J.H. DeVan

The Effect of an Oxide Dispersion on the Critical Al Content in Fe-Al Alloys

Oxide-dispersed iron aluminides with Al contentsranging from 10 to 28 at.% were oxidized in air attemperatures between 600 and 1300°C in order todetermine the critical Al content necessary to form a protective, external alumina scale. AY2O3-Al2O3dispersion, performed similarly to anAl2O3 dispersion, indicated littleeffect of Y on the critical Al content. Compared toingot-processed iron aluminides, the addition of an oxide dispersion reduced thecritical Al content at 900°C. This reduction isattributed to the finer grain size in theoxide-dispersed Fe-Al.

Corrosion of ferrous alloys exposed to thermally convective Pb-17 at. % Li

Abstract A type 316 stainless steel thermal convection loop with type 316 stainless steel coupons and a Fe-9Cr-1Mo steel loop containing Fe-12Cr-1MoVW steel specimens circulated molten Pb-17 at% Li at a maximum temperature of 500°C. Specimens were exposed for more than 6000 h. Mass loss and surface characterization data were compared for these two alloys. At any particular exposure time, the corrosion of type 316 stainless steel by Pb-17 at% Li was more severe, and of a different type than that of similarly exposed Fe-12Cr-1MoVW steel. The austenitic alloy suffered nonuniform penetration and dissolution by the lead-lithium, whereas the Fe-12Cr-1MoVW steel tended to be more uniformly corroded. The presence of a ferritic layer on the type 316 stainless steel, and its susceptibility to spalling during specimen cleaning, were shown to be important in evaluating the data for this steel and in comparing corrosion losses for the two types of alloys. A model for the nonuniform penetration of type 316 stainless steel by Pb-17 at% Li was suggested.

Oxidation of refractory metals in air and low pressure oxygen gas

Abstract Oxidation rates and effects of oxidation on mechanical properties of several refractory metal alloys were determined under a variety of temperature/pressure conditions. The alloys investigated were V–4Cr–4Ti, V–5Cr–5Ti, Nb–1Zr, Nb–1Zr–0.1 C, Ta–8W–1Re–0.7Hf–0.025C and Mo–46Re. Although none of these alloys form protective surface oxides, oxidation behavior and accompanying effects on mechanical behavior are quite different depending on whether oxidation results in external film formation, dissolution or internal oxidation. Data are presented on oxidation rate, microstructural development and effects on mechanical properties for the various alloys.

Reactions of oxygen with VCrTi alloys

Abstract Fusion reactors have been proposed with a vanadium alloy as the structural/containment material. However, vanadium has a significant affinity for interstitial contamination that could deleteriously affect its mechanical properties. The effects of oxygen pick-up in air and low pressure oxygen environments were investigated at 400–500°C for two Vue5f8Crue5f8Ti alloys. As expected the studies showed that the room temperature tensile ductility is reduced by exposure to air or low pressure oxygen environments. However, the magnitude depends upon processing history and subsequent heat treatment. Possible embrittling mechanisms such as grain boundary weakening or weakening of near-boundary regions are discussed.

A determination of the solubility of lithium oxide in liquid lithium by fast neutron activation

The solubility of lithium oxide in liquid lithium has been measured in the temperature range of 195 to 734°C. A direct sampling method was employed in which nickel sampling tubes fitted with 2 μm pore size filters were used to take samples of the oxide-saturated lithium. The samples, still contained in their sampling tubes, were analyzed for oxygen content by a fast neutron activation method. A value for the oxygen blank attributable to the sampling tubes was derived from a regression analysis of the individual solubility data points. The resulting solubility values are considerably lower than those reported by previous investigators and can be represented by the equations In S(mol% Li2O) = 6.054–6669/T, log S∗(wppm oxygen) = 6.992–2896/T. The results indicate that removal of oxygen from lithium to a level of about 7 wppm (the solubility at 200°C) by cold trapping or by filtration should be feasible.

Temperature limits on the compatibility of insulating ceramics in lithium

Abstract CaO and AlN are candidates for electrically insulating coatings in a lithium-cooled fusion reactor. Bulk specimens of AlN+0.04 wt%Y and single-crystal CaO have been exposed to lithium in 1000 h isothermal capsule tests at 500–800 °C to determine the maximum temperature at which acceptable compatibility is likely. A large increase in the amount of mass loss of AlN was observed between 600 and 700 °C. At 700 °C, the amount of dissolution was reduced when a Mo capsule was used instead of a V alloy test capsule. High mass losses for single-crystal specimens of CaO were observed after exposure at 600 °C. In this case, changing to a Mo test capsule or adding Ca or O to the lithium did not consistently show a beneficial effect. At 700 °C, neither doping the Li with Ca or O significantly altered the high mass losses. These results suggest that CaO may be limited to exposure temperatures of less than 600 °C but that AlN may be able to operate above 600 °C. Because some designs call for operating temperatures of 750 °C, other compositions, such as Er 2 O 3 and Y 2 O 3 , also are being evaluated. Preliminary results show promise for these oxides after exposure at 800 °C.

Effects of oxygen and hydrogen at low pressure on the mechanical properties of V–Cr–Ti alloys☆

Abstract Exposure of V–Cr–Ti alloys to low oxygen partial pressures at high temperature results in oxygen absorption and internal oxidation. Characterization of a V–4Cr–4Ti alloy after oxidation at 500°C revealed a microstructure with ultrafine oxide precipitates in the matrix and along grain boundaries. Heat treatment at 950°C following oxidation resulted in large TiO x precipitates in the matrix and grain boundaries. Tensile ductility was reduced by exposure to low-pressure oxygen under the temperature and pressure conditions. However, heat treatment at 950°C following oxidation was generally effective in recovering ductility irrespective of initial annealing treatment or grain size. Without increases in oxygen, >500 wppm hydrogen was required to cause significant decreases in tensile elongation. When oxygen was added either during or prior to hydrogen exposure, significant embrittlement occurred with 100 wppm hydrogen. Because of this synergism with hydrogen, oxygen pick-up remains a major concern for V–Cr–Ti alloys in fusion reactor applications.

Compatibility of structural materials with fusion reactor coolant and breeder fluids

Abstract Fusion reactors are characterized by a lithium-containing blanket, a heat transfer medium that is integral with the blanket and first wall, and a heat engine that couples to the heat transfer medium. A variety of lithium-containing substances have been identified as potential blanket materials, including molten lithium metal, molten LiF-BeF2, Pb-Li alloys, and solid ceramic compounds such as Li20. Potential heat transfer media include liquid lithium, liquid sodium, molten nitrates, water, and helium. Each of these coolants and blankets requires a particular set of chemical and mechanical properties with respect to the associated reactor and heat engine structural materials. This paper discusses the materials factors that underlie the selection of workable combinations of blankets and coolants. It also addresses the materials compatibility problems generic to those blanket-coolant combinations currently being considered in reactor design studies.

Thermal-gradient mass transfer in lithium-stainless steel systems

Abstract The corrosion of type 316 stainless steel by flowing Li was studied as a function of time in thermal-convection loops. After a transient period, the corrosion rates were observed to be constant with time with values ranging from 10 to 20 mg/m 2 h. Preliminary analysis indicated that the corrosion rate is controlled by the diffusion of Fe through the liquid Li boundary layer. The deposition processes involved the formation of crystals of nearly pure Cr in the cold legs of the loops.

Compatibility of stainless steel with Pb-17 At. Pct Li

The corrosion of type 316 stainless steel and Sandvik HT9 by static Pb-17 at. pct Li between 300 and 500 °C was studied. The resulting weight losses were significantly greater than those of these steels in lithium. The corrosive attack was very uniform, and the roomtemperature tensile properties of the steels were unaffected by the exposure. The application of molten Pb-17 at. pct Li as a tritium-breeding fluid in conjunction with ferrous alloys in a fusion reactor may be limited to 400 °C or below.