The materials used for this work were two sorts of reactor pressure vessel (RPV) steels, which contain different amounts of phosphorous (P), namely 0.011 and 0.002 wt%. The specimens for Charpy V-notch (CVN) impact test, Auger electron spectroscopy (AES), and tensile test were thermally aged at 400, 450, and 500°C for 1000, 3000, and 5000 h. After the thermal aging, the AES specimens were broken in the AES chamber to measure the P concentration at grain boundaries. The AES measurements for as-received specimens were carried out following hydrogen charging so that grain boundary facets were available even without P segregation. The AES measurements revealed that the peak height ratio (PHR) of P/Fe at the grain boundaries of the high-P steel were 0.066, 0.141, and 0.120 in the specimens aged at 400°C for 3000 h, 450°C for 3000 h, and 500°C for 1000 h, respectively. The ductile-brittle transition temperature (DBTT) was measured for the aged specimens, and the ?DBTT of 15K was observed only for the specimen aged at 450°C for 3000 h, although no changes in the hardness and tensile properties were observed. The grain boundary fracture ratio (GBFR) increased with increasing the PHR of P/Fe. Grain boundary fracture mode was located at the area close to the V-notch root. There was a good relationship among PHR, GBFR, and DBTT, indicating directly that the shift in the DBTT was due to grain boundary embrittlement caused by P segregation.

Microstructural observation was performed on an Fe-0.3wt.%Cu alloy using a cross-sectional method. The specimens were solution treated and aged at 893K for 22.5h, followed by proton irradiation up to a fluence of 3x1021H+/m2(0.2 dpa at a peak position) at 353K. A band of structure, 0.8?m wide, was observed in a region 6.5 ?m beneath the irradiated surface. The band consisted of a high density of black spot damage, which average size and number density at the center of the band was determined to be 2.3nm and 2.2x1026/m3, respectively. Besides the black spot damage, a number of small structures (3nm) were observed in the defect band at strong two beam condition, while precipitates having F.C.C structure with a lattice constant of 10.16A and large precipitates (6nm) causing halo rings corresponding to F.C.C. copper were observed at the region in and around both sides of the band. Taking in account results of micro vickers hardness and positron life-time measurements in Fe-Cu alloys, radiation hardening observed in Fe-0.3wt.%Cu alloy is attributed mainly to the radiation damage structures, such as dislocation loops and microvoids. However, It is pointed out that interaction between copper and carbon plays an important role on the microstructural evolution under irradiation and the resultant hardening in Fe-C-Cu alloys.

Hardness measurements and microstructure examinations of Fe(-C)-Cu-Ni model alloys were performed following 1 MeV proton irradiation below 80°C. Microstructural examinations by transmission electron microscope (TEM) were carried out by means of a cross section method. A band of damage structures, parallel to the irradiated surface, was observed at a depth of 6.5 ? m in agreement with calculation based on the TRIM code. TEM observation revealed that the band consisted of high density of small black spots, which were considered to be interstitial-type dislocation loops. The amount of irradiation hardening increased with increase in copper concentration. An addition of 0.6wt%Ni to Fe-Cu alloys further increased the hardening, although the effect was reduced with increasing copper concentration. Irradiation hardening of pure iron was also significantly increased by the addition of nickel. The size and number density of the spot-like structures in Fe-Cu alloys decreased and increased, respectively, with addition of nickel. Three recovery stages were found in Fe-Cu-Ni alloys during post-irradiation isochronal annealing to 675°C: the stages are at around 150°C, 400°C and 600°C. The first stage was only observed in nickel-containing alloys, while the third stage was only observed in copper-containing alloys. After annealing to 375°C, the density of spot-like structures decreased in Fe-Cu-Ni alloy but increased in Fe-Cu alloy, while for both the size of spots increased.

The materials used were Ti-50.0, 50.5 and 51.0at%Ni alloys which were cold rolled and aged at 400°C for 1 hr. Neutron irradiation was performed in the Japan Materials Test Reactor (JMTR) up to a dose of 1.2 x 1024n/m2 at cooling water temperature (about 60°C), which meant the specimens were irradiated in the parent phase (B2). After the irradiation, micro-Vickers hardness measurements, microstructure observations by transmission electron microscope (TEM) and positron annihilation spectrometry (PAS) measurements were carried out. Differential scanning calorimetry (DSC) measurements were taken at temperatures between -120°C and 120°C. After the irradiation, all the alloys show extremely high irradiation hardening: the estimated increase in the yield stress of Ti-50Ni alloy is almost 1GPa. Although the hardness of the Ti-Ni alloys depends on the alloy composition in the unirradiated condition, all the alloys reach the same hardness value after the irradiation. DSC measurements revealed that the martensitic transformation was completely suppressed by the irradiation in all the alloys. PAS study revealed that no structural vacancies existed in the alloys before the irradiation, and that vacancies were formed after the irradiation. Microstructural observations indicate that disordered regions, which were considered to be amorphous, were homogeneously distributed in the irradiated parent ordered phase, accompanied by an appearance of the halo ring on the diffraction pattern. The irradiation effects disappeared following a post-irradiation anneal at 250°C for 1 hr. The recovery of the martensitic transformation by post-irradiation annealing is attributed to the migration of vacancies that causes the reordering. It is expected that Ti-Ni alloys are potential self-restorative materials for the irradiation above 250°C where vacancies are mobile.

Positron lifetime and micro-Vickers hardness were measured on well annealed model alloys, Fe-C(0%, 0.2%, 0.35%)-Cu(0%, 0.15%, 0.3%), after 1MeV proton irradiation with a dose of 3x1017 /cm2 below 80°C. Longer lifetime, ranging from 310 to 360 ps, component appears and gives evidence of formation of microvoids containing about 10 vacancies. The longer lifetime decreases with increasing copper content and suggests smaller microvoids for Fe-Cu alloys. The longer lifetime increases with annealing temperature up to 400°C in pure Fe, but exhibits decrease around 300°C in Fe-C-Cu alloys. This decrease indicates reduction in effective size of microvoid around 350°C. Irradiation hardening is accelerated by copper but retarded by carbon. Post-irradiation anneal hardening is revealed at about 150°C and 350°C in Fe-C and Fe-C-Cu alloys. In Fe-Cu alloys, however, a single narrow hardening peak is observed around 350°C. The irradiation hardening in Fe-C alloy anneals out around 550°C, while that in alloys containing Cu makes complete recovery at about 650°C.

The effect of nickel addition on irradiation-induced hardening has been investigated for reduced-activation martensitic steel (RAMS). Specimens were irradiated in the JMTR at 80°C and 220°C up to 0.15 dpa. There was no significant difference in the tensile properties between the steels with and without Ni addition after the irradiation at 220°C, while in the case of the irradiation below 170°C, the Ni-added RAMS showed a more than four times larger irradiation hardening than the steel without Ni addition. The recovery behavior of the irradiation hardening of Ni-added steel on post-irradiation annealing showed two-step recovery behavior; the first step was around 200°C and the second one was around 350°C, while only the single step of recovery was observed around 350°C in the steel without Ni addition. The mechanism of the recovery process of the tremendous irradiation hardening in Ni-added steel is discussed along with the behavior of vacancies, carbon atoms and their complexes investigated by means of positron annihilation lifetime spectrometry.