FRACTURE AND DUCTILE-TO-BRITTLE TRANSITION CHARACTERISTICS OF MOLYBDENUM BY IMPACT AND STATIC BEND TESTS

In order to investigate the relationship between the impact and static bending behaviors at low temperatures for molybdenum having different grain structures and different grain boundary strengths, the impact and static bend tests were performed for miniaturized specimens of five kinds of molybdenum. The pure molybdenum with or without carbon addition and the TZM alloy had a fine and equiaxed grain structure, and the doped-molybdenum with or without carbon addition had a coarse and elongated grain structure. The total absorbed energy, total deflection, yield and maximum stresses by the impact bend test, and the bend angle, yield and maximum stresses by the static bend test were calculated and compared with each other. The fracture and ductile-to-brittle transition characteristics by both tests were discussed from the viewpoints of the grain structure and the grain boundary strength. The results are summarized as follows. (1) Fracture characteristics by the impact and static bend tests are essentially the same. The difference in critical stress representing the low-temperature fracture strength between the materials is due to the difference in grain structure and grain boundary strength. (2) DBTT by the impact bend test depends apparently on the critical stress of the material, whilst DBTT by the static bend test depends not only on the critical stress but also on the yield stress. (3) DBTT by the impact bend test is higher by 90-175 K than that by the static bend test. This result is principally due to the increase of the yield stress at a given temperature which results from the increase of the strain rate. The degree of the DBTT increase differs between materials. This result is due to the difference in yield stress resulting from the difference in grain structure.