The fracture toughness and toughening mechanisms of wrought low carbon arc cast, oxide dispersion strengthened, and molybdenum-0.5 pct titanium-0.1 pct zirconium molybdenum plate stock

The high-temperature strength and creep resistance of low carbon arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened (ODS) molybdenum, and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have attracted interest in these alloys for various high-temperature structural applications. Fracture toughness testing of wrought plate stock over a temperature range of - 150 degrees C to 1000 degrees C using bend, flexure, and compact tension (CT) specimens has shown that consistent fracture toughness results and transition temperatures are obtained using subsized 0.5T bend and 0.18T disc-CT specimens. Although the fracture toughness values are not strictly valid in accordance with all ASTM requirements, these values are considered to be a reasonable measure of fracture toughness. Ductile-to-brittle transition temperature (DBTT) values were determined in the transverse and longitudinal orientations for LCAC (200 degrees C and 150 degrees C, respectively), ODS (< room temperature and - 150 degrees C), and TZM (150 degrees C and 100 degrees C). At test temperatures > DBTT, the fracture toughness values for LCAC ranged from 45 to 175 - MPa root m, TZM ranged from 74 to 215 MPa root m, and the values for ODS ranged from 56 to 149 MPaVm. No temperature dependence was resolved within the data scatter for fracture toughness values between the DBTT and 1000 degrees C. Thin sheet toughening is shown to be the dominant toughening mechanism, where crack initiation/propagation along grain boundaries leaves ligaments of sheetlike grains that are pulled to failure by plastic necking. Specimen-to-specimen variation in the fraction of the microstructure that splits into thin sheets is proposed to be responsible for the large scatter in toughness values at test temperatures > DBTT. A finer grain size is shown to result in a higher fraction of thin sheet ligament features at the fracture surface. As a result finer grain size materials such as ODS molybdenum have a lower DBTT.