Crack Generation and Propagation Characteristics of Molybdenum Single Crystals

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Tensile tests at 77, 173 and 293 K were performed on molybdenum single crystals having axial-orientations near the ⟨011⟩-direction, which had been prepared by the secondary recrystallization method. The effect of “island grains” at the crystal surface and/or in the bulk, and the effect of test temperature on the crack generation and propagation characteristics were investigated.<BR>At 77 and 173 K, cracks were generated from “island grains” for all of annealed or polished specimens and for most of carburized specimens. The plane on which the cracks propagated was initially the {011}-plane and afterwards changed to the {112}-and/or {001}-planes, though the {011}-plane component just around the crack generation sites tended to diminish or vanish in polished or carburized specimens. The fracture strength of annealed or polished specimens depends on the size of “island grain” from which the crack was generated. On the other hand, the strength of carburized specimens was appreciably enhanced by addition of a small amount of carbon and was apparently not size-dependent.<BR>At 293 K, cracks which were not propagated were observed at the interfaces between the “island grains” and the matrix in annealed or polished specimens. However, any “island grains” were not responsible at all for the final failure of the crystals. All the specimens failed after having shown considerably high ductility.

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