Effects of lead on the sustained-load cracking of AlMgSi at ambient temperatures

Abstract

The effects of heavy metal impurity atoms (e.g. lead) on the room temperature sustained-load cracking susceptibility of AlMgSi alloys have been studied. Impurity levels in the range less than 10–550 wt. ppm were tested, while microstructures were varied by utilizing homogenization treatments of either 450°C for 12 h or 570°C for 2h followed by solution treatment and aging at 175°C. The rate of crack growth, measured by a d.c. potential drop technique on bolt-loaded double-cantilever-beam specimens, increased with increasing stress intensity, increasing grain size and increasing lead level. Fracture under sustained load was intergranular, although the details of the intergranular fracture surfaces were primarily affected by the amount of lead in the alloy and the stress intensity level K. Low lead alloys exhibited proportionally more intergranular microvoid coalescence fracture at an equivalent K than did the high lead alloys which exhibited predominantly low ductility intergranular failure. Microanalysis by a relatively new technique, laser microprobe mass spectroscopy, was utilized to detect the presence of lead on the surfaces of newly fractured specimens, as well as in subsurface cracks. Details of fracture initiation were additionally studied on smooth polished tensile specimens strained at different rates to different levels of strain. It is shown that grain boundary accommodation of slip becomes the predominant deformation mode at low strain rates in the polished tensile specimens, regardless of lead level. The dependence of lead embrittlement on the imposed strain rate is also discussed.