Stress-corrosion cracking of a uranium 6-weight percent niobium alloy

Abstract

The stress-corrosion cracking (SCC) behavior of U--6 wt percent Nb in three different heat-treated conditions was investigated. The three heat treatments were: (a) solution quenched (the nonaged condition); (b) solution quenched and aged 6 hours at 250/sup 0/C (the underaged condition); and (c) solution quenched and aged 80 minutes at 600/sup 0/C (the overaged condition). The SCC tests utilized smooth, dead-weight loaded, static tensile specimens exposed to nitrogen-saturated or oxygen-saturated water containing 50 ppM chloride ions. The alloy in the solution-quenched condition was not susceptible to SCC when loaded to 90 percent of yield strength in either the oxygen or nitrogen-saturated chloride solutions. The underaged material (6 hours at 250/sup 0/C) was susceptible to SCC in oxygen or nitrogen-saturated water containing 50 ppM chloride ions. The underaged material was more susceptible to cracking in the oxygen-saturated solution than in the nitrogen-saturated solutions. The SCC in the underaged material proceeded by a principally intercrystalline fracture mode with the overload condition being principally transgranular. The overaged material also exhibited SCC, but to a lesser extent than the underaged material. Here again, cracking was more pronounced in the oxygen-saturated solution than in the nitrogen-saturated solution. The overaged material fractured by a combined SCC mode and mechanical-cracking mode, i.e., stress-corrosion cracks were growing at the same time corrosion was reducing the cross-sectional area of the specimen. The importance of knowing alloy characteristics relates to considerations of design, environmental behavior, and economics.