Microindentation of aluminum
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I. INTRODUCTION
IN the last decades, there has been considerable interest in severe plastic deformation (SPD) that can achieve grain size refinement in metallic alloys. The highly deformed materials appear to retain high levels of internal strain and high dislocation densities, and grain boundaries have been described as being in nonequilibrium state.[1] Such materials display unique features such as high strength with ductility, superplasticity at moderate temperatures, and high strain rates. Of the existing SPD processes, equal channel angular extrusion (ECAE) and high pressure torsion (HPT) are the most versatile and potent processes for fabricating materials of ultrafine structure in a large quantity. In ECAE, a rod-shape sample is extruded repetitively through a die with equal channels.[2–6] In HPT, a thin disk is subjected to a high pressure and concurrent torsion straining.[7] It is, however, generally believed that the ECAE process can be scaled relatively easily to produce large bulk samples[8] and is amenable to simplification. The ECAE process appears to be effective with aluminum alloys and makes it possible to produce equiaxed grains of submicrometer sizes that give superplastic ductility at elevated temperatures[9] and the as-extruded aluminum alloys can be formed into domes[10] or rolled into thin sheets without a significant loss in their superplastic properties.[11] In most cases, the microstructure has been described as “submicron polycrystals” with no specific analysis, and the mechanical behavior under static and dynamic loading has been evaluated mostly by using tensile and compression tests.[12–16] The understanding of the fundamental mechanisms controlling the mechanical deformation as well as the evolution of the microstructure, however, is still at a very early stage. The microindentation is a very simple and convenient method for determining time-independent plastic flow in materials. An indenter is pushed onto the surface of a sample FUQIAN YANG, Assistant Professor, LINGLING PENG, Postdoctoral Student, and KENJI OKAZAKI, Full Professor, are with the Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506. Contact e-mail: [email protected] Manuscript submitted March 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
to evoke plastic flow in indentation test, both the load and the displacement of the indenter are recorded. Based on the load-displacement curve and the impression profile, the mechanical response of a material can be evaluated. The microhardness, HV, being used to describe the plastic behavior of a material, is defined as the ratio of the indentation load to the contact area between the indenter and the sample. That is (for the Vickers indenter), HV 2F sin f>D2
[1]
where F is the indentation load, is a half of the included angle between opposite faces of a pyramid (68 deg), and D is the diagonal length of the impression profile. Despite the advantage of the indentation technique as a method to characterize local material behavio
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