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.     

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.     

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.     

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 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.     

Improving resistance to dynamic embrittlement and intergranular oxidation of nickel based superalloys by grain boundary engineering type processing

Abstract

Polycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s^?1 under four-point bending conditions at 650°C in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be 'dynamic embrittlement', i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is particularly slow for special coincident site lattice (CSL) grain boundaries, bending and oxidation experiments were carried out using specimens that underwent successive steps of deformation and annealling, i.e. grain boundary engineering. It has been shown that an increase in the fraction of special CSL grain boundaries yields a higher resistance to both intercrystalline oxidation and hold time cracking by dynamic embrittlement.     

HRTEM study of [001] low-angle tilt grain boundaries in fiber-textured BaTiO3 thin films

Low-angle tilt grain boundaries in [001] fiber-textured BaTiO₃ thin films were investigated by high-resolution transmission electron microscopy. Extensive observation revealed a very high density of low-angle tilt grain boundaries in the film. The low-angle tilt grain boundaries can be described as periodical arrays of dislocations on {100} and {110} boundary planes. The boundaries with (100) plane on {100} planes are composed of perfect dislocations with Burgers vectors b = a < 100 > (a = lattice constant of BaTiO₃: 0.3992 nm), while the boundaries with (110) plane on {110} planes are composed of the dissociated dislocations with Burgers vectors a/2 < 110 >. It was thus found that the difference in the boundary plane leads to different dislocation structures along the low-angle grain boundaries.     

Grain boundary segregation in Ni-base (718 plus) superalloy

Abstract

Segregation of phosphorus and molybdenum to grain boundaries in a commercial grade Ni-base (718 plus) superalloy 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 grain boundaries increase in linear manner with the square root of aging time.     

An examination of the linkage of cleavage cracks at grain boundaries

Abstract

Grain boundaries resist the propagation of cleavage cracks in polycrystalline materials, and 3D geometrical models have been used to predict the accommodation required at a grain boundary as a crack propagates from grain to grain. This paper describes how focused ion beam (FIB) microscopy, which provides topographic and crystallographic contrast imaging and allows ion milling to be undertaken at selected areas of interest, can be used to investigate these local fracture events. Results of low temperature fracture of polycrystalline bcc Fe–3%Si and hcp zinc are presented. The interactions between these results and the geometrical modelling are briefly discussed.     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with Σ3〈011〉, Σ5〈001〉 and Σ9〈011〉 coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}〈001〉 (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.The Σ5〈001〉 and Σ9〈011〉 twist boundaries showed higher migration speed than Σ3〈011〉 twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (Δθ) from the ideal orientation relationship for a coincidence boundary. The increase of Δθ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the Σ9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed.     

Grain boundary plane distributions and single-step versus multiple-step grain boundary engineering

The ‘five-parameter’ (i.e. both misorientation and grain boundary plane) distribution in type 304 austenitic stainless steel has been measured and evaluated for an ‘as-received’ (AR) specimen and specimens undergoing both single-step grain boundary engineering processing (SSGBE) and multiple-step grain boundary engineering processing (MSGBE) comprising three iterations. The results showed that the fundamental requirement for twinning-related GBE is to maximise concomitantly the proportion of both Σ3 and Σ9 boundaries, which in turn supports the development of special planes in the grain boundary network. 1 1 0 and 1 1 1 tilt and twist boundaries play a key role in the formation of ‘special’ grain boundary planes. MSGBE added increased proportions of Σ3 boundaries and resulted in development of different characteristics in the planes distribution compared to SSGBE. These modifications are likely to result in improved grain boundary properties after MSGBE compared to SSGBE.     

Method for determining the elastic modulus at grain boundaries for polycrystalline materials

Abstract

On the basis of the model of non-equilibrium grain boundary segregation induced by tensile stress, a set of kinetic equations is derived to formulate this process. These kinetic equations allow excellent simulation of the grain boundary segregation of phosphorus and sulphur observed in steels subjected to low tensile stresses. In the present paper, based on such a widely approved model, a new approach is proposed to quantify the elastic modulus at grain boundaries for polycrystalline materials. Using the observation of Misra, the grain boundary elastic modulus E _gb = 2.03 × 10⁹ Pa at 883 K for Cr-Mo-V-2.6Ni steel is obtained for the first time. This result shows excellent agreement with the local elastic constants simulated theoretically by Kluge et al., and indicates that the grain boundary elastic modulus for a polycrystalline material is much lower than the commonly assumed value.     

Relationship between coincidence site lattice,boundary plane indices,and boundary energy in nickel

Abstract

This paper reports an analysis both by coincidence site lattice (CSL) categorisation and interface–plane categorisation of a large data set of grain boundary geometry in nickel. The analysis showed that whereas two-thirds of Σ3 and Σ9 CSLs were classed as having ‘special’ geometry (i.e. related to low energy), very few other CSLs had ‘special’ geometries. It was found that for Σ3s there was an empirical relationship connecting the frequency of occurrence of specific planes (mostly asymmetrical tilt types on the 011 zone), the average interplanar spacing at the boundary (d(eff)), and the boundary energy. Σ3 boundaries having a planar coincidence site density (PCSD) of unity were much more poorly represented in the data set than Σ3s having PCSD = 3, indicating the far greater importance of boundary planes than the PCSD. The occurrence of ‘special’ planes at Σ9 boundaries is considered to be partly a geometrical consequence of their contiguity with Σ3s.     

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.     

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.     

Production of very fine grained AISI 304 steel with high special grain boundary density by grain boundary engineering

Abstract

AISI 304 stainless steel was subjected to grain boundary engineering by applying cycles of calibre rolling and subsequent heat treatment. After three cycles the grain size started to decrease, and after the fourth cycle a very fine grained material having high fraction of special grain boundaries was produced. Due to the short heat treatment at 850°C, only partial recrystallization occurred after the first three cycles, which was proven by the large amount of low angle boundaries. The stored elastic strain energy helped the grain boundary movement and the formation of annealing twins in the fourth cycle, which caused the formation of very fine grained structure with a large amount of special grain boundaries.     

The roles of grain orientation and grain boundary characteristics in the enhanced superelasticity of Cu71.8Al17.8Mn10.4 shape memory alloys

Through texture and grain boundary control by continuous unidirectional solidification, the continuous columnar-grained polycrystalline Cu_71.8Al_17.8Mn_10.4 shape memory alloys were prepared and possess a strong 〈0 0 1〉 texture along the solidification direction and straight low-energy grain boundary. The alloys show excellent superelasticity of 10.1% improved from 3% for ordinary polycrystalline counterpart and with a tiny residual strain of less than 0.3% after unloading. There are some reasons for the enhanced superelasticity: (1) The martensitic transformation of all grains with strong 〈0 0 1〉-oriented texture occur at the same time under the tensile loading, which can avoid the significant stress concentration problem and transformation strain incompatibility at the grain boundaries due to the high elastic anisotropy in ordinary polycrystalline alloy. (2) High phase transformation strain can be obtained along 〈0 0 1〉 grain orientation. (3) Straight low-energy grain boundary and the absence of grain boundary triple junctions of continuous columnar-grained polycrystals can significantly reduce the blockage of martensitic transformation at the grain boundaries. These results provide a reference to structure design of high-performance polycrystalline Cu-based shape memory alloys.     

The effect of solutes on grain boundary mobility during recrystallization and grain growth in some single-phase aluminium alloys

The effect of solutes (Si, Mn, Mg) in quantities typical of commercial aluminium alloys, on grain boundary mobility in aluminium, has been investigated with in situ annealing and electron backscattered diffraction in the SEM, and grain growth experiments. The in situ experiments provided information on the migration of the high mobility tilt boundaries of misorientations close to 40°〈1 1 1〉. Grain growth experiments were used to investigate boundary migration in alloys of high solute content (1-5wt%Mg), and a comparison between the in situ and bulk experiments is made. The relationship between boundary velocity and driving pressure was found to be linear in all cases, and the activation energies for boundary migration were higher than those controlled by lattice diffusion of the solutes at higher solute concentrations.     

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.     

Oxygen diffusion blocking of single grain boundary in yttria-doped zirconia bicrystals

Yttrium-doped zirconia bicrystals with [001] symmetric tilt Σ 5 grain boundaries were fabricated by a diffusion bonding technique, and oxygen diffusion behavior across the grain boundary was measured by secondary ion mass spectrometry (SIMS), tracing ¹⁸O isotope. It was found that the ¹⁸O fraction across the boundary exhibited explicit decrease around the boundary plane, indicating that the oxygen-diffusion is blocked by the presence of the Σ 5 grain boundary. This is the first experimental detection of oxygen diffusion blocking at a single grain boundary in zirconia ceramics. From high-resolution transmission electron microscopy observations and energy dispersive X-ray spectroscopy analysis, neither amorphous layers nor Si impurity segregation were found at the grain boundary. The grain boundary blocking effect of the Σ 5 boundary must be an intrinsic feature arising from its core structure and yttrium solute segregation of the grain boundary.