The role of stress state on the fracture toughness and toughening mechanisms of wrought molybdenum and molybdenum alloys

Rolling unalloyed molybdenum, molybdenum alloys, and Oxide Dispersion Strengthened (ODS) molybdenum into sheet produces microstructures with elongated, pancake shaped grains that can result in anisotropic mechanical properties. In this work, unalloyed molybdenum, molybdenum alloys, and ODS molybdenum are rolled to thinner sheet and then subjected to tensile and fracture toughness testing and examination of the toughening mechanism. The ductile laminate toughening mechanism observed for wrought molybdenum results from a lower toughness in the short-transverse orientation that leads to separation of the layers of sheet-like grains of the microstructure along the grain boundaries in the regions of stress concentration. This splitting of the microstructure results in the formation of ligaments of grains, or non-constrained laminates, that are stretched to failure under a plane stress-state with large amounts of plastic deformation. The thinner specimens exhibit higher fracture toughness values and lower Ductile to Brittle Transition Temperature (DBTT) values than for thicker specimens machined from thicker starting material from the same alloy. The lower constraint of the thinner specimens tested in this work results in higher toughness and lower DBTT values. The finer grain size, finer precipitate size, and state of plane stress achieved for the thinner sheet specimens appears to enhance the ductile laminate toughening to result in higher fracture toughness and lower DBTT values. The detrimental effect of crack initiation from brittle carbides, oxides, and second phases is also observed to be diminished under a stress-state of plane stress.