Mechanical behaviour and microstructure of fatigued aluminium single crystals

Abstract

For many decades the fatigue behavior of metallic materials has been a subject of intensive studies. Most profoundly investigated are Cu single crystals. A prominent feature of mechanical behavior in cyclically deformed pure Cu, Ni and other fee metals is the occurrence of a plateau in the cyclic stress-strain curve. A characteristic microstructure developing in these metals under cyclic loading exhibits bands of localized strain referred to as persistent slip bands (PSBs). Scarce experimental data on the mechanical response of Al to cyclic loading indicate that at room temperature the behavior of this material differs from that of Cu and Ni. This motivated the present study aiming to obtain more data and to get a better insight into the properties of Al under tension-compression loading. High purity Al single crystals oriented for single slip were tested under cyclic tension-compression conditions at two temperatures (T = 298 K and T = 78 K). For aluminum, the room temperature corresponds to a relatively high homologous temperature of about 0.3. Given the large stacking-fault energy of Al, its fatigue behavior at this temperature can be placed in the category of high-temperature behavior of pure fcc metals. At the liquid nitrogen temperature the dislocationmore » motion tends to become more planar which leads to dislocation structures resembling those found in Cu and other pure fcc metals cyclically deformed at room temperature. A plateau in the cyclic hardening curve associated with the occurrence of the PSBs was found in the fatigued specimens, which is at variance with the cyclic hardening behavior for relatively homogeneous room temperature deformation in tension-compression.« less