Influence of vanadium on grain boundary segregation of phosphorus in iron and iron-carbon alloys

The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.     

Non-equilibrium segregation of solutes to grain boundary

Mechanisms for the non-equilibrium segregation of solutes to static grain boundary during cooling (quenching-induced segregation) and to moving grain boundary during recrystallization (moving-induced segregation) are proposed. For quenching-induced segregation, in consideration of the local equilibrium among vacancies, solute atoms and vacancy-solute atom complexes, as well as the influence of equilibrium grain-boundary segregation, the theoretical dynamic formulae for this non-equilibrium segregation have been derived on the basis of the vacancy-dragging mechanism. Theoretical calculations have been carried out for the non-equilibrium segregation of boron to austenitic grain boundaries during isothermal holding and continuous cooling after heating at high temperature; the results agree well with those obtained from experiments. The model has also successfully explained the different behaviours of boron segregation during cooling in -Fe and in -Fe. For moving-induced segregation, based on the interaction between dislocations and the moving boundaries during recrystallization, a dislocation relaxation and widening grain-boundary mechanism of solute segregation on moving boundaries is proposed. Applying this model, we have calculated the boron segregation on moving boundaries during recrystallization in Fe-3% Si alloy; the results of these calculations agree with experimental results.     

Non-equilibrium segregation of solute to grain boundary

The behaviour of boron segregation to grain boundaries in Fe-3%Si has been studied by means of particle tracking autoradiography. The results indicate that (i) the binding energy between boron atoms and grain boundaries is 55.7±1.7 kJ mol^–1; (ii) in contrast to the nature of boron segregation in -Fe, no observable non-equilibrium segregation of boron to grain boundaries exists in Fe-3%Si alloy during cooling and isothermal holding.     

Universal features of grain boundary networks in FCC materials

Grain boundary character distributions and triple junction distributions have been determined for 70 experimental microstructures, comprising aluminum-, copper-, austenitic iron- and nickel-based alloys in a wide variety of processed states. In these FCC metals, the fraction of coincidence site lattice (CSL) boundaries ranges from about 12% (as for a random Mackenzie distribution) to values as high as 75%. Despite wide variations in composition, processing, and grain size, we find that the grain boundary character distribution and triple junction distributions of these materials have striking similarities, and can be described by just a few parameters. This universality arises due to the highly non-random laws that govern the assembly of the grain boundary network, and due to the kinematic limitation that CSL boundaries arise primarily through twinning.     

Interactive grain boundary segregation of nickel and antimony in iron

Abstract

A study of the grain boundary segregation of nickel and antimony in iron is reported in the present paper. It is shown by the results that antimony segregation increases as the bulk nickel and antimony concentrations increase. However, once the solubility limit for antimony in iron is exceeded, the amount of segregation remains essentially constant. Segregation of nickel in iron increases as the bulk concentration of nickel increases and as the bulk concentration of antimony increases. The last effect is observed only when a certain level of antimony is reached, a level that depends on the concentration of nickel. Small additions of antimony, even though they cause an increase in segregation of antimony, do not cause an increase in segregation of nickel and, once the solubility limit for nickel in Fe–Sb alloys is exceeded, the segregation of nickel reaches a plateau. It is only between these two regimes that segregation of nickel is affected by changes in the concentration of antimony. All these results can be explained based on the mutual effects that nickel and antimony have on the solubility of each in iron. The results are not consistent with models based on cosegregation. Finally, other data in the literature are examined; all these data can be explained by an argument based on solubility changes.

MST/795     

Kinetics of grain boundary segregation during recrystallisation annealing

Abstract

In batch annealing and continuous annealing processes, both recrystallisation and grain boundary segregation can occur. In this paper, a simple model is derived which explores the interaction of the boundary migration and segregation processes and considers the application to phosphorus segregation during the annealing of interstitial free steels. The model considers both segregation to a migrating boundary and the segregation which occurs during continuous cooling after the holding period during the anneal cycle.     

A new computer model of grain boundary segregation

Improvements have been made to an earlier model of grain boundary segregation. The new model includes an influx of vacancies into the region of the material considered by the model, and accounts for the effect of a grain boundary on defect-binding energies. The model predictions for boron segregation in steel are compared with the original model.     

A new model for radiation-induced grain boundary segregation with grain boundary movement in concentrated alloy system

We have developed a new model for radiation-induced grain boundary migration (RIGM) and radiation-induced segregation (RIS) for austenitic iron-chromium-nickel alloy system. It was assumed that the RIS was induced by diffusional and annihilation processes of excess point defects at the grain boundary, and the RIGM occurred due to rearrangement process of atoms on one of the interfacial planes by annihilation of point defects. The calculated results indicated that the region of RIS was enlarged by the RIGM and asymmetrical concentration profiles were observed around the migrated grain boundary. The present model could explain the RIS behavior with or without grain boundary migration as comparing with our previous experimental results.     

Relation of ductile-to-brittle transition temperature to phosphorus grain boundary segregation for a Ti-stabilized interstitial free steel

Equilibrium grain boundary segregation of phosphorus in a Ti-stabilized interstitial free (IF) steel is measured using Auger electron spectroscopy (AES) after the specimens are aged for adequate time at different temperatures between 600 and 850 °C. Based on the experimental data of equilibrium grain boundary segregation along with the McLean equilibrium segregation theory, the free energy of segregation of phosphorus is evaluated to be ∼44.8 kJ/mol, being independent of temperature. With the AES results being combined with the ductile-to-brittle transition temperatures (DBTTs) determined by impact tests, a relationship between DBTT and phosphorus boundary concentration is established. Predictions with the relationship indicate that cold work embrittlement may be severe if the steel is annealed at relatively low temperatures after cold rolling.     

Grain-boundary segregation and grain growth in nanocrystalline substitutional solid solution alloys

A model for describing solute segregation at grain boundaries has been developed for substitutional solid solution alloys,which integrates multiple factors from atomic to microstructural scales.A concept of mo-lar Gibbs free energy of segregation was introduced to evaluate the segregating capability of the solute elements in a closed system,through which the influences of grain boundary structure,grain size,ma-terial composition,and external conditions were described.Based on the evaluation of various energy forms related to solute segregation and grain growth processes,the nature of the thermal stabilization of nanograin structures by solute segregation was disclosed.A criterion for the destabilization of nanostruc-tures,which is determined by the competition of the change rates between the molar Gibbs free energy of segregation and the total energy of grain boundaries with grain size,has been proposed.This study provided guideline to achieve high-temperature stability of nanograin structures of solid solution alloys even for the weakly segregating nanocrystalline systems.     

Some aspects of the anisotropy of grain boundary segregation and wetting

A recently developed model of grain boundary (GB) segregation, in terms of the five macroscopic parameters of GB orientation, has been exercised to explore the anisotropy of GB segregation. The five macroscopic GB orientation parameters are defined by means of the orientations of the two crystallographic planes that terminate the crystals on either side of the GB, and a twist angle. Some important conclusions include the following: (a) the composition of a boundary depends on all five parameters of GB orientation, (b) the segregation profile across a GB depends on the two planes which terminate the adjacent crystals, (c) the composition profile across GB’s terminated by identical crystallographic planes is symmetric, but is asymmetric when GB’s are terminated by different planes, and (d) the strength of the segregation on one side of a GB influences the extent of segregation on the other. Some experimental results on Nb-doped TiO₂ are presented in order to verify above predicted trends. In addition, it is shown that the model predicts the possibility of anisotropic GB wetting transitions as two-phase coexistence is approached.     

The five parameter grain boundary character distribution of polycrystalline silicon

The purpose of this paper is to describe the five-parameter grain boundary character distribution (GBCD) of polycrystalline silicon and compare it to distributions measured in metals and ceramics. The GBCD was determined from the stereological analysis of electron backscatter diffraction maps. The distribution of grain boundary disorientations is non-random and has peaks at 36°, 39°, 45°, 51°, and 60°. The axis-angle distribution reveals that most of the grain boundaries have misorientations around the [111], [110], and [100] axes. The most common grain boundary type (30 % number fraction) has a 60° misorientation around [111] and of these boundaries, the majority are twist boundaries. For other common boundaries, symmetric tilt configurations are preferred. The grain boundary character distribution of Si is distinct from those previously observed for metals and ceramics. The measured grain boundary populations are inversely correlated to calculated grain boundary energies available in the literature.     

Effect of ternary solute interaction on interfacial segregation and grain boundary embrittlement

Effect of ternary solute interaction on interfacial segregation and grain boundary embrittlement in an M–I–J system is modeled on the basis of combined Guttmann and Rice–Wang approaches. It is clearly shown that repulsive I–J interaction strengthens interfacial segregation of the impurity I, suppresses segregation of the solute J, and substantially enhances intergranular embrittlement. Attractive interaction exhibits an opposite effect. Generally, the effect of the ternary interaction is weaker than that of the binary one. Although there are only rare experimental data in this respect, their comparison to model calculations shows a very good agreement.     

First-principles study on the effect and magnetism of iron segregation in Cu grain boundary

The atomic configurations and electronic structures of iron on CuΣ5 symmetrical tilt grain boundary (GB) have been studied based on the density functional theory. Different segregation positions of iron are considered. A weak tendency of iron segregating to GB is arrived due to the segregation energy. In addition, iron segregation shows a cohesion strengthening effect of Cu GB according to Rice–Wang model, which is mainly contributed by the charge redistribution. Finally, an enhancement of the local magnetic moment of iron in Cu GB or bulk or surface is explored due to larger atomic volume than the FCC iron crystal and the Cu atoms surrounding iron are slightly polarized by the doped iron. This study can enrich the understanding of the effects of iron on the cohesion of Cu–Fe alloy and also might supply an indirect guidance to expand the application of Cu–Fe alloy in electronic device manufacture field.