Shenhua Song

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

Quenching and tempering-induced molybdenum segregation to grain boundaries in a 2.25Cr–1Mo steel

Abstract Molybdenum is one of the major alloying elements in low-alloy structural steels and is beneficial for mitigating temper embrittlement. Enhancement of the grain boundary cohesion by molybdenum segregation is one of the mechanisms for molybdenum to alleviate the temper embrittlement. To understand clearly the segregation behaviour of molybdenum in low-alloy structural steels, examination of molybdenum grain boundary segregation during quenching and tempering in a 2.25Cr–1Mo steel is conducted experimentally by virtue of field emission gun scanning transmission electron microscopy. There is some molybdenum segregation during quenching. Supersaturated vacancies created by quenching play a certain role in the segregation. Combined equilibrium and non-equilibrium segregation of molybdenum is the nature of segregation in the tempered steels.

Effect of rare earths on the carburization of steel

The effects of rare earths (RE) on the carburization of steel are examined with four 0.2%C steels doped and undoped with RE. Clearly, both RE in carburizer and RE in steel may accelerate the carburizing process. RE in carburizer is more effective at enhancing carburization than RE in steel. The mechanism for this enhancement of carburization is that the RE oxide enhances the medium-sample interface reaction during carburizing.

Temper embrittlement of a CrMo low-alloy steel evaluated by means of small punch testing

Abstract Temper embrittlement of a 2.25Cr1Mo steel is evaluated by use of small punch testing with disc specimens. Clearly, a ductile-to-brittle transition temperature of the steel may be determined by this test, but the value obtained is much lower than that determined by the standard Charpy test. The steel exhibits some temper embrittlement when tempered at 400°C for 86 days.

Effect of boron on phosphorus-induced temper embrittlement

Combined equilibrium and non-equilibrium grain boundary segregation of solute atoms in dilute ternary alloys is modelled through consideration of site competition between two solutes. Model predictions are made for a low-alloy steel containing boron. The predicted results indicate that the kinetics of phosphorus segregation are dramatically facilitated by quenched-in vacancies, and the magnitude of the segregation, however, is substantially suppressed by the competition of boron with phosphorus for segregation sites, and in turn the phosphorus-induced embrittlement may be alleviated.

Combined effects of cerium and boron on the mechanical properties and oxidation behaviour of Ni3Al alloy

The combined effects of cerium and boron additions on the room-temperature tensile properties and high-temperature oxidation behaviour of Ni3Al alloys: alloy 1 doped with cerium, alloy 2 doped with boron, and alloy 3 doped with both cerium and boron. The strength, ductility and oxidation behaviour of the alloy are more effectively improved by combined cerium and boron additions than by cerium or by boron addition alone. Of the three alloys, alloy 3 exhibits comprehensively the best mechanical properties and oxidation resistance. Alloy 2 presents the better mechanical properties than alloy 1; nevertheless, alloy 1 has the better oxidation resistance than alloy 2.

Irradiation-induced silicon segregation in ferritic steels

Abstract Irradiation-induced segregation (IRS) of Si in ferritic steels has been modelled using a non-equilibrium segregation based method relying on describing the motion of point defect-solute complexes to grain boundaries. The model is extended from the earlier versions to account more accurately for composition changes occurring on the grain boundary plane and to account more properly for the temperature dependence of dislocation density. The results of the model indicate strong peaking of IRS at certain critical temperatures. The peak positions move as the irradiation conditions are altered and as certain microstructural features like grain size and dislocation density are changed. The model is applied in this paper to provide explanation for observed DBTT shifts after neutron irradiation of ferritic steels.

A new view on the temperature-time dependence of temper embrittlement

It is well known that grain boundary segregation of certain impurities like P, S, Sb and others gives rise to temper embrittlement of low-alloy steels. It is important to understand the mechanism of temper embrittlement in some detail so as to predict the embrittlement for low-alloy steels. It is often assumed in the treatments of temper embrittlement that the impurity segregation to grain boundaries is brought about by an equilibrium segregation mechanism [1-4]. In 1950, Jaffe and Buffum [5], using standard Charpy specimens with a V-notch, evaluated the degree of embrittlement, as determined by the shift in ductile-brittle transition temperature (DBTT), of a certain heat of SAE 3140 steel with composition shown in Table I. The material was austenitized for 1 h at 900 °C, water-quenched to give 100% martensite with a prior austenite grain size of ASTM 8 (22/~m), then tempered for l h at 675°C and again waterquenched. This heat-treatment was known to give a low DBTT of -83 °C and thus this became its unembrittled reference state. The material was then given isothermal holding treatments for between 10 min and 200 h at certain temperatures in the temperature range 375-650 °C. The shift in DBTT from the reference state gave the measure of embrittlement and, when plotted as constant embrittlement curves as a function of time at temperature, gave the T t embrittlement diagram shown in Fig. 1. In a more detailed study of the double nose present in Fig. 1, Buffum and Jaffe [6] discovered that the DBTT in the embrittled state was microstructure-dependent to the extent of 2 °C per Rockwell C hardness number increase, i.e. the DBTT increased as the material became harder. The DBTT shown in Fig. 1 were corrected to constant hardness and the T t diagram, as shown in Fig. 2 [7], exhibited only one nose. This T-t embrittlement diagram stands for that component at constant microstructure and grain size. In practice, most commercial structural steels may not undergo the toughening treatment around 650 °C after quenching and before tempering in view of the requirement of strength. Therefore, we

Neutron energy spectrum and temperature effects on freely migrating defect concentrations and grain boundary segregation in α-Fe

Molecular dynamics (MD) calculations of cascades in α-Fe are used to predict the concentration of freely migrating defects. The single self-interstitial atoms (SIAs) are recognised to be important because they are more remote from the cascade and their migration energies are less than those of single vacancies. We are therefore able to estimate the enhanced concentration of freely migrating interstitial defects as a function of: (1) neutron energy spectrum, and (2) temperature. We compare our predictions with those obtained by diffusion experiments. The algorithms describing the temperature effect for a given neutron energy spectrum are incorporated in the models for radiation induced grain boundary segregation of P in α-Fe to produce an improved prediction of the temperature dependence of the segregation. Comments will be made on the implications of the new P segregation predictions for ferritic steels and on the importance of the low energy part of the neutron energy spectrum in determining freely migrating defect populations.

Radiation-induced inter-granular segregation in first wall fusion reactor materials

Experimental evidence for phosphorus segregation at grain boundaries in steels is presented. Theories for irradiation-induced inter-granular segregation are described. Non-equilibrium segregation (NES) and rate theory approaches have similar success in predicting phosphorus behaviour in the practically important temperature range although site competition and micro-structural effects are better accounted for by the NES theory. The need for better data on diffusion constants and point defect-impurity binding energy is emphasised.