In-Situ Fracture Studies and Modeling of the Toughening Mechanism Present in Wrought Low-Carbon Arc-Cast Molybdenum, Titanium-Zirconium-Molybdenum, and Oxide-Dispersion-Strengthened Molybdenum Flat Products

Abstract

In-situ testing, ultrasonic C-scans, and metallography were used to show that a crack-divider delamination form of thin-sheet toughening occurs in wrought low-carbon arc-cast (LCAC) unalloyed molybdenum, oxide-dispersion-strengthened (ODS) molybdenum, and a wrought molybdenum alloy with 0.5 pct Ti and 0.1 pct Zr additions by weight (TZM), at temperatures greater than or equal to the ductile-to-brittle transition temperature (DBTT). Cracking along boundaries relieves mechanical constraints and frees ligaments that may plastically stretch to produce toughening. Anisotropy in fracture toughness with lower values in the short-transverse (ST) direction is shown to produce the crack-divider delaminations at the crack tip in the longitudinal (LT) and transverse (TL) orientations. The delamination zone increases with increasing stress-intensity factors to sizes significantly larger than the plastic zone; this leads to large increases in fracture toughness by the thin-sheet-toughening mechanism. Fracture in ODS Mo alloys proceeds mainly along grain boundaries to produce small ligaments that exhibit ductility for both LT and TL orientations, resulting in a lower DBTT and higher toughness values at lower temperatures than those observed in LCAC molybdenum and TZM. A combination of grain-boundary fracture and cleavage is prevalent in LCAC molybdenum and TZM. The predominant tendency for microcracking along grain boundaries to leave fine, ductile ligaments in ODS molybdenum can be attributed to a fine-grained microstructure with a ≈1- to 2-μm thickness of sheetlike grains. The presence of mixed grain-boundary fracture and cleavage in LCAC molybdenum and TZM can be attributed to a microstructure with a larger thickness of sheetlike grains (4 to 15 μm).