Fatigue crack propagation due to torsional loads in a cast Al–Si alloy: Understanding the competing influence of defects and the microstructure using synchrotron 3D observations

This paper focuses on the crack propagation mechanisms under pure torsional loads of a cast Al-Si alloy. This article is the second part of recent work concerning the characterisation of torsional fatigue crack formation mechanisms using 3D synchrotron observation techniques. While the aim of the first part was to investigate the crack initiation mechanisms, this second article is focused on the crack growth mechanisms. For this, synchrotron observations including X-ray tomography and Diffraction Contrast Tomography were used i) to obtain the full 3D microstructure and ii) to perform 3D in-situ monitoring of the crack growth. It is shown that there are two coexisting crack propagation modes: a shear-mode and an opening-mode. Shear-mode cracks, which are crystallographically controlled, propagate predominantly on the planes of maximum shear stress corresponding to the slip system with the highest Schmid factor. The growth rates of those cracks can be relatively high (da/dN ≈ 1 0 − 8 m/cycle) but frequent and long-lasting arrests at grain boundaries are observed. These crack arrests are governed mainly by low Schmid factors in adjacent grains and large misorientation angles (defined by tilt and twist angles). Unlike shear-mode cracks, cracks propagating in an opening-mode propagate steadily and are not significantly hindered by the surrounding microstructure. They initiate only from pores and do not initiate in defect free zone of the aluminium matrix. They propagate on plane of maximum normal stress with lower growth rates (da/dN ≈ 1 0 − 9 m/cycle) and have a near-threshold stress intensity range observed to be approximately Δ K I ≈ 1 M P a m .