Effects of test temperature and grain size on the Charpy impact toughness and dynamic toughness (KID) of polycrystalline niobium

The effects of changes in test temperature (-196 degreesC to 25 degreesC) and grain size (40 to 165 mum) on the dynamic cleavage fracture toughness (K-ID) and Charpy impact toughness of polycrystalline niobium (Nb) have been investigated. The ductile-to-brittle transition was found to be affected by both changes in grain size and the severity of stress concentration (i.e., notch vs fatigue-precrack). In addition to conducting impact tests on notched and fatigue-precracked Charpy specimens, extensive fracture surface analyses have been performed in order to determine the location of apparent cleavage nucleation sites and to rationalize the effects of changes in microstructure and experimental variables on fracture toughness. Existing finite element analyses and the stress field distributions ahead of stress concentrators are used to compare the experimental observations with the predictions of various fracture models. The dynamic cleavage fracture toughness, K-ID, was shown to be 37 +/- 4 MPa rootm and relatively independent of grain size (i.e., 40 to 105 mum) and test temperature over the range -196 degreesC to 25 degreesC.