X-ray topographic fractography of single crystals of molybdenum and niobium
Abstract
The semibrittle (001) cleavage fracture at 77 K of Mo and outgassed Nb single crystals was related to their fracture toughness. Berg-Barrett X-ray reflection topography and White Beam Synchrotron Fractography were used. Calibrated electropolishing in layers followed by Synchrotron Fractography revealed that under the conditions which existed at the tip of the running crack, Mo underwent general yielding which resulted in a plastically curved lattice. This result correlates with the substantial fracture toughness for Mo measured previously. On the other hand, double (112) slip was observed for Nb. The plastic zone associated with the crack tip is shown to have a structure typical of shock loaded materials, consisting of arrays of screw dislocations, micro- and macrotwins. This result correlates with the low fracture toughness of Nb at 77 K. It is proposed that these extremes in cleavage behavior for Mo vs Nb are rationalized in terms of the capacity of a bcc structure to undergo a slip to twinning transition at high strain rates.
- Publication:
-
Masters Thesis
- Pub Date:
- December 1982
- Bibcode:
- 1982MsT.........12H
- Keywords:
-
- Crack Propagation;
- Fracture Strength;
- Molybdenum;
- Niobium;
- Screw Dislocations;
- Single Crystals;
- Toughness;
- Twinning;
- Electropolishing;
- Shock Loads;
- Stacking Fault Energy;
- Strain Rate;
- Stress Distribution;
- Topography;
- X Ray Analysis;
- Solid-State Physics