Characterization of specimens exposed in a Li loop

A monometallic V-4Cr-4Ti thermal convection loop was run for 2355 h with a peak temperature of 700 C (973 K) and Li flow rate of 2-3 cm/s. Specimens of V-4Cr-4Ti exposed in the hot and cold legs of the loop and tensile tested in vacuum at 500 C (773 K) showed an increase in the 0.2% yield and ultimate tensile strengths and a decrease in the serration amplitude with decreasing exposure temperature in the loop. However, only minor changes in ductility were measured. With the higher temperature exposures, a decrease in Vickers hardness was measured, but little change in the grain size was observed. Characterization of the microstructure after exposure at 627 C (900 K) in the loop showed an increase in the density of Ti- and N-rich grain boundary and matrix precipitates near the specimen surface after exposure corresponding to an increase in the hardness in the near-surface region. Two-layer V/Y₂O₃ coatings on V-4Cr-4Ti substrates also were exposed in the loop, and initial room temperature characterization was conducted.