Correlation between Hydrogen Distribution in V-4Cr-4Ti Alloy and Impact Strength

Full text of publication follows: Hydrogen embrittlement of vanadium alloys has been extensively studied. In such studies, it is common to examine the correlation between mechanical properties and average hydrogen concentration. Hydrogen distribution in vanadium alloys, however, is not necessarily uniform, and hence it is also important to understand the influence of hydrogen distribution on mechanical properties. From these viewpoints, the correlation between impact strength of V-4Cr-4Ti alloy and hydrogen distribution was investigated in the present study. Impact specimens (1.5 x 1.5 x 20 mm) with V-notch were prepared from two types of sheets of a V-4Cr-4Ti alloy (NIFSHEAT- 2) with different extent of cold rolling. The thickness of one sheet was 4.0 mm and the extent of thickness reduction by cold rolling was 96 %, while those of another sheet were 6.6 mm and 93 %, respectively. After recrystallization annealing at 1273 K for 2 h, some of these specimens was loaded with hydrogen up to 1500 appm, and the influence of hydrogen on impact strength was examined at 173 K. Mixture gas of deuterium and tritium was also introduced into the specimens, and the distribution of hydrogen isotopes in the alloy was observed by imaging plates (IP). No noticeable influence of hydrogen on the impact strength was observed for the specimens prepared from the thinner sheet. On the other hand, the absorbed energy of specimens prepared from the thicker sheet significantly decreased by hydrogen loading, although ductile behavior was still observed even after loading. The extent of reduction of cross section during deformation also became smaller by hydrogen loading. The observations with IP showed that hydrogen was concentrated in band-like regions where Ti-rich precipitates were distributed in high density. The difference in hydrogen concentration between such band-like regions and matrix was larger for the specimen prepared from the thicker sheet; the fraction of hydrogen trapped by the precipitates was higher for this type of specimen. These observations indicated that the interaction between the precipitates and hydrogen played key roles in the above-mentioned change in the impact strength induced by hydrogen. (authors)