R.G. Faulkner

Non-equilibrium grain-boundary segregation in austenitic alloys

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.

Grain boundary segregation under neutron irradiation in dilute alloys

Irradiation-induced segregation mechanisms are classified into solute-point-defect complex type and inverse Kirkendall type. For solutes that have a strong interaction with interstitials in a dilute alloy, the complex effect plays an important part in the segregation. Our earlier model describing solute grain boundary segregation during neutron irradiation in dilute binary alloys, based on the complex effect mechanism, is modified by considering the irradiation-enhanced solute diffusion and the long-range recombination effect of freely migrating point-defects, and expanded to evaluate solute segregation in dilute ternary alloys through consideration of solute-solute competition for segregation sites. Applications of the model to predictions of P grain boundary segregation in neutron irradiated alpha-Fe and Fe-B-P and Fe-C-P alloys indicate that the model has reasonable validity

Validation of the shear punch-tensile correlation technique using irradiated materials

In previous studies on a variety of unirradiated materials, a linear relationship was developed between uniaxial tensile strength and effective shear strength, as determined from the shear punch test (SPT). Using the same data, a correlation was also developed to predict tensile uniform elongation from shear punch data. Validation of both correlations using a new database on both irradiated and unirradiated materials has been completed successfully.

Radiation-induced segregation in HT-9 martensitic steel

Abstract Miniature notched-bar specimens of normalized and tempered HT-9 were neutron irradiated to ∼13 dpa and broken at liquid nitrogen temperatures in a UHV chamber. Fracture surfaces were analyzed using scanning Auger electron spectroscopy. Following irradiation at 410°C, the fracture surface contained a small number of large relatively smooth facets, which are thought to be prior austenite grain boundaries. Strong segregation of Ni, Cr, Si, and P was detected at these surfaces, the remainder of the fracture surface showing no evidence of segregation. At irradiation temperatures of 520°C and 565°C, there was relatively little segregation and none was found in thermal controls.

Shear punch testing of 59Ni isotopically-doped model austenitic alloys after irradiation in FFTF at different He/dpa ratios

In this last of a series of papers describing the evolution of microstructure, void swelling and mechanical properties of model austenitic alloys in response to differences in helium/dpa rates, shear punch testing is used to assess the relative effect of helium generation ratios and various important material and environmental variables. Shear punch data confirm the general trends observed in earlier tensile data derived from the 59Ni isotopic doping experiment. There is a convergence to a common saturation level of yield strength that depends on alloy composition, temperature and displacement rate, but not on starting condition. The approach to saturation can be sensitive to helium/dpa ratio, however, and may depend on the starting state. For reasons not yet known, shear punch tests appear to be more sensitive to such transient differences than are tensile tests.

Chemical volume diffusion coefficients for stainless steel corrosion studies

AbstractChemical diffusion coefficients for chromium in austenitic and ferritic steels are determined using diffusion couples studied by electron probe microanalysis techniques. The average chemical volume diffusion coefficient, for the composition range 14 to 28 at % chromium, for ferritic AISI 446 in the temperature range 800 to 1000° C is:

Radiation-induced grain boundary segregation in dilute alloys

\tilde D = 0.15\left( {\frac{{ + 0.54}}{{ - 0.12}}} \right) \exp \left( {\frac{{ - 210( \pm 15)}}{{RT}}} \right) cm^2 \sec ^{ - 1}

Modelling the effects of alloying elements on precipitation in ferritic steels

and for austenitic AlSl 310 in the temperature range 800 to 1200°C is: