiation temperature. The occurrence of boundary migration strongly depended upon orientation relationship between boundary interfaces. It is suggested that grain boundary migration under irradiation remarkably occurs in the alloys in which solute enrichment is taken place at the grain boundary as a result of the flow of radiation-introduced point defects into grain boundary and that their magnitude depend upon net flow of point defect, especially that of under-sized interstitial atoms.     

Power frequency magnetic properties and aging of 4130 steel

Cr–Mo steels are utilized in large, high-speed rotating machines where the mechanical stress requirements limit available soft magnetic laminate choices. Because this is currently a niche application, the magnetic properties of these steels are relatively undocumented. This paper presents the magnetic hysteresis behavior of a quenched and tempered 4130 steel at alternating frequencies up to 1200 Hz and temperatures up to 100 °C. The high coercivities and core losses are contrasted with a 3.2%Si–Fe alloy. “Aging” of this behavior over time of cyclic field application was not observed in 300 h. However, surface embrittlement was observed. Designers should be aware that cyclic magnetic fields, even in the absence of temperature excursions and mechanical stress, can lead to a relaxation of the 4130 microstructure and possible deterioration of yield strength.     

Precipitates in electrical steels

Precipitates heavily influence the magnetic properties of electrical steels, either as a key controlled requirement as part of the manufacturing process or as an unwanted harmful residual in the final product. In this current work copper-manganese sulphides precipitates are the primary inhibitor species in the conventional grain-oriented (CGO) steels examined and grain boundary pinning is effective at a mean precipitate size of 30–70 nm. The growth of CuMnS has been studied and the results show that a precipitate size above ∼100 nm allows the onset of secondary recrystallisation in the heating conditions applied.     

Sr偏析Al晶界结构的第一性原理计算

采用基于密度泛函理论和局域密度近似的第一性原理赝势方法,计算了纯Al晶界和杂质Sr偏析Al晶界的原子结构和电子结构.结果表明Sr偏析引起了晶界膨胀和晶界处电子密度的大幅度降低,从而导致晶界结合力的减弱.这应为Sr杂质偏析引起的Al晶界脆化的主要根源所在. 关键词： Al晶界 Sr 杂质偏析 第一性原理计算     

Low core loss non-oriented silicon steels

Low core loss non-oriented silicon steels are produced with high (Si+Al) content to reduce eddy current losses. However, high alloy content has detrimental effect on mechanical properties, saturation polarization and thermal conductivity. A new generation of medium and, particularly, low core loss non-oriented silicon steels was developed, with lower alloy content than the conventional grades, based on improved purity and texture. The development allowed the production of new low loss grades, with maximum core loss (W_1.5/50) of 2.30 W/kg at 0.50 mm and 1.95 W/kg at 0.35 mm, with high permeability (J₅₀ of 1.7 and 1.72 T, respectively). Texture improvement was based on hot band structure control and higher boundary mobility. Large hot band grain size and low [1 1 0]∥RD fiber fraction in the hot band texture contribute to reduce the intensity of [1 1 1]∥ND and slightly increase the intensity of [0 0 1]∥RD in the final product. Higher grain boundary mobility and/or a two-stage cold rolling process, with an intermediate annealing, increase the fraction of [0 0 1]∥RD and reduce the fraction of [1 1 1]∥ND on recrystallization and lead to favorable texture evolution on grain growth.     

Atomic and electronic structures of a grain boundary in iron with impurity segregation

We present a short review on our current investigations of the atomic and electronic structures of a grain boundary in iron. Atomic structures of grain boundaries were simulated and the local electronic densities of states were calculated in the simulated structure. When phosphorus impurity atoms segregated at the grain boundaries in iron, trigonal prismatic FeP clusters were formed. Segregated boron atoms tended to stay at the central site of polyhedra constructed by host atoms in the grain boundaries. The non-bonding states of the iron atom at the grain boundary disappear by forming a strong bonding orbital with the orbital of the segregated impurity atom. This bonding orbital is formed in a Fe3d host band in the case of a boron impurity. On the other hand, the bonding orbital is formed at lower energies for the phosphorus impurity and is less-mixed with the Fe3d host band. Non-bonding states are formed around the Fe₉P clusters. These can give a qualitative explanation for the embrittlement of the impurity segregated grain boundary. Finally, we can explain from the viewpoint of the electronic structure why the interstitial impurity is the only cohesive enhancer.     

Atom probe tomography of 15Kh2MFA Cr–Mo–V steel surveillance specimens

An atom probe study has been performed on 15Kh2MFA base and 10KhMFT weld metal surveillance specimens from a VVER-440/213C reactor to investigate the mechanisms that produce embrittlement in low copper materials during service. The composition of the base metal was Fe-0.06 at.% Cu, 3.1% Cr, 0.34% V, 0.46% Mn, 0.35% Mo, 0.07% Ni, 0.34% Si, 0.74% C, 0.025% P, and 0.028% S. The base material was characterized after thermal aging for 10 years at 295°C and after neutron irradiation at 270°C for 10 years to a fluence of 1.0×10²⁵ m⁻² (E>0.5 MeV). The ductile-to-brittle transition temperatures (DBTT) of the base metal were −49, −70 and 141°C, for the unirradiated, thermally aged and neutron irradiated conditions, respectively. The composition of the weld metal was Fe-0.05 at.% Cu, 1.46% Cr, 0.22% V, 1.11% Mn, 0.29% Mo, 1.17% Si, 0.17% C, 0.02% P, and 0.029% S. The weld material was characterized after tempering for 18 h at 690°C plus a simulated stress relief treatment of 43.5 h at 680°C, after thermal aging for 5 years at 295°C, and after neutron irradiation at 275°C for 5 years to a fluence of 5.2×10²⁴ m² (E>0.5 MeV). The DBTTs were 7, 11 and 123°C, respectively, for these three conditions. A high number density of ultrafine manganese- and silicon-enriched regions was observed in both neutron-irradiated materials. Phosphorus segregation was observed at the VC-matrix interface and at grain boundaries.     

Effect of manganese on grain boundary segregation of sulfur in iron

The ASED-MO theory was used to study the electronic effects of S and the S-Mn couple upon the chemical embrittlement of Fe grain boundaries. The results obtained for S alone in a model of grain boundary (GB) are consistent with its observed behavior as a chemical embrittling agent. It was found that the total energy of the cluster decreases when the S atom is located at the GB. When S segregate at the Fe GB containing Mn, the embrittlement process was modified. The crystal orbital overlap population (COOP) curves gives a measure of Fe-Fe bond weakening due to the segregated atoms at the GB. Our calculations show that Mn behaves as a weak embrittler on the Fe GB. The Fe-Mn bonds were strengthened, while Fe-Fe bonds of the capped trigonal prism of the GB (CTP) were weakened. On the other hand, when S segregate at the Mn/Fe cluster, some metallic bonds were resistant to chemical embrittlement.     

Probe Mössbauer spectroscopy of the grain boundaries of a Mo-O nanocrystalline system obtained by mechanical alloying

The localization of Fe atoms in the process of mechanical alloying of a Mo powder composite with 8 at % O at boundaries of the bcc Mo grains has been investigated by Mössbauer spectroscopy on impurity ⁵⁷Fe isotope atoms (1 at %), X-ray diffraction, and Auger spectrometry. The process begins with the formation of a nanostructure (～10 nm) in bcc Mo and ends with the formation of a bcc supersaturated solid solution with O atoms at interstitial positions and Fe atoms at substitutional positions. The presence of oxygen in the boundaries of bcc Mo grains leads to an extraordinarily large isomer shift (2 mm/s with respect to α-Fe) for the grainboundary component in the Mössbauer spectrum. This circumstance makes it possible to consider ⁵⁷Fe-O complexes as new probes for studying grain boundaries of powder nanocrystalline materials. As a result, the following three structural components have been identified in the mechanically activated system: a grain boundary and distorted near-boundary regions with the common name interface and a grain with the perfect (defect-free) structure. For powder nanocrystalline (～10 nm) materials subjected to intense mechanical treatment in a planetary ball mill, the widths of the unrelaxed grain boundary and interface average over the entire volume of particles have been experimentally estimated as 0.2 and 1 nm, respectively.     

Quasi-static intergranular brittle fracture: Dynamic embrittlement

A mode of brittle fracture is described which is fundamentally different from the rapid transgranular cleavage or intergranular decohesion that is usually associated with that term. It involves stress-induced diffusion of surface-adsorbed embrittling elements along grain boundaries, and it occurs by slow, step-wise crack growth, the rate of which can, in principle, be calculated from the knowledge of the relevant intergranular diffusion coefficient, the stress profile at the crack tip and the dependence of the stress for grain-boundary decohesion on the concentration of the embrittling element. This mode of fracture is postulated to be possible in any high-strength alloy with a low-melting-point element adsorbed on the surface if the applied stress is high enough. Known examples include the brittle type of stress-relief cracking in steels, tin-induced cracking of Cu-Sn alloys, oxygen-induced cracking of iron-, copper-, and nickel-based alloys, and the group of phenomena known as liquid-metal embrittlement and solid-metal embrittlement.The paper is dedicated to Dr. Frantiek Kroupa in honour of his 70^th birthday.This work is supported by National Science Foundation Grant CMS 95-03980.