The Role of Oxygen Uptake and Scale Formation on the Embrittlement of Vanadium Alloys

Abstract

Vanadium alloys are of interest in fusion energy systems, however, their environmental durability is a major concern. Specimens of V–4Cr–4Ti were exposed to air and oxygen (105,Pa), low pressure (10−3−10−6 Pa) oxygen and high purity He environments (105−101 Pa) it at 500−700°C in order to characterize the surface oxide, determine oxidation kinetics and quantify effects on mechanical properties at 25 and 600°C. At low oxygen pressures (PO_2.10−5 Pa), linear reaction kinetics were measured for exposures up to 2000 hr and the data were used to develop a mathematical expression for the oxidation rate as a function of temperature and oxygen pressure. At higher pressures, linear–parabolic reaction kinetics were associated with high oxygen uptake and the formation of an external oxide layer. Room-temperature and 600°it C tensile ductility was reduced by these exposures, but specimens which formed an external oxide were found to retain some tensile ductility after exposure. However, similar specimens with an external oxide that were subsequently annealed for 2000 hr at 700°C became severely embrittled demonstrating that a surface oxide will not prevent degradation of this refractory alloy. Exposures in He were performed to determine the effect of total gas pressure on oxygen uptake.