Dynamic fracture toughness of Al–7Si–Mg (A357) aluminum alloy Nikolaos D. Alexopoulos a,⇑ , Antonis Stylianos a , John Campbell b a University of the Aegean, Department of Financial Engineering, 821 00 Chios, Greece b University of Birmingham, Department of Metallurgy and Materials, Edgbaston B15 2TT, UK article info Article history: Received 9 January 2012 Received in revised form 15 November 2012 Available online 29 November 2012 Keywords: Mechanical characterization Aluminum alloys Casting Age hardening Impact Fracture abstract Impact behavior related to crack initiation and growth of cast A357 (Al–7Si–0.7Mg) alloy was experimentally investigated. Dynamic fracture toughness for various artificial aging condi- tions were evaluated by exploiting instrumented impact Charpy V-notch experiments; impact tests were preferred instead of the standard fracture toughness tests in order to have a fast assessment of the effect of artificial aging on the fracture toughness behavior. Charac- teristic points of impact load–deflection curves related with cracking were identified and post-correlated with different artificial aging conditions. Absorbed impact energies for stable and unstable crack growth were calculated for 28 different aging conditions; resulting crack- ing velocity at the unstable crack regime (rebound compliance) was also calculated and it was found that rebound compliance is inversely related to ductility. Cracking initiation load values were identified by exploiting the compliance changing rate method; both stress inten- sity factor and J-integral analysis were employed to calculate critical dynamic fracture toughness values. They were found to be slightly higher than corresponding quasi-static properties and they are both changing with varying aging conditions. Limitations and results of both notch-fracture toughness analyses were discussed by taking into account the mate- rial aging condition. Performed scanning electron microscopy on the fracture surfaces revealed the fracture mechanism that is correlated with respective microstructure. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Cast alloy Al–7Si–0.7Mg (A357) is a precipitation- hardened aluminum alloy that exhibits excellent casting characteristics, low cracking tendency during solidification, and relatively high strength. For many decades now, it is widely used to produce complex-shape aeronautical and automotive components. Mechanical properties of these cast aluminum alloys are governed by two main parameters that will be shortly explained underneath: (a) those that are dictated by the casting process itself (pores, solidification) and (b) those that are influenced by the physical metallurgy (chemical composition, heat treatment) of the alloy. For the case of the first one, tighter controls currently ap- plied during the casting process as well as the advancements of the casting processes have resulted in a reduction of structural defects in castings, e.g. pores and oxide bifilms, which seriously degrade mechanical properties (Campbell, 2003; Nyahumwa et al., 2001; Tiryakiog ˘lu et al., 2003). However, a thorough understanding of the physical metal- lurgy background of the precipitation-hardened cast alumi- num alloys indicated that chemical composition (Alexopoulos and Pantelakis, 2003), solidification rate (Alexopoulos and Tiryakiog ˘lu, 2009) and heat treatment (Alexopoulos and Pantelakis, 2004a) are the key parameters to alter their mechanical properties. Cast Al–Si–Mg alloys are dispersion hardened by the eutectic Si particles reinforc- ing the aluminum matrix (Caceres et al., 1996; Hunt et al., 1991). It is well known that ductility and fracture of these al- loys are strongly affected by the size, distribution, and mor- phology of the Si particles (Lloyd, 1995, 1991; Poole and Dowdle, 1998). An additional factor that contributes to the mechanical behavior and fracture of these alloys is the presence of the secondary precipitation-hardening particles stoichiometrically analogous to the Mg 2 Si (b 00 phase) (Hahn 0167-6636/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechmat.2012.11.005 ⇑ Corresponding author. Tel.: +30 2271 035430; fax: +30 2271 035429. E-mail address: nalexop@aegean.gr (N.D. Alexopoulos). Mechanics of Materials 58 (2013) 55–68 Contents lists available at SciVerse ScienceDirect Mechanics of Materials journal homepage: www.elsevier.com/locate/mechmat