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Lecture

The brittle-to-ductile transition in cold rolled tungsten: Low-temperature toughness opens a new era in industrial application of tungsten

Thursday (27.09.2018)
11:45 - 12:00 S1/01 - A1
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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 [1], 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 [2], possible mechanisms for polycrystalline W are still controversially discussed [3].

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.


References

[1] Lassner E, Schubert W-D. Tungsten. New York: Springer; 1999. doi:10.1007/978-1-4615-4907-9.

[2] Giannattasio A, Roberts SG. Strain-rate dependence of the brittle-to-ductile transition temperature in tungsten. Philosophical Magazine 2007;87(17):2589–98. doi:10.1080/14786430701253197.

[3] Reiser J, Hoffmann J, Jäntsch U et al. Ductilisation of tungsten (W). International Journal of Refractory Metals and Hard Materials 2016;54:351–69. doi:10.1016/j.ijrmhm.2015.09.001.

 

Speaker:
Carsten Bonnekoh
Karlsruhe Institute of Technology (KIT)
Additional Authors:
  • Dr. Jan Hoffmann
    Karlsruhe Institute of Technology (KIT)
  • Dr. Andreas Hoffmann
    Plansee SE
  • Dr. Jens Reiser
    Karlsruhe Institute of Technology (KIT)