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 |
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Authors: | B V Cockeram |
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Affiliation: | (1) the Bechtel-Bettis Atomic Power Laboratory, 5122-0079 West Mifflin, PA |
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Abstract: | 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 °C to 1000 °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 °C and 150 °C, respectively), ODS (<room temperature and −150 °C), and TZM (150 °C
and 100 °C). At test temperatures > DBTT, the fracture toughness values for LCAC ranged from 45 to 175 MPa√m, TZM ranged from
74 to 215 MPa√m, and the values for ODS ranged from 56 to 149 MPa√m. No temperature dependence was resolved within the data
scatter for fracture toughness values between the DBTT and 1000 °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. |
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