| Tensile superplasticity of YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}rm 7-x{rcub}{dollar} and of YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}rm 7-x{rcub}{dollar}/ 25 vol%Ag composites was investigated. Tensile elongations close to 100% were achieved for fine grained (0.6-0.9 {dollar}mu{dollar}m) microstructures in the temperature range of 800-875{dollar}spcirc{dollar}C, at a strain rate 6 {dollar}times{dollar} 10{dollar}sp{lcub}-6{rcub}{dollar} to 10{dollar}sp{lcub}-3{rcub}{dollar}/sec. It is suggested that the dominant superplastic deformation mechanism is grain boundary sliding, accommodated and controlled by interface reaction, characterized by a stress exponent of n = 2, a grain size exponent of p = 1.5 and an activation energy of Q{dollar}sb{lcub}rm sp{rcub}{dollar} = 515 {dollar}pm{dollar} 100kJ/mol. Superplastic deformation was accompanied by grain growth. Both static and dynamic grain growth exhibited cubic growth kinetics with apparent activation entalpies of 512 and 437 kJ/mol, respectively. These results are in accordance with the deformation results and are consistent with the apparent activation energy for self diffusivity of yttrium along grain boundaries. Addition of 25vol% silver resulted in a greater strain-to-failure. The deformation parameters of the composite were the same as those of the monolithic material, suggesting the same deformation mechanism. A weaker time dependence resulted for both static and dynamic grain growth, yielding a grain growth exponent of 4. Critical temperature T{dollar}sb{lcub}rm c{rcub}{dollar}, normal state resistivity and critical current density J{dollar}sb{lcub}rm c{rcub}{dollar} measurements showed an overall improvement in current carrying capability after deformation. As a result of grain growth and better intergrain coupling J{dollar}sb{lcub}rm c{rcub}{dollar} increased over six fold with the large strains, nevertheless remaining within typical values found for bulk materials. |
