Impact characteristic and crack propagation mechanisms of large-size molybdenum alloy single crystal and polycrystal

The impact characteristic and crack propagation mechanism for large-size Mo-3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo-3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo-3Nb single crystals is lower than that of Mo-3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo-3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo-3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation.