Dependence of dynamic fracture resistance on crack velocity in tungsten: Part II. bicrystals and polycrystals
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I.
INTRODUCTION
THEmorphology of surface and subsurface deformation on {100} in tungsten has been examined in the preceding paper, Part 1,1 and has been correlated with the local crack propagation velocity. An important feature in crack propagation in polycrystals is the effect of the grain boundaries on the deformation and fracture processes at the crack tip. This is particularly important in materials in which microcracks form in the interior of grains, and in which the grain boundaries are known to form strong barriers to slip. This would not be important in materials where slip dislocations can readily extend across grain boundaries, so that a more or less homogeneous plastic flow across many grains prevails. We showed in Part I that at the crack tip in single crystals, dislocations in more than one slip systems were activated. Thus, impediments to dislocation activation in multiple slip systems, as those giving rise to predominantly planar slip in the grain boundary region, may significantly reduce the ability of dislocation processes to relax the crack tip stresses. This indirectly facilitates the initiation of a crack across the grain boundary. We have examined the effect of a single, well defined, grain boundary of twist misorientation about (100), and ascertained its effect on crack propagation speed and energy dissipation. Our work is then extended to find the contfibution of grain boundaries to crack propagation resistance in polycrystals. The cracking of the grain boundaries as the angle of misorientation increased was discussed by us in a previous publication. 2 Here, we wish to examine the energy dissipation along the path of the main cleavage crack, since the joining of cracks across grain boundaries must also constitute an important energy dissipation mechanism.
II.
introduced by slowly wedging open the DCB specimen in an Instron Testing Machine. Crack initiation in these polycrystals was evidenced by a small load drop, corresponding to the occurrence of "Pop-in." These specimens were then tested in the impact tester in the same manner as the bicrystals and single crystals. Fractured specimens were examined with optical and scanning electron microscopy. Dynamic fracture surface energy was extracted through the analysis of Bums and Webb 3 by means of the measured crack velocities along the crack path, especially at the vicinity of the grain boundary region.
III.
EXPERIMENTAL RESULTS
A micrograph showing the quality of the bicrystals obtained by our techniques is shown in Figure 1. Some of the bicrystals showed subgrain structure close to the outside surface of the crystals. These were largely removed during the shaping of the crystals into the geometry of the DCB specimens. Since the crack propagated along the midsection of the specimens, the influence of subgrains is believed to be small.
EXPERIMENTAL PROCEDURE
The procedure for the growth of tungsten bicrystals of twist misorientation about (100) was reported previously. 2 DCB specimens were made and tested in the same manner as shown in P
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