Fractographic features of failure in polycrystalline chromium on uniaxial stretching

Abstract

1. The main fractographic feature in the brittle failure of chromium is cleavage crack branching. The cracks deviate from the initial direction by ±35°. 2. The crack length at branching ranges from 60 to 80 μm. The crack lengths at branching can differ by a factor of four even in a single specimen if several cracks are generated. 3. In completely brittle failure, cracks branch in cast chromium with grain size 400 μm. Branching is not observed in recrystallized chromium with grain sizes of 36 and 14 μm. 4. Deformation either preliminary or directly preceding failure favors branching, e.g., when a specimen is loaded in the brittle-plastic transition region. 5. A cleavage crack passes through the porous part of a specimen produced by plastic strain and causes pore coalescence, i.e., the porosity causes a change in the failure mechanism. 6. The cracking resistance has been estimated from the branching as follows: in coarsegrained cast chromium 2 J/m2 (−50°C), recrystallized 19.5 J/m2 (150°C), and deformed 39.5 J/m2 (200°C), 36 J/m2 (300°C), and 58 J/m2 (400°C).