P.E.J Flewitt

Properties and evolution of sessile interstitial clusters produced by displacement cascades in α-iron

Recent molecular dynamics (MD) studies of displacement cascades in α-iron have shown that although most self-interstitial atoms are created in the form of single defects or glissile clusters, sessile clusters are also formed during the quenching stage of a cascade. Statistics on the size and number of sessile interstitial clusters have been determined as a function of cascade energy and irradiation temperature. The thermal stability of three common forms of sessile cluster has been investigated by MD, and the lifetime and activation energy for transforming into glissile form have been estimated. Most sessile clusters transform within a few hundred ps at about 500 K with an activation energy of between 0.35 and 0.5 eV. This suggests a re-assessment of the interpretation of recovery stage II in iron. One type of cluster having a compact three-dimensional configuration of high symmetry is found to be stable for at least 500 ps at temperatures up to 1500 K. The interaction of sessile clusters with point defects and glissile clusters has been studied, and the possibility of their subsequent transformation into glissile clusters is discussed.

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.

The influence of strain on defect generation by displacement cascades in α-iron

Abstract Although there have been numerous computer-simulation studies of the damage created by displacement cascades in metals, little attention has been paid to the influence of stress on defect generation in the primary cascade process. In the present work, molecular dynamics (MD) has been used to investigate defect production and clustering by displacement cascades in α-iron under uniform tensile strain. Cascades of 10 keV primary-recoil energy have been simulated in a single crystal with strain of 0%, 0.1%, 0.5% or 1% applied along a 〈1 1 1〉 axis, with at least four events modelled for each condition. The results show that the number of interstitial and vacancy defects is smaller in the strained models, particularly for the strain of 0.1%. This decrease in the number of defects is possibly related to the effect of strain on self-interstitial motion, leading to enhanced recombination with vacancies. The number of interstitials in clusters is approximately independent of applied strain, but the fraction of interstitials aligned parallel to the strain axis increases with increasing strain. The effect is small at 0.1% strain, but most single interstitials are created in this orientation at 0.5% and all single and clustered interstitials are aligned at 1%. These results are discussed in terms of the influence of strain on defect formation energy and the mobility of interstitials.

Residual stresses in a multi-pass CrMoV low alloy ferritic steel repair weld

Abstract A trial part through-section excavation manual metal arc repair weld, designed to minimise residual stresses was prepared using a critical combination of weld metal strength, bead size and deposition pattern. The repair was in a low alloy ferritic CrMoV steel plate, and the weldment was made using a lower strength C–Mn steel weld metal. Residual stresses have been measured to a high spatial resolution across the weldment using the X-ray diffraction technique with CrKα X-radiation. Peak tensile stresses have been identified in the heat affected zone in the parent plate adjacent to the weld metal. Further measurements have been made using the rosette strain gauge centre hole relaxation and deep hole drilling techniques. The measured residual stress profiles are discussed with respect to comparisons between techniques, the range of microstructures within the weldment and predictions of a finite element model for this particular multi-pass repair weld.

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.

Grain boundary phosphorus and molybdenum segregation under irradiation and thermal conditions in a 2.25Cr1Mo steel

Segregation of P and Mo to prior austenite grain boundaries in a 2.25Cr1Mo steel subjected to neutron irradiation and ageing is examined using field emission gull scanning transmission electron microscopy (FEGSTEM) with energy dispersive X-ray microanalysis. The steel samples are irradiated and aged at around 270 and 400 degrees C, respectively. The irradiation dose rate and dose are about 1.05 x 10(-8) dpa s(-1) and 0.032 dpa for 370 degrees C-irradiation, and 1.75 x 10(-8) dpa s(-1) and 0.13 dpa for 400 degrees C-irradiation. The FEGSTEM results indicate that, during 270 degrees C-irradiation, there is no irradiation effect for P segregation and some effect for Mo segregation with the Mo boundary concentration decreasing. During 400 degrees C-irradiation, there is a slight irradiation effect for P segregation with the P boundary concentration increasing and some irradiation effect for Mo segregation with the Mo boundary concentration decreasing. The 400 degrees C-ageing does not enhance Mo segregation as compared with the 270 degrees C-ageing, but it enhances P segregation

Irradiation-induced embrittlement of a 2.25Cr1Mo steel

Irradiation-induced embrittlement of a 2.25CrlMo is investigated by means of small punch testing and scanning electron microscopy (SEM). The ductile-brittle transition temperature (DBTT) determined by the small punch test is much lower than that determined by the standard Charpy test. There are some irradiation-induced embrittlement effects after the steel is irradiated at about 270 degrees C for 46 days with a neutron dose rate of 1.05 x 10(-8) dpa s(-1) and at about 400 degrees C for 86 days with a neutron dose rate of 1.75 x 10(-8) dpa s(-1). In addition, there is some temper embrittlement after the steel is aged at about 400 degrees C for 86 days

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.