Lead-induced solid metal embrittlement of an excess silicon Al−Mg−Si alloy at temperatures of −4°C to 80°C

Abstract

It is shown that minute (e. g., <500 ppm) additions of heavy metal impurities (e. g., Pb, Bi) can induce sustained load cracking at ambient temperatures in Al−Mg−Si alloys. This article describes experiments designed to elucidate the role of Pb level, test temperature, strain rate, and stress state on the observed cracking phenomena. Sustained load cracking was observed at temperatures as low as −4°C in either air or vacuum with the rate of crack growth and the apparent threshold value for crack growth being strongly influenced by the amount of lead (i.e., internal) in the alloy. The fracture mode was strongly affected by the test temperature, Pb level, strain rate, and the imposed stress intensity level,K. Fracture in the low-lead (i.e., <10 ppm) alloys was predominantly by intergranular microvoid coalescence (IGMVC), while fracture in the higher lead alloys was predominantly by low ductility intergranular fracture (LDIGF) when the crack-tip strain rate was sufficiently low. High-resolution scanning electron micrographs taken from LDIGF surfaces suggested minimal deformation, while surface analyses of these surfaces performed using both laser microprobe mass spectroscopy (LMMS) and high-resolution scanning auger microscopy indicated that lead was primarily responsible for the LDIGF cracking phenomenon. Lead was observed both on the surfaces of fractured specimens as well as in subsurface cracks not contiguous with the macroscopic cracks. An external supply of Pbvia the application of either solid Pb or a Pb−Bi alloy to the external surfaces of specimens held under sustained load promoted LDIGF, decreased thresholdK’s, and increased the rate of crack growth. The phenomena and possible mechanisms of heavy metal impurity-induced cracking at ambient temperatures in aluminum alloys are discussed.