Body-centered cubic (bcc) materials behave brittle and shatter like glass, below the brittle-to-ductile transition temperature (DBTT). This restricts the applicable operation temperatures of all bcc structure materials. Consequently, our model material tungsten (W), with its very high DBTT , is predominantly used as functional material so far. Despite, gliding of the screw dislocation has been revealed as controlling mechanism for the brittle-to-ductile transition (BDT) in single crystalline W one decade ago , possible mechanisms for polycrystalline W are still controversially discussed .
In the present study, we attempt to elucidate the nature of the BDT in polycrystalline W and the effect of lattice defects on the DBTT. Various W sheets were produced by cold rolling. Comprehensive electron microscopy-based analysis, X-ray diffraction experiments, and thoroughly designed fracture toughness tests were performed for an in-depth study on fracture behavior vs. microstructure correlation.
A world-record breaking DBTT of 210 K, −65 °C, was achieved in fracture toughness experiments. Our analysis identified (1) a small grain size along the crack front and (2) a high dislocation density as desirable properties to achieve low DBTTs and high toughness at low temperatures. We conclude that severe cold rolling qualifies W to nucleate dislocations at the crack tip in a significant number (1) by grain boundary sources and (2) by dislocation multiplication. Since cold rolling can easily be extended to an industrial-scale fabrication, we predict a new era in the application of polycrystalline W.
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