Effect of test temperature on the dynamic torsional deformation behavior of two aluminum-lithium alloys
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INTRODUCTION
THE addition of the lightest alloying element of lithium to aluminum alloys, viz., Al-Li alloys, has many advantages. For example, when 1 wt pct Li is added to aluminum, the elastic modulus increases by 6 pct, and the strength and stiffness are far superior to commercial aluminum alloys. Many studies have been made on the application of Al-Li alloys to structural materials for transportation systems.[1–4] Al-Li alloys also generate special attention because of their high potential to be applied as armor plates for warfare vehicles, which require mobility, transportability, and protective ability.[5] Considering the application areas of structural materials, failure or fracture of the structures is more likely to occur under conditions of dynamic loading than under static or quasistatic conditions. Thus, studies on dynamic deformation are very important with respect to the selection of structural materials and the design of structures. However, because research on dynamic fracture and deformation of Al-Li alloys has only recently been conducted, the research results are not yet sufficient.[5,6] On the other hand, it is widely known that the mechanical properties of Al-Li alloys are greatly improved at temperatures lower than room temperature;[7–20] therefore, much attention has been paid to their application in structural materials requiring excellent cryogenic properties. Studies to understand the mechanisms involved in the improvement of material properties at low temperatures have focused on the following three areas: (1) the decrease of the mobility of impurities segregated along grain boundaries,[9,10,11] (2) the delamination toughening effect,[7,16,17] and (3) the modification of deformation behavior.[12,13,15,18,20] However, there CHANG GIL LEE, Senior Researcher, is with the Materials Engineering Department, Korea Institute of Machinery and Materials, Changwon, 641-010 Korea. KI JONG KIM, Research Assistant, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790784 Korea. KYUNGMOX CHO, Associate Professor, is with the Department of Metallurgical Engineering, Pusan National University, Pusan, 609-735 Korea. Manuscript submitted March 19, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
is still much controversy over the rational explanation for the improvement of the cryogenic properties of Al-Li alloys. For a clarification of the factors involved and their correlation, a basic understanding of the deformation characteristics of Al-Li alloys and their fracture mode is needed. In the present study, torsional tests under dynamic loading were conducted using a torsional Kolsky bar[21,22,23] to investigate the dynamic deformation characteristics of Al-Li alloys. The materials used were two representative Al-Li alloys, i.e., 2090 and 8090 Al alloys. They were under dynamic torsional tests both at room temperature and a low temperature (2196 7C), and their dynamic deformation behavior was comparatively analyzed. I
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