Anisotropic fracture behaviour and brittle-to-ductile transition of polycrystalline tungsten

Abstract

The fracture behaviour of polycrystalline sintered and rolled tungsten rods was investigated from −150°C to 950°C by means of three-point bending tests and electron microscopy where special attention was drawn to the influence of the microstructure. This thorough investigation demonstrates the positive impact of the crystallographic and grain shape anisotropy in tungsten. Specimens extracted along the rolling direction exhibit twice as high fracture toughnesses and a significantly reduced brittle-to-ductile transition temperature than the other two investigated orientations. Furthermore, these specimens show a change in their fracture mode from transgranular to intergranular fracture with crack deflection occurring around 270°C. In an in situ SEM fracture test, the origin of this crack deflection could be clarified. Finally, a fracture mechanics model is presented which predicts correctly the transition between the two fracture modes and which gives an energy criterion suitable to interpret experimental fracture results.

Keywords:

• fracture toughness
• anisotropy
• polycrystalline tungsten
• brittle-to-ductile transition
• fracture mechanics modelling

Acknowledgements

The research was supported by the German Science Foundation DFG under Grant No. WE 4256/3-2 and the Research and Development Nuclear Fusion Programme of the Forschungszentrum Karlsruhe. The authors wish to thank R. Moenig and P. Gruber for their help and support in carrying out the in situ experiments and Plansee Metall GmbH for supplying the tungsten material. Very valuable discussions with P. Gumbsch on the fracture and the BDT in tungsten are gratefully acknowledged.