Grain boundary segregation of phosphorus and molybdenum in A533B steel

Abstract

Phosphorus and molybdenum segregation to grain boundaries in a commercial grade A533B steel subjected to a variety of heat treatments has been examined using a field emission gun scanning transmission electron microscope (FEGSTEM) with energy dispersive X-ray micro-analysis. The results indicate that P and Mo concentrations at prior austenite grain boundaries increase with aging time. This follows the prediction of McLean's equilibrium segregation model, when modified to take account of the interaction energy between phosphorus and molybdenum.     

Grain boundary segregation of phosphorus and molybdenum in 2·25Cr–1Mo steel

Abstract

Segregation of phosphorus and molybdenum to grain boundaries in a commercial grade of 2·25Cr–1Mo steel subjected to different heat treatments has been examined using a field emission gun scanning transmission electron microscope with energy dispersive X-ray microanalysis. The results indicate that P and Mo concentrations at prior austenite grain boundaries increase with aging time. This follows the prediction of McLean's equilibrium segregation model, when modified to take account of the interaction energy between phosphorus and molybdenum.     

Grain microstructure evolution and grain boundary junction engineering

Abstract

The grain microstructure evolution in the course of two dimensional (2D) grain growth is considered in greater detail, taking into account the influence of grain boundary triple junctions. It is shown that there are two limiting regimes of grain growth in polycrystals: the first one is associated with the situation when the kinetics of grain growth are controlled by the motion of grain boundaries, while the second one is defined by the motion of grain boundary triple junctions, i.e. when the mobility of triple junctions determines the kinetics of grain growth. A generalised theory of 2D grain growth including a limited triple junction mobility is presented. The theoretical predictions are compared with results of computer simulations by a virtual vertex model. We introduce a new branch of grain boundary engineering, namely, grain boundary junction engineering that utilises junction properties for microstructure control.     

Grain boundary engineering: an overview after 25 years

Abstract

In 1984, 'grain boundary design', later known as 'grain boundary engineering (GBE)', was proposed. The central premise of GBE is that specific thermomechanical treatments, mainly on face centred cubic materials which readily form annealing twins, can be used to improve resistance to various forms of intergranular degradation such as corrosion, cracking or embrittlement. Engagement with the concept has accelerated in recent years. This overview charts the progress of GBE from its inception 25 years ago to the present day, including suggestions of key topics for ongoing or future research. These topics comprise confirmation of which boundaries are 'special' in terms of crystallography and properties, optimisation of processing regimes, new approaches to GBE in systems without annealing twinning and incorporation of connectivity metrics, especially in three dimensions.     

Grain boundary crystallography in two Fe-C-P alloys

Abstract

Misorientation, grain growth and brittle fracture were investigated in two iron - carbon alloys containing 0.06 wt-% phosphorus (0.06P) and 0.12 wt-% phosphorus (0.12P) after selected heat treatment schedules. A 'fracture surface serial sectioning' technique was devised and combined with misorientation measurements to reconstruct specimens after fracture. Anomalous grain growth occurred in the 0.06P specimen only, after 1000°C annealing. This was attributed to the inhomogeneous distribution of phosphorus at the interfaces. No evidence was found for the direct influence of misorientation angle distributions or coincidence site lattice distributions on anomalous grain growth. The proportion of Σ3s increased greatly after annealing at 1000°C, attributed to the twinning that developed in the austenite range. There was strong evidence that Σ3s were in general more resistant to brittle fracture than were random boundaries. It is suggested that alloys of this type could be 'grain boundary engineered' to improve fracture resistance.     

Distribution of misorientations and grain boundary planes in grain boundary engineered brass

Abstract

The present paper reports the application of a five parameter determination of grain boundary types to grain boundary engineered α brass. Approximately 20 000 grains constituted the total sample population, giving rise to more than 77 000 grain boundary line segments. This is the first time that the orientation of a large sample population of grain boundary planes has been measured in a grain boundary engineered material. The most important findings of the investigation were that the distribution of planes showed a prevalence of 〈 110 〉 tilt boundaries, especially asymmetric tilt types, and the presence of 〈 111 〉 twist boundaries. This distribution is a consequence of the low energy of these boundary types. Furthermore, more than three-quarters of boundaries could be considered to be 'potentially special'. The presence of these boundaries greatly fragmented the grain boundary network. This fragmentation is probably a key factor in the development of superior properties in a grain boundary engineered material.     

Grain boundary segregation of phosphorus,manganese, and carbon in boiler shell weld material

Abstract

Submerged arc weld materials have been employed in a study of the effects of manganese and carbon on phosphorus segregation and intergranular embrittlement. The equilibrium grain boundary segregation behaviour of these elements during aging has been studied in two different boiler shell weld materials, which differ mainly in the manganese concentration and operating temperature during service. The materials have seen temperatures above 300C during operation of the boiler in service, at which temperature thermally induced segregation and embrittlement occurs. A new co-segregation model has been compared with the existing site competition model. The microstructure shows fine and coarse grained regions. The effects of manganese and carbon on the grain boundary segregation of phosphorus have been examined. Thermally induced grain boundary segregation during full service life up to 50 years as a function of temperature is described. To evaluate the free matrix concentration of a given element, equilibrium thermodynamic software was used to allow for the tendency to form precipitates within the alloy matrix. The predicted results reveal the dependence of the grain boundary concentration on temperature and show that manganese and carbon decrease the phosphorus segregation by site competition. The final segregation consists of non-equilibrium and equilibrium segregation, which occur during quenching after welding, post-weld heat treatment, and service. The microstructure has been investigated by optical microscopy and transmission electron microscopy to show carbide formation at the grain boundaries and intergranular precipitation of MnS. Preliminary analysis of the grain boundary has been made and the results compared with theoretical segregation predictions.     

Theory of boundary diffusion controlled grain rotation in a 'bamboo' structure

Abstract

The theory of grain boundary diffusion controlled rotation of an orthogonal bicrystal about its common boundary has been extended to the case of cylindrical geometry. The analysis for this simple 'bamboo' geometry enables predictions to be made with a higher level of certainty than is usual for other diffusion controlled processes. Since bamboo structures are easy to fabricate, this suggests an indirect method of estimating boundary diffusion coefficients based on experimental measurement of rotation rates. A numerical analysis is presented and the dependence of the rotation rate on bending moment and wire radius is determined. The variation of the local stress and diffusion fluxes over the boundary is calculated. The conditions where experimental measurements are likely to provide a viable method of estimating grain boundary diffusion coefficients are predicted.     

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.     

Effect of Hf ion implantation on grain growth in Ni nanocrystalline electrodeposits

Abstract

The microstructural stability of Ni nanocrystalline electrodeposits was investigated to verify general principles underlying the suppression of grain growth by microalloying with elements of very low solid solubility. Hf ions at 300 keV energy were implanted in Ni nanocrystalline foils at low (5·8 × 10¹⁵ ions cm^?2) and high (3·0 × 10¹⁶ ions cm^?2) doses. Their effects on grain growth at 550°C were studied in situ by transmission electron microscopy at 1·25 MeV and by selected area electron diffraction. Grains roughly doublled in size during implantation, but grain growth during subsequent heat treatment was dose dependent and significantly less than in specimens without implantation. Observation on implanted Ni single crystals revealed clustering and the formation of fine Ni₅Hf precipitates. A possible mechanism of grain growth suppression is discussed.     

Ferrite/pearlite band formation in hot rolled medium carbon steel

Abstract

The influence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.     

Nuclei for heterogeneous formation of graphite spheroids in ductile cast iron

Abstract

The crystal structure and composition of nuclei for graphite spheroids in a ductile cast iron containing small amounts of magnesium and traces of aluminium have been studied in a transmission electron microscope equipped with EDX and parallel electron energy loss spectrometer and in an electron microprobe analyser. The particles were identified as Al–Mg–Si nitrides, having a trigonal superlattice crystal structure derived from a hexagonal AlN type fundamental cell. The superlattice can be indexed according to a hexagonal Bravais lattice, with parameters a=0.544 nm and c=0.482 nm. The parameters of the fundamental cell are a _f=0.314 nm and c _f=0.482 nm, deviating only 1–3% from the parameters of hexagonal AlN. Based on the compositional analysis, the chemical formula of the nitride particles is suggested to be AlMg_2.5Si_2.5N₆.     

Grain boundary microstructure and fatigue crack growth in Allvac 718Plus superalloy

The correlation between grain boundary microstructure and fatigue crack growth with hold-times was investigated for two conditions of the superalloy Allvac 718Plus; a Standard condition with the recommended distribution of grain boundary phases and a Clean condition with virtually no grain boundary phases. Fatigue testing was performed at 704 °C using 10 Hz cyclic load with intermittent hold-times of 100 s at maximum tensile load. Microstructural characterization and fractography were conducted using scanning- and transmission electron microscopy techniques. Auger electron- and X-ray photoelectron spectroscopy techniques were used for oxide analyses on fracture surfaces. It was found that in the Standard condition crack growth is mostly transgranular for 10 Hz loading and intergranular for hold-times, while for the Clean condition crack growth is intergranular in both load modes. The lower hold-time crack growth rates in the Standard condition are attributed to grain boundary δ-phase precipitates. No effect of δ-phase was observed for 10 Hz cyclic loading crack growth rates. Two different types of oxides and oxide colours were found on the fracture surfaces in the Standard condition and could be correlated to the different loading modes. For cyclic loading a bright thin Cr-enriched oxide was dominate and for hold-times a dark and slightly thicker Nb-enriched oxide was dominant These oxide types could be related to the oxidation of δ-phase and the matrix respectively. The influence of δ-phase precipitates on crack propagation is discussed.     

Creep damage and grain boundary precipitation in power plant metals

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for creep damage such as grain boundary cavities and microcracks. Monte Carlo based grain boundary precipitation kinetics is combined with continuum creep damage mechanics (CDM) to model both the microstructural evolution and creep behaviour in power plant metals. It is found that grain boundary precipitates, such as M₂₃C₆ in most Cr containing ferritic steels, are harmful to the creep properties of the material, in line with experimental observations. It is also found that to improve the creep behaviour of the material, means should be found either to increase the proportion of MX type particles, such as VN, or to decrease or remove the larger grain boundary precipitates, such as M₂₃C₆. Hafnium has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study the effect of hafnium on the grain boundary precipitation kinetics. It is found that the implantation of hafnium to the steel completely prohibits the formation of the common grain boundary M₂₃C₆ particles. Instead, two new types of precipitates are formed. One is hafnium carbide, which is an MX type precipitate, and is very small in size and has a much higher volume fraction as compared with the volume fraction of VN in conventional power plant ferritic steels. The other is Cr- and V-rich nitride of formula M₂N. CDM modelling shows that implantation of hafnium can markedly improve the creep property of the material. In addition, the replacement of M₂₃C₆ with hafnium carbide increases the concentration of Cr in the matrix and is expected to improve the intergranular corrosion resistance of the material.     

Grain boundary precipitation in Inconel 718 and ATI 718Plus

Grain boundary precipitates in Inconel 718 and ATI 718Plus are important to control during hot working processes, since they can control the grain size. Precipitating excessive or insufficient amounts can be detrimental to the final component. Therefore, it is important for manufacturers to understand the formation and kinetics of grain boundary precipitation and the effect this has on mechanical properties. This review considers the background of grain boundary precipitation, including the effect of the thermal stability of γ′ and γ^″ phases. In addition, the effect of stress on the grain boundary phases and their precipitation kinetics in different conditions are also included. Also, the impact of grain boundary precipitation on the mechanical properties is explored.

This review was submitted as part of the 2017 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged     

Experimental study on the characteristic of grain-boundary segregation of phosphorus in Ni-base superalloy

In virtue of Auger electron spectroscopy, the levels of grain-boundary segregation of phosphorus in Ni-based superalloy after solution-treated at 1200 °C for 9.8 h and at 1020 °C for 68.5 h and then aged at 720 °C all for 2 h are measured. It is found that the levels increase with the increase in solution treatment temperature. The measurement result is analysed with the laws of equilibrium segregation and non-equilibrium segregation. Based on the analysis, the characteristic of non-equilibrium grain-boundary segregation of phosphorus in the superalloy is confirmed for the first time.     

Grain growth in IN718 superalloy fabricated by laser additive manufacturing

ABSTRACT

The grain growth mechanism of IN718 superalloy fabricated by selective laser melting (SLM) was studied. Epitaxial growth with the same crystallographic orientation or rotating by 90° across the melting pool boundary and competitive growth in the same melting pool were observed. Either of the two patterns of epitaxial growth can maintain the same grain across the melting pool boundary. Competitive growth is determined by both the heat flow direction and preferred crystallographic orientation. In SLM, the grains grow along the preferred crystallographic orientation owing to a high solidification rate. The smaller the deviation angles between the heat flow direction and the preferred crystallographic orientation, the faster the grain growth rate.     

Effects of solidification rate on segregation and fluid flow tendency in mushy zone of directionally solidified superalloy Inconel 718

Abstract

The microstructure and composition of the interdendritic liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates between 2 and 100 μm s^?1 have been investigated by SEM and EDAX techniques. The interdendritic liquid segregation profiles along the mushy zone are presented. The liquid density difference and Rayleigh number in the interdendritic liquid were calculated and analysed as well. It was found that when the solidification rates increased in the range 10–70 μm s^?1, segregation of Nb decreased, but segregation of Mo was most serious at 20 μm s^?1. The liquid density difference increased the most for rates from 20 to 40 μm s^?1 as temperature decreased. The maximum relative Rayleigh number was highest at 10°C below the liquidus temperature at 20 μm s^?1, which indicated the conditions where fluid flow most easily occurred for Inconel 718. The relative Rayleigh number synthetically considers the factors affecting fluid flow and can give a reasonable prediction for fluid flow tendency.     

Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces

Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as ‘segregation engineering’ solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing.     

Microstructure and mechanical properties of nickel based superalloy IN718 produced by rapid prototyping with electron beam melting (EBM)

Abstract

A nickel alloy of a composition similar to that of the nickel based superalloy Inconel alloy 718 (IN718) was produced with the electron beam melting (EBM) process developed by Arcam AB. The microstructures of the as processed and heat treated material are similar to that of conventionally produced IN718, except that the EBM material showed some porosity and the δ phase did not dissolve during the solution heat treatment because the temperature of 1000°C apparently was too low. Mechanical testing of the layer structured material, parallel and perpendicular to the built layers, revealed sufficient strength in both directions. However, it showed only limited elongation when tested perpendicular to the built layers due to local agglomerations of pores. Otherwise, data for the hardness, Young’s modulus, 0·2% yield tensile strength and ultimate tensile strength match those recommended for IN718.