Mechanical Properties, Dislocation Density and Grain Structure of Ultrafine-Grained Aluminum and Aluminum-Magnesium Allo
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UCTION
IT is now well established that the mechanical properties of various metals can be improved by severe plastic deformation (SPD) processes such as equal channel angular pressing (ECAP).[1] As the material becomes ultrafine grained (UFG) or nanocrystalline, a significantly enhanced strength combined with a relatively high ductility is reported for monotonic tests.[2–6] Under cyclic loading, UFG materials often show superior behavior in comparison to their conventionally grain (CG) sized counterparts. Except from a decreased fatigue life at the technologically relatively unimportant loading condition of high total strain amplitudes,[7,8] UFG materials exhibit significantly improved fatigue properties under stress-dominated loading conditions, compared to the CG materials.[8–11] It is also known that the impurity content of a material influences the microstructure, which can be obtained by SPD. In the literature, it is described that, with increasing Mg addition to Al[12] or Al-Sc alloys,[13] a finer grain size is achieved. Consequently, the mechanical properties are also affected. In particular, during cyclic loading, the microstructural stability plays a key role for enhanced fatigue properties (e.g., References 14 and 15). In the present work, the influence of the amount of the alloying element magnesium on the microstructure of aluminum and on the mechanical properties is investigated. For comparison, commercially pure aluminum has been investigated. Hardness tests and fatigue experiments J. MAY, M. DINKEL, D. AMBERGER, H.W. HO¨PPEL, Dr., and M. GO¨KEN, Professor Dr., are with the Department of Materials Science and Engineering, Institute I: General Materials Properties, Friedrich-Alexander University Erlangen-Nu¨rnberg, 91058 Erlangen, Germany. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Ultrafine-Grained Materials: from Basics to Application’’, which occurred September 25-27, 2006 in Kloster Irsee, Germany. Article published online April 11, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
were carried out on materials in the UFG and CG conditions.
II.
EXPERIMENTAL
High-purity aluminum and five different aluminum alloys have been investigated. The high-purity aluminum with an aluminum content of 99.999 pct is further denoted as Al 99.999. For a proper investigation of the influence of different amounts of impurities, AlMg0.5, AlMg1, AlMg1.5, and AlMg2 have been produced by alloying the high-purity aluminum with 0.5, 1.0, 1.5, and 2.0 wt pct magnesium. These alloys and the Al 99.999 were cast in the group of Professor Gottstein at the Institute of Physical Metallurgy and Metal Physics, RWTH (Aachen, Germany). For comparison, also an aluminum alloy of commercial purity has been used, similar to an AA1050 alloy and with Fe and Si as main impurities. This alloy is denoted as CP-Al 99.5. For details on the chemical composition of this material, the reader is referred to Reference 16. All materials have been investigated in a CG state and in an
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