Quench sensitivity of toughness in an Al alloy: Direct observation and analysis of failure initiation at the precipitate-free zone

An analysis of toughness in 6156 Al-Mg-Si-Cu sheet was performed using enhanced Kahn tear tests on samples quenched at different rates. Crack initiation energies were hardly affected by changing the water quench temperature from 20 degrees C to 60 degrees C; however, a significant reduction was evident on air cooling. Crack propagation energy was reduced for both 60 degrees C water-quenched and air-cooled materials. Observation of failure initiation through synchrotron radiation computed tomography for the 60 degrees C water-quenched material revealed failure ahead of the crack tip of grain boundaries oriented at 45 degrees to the main loading axis, and crack "tongues" extending into the material ahead of the main crack. Failure was predominantly intergranular. Fractographic assessment revealed predominantly voiding and shear decohesion in the 20 degrees C water-quenched material. With the aid of the new findings, past models on the influence of precipitate-free zone parameters on toughness were revised. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.