Fracture surfaces of complex metallic materials : roughness indices, scaling and universality

Surfaces de rupture d’alliages métalliques complexes : indices de rugosité, lois d’échelle et universalité

Abstract

The morphology of fracture surfaces in complex metallic alloys is analysed. The simultaneous use of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) allows the measurement of the universal roughness exponent ζ_┴ = 0.78 over five decades of length scales (0.5 nm - 0.5 µm). Furthermore, a small length scales regime (1 nm -1 µm) is shown to be characterized by a roughness index ζ_┴QS ≅ 0.5. On the other hand, cracks fronts stopped during their propagation at pinning microstructural obstacles in two different metallic alloys are analysed, and their « in-plane » roughness index is determined for the first time. In the case of the 8090 Al-Li alloy, which has a very anisotropic microstructure, the roughness of the tensile crack front propagating along the grains length is equal to ζ ≅ 0.60. On the other hand, the roughness of the purely three-dimensional fatigue crack front in the Super-α₂ Ti₃Al-based alloy is equal to ζ ≅ 0.54.