Anisotropic FCGR behaviour in hot-rolled Mg-3Al-0.5Ce alloy

In this work, anisotropic fatigue crack growth rate (FCGR) behaviour in a hot-rolled Mg-3wt%Al-0.5wt%Ce alloy was investigated using compact tension (CT) specimens with notch (an) parallel to the rolling direction (RD) and transverse direction (TD). The FCGR tests were conducted at a constant load ratio (R = 0.1) and maximum stress intensity factor (KMax = 15.6 MPa√m) to investigate the crack closure effect. For both constant R and KMax conditions: (i) The load-displacement curves for every loading cycle were linear for an ∥ to RD and TD, indicating no crack closure; (ii) The FCGR was found to be lower for an ∥ RD than an ∥ TD over the entire stress intensity factor range (ΔK). The hot-rolled sample contained long-aligned Al11Ce3 intermetallic phase within grain boundaries that are elongated along RD. During the FCGR test, {101¯2}〈101¯1〉 extension twins (ET) with lamellae ∼⊥ and c-axis ∼∥ to these elongated intermetallics along RD developed irrespective of the notch orientation. During the loading cycle, these intermetallics along RD generate back-stresses, reducing the in-plane tensile stress ∼∥ and ∼⊥ to crack-tip to ∼0 for an ∥ to RD and TD, respectively. Hence, lenticular ET ∼⊥ and ∼∥, with c-axis ∼∥ and ∼⊥ to crack path activates, leading to trans and inter lamellar crack for an ∥ to RD and TD, respectively, and anisotropic FCGR. Translamellar crack in an ∥ RD reduces the FCGR due to plastic energy dissipation as perceived by comparatively more geometrically necessary boundaries (GNBs). On the other hand, faster FCGR was obtained for an ∥ TD due to interlamellar cracking. Thus, the crack growth through the matrix-ET interfaces was favoured due to strain incompatibility. The Fractography for an ∥ RD shows smaller elongated grooves along crack propagation, which indicates crack arrest. However, larger elongated grooves for an ∥ TD indicated easy crack propagation due to favourable interlamellar crack.