Influence of an electric field on the plastic deformation of fine-grained Al 2 O 3
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INTRODUCTION
THE external parameters usually considered in the mechanical properties of materials are temperature, pressure (or stress), and time. Usually neglected are the effects of electric and magnetic fields. However, in many cases, such fields can exert a significant influence, especially when applied in conjunction with the usual parameters. For example, it was found that the application of an external electric field of ,1 kV/cm (the specimen being one electrode of a DC electrostatic circuit) during the superplastic deformation (SPD) of the 7475 Al alloy produced the following effects:[1–5] (a) reduced the flow stress by 10 to 20 pct; (b) increased slightly the strain-rate hardening exponent m 5 ln s/ ln «˙ ; (c) increased the width of the dispersoid-free zone at the grain boundaries and changed its composition; (d) retarded grain growth; and (e) significantly reduced cavitation. Similar effects of an electric field were obtained during the SPD of an Al-SiC particulate composite.[6] In contrast to the reduction in flow stress during SPD of the Al alloys, an electric field reduced the creep rate (i.e., increased the flow stress) during SPD of the Zn-5 wt pct Al alloy.[7] In the case of ceramics, an electric field of ,1 kV/cm significantly reduced the flow stress and increased the ductility of polycrystalline NaCl[8–13] and fine-grained 3Y-TZP (yttria-stabilized tetragonal zirconia polycrystals).[14] It also retarded grain growth in these materials.[13,14] The mechanisms responsible for the electric field effects J. CAMPBELL, Research Associate, Y. FAHMY, Senior Research Associate, and H. CONRAD, Professor Emeritus, are with the Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695-7907. Manuscript submitted November 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
have yet to be fully resolved. In the case of metals, they appear to be related to the charge on vacancies and the charge at the surface produced by the field.[1] In the case of NaCl, at low homologous temperatures, the field appears to enhance cross-slip by a reduction in stacking-fault energy;[11] at high temperatures, the major effect seems to be related to the space charge at dislocations and grain boundaries.[10] This also appears to be responsible for the effect of the field on the SPD of fine-grained 3Y-TZP.[13] The present investigation was undertaken to determine the influence of an electric field on the plastic deformation of fine-grained Al2O3. A reduction in the flow stress and possibly an increase in ductility caused by an electric field could be of interest in the forming of this ceramic material. II. EXPERIMENTAL The Al2O3 starting material was the same reactive-grade powder doped with ,300 ppm MgO used in the studies in the companion article.[15] Dog-bone shaped tensile specimens (Figure 1) with gage dimensions of 14 3 3.4 3 2 mm were fabricated from the spray-dried powder by dry pressing (207 MPa) and then sintering in air for 1 hour at temperatures of 1550 8C to 1625 8C. The density
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