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A commercial grade 12%CrMo VNb ferritic/martensitic stainless steel in the form of parent plate and high-nickel off-normal weld material has been fast neutron irradiated to equivalent damage levels of 33 and 50 dpa at 400 and 465°C respectively. The microstructural and microchemical changes induced in the irradiated material, together with as-tempered and thermal control material, have been determined to high resolution by conventional transmission electron microscopy and the use of a field emission gun scanning transmission electron microscope (FEGSTEM). Equilibrium (co )segregation of chromium, molybdenum and phosphorus was detected at boundary planes in thermally aged material, with greater enrichment at the higher ageing temperature. The relative magnitudes of apparent phosphorus segregation at the two temperatures were in accordance with McLean's model governing the kinetic approach to equilibrium. The electron probe I segregant interaction was modelled in an attempt to deconvolute true segregant concentrations from derived concentration profiles: these 'deconvoluted' concentrations approximated those predicted by McLean's model. The net effects of irradiation on parent plate interfacial microchemistry were found to be to: (i) inhibit the (co )segregation of chromium, molybdenum and phosphorus, (ii) cause chromium depletion from adjacent to boundary planes, (iii) cause enrichment of silicon at prior austenite and lath boundaries during irradiation at 400°C and (iv) cause enrichment of nickel at lath boundary planes only, at both temperatures. The radiationinduced precipitates ~C and G phase, both nickel- and silicon-rich, were observed. The fully martensitic off-normal weld metal transformed to a duplex austenite!ferrite structure during irradiation at 465°C; in contrast the thermal control was at least metastable. The transformation was thought to be a martensitic reversion, facilitated by radiation-generated dislocation loops acting as nucleation sites. The austenite was heavily voided (-15 vol.%); the ferrite was relatively void-free. Depletion of the oversized solutes chromium, manganese and molybdenum and enrichment of nickel, silicon, aluminium and traces of titanium were detected at void interfaces in the austenite: little segregation could be discerned at voids in the ferrite. Overall, the results within this work and in comparison to previous studies highlight the sensitivity to initial composition, microstructure and heat treatment that the 12%Cr ferritic/martensitic steels display in their response to irradiation.

The antiferromagnetic structure of the intermetallic compound U₁₄Au₅₁ has been determined from neutron polarimetric measurements and refined by combining these data with integrated intensity measurements. The structure was found to be non-collinear with the U moments confined to the a- b plane. The moments of the U atoms in each of the two sets of sixfold sites are arranged hexagonally with rotations of 60° between them and the two sets are rotated with respect to each other by 50°. The third (twofold) set of U atoms has no ordered moment. These conclusions are in disagreement with a previous determination of the magnetic structure from powder data which gave a collinear structure with moments parallel to the c-axis. Magnetization measurements made on single crystals in the temperature range 300-2K can be understood in terms of a transition to a non-collinear easy plane antiferromagnetic structure stable below 22K. Polarized neutron measurements have been used to determine the contribution of each of the U sites to the susceptibility between 22 and 2K. These show that of the two sixfold U sites, that with the smaller ordered moment contributes more than half of the total susceptibility. The twofold site, which is characterized by a small U-U separation, makes the smallest contribution.

The present study was concerned with the variation of strength with cooling rates in vanadium and niobium steels continuously cooled from austenite. The range of cooling rates varied from 0.05K. s-l to 500K-s⁻¹ approximately. It was found that a peak hardening was observed in the high vanadium steels (at = 2K.s⁻¹), whilst the low vanadium and niobium steels produced a less definite peak. An. attempt was also made to explain the variation of strength through an investigation into the strengthening mechanisms in steel. Finally, efforts were made to study changes in the microstructure in relation to cooling rates. To gain an understanding of the effects of vanadium and niobium additions to the strength and microstructure of low carbon steels, a study of the literature particularly relevant to the present study was carried out. For instance, the relationship between structure and mechanical properties was studied. Also, the mechanism of formation of microstructural components observed in the material was reviewed. Before a study of the findings was attempted, the instruments used and the experimental methods developed in the present study, were described. Strengths and weaknesses of these methods were pointed out. Following the observation of a peak hardening in the steels, interesting results were obtained on aspects which could explain the variation of strength with cooling rates in vanadium and niobium steels. For instance, two distinct groups of precipitates and a near constant of 10¹⁰ dislocations. cm/cm³ were found in vanadium and niobium steels. The study of the microstructure also revealed interesting features. In particular, it was observed that the peak hardening was obtained in a ferritic matrix. It was also shown that the solution treatment influences not only the precipitation hardening but also both the macrostructure and the microstructure of steels. (In this study, microstructure refers to features observed inside the grains, whereas macrostructure is generalized to include grain structure). The present study terminates with a discussion of the various strengthening mechanisms in the steels investigated. The strength of the materials was measured by means of hardness tests which were empirically related to the yield strength. The peak was thus ascribed to precipitation hardening. The largest increase in the height of the peak for a given amount of vanadium and niobium addition, corresponded to steels of a stoichiometric composition of (Nb, V) carbonitride. On the other hand, manganese was observed to reduce knd to spread the peak evenly. Twinning was found to be present in vanadium steels and was observed for the first time in niobium steels. The contribution to strength from microtwins was appraised, but they appeared to have negligible effect.

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