On the Micromechanisms of Fatigue Crack Propagation in Aluminum Cast Alloys

• Published in: Key engineering materials 2014-01, Vol.592-593, p.393-396
• Main Authors: Michels, Wilhelm, Giertler, Alexander, Krupp, Ulrich, Siegfanz, Stefanie
• Format: Article
• Language: English
• Subjects: Aluminum base alloys; Cast alloys; Crack propagation; Cracks; Electron back scatter diffraction; Eutectics; Fatigue failure; Spheroidizing
• ISSN: 1013-9826; 1662-9795; 1662-9795
• DOI: 10.4028/www.scientific.net/KEM.592-593.393

Specimens of the common cast alloy Al-7Si-0.3Mg (A356) were solution heat treated at different temperatures and times to modify (i) the precipitation strengthening effect in the aluminum solid solution phase and (ii) to spheroidize the eutectic silicon. Just 15 minutes of solution heat treatment at a temperature of 540°C are sufficient to reach the desired effect. Cyclic loading experiments revealed an increase in fatigue life as compared to specimens heat treated at lower temperatures. In particular in the HCF regime, fatigue cracks that were originally initiated at pores follow crystallographic slip bands under shear control (mode II) as it was proven by automated EBSD (electron back scatter diffraction) measurements. The smoothly polished surface of the fatigue specimens was observed continuously by a long-distance microscope and discontinuously by SEM (scanning electron microscopy) to identify the fatigue crack propagation mechanisms. It was shown that da/dN decreases strongly when the crack tip interacts with the eutectic areas. Obviously, the blocking effect of the eutectic silicon particles makes the crack leaving the straight slip-band path.