Equilibrium grain boundary segregation is assessed in terms of its measurement, theory and industrial significance. Aspects of the phenomenon of this segregation, namely the localisation, chemical system dependence, response to environmental parameters, etc., are discussed in relation to the measurement techniques for grain boundary segregation. These techniques-the interfacial energy approach, AES, XPS, STEM with X-ray analysis and the atom-probe FIM-are analysed in terms of their unique contributions. The theories of segregation are presented in a unified framework which uses analogues of the theories for gas adsorption on free surfaces. The cases of limited and unlimited numbers of sites are treated. Thus, McLean's segregation theory is seen to be the counterpart of Langmuir adsorption and a self-interacting segregant is seen to follow the analogue of the Fowler theory. The Fowler theory is extended to ternary systems and accurate methods of estimating the value of the relevant fixed parameters are presented. The kinetics of segregation and desegregation are then outlined. A range of material aspects affected by grain boundary segregation are described.

https://iopscience.iop.org/article/10.1088/0305-4608/10/6/006/pdf

Grain boundary segregation

The theory of non-equilibrium grain-boundary segregation is discussed with particular reference to recent ideas and data relating to boron grain-boundary segregation in Type 316 austenitic steel. The kinetics of the non-equilibrium grain-boundary segregation process are considered in depth and a model is developed which, it is hoped, will more realistically describe the magnitude and extent of the process. Reasonable agreement is found between the predictions of the model and experimental evidence for non-equilibrium boron, aluminium and titanium segregation to grain boundaries in austenitic steels. The model predicts, generally, that elements with large misfits with the matrix atoms will segregate most. Larger grain sizes lead to greater grain-boundary segregation. Also, the two critical heat-treatment parameters in non-equilibrium segregation are the solution-treatment temperature and the cooling rate from the solution-treatment temperature. Predictions of the worst combinations of these parameters for maximum non-equilibrium segregation to grain boundaries in austenitic steels are presented.

Non-equilibrium grain-boundary segregation in austenitic alloys

The influence of deformation-induced excess vacancies on segregation and on precipitation is discussed within a phenomenological framework. During high temperature deformation, vacancies are mainly generated by the non-conservative movement of thermal jogs. Sinks for these vacancies are dislocations and grain boundaries. The excess vacancy concentration increases with strain rate and inverse temperature. Stronger effects are expected in f.c.c. materials because of their comparatively low diffusivities; in austenitic steels, the concentrations of deformation-induced vacancies are predicted to be significant, even at temperatures as high as 0.75 of the melting temperature. At the high strain rates (> 10 s −1 ) used in industrial rolling, the vacancy concentration may attain levels 2–3 orders-of-magnitude higher than the equilibrium one. One consequence of the presence of deformation-induced vacancies is the non-equilibrium segregation of boron observed e.g. in austenitic Nb-B steels, where it stimulates the nucleation of Nb borocarbonitrides.

Modelling the effect of deformation-induced vacancies on segregation and precipitation

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel—I. Large scale segregation behaviour

Microstructural stability of modified 9Cr–1Mo steel during long term exposures at elevated temperatures

The distribution of boron in samples of Type 316 austenitic steel (containing 18 ppm by weight of B), cooled at a rate of either 50 or 500 degC/s from solution-treatment temperatures in the range 900–1350°C, has been determined using an optical microscope autoradiographic technique. Segregation of the boron to the grain-boundary regions was detected in the specimens cooled at the slower rate from the higher solution-treatment temperatures, the extent of the intergranular segregation diminishing with decreasing solution-treatment temperature; grain-boundary segregation of the boron was not detected in the specimens cooled at 500 degC/s irrespective of the solution-treatment temperature. A theoretical model is proposed to account for this non-equilibrium grain-boundary segregation of boron during cooling, based on the existence of mobile vacancy-boron complexes. The predictions of the model are in good agreement with the experimental observations if realistic values for the vacancy-boron binding ene...

A. M. Stoneham

Papers

The segregation of boron to grain boundaries in solution-treated Type 316 austenitic stainless steel