Plastic deformation kinetics of fine-grained alumina
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INTRODUCTION
II. EXPERIMENTAL
POLYCRYSTALLINE alumina (a-Al2O3) is generally considered to be a brittle material. However, it can exhibit reasonable ductility at high homologous temperatures when the grain size is ,5 mm. A listing of the studies into the plastic deformation kinetics of fine-grained Al2O3 is presented in Table A1 of the Appendix. The results are considered in terms of the constants in the Weertman–Dorn (W–D) relation for the plastic deformation of materials at high homologous temperatures: mb s «˙ 5 A kT m
n
p
1212 b d
D
[1]
where «˙ is the strain rate, s the applied stress, m the shear modulus, b the Burgers vector, D 5 D0 exp (2Q/RT ) the appropriate diffusion coefficient, and A, n, and p constants. The terms kT and RT have their usual meaning. From Table A1, it is seen that the parameters n, p, and Q have a range in values depending on the material and test conditions, indicating various rate-controlling mechanisms. Furthermore, a relatively smaller number of tests have been conducted in tension compared to compression and bending; only a few stress relaxation, (SR) tests were performed. It has been determined that the plastic deformation behavior of Al2O3 in tension differs from that in compression and flexure bending.[1–4] Moreover, stress relaxation offers a means for determining the plastic deformation kinetics at a constant structure, i.e., it is an “isostructure” test.[5,6] Therefore, the objective of the present investigation was to obtain definitive data on the plastic deformation kinetics of finegrained Al2O3 in uniaxial tension, employing stress relaxation as the principal test mode (along with constant strain rate and strain rate cycling tests) for the purpose of evaluating the rate-controlling mechanism. Moreover, the results of these tests provide the reference against which the influence of an electric field on the plastic deformation of the same material was evaluated.[7]
J. CAMPBELL, Postdoctoral 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
The Al2O3 starting material was a reactive-grade powder doped with MgO and had an average crystal size of 0.4 mm. It was provided by Alcoa and had the following composition in wt pct as determined by the supplier. Al2O3 Na2O
SiO2
MgO
ZrO2 Fe2O3
CaO
B2O3
99.8 0.065* 0.08* 0.025 to 0.04* 0.01** 0.03** 0.004** 0.035* *Batch composition. **Nominal composition.
Dog-bone shape tensile specimens with gage dimensions 14 3 3.4 3 2-mm thick were fabricated from the spraydried powder by dry pressing (207 MPa) and then sintering in air for 1 hour at 1550 8C to 1625 8C. The density of the as-sintered specimens ranged from 97 to 98 pct and the mean linear intercept grain size from ,1.4 to 2.9 mm. As an example, the microstructure of a specimen sintered for 1 hour at 1550 8C is shown in Figure 1 and the
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