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 °C to 60 °C; however, a significant reduction was evident on air cooling. Crack propagation energy was reduced for both 60 °C water-quenched and air-cooled materials. Observation of failure initiation through synchrotron radiation computed tomography for the 60 °C water-quenched material revealed failure ahead of the crack tip of grain boundaries oriented at 45° 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 °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.