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

Analysis of toughness in 6156 Al-Mg-Si-Cu sheet has been performed using enhanced Kahn tear tests on samples quenched at different rates. Crack initiation energies were hardly affected by changing 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 (SRCT), 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 have been revised.

[1]  L. Davin,et al.  Room temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield strength development , 2005 .

[2]  Marco J. Starink,et al.  Improvements in quench factor modelling , 2003 .

[3]  J. Knott,et al.  Fundamentals of Fracture Mechanics , 2008 .

[4]  A. Deschamps,et al.  On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy , 2003 .

[5]  J. Hatch,et al.  Aluminum: Properties and Physical Metallurgy , 1984 .

[6]  J. Hosson,et al.  On the effects of thermomechanical processing on failure mode in precipitation-hardened aluminium alloys , 2002 .

[7]  A. Vasudévan,et al.  Grain boundary ductile fracture in precipitation hardened aluminum alloys , 1987 .

[8]  I. Sinclair,et al.  Evolution of voids during ductile crack propagation in an aluminium alloy sheet toughness test studied by synchrotron radiation computed tomography , 2008 .

[9]  E. Hornbogen,et al.  Fracture toughness of precipitation hardened alloys containing narrow soft zones at grain boundaries , 1977 .

[10]  I. Sinclair,et al.  Microstructural effects on fracture toughness in AA7010 plate , 2000 .

[11]  T. Kawabata,et al.  Ductile fracture in the interior of precipitate free zone in an Al-6.0%Zn-2.6%Mg alloy , 1976 .

[12]  Wolfgang Ludwig,et al.  Penetration of liquid gallium into the grain boundaries of aluminium: a synchrotron radiation microtomographic investigation , 2000 .

[13]  A. Deschamps,et al.  A model for predicting fracture mode and toughness in 7000 series aluminium alloys , 2004 .

[14]  A. Reynolds,et al.  Correlation of grain-boundary precipitates parameters with fracture toughness in an Al–Cu–Mg–Ag alloy subjected to long-term thermal exposure , 1998 .