High-Temperature Deformation of Yba 2 Cu 3 07-8 With Ag Additions
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HIGH-TEMPERATURE DEFORMATION OF YBa 2 Cu 3 07-8 WITH Ag ADDITIONS J.L. ROUTBORT,* K.C. GORETTA** and J.P. SINGH** *Materials Science Division, **Materials and.Components Technology Division Argonne National Laboratory, Argonne, IL 60439 ABSTRACT The steady-state flow stress of YBa 2 Cu 3 OT7 containing 15 to 30 vol.% Ag has been measured in air at nearly constant compressive strain rates between 5 x 10-6 and 1 x 10-4 s-1 from 830 to 900'C. Addition of Ag dramatically decreases the flow stress compared to that of the pure superconductor, but the stress exponents and the activation energy for deformation remain unchanged. INTRODUCTION Second phase Ag additions have been observed to increase density of YBa2Cu 3 07_8 (123) and result in enhanced room-temperature fracture toughness, strength and critical current density (Jc) [1]. Increases in Jc have also been obtained by the texture which can be induced by plastic deformation [2]. Plasticity is, therefore, being considered as a method of superconductor fabrication. Furthermore, high-temperature deformation processes are usually controlled by diffusional processes and therefore can be used to obtain inferences on diffusion. It is therefore important to investigate the effect of Ag additions on the high-temperature plasticity of YBa 2 Cu307-8. There have been several investigations of deformation of 123 [3-5] and while the results all agree that high-temperature plasticity occurs, the interpretations of the experiments are not in accord. The most recent study [5] was performed on fully dense material in the temperature range of 870'C to 980'C. The conclusion was that steady-state strain rate, c, was given by
S- Us P(O2 )-0. 48 ±0.06 exp -(800 ± 100 kJ/RT),
(1)
where a• is the steady-state stress and P(0 2 ) is the oxygen partial pressure; however, the activation energy (800 kJ/mol) may also depend on P(0 2 ). This description indicates that deformation is controlled by diffusion. The experiments presented here concentrated on the effect of Ag additions on the plasticity of the 123 superconductor. EXPERIMENTAL The composites of 123 containing Ag used in this investigation were the same as those used for previous fracture and Jc experiments [1]. A typical microstructure of a 20 vol.% Ag composite is shown in Fig. 1. The density of the composite increased from 84% of theoretical for the 15 vol.% Ag composite to -95% for the 20 and 30% composites. The grain size of the composites, approximately 15 gim, was smaller than that of the previously studied pure 123 [5]. Samples were deformed in direct compression in air at a nearly constant strain rate in an identical manner as has been described [5].
RESULTS A typical stress versus strain (e) curve measured at 8701C is shown in Fig. 2. In this particular test, the strain rate was changed. The steady-state stress, Mat. Res. Soc. Symp. Proc. Vol. 169. @1990 Materials Research Society
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as, is defined as the constant stress which develops at a constant c when the work-hardening rate (do/de) becomes zero. The stress exponent
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