The mechanical properties and fracture mechanisms of wrought low carbon arc cast (LCAC), molybdenum–0.5pct titanium–0.1pct zirconium (TZM), and oxide dispersion strengthened (ODS) molybdenum flat products

Abstract

Molybdenum alloys such as low carbon arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened (ODS) molybdenum, and molybdenum–0.5pct titanium–0.1pct zirconium (TZM) molybdenum are of interest for structural applications at high temperatures, but these alloys are poorly characterized with respect to fracture toughness and the ductile to brittle transition temperature (DBTT) in the presence of a notch. Both tensile and fracture toughness testing of these flat rolled molybdenum alloys at temperatures above the DBTT are shown to produce a ductile laminate fracture mechanism, where cracks initiate along grain boundaries in the region of triaxial stresses to leave ligaments of sheet-like grains that are stretched to failure with a high degree of plasticity. The DBTT determined from toughness testing is 50–200 °C higher than determined from tensile testing, which shows the constraining effect of the notch. Use of the J-integral test method provided a more consistent and accurate measure of fracture toughness values at temperatures above the DBTT where large amounts of plasticity are observed. A transition was observed from toughness values between 5.8 and 29.6 MPa√m at temperatures below the DBTT to toughness values between 45 and 175 MPa√m for LCAC, 40–215 MPa√m for TZM, and 53–205 MPa√m for ODS. The variation in fracture toughness values at temperatures > DBTT is shown to correlate with size and number density of the ductile laminate features, where high fracture toughness values result from a fine laminate spacing. Since a finer grain size results in a smaller laminate size, the lower DBTT observed for fine grained ODS molybdenum can be understood in terms of the ductile laminate failure mode.