Over the past several years, there has been considerable interest in phenomena that take place at nanometer (10 Ϫ9 m) length scales, and that may lead to novel mechanical properties in structural materials. As a result of this interest, a number of synthesis techniques that are capable of producing structural materials with grain sizes of 10 to 500 nm have been developed. Primary among these synthesis techniques is cryomilling, which consists of mechanical alloying/milling under liquid nitrogen. Furthermore, cryomilling is capable of producing bulk materials in large quantities. [1,2,3] Structural materials produced by cryomilling may be divided into ultrafine grained materials, which exhibit grain size larger than 200 nm, and bulk nanostructured materials, which are characterized by grain sizes less than 200 nm.In a very recent article, [4] the mechanical behavior of cryomilled Al-7.5 pct Mg alloy that exhibited an ultrafine grained microstructure with an approximate size of 300 nm was reported. The data of this alloy at room temperature, when plotted in terms stress-strain curves, have shown that the tensile behavior of the alloy is characterized by high strength, low strain hardening, and reasonably high ductility, compared to conventional 5083 Al. [4] Further, after yielding, there are well-defined stress drops. It has been suggested that a possible source for these stress drops is deformation twinning. In order to examine the possibility of deformation twinning, tests need to be performed under the following conditions: low temperature and small grain size. A decrease in temperature tends to favor twinning over slip, even in metallic systems with a high stacking fault energy such as Al (fcc). Also, when grain size is decreased, the twinning stress may become smaller than the stress for slip. This possibility is supported by the observation [5] that the twinning stress for Cu-15. at pct Zn increased linearly with d Ϫ1 and not with d Ϫ0.5 , which characterizes the grain size dependence of the stress for slip. In addition, deformation twinning was observed in nanostructured Al powers after cryomilling process using a high resolution electron microscope. [6,7] This article reports preliminary results on the mechanical behavior of nanostructured Al-7.5 pct Mg that was processed by cryomilling. The mechanical behavior of the alloy was investigated by performing tensile tests in liquid nitrogen.The primary purpose of the investigation was to determine if deformation twinning is operative during deformation.The extruded cryomilled nanostructured Al-7.5 pct Mg alloy used in the present investigation was manufactured at the Rocketdyne Division, Boeing Company (Canoga Park, CA), using the following procedures. [4] A mixture of pure aluminum powders and high-magnesium aluminum alloy powders was mechanically milled in liquid nitrogen. The milled powders were packed in aluminum cans in an inert atmosphere before hot degassing under vacuum and consolidated by hot isostatic pressing. The consolidated billets were finally ex...
