J.R. DiStefano

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 VCrTi 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.

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.

Review of alkali metal and refractory alloy compatibility for Rankine cycle applications

The principal corrosion mechanisms in refractory metal-alkali systems are dissolution, mass transfer, and impurity reactions. In general, niobium, tantalum, molybdenum, and tungsten have low solubilities in the alkali metals, even to very high temperatures, and static corrosion studies have verified that the systems are basically compatible. Loop studies with niobium and tantalum based alloys have not indicated any serious problems due to temperature gradient mass transfer. Above 1000 K, dissimilar metal mass transfer was noted between the refractory metals and iron or nickel based alloys. The most serious corrosion problems encountered are related to impurity reactions associated with oxygen.

Oxidation and its effects on the mechanical properties of Nb-1Zr

Abstract The alloy Nb–1Zr is a high temperature material that is often considered for applications where strength and resistance to alkali metals are required. However, these favorable properties can be compromised because it lacks oxidation resistance in many environments. After reviewing the oxidation behavior of Nb–1Zr, additional studies were conducted in high temperature, low pressure gaseous oxygen environments to determine the effects of oxidation on the tensile properties and hardness of Nb–1Zr for a specific space system application. Strengthening and loss of ductility were found to occur from room temperature to 900°C following low pressure oxidation in vacuum or argon at 750°C and 900°C, respectively. In general, oxygen that is associated with zirconium as extremely fine zirconium–oxygen zones is responsible for the strengthening/embrittlement that is observed. Heat treatments that result in non-coherent ZrO 2 precipitates can neutralize the effects of oxygen on mechanical properties provided the total oxygen content is below that required to convert all of the Zr to oxide.

Effect of hydrogen and oxygen on the tensile properties of V–4Cr–4Ti

Abstract Flat subsized tensile specimens of the vanadium alloy V–4Cr–4Ti were loaded with different amounts of pure hydrogen in order to study its effect on room temperature tensile properties. It was found that, apart from a slight influence of pretreatment, hydrogen can be tolerated up to about 2.5 at.%, whereas higher hydrogen contents lead to catastrophic failure. It is suggested that this behavior is attributed to coexistence of dissolved hydrogen and brittle hydride at room temperature. In addition, some measurements were made with specimens that had been loaded with about 850 wppm oxygen from a low oxygen partial pressure at 500°C (773 K), prior to hydrogen exposure. In this case room temperature ductility, starting from a decreased level, suffered severe deterioration by hydrogen concentrations of much less than 1 at.%. Apparently, in this case, embrittlement is concentrated at the near-surface grain boundaries, and stresses can no longer be absorbed if the matrix gets hardened by the addition of hydrogen. Thus, under the given conditions, oxygen and hydrogen show a strong synergistic effect on the tensile properties of this material.

The effect of mercury on the fatigue behavior of 316 LN stainless steel

Abstract Design of the mercury target system components for the Spallation Neutron Source (SNS) requires data on high- and low-cycle fatigue behavior, and the program in progress includes determining the effects of mercury on the fatigue behavior of type 316 LN stainless steel, the primary material of choice for the target vessel. Uniaxial, load-controlled fatigue tests with R =−1 and (minimum stress/maximum stress) have been conducted in air and mercury at room temperature employing constant amplitude, sinusoidal loading at frequencies from 0.1 to 700 Hz. Stress amplitude versus fatigue life ( S – N curves) data at 10 Hz for both air and mercury show a sharp knee at approximately 1 million cycles indicating a fatigue endurance limit in either air or mercury around 240 Mpa. Mean stress ( R =0.1) lowers the endurance limit to 160 MPa. At relatively low frequency, both frequency and environment (mercury) had some impact on fatigue life of type 316 LN stainless steel at high-stress levels (i.e., stresses considerably above the apparent fatigue limit). Although testing at a high frequency of 700 Hz, showed a decrease in fatigue life in air compared with that at 10 Hz, a significant increase in specimen temperature was observed in air due to self-heating. No pronounced effects of waveform have yet been found, but data are limited.

Influence of mercury environment on the fatigue behavior of spallation neutron source (SNS) target container materials

Abstract The high-cycle fatigue behavior of 316 LN stainless steel (SS), the prime candidate target-container material for the spallation neutron source (SNS), was investigated in air and mercury at frequencies from 10 to 700 Hz with a R ratio of 0.1. A decrease in the fatigue life of 316 LN SS in air was observed with increasing frequency. However, little influence of frequency on fatigue life was found in mercury. An increase in the specimen temperature at 700 Hz seems to be the main factor that contributed to the decrease of the fatigue life in air, relative to that at 10 Hz. However, because of the cooling effect of mercury, only a small temperature increase was found at 700 Hz, and, therefore, there was little frequency influence in mercury. At 10 Hz, a shorter fatigue life of 316 LN SS was measured in mercury than in air at stresses greater than yield strength, which may have resulted from liquid metal embrittlement (LME). At lower stresses, no difference in fatigue lives between mercury and air was detected at 10 Hz. At 700 Hz, the fatigue life in mercury was longer than in air. The fatigue endurance limit measured at both frequencies in mercury and in air was approx. 350 MPa.

Recent Progress Addressing Compatibility Issues Relevant to Fusion Environments

Abstract Current compatibility research in the U.S. focuses on two topics: dual- or multi-layer electrically-resistant Y2O3/vanadium coatings in a V-Li blanket concept and SiC composites with a Pb-Li coolant. The compatibility issue for multi-layer coatings includes the ceramic insulating layer and the metallic vanadium alloy layer. Characterization of Y2O3 coatings after exposure to Li shows significant changes in the microstructure. Initial static capsule results for V-4Cr-4Ti alloys in Li at 800°C showed unexpected small mass gains. Capsule tests of monolithic SiC in Pb-17Li showed no mass change and no wetting after 1000h at 800°C and only limited wetting after 1000h at 1100°C. Chemical analysis of the Pb-Li after the tests did not detect Si to the detectability limit of 30ppma (5wppm). In both liquid metal systems, loop tests with a representative temperature gradient are needed to truly determine compatibility limits.