In-Situ Fracture Studies and Modeling of the Toughening Mechanism Present in Wrought Low-Carbon Arc-Cast Molybdenum, Tit
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INTRODUCTION
MOLYBDENUM alloys are used in applications where high-temperature strength and creep resistance are needed, but poor oxidation resistance limits their use to applications in which a controlled atmosphere can be maintained.[1–4] Molybdenum has a bcc structure and, similar to the majority of bcc metals, exhibits a ductileto-brittle transition temperature (DBTT). Also, its fracture properties are sensitive to temperature,[5] interstitial content,[5] grain size,[5,6] slip or deformation mechanisms,[7–9] grain-boundary character,[10–13] and microstructure.[14–25] Studies of molybdenum alloys have generally used unnotched tensile or bend specimens, in which large amounts of ductility typically are measured near a homologous temperature of 0.1 (17 C) or greater to define the DBTT. Recent work has shown that the use of fracture toughness testing results in a higher DBTT (100 C to 200 C) than that obtained from tensile specimens, for both wrought B.V. COCKERAM, Senior Scientist, is with the Bechtel-Bettis Atomic Power Laboratory, West Mifflin, PA 15122-0079. K.S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238. Contact e-mail: [email protected]. Manuscript submitted November 6, 2007. Article published online June 19, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
unalloyed low-carbon arc-cast (LCAC) molybdenum and a wrought molybdenum alloy with 0.5 pct Ti and 0.1 pct Zr additions by weight (TZM). This can be ascribed to the constraining effect of the precrack.[14–16] Higher fracture toughness values at lower temperatures and lower toughness-based DBTT values are observed for wrought molybdenum alloys that have a finer grain size, such as oxide-dispersion-strengthened (ODS) molybdenum.[14–16] The wrought LCAC molybdenum, TZM, and ODS molybdenum alloys used in these studies have microstructures that consist of elongated, pancake-shaped grains. These recent studies[14–16] have shown that one of the fracture mechanisms in wrought molybdenum-based alloys involves the formation of thin sheet ligaments,[26,27] which leads to an enhancement of the fracture resistance of Mo-based alloys[14–16] via a ductile-laminate toughening mechanism similar to that observed for Al-based alloys,[26–28] steels,[29] lamellar TiAl-based alloys,[30] and metal-matrix composites.[31] The overall material toughness goes up, because fracture at the grain boundaries allows the grain interiors to deform more freely, thereby increasing the plastic energy absorbed by the material prior to complete crack propagation across the sample. The enhancement in fracture toughness that results from thin-sheet toughening is produced by relaxing the VOLUME 39A, SEPTEMBER 2008—2045
triaxial stresses near the crack tip and increasing the critical strain at fracture.[26,27] The theoretical maximum increase in fracture toughness that can be achieved through thin-sheet toughening has been estimated to be pffiffiffi [26,27] but higher levels of fracture a factor of 3, toughness enhancement have been obtained in ODS molybd
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