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 À150 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.

Influence of impurities on the fracture behaviour of tungsten

“…Therefore, only the simple occurance of grain boundary fracture in tungsten is not a hint for impurity induced embrittlement. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 a The amount of inter-/transgranular fracture could not be determined due to the extremely small grain sizes and also due to varying amounts in the different directions of a HPT sample [17] b The fracture toughness range is mainly dependent on the crack orientation of the samples with respect to the rolling direction [18,19] c Due to the their microstructures and fracture morphologies the fracture toughness values are similar to technically pure tungsten in the same conditions d Values vary extremely depending on the direction of the crack propagation in a high pressure torsion sample [17] AES-spectrum of a severe plastically deformed tungsten single crystal which was subsequently recrystallized at 1200 • C for an hour in a vacuum furnace (a). The O-peak at 510eV represents the adsorbed oxygen after fracturing the sample in the analysis chamber.…”

Recent progress in research on tungsten materials for nuclear fusion applications in Europe

“…It is known that tungsten features an anisotropic fracture behaviour in the case of the presence of elongated grains, e.g., after rolling, forging and deformation by HPT [55,56]. Additionally, the fracture resistance may increase with crack propagation, which implies that it is not always possible to characterize the material's toughness with one single value such as plane strain fracture toughness K IC or critical energy release rate G IC .…”

“…In a later stage of testing and in the case of predominant intercrystalline failure, the increase of the R-curve depends on crack bridges appearing, which connect the crack flanks and fail by plastic deformation, thus consuming more energy and leading to an extrinsic toughening. Experimental details are given in [55]. There, temperature-dependent fracture experiments using technically pure tungsten featuring recrystallized (globular) and deformed (elongated grains) microstructures are described.…”

A brief summary of the progress on the EFDA tungsten materials program

“…Within the present EFDA programme, commercial grade tungsten materials are used to establish a database for future recording of the progress in material development. As an example, fracture toughness of a round blank of pure tungsten where the specimens were oriented longitudinally (L-R), radially (R-L) and circumferentially (C-R) was investigated intensively (see [18,19]). …”