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

Abstract

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 (a_n) 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 (K_Max = 15.6 MPa√m) to investigate the crack closure effect. For both constant R and K_Max conditions: (i) the load-displacement curves for every loading cycle were linear for a_n ∥ to RD and TD, indicating no crack closure; (ii) the FCGR was found to be lower for a_n ∥ RD than a_n ∥ 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, $\left\{10\overline{1}2\right\}〈10\overline{1}1〉$ extension twins (ET) with lamellae ∼⊥ and c-axis $\sim \parallel$ 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 $\sim \parallel$ and $\sim \perp$ to crack-tip to $\sim 0$ for a_n ∥ to RD and TD, respectively. Hence, lenticular ET $\sim \perp$ and $\sim \parallel$, with c-axis $\sim \parallel$ and $\sim \perp$ to crack path activates, leading to trans and inter lamellar crack for a_n ∥ to RD and TD, respectively, and anisotropic FCGR. Translamellar crack in a_n ∥ 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 a_n ∥ TD due to interlamellar cracking. Thus, the crack growth through the matrix-ET interfaces was favoured due to strain incompatibility. The Fractography for a_n ∥ RD shows smaller elongated grooves along crack propagation, which indicates crack arrest. However, larger elongated grooves for a_n ∥ TD indicated easy crack propagation due to favourable interlamellar crack.