JOURNAL OF MATERIALS SCIENCE 32 (1997) 1409 — 1414 Computer-modelled deformation mechanism maps for hot uniaxially pressed Bi-2223 superconductor M. P. JAMES, B. A. GLOWACKI Interdisciplinary Research Centre in Superconductivity, University of Cambridge, Cambridge CB3 OHE, and Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK A. TAMPIERI, G. CELOTTI Research Institute for Ceramics Technology - CNR, Via Granarolo, 64-48018 Faenza (RA), Italy A deformation mechanism map represents the densification behaviour of a green powder body. A map is created by evaluating the rate-equations for five types of densification mechanisms: yield, boundary diffusion, volume diffusion, power-law creep, and Nabarro —Herring creep. These rates are summed where appropriate and integrated to give the density at a given pressure, temperature and time; this can then be plotted on a map. Such maps have been created for hot-pressed Bi-2223, using the HIP 6.0 computer software. Four temperature—density deformation maps were created at different pressing pressures and for two typesof powder:asolid-statereacted powder and apyrolysed organicprecursor powder. The resulting maps are compared and discussed in relation to a set of experimental results. 1. Introduction The low critical current in bulk superconductors, es- pecially in strong magnetic fields, is the main limita- tion to be overcome before they can be utilized in major applications such as superconducting magnetic shielding and current leads. The two major causes of the low J  behaviour in bulk polycrystalline Bi-2223 superconductors are weak links at the grain bound- aries [1], and flux line movements within the grains arising from severe flux creep [2]. Across most high-angle grain boundaries, the flow of transport current is severely limited, especially in the presence of a magnetic field. Grain-boundary weak links are not clearly understood; however, dis- turbed crystal structure, chemical deviation, the pres- ence of secondary non-superconducting phases, the presence of microcracks and anisotropy of the elec- tronic properties at high-angle boundaries of the crys- tals, can all contribute to low values of critical current of the intergrain region, and thus of the ceramic as a whole. It is well known that high density and a high degree of grain alignment improve the weak links between grains [3]. Hot pressing has been used to increase the density of bulk Bi-2223 samples [4], and also to increase the texture by increasing grain alignment [5, 6], both of which increase critical current values. Hot pressing increases density by increasing the stresses present at the particle contacts, adding plastic yielding and power-law creep to the pressureless diffusional densifi- cation mechanisms. The modelling of densification and the development of rate equations for the different mechanisms has been described in the literature [7]. The modelling of densification and the mechanisms by which different powders undergo densification at different temper- atures, pressures and for different periods of time can provide information to simplify the optimization of the many hot-pressing process variables. This in- formation may be presented in the form of a deforma- tion mechanism map [8]. A deformation mechanism map is a graphical representation of the rate equations for the diffusional densification mechanisms, such as boundary diffusion and volume diffusion, together with the rate equations for power-law creep and Nabarro—Herring creep and the equation for yield. The contributions to the densification rate from each of the mechanisms is evaluated and, as they are inde- pendent, the rates are summed to give the total densifi- cation rate. This is then integrated to get the density at a given pressure, temperature, and time. The density can then be plotted against either pressure or temper- ature, often with time contours. Such a map has been used to predict the densification of YBa  Cu  O  [9]. To enable deformation maps to be constructed accu- rately and efficiently, the HIP 6.0 program has been written [10] to aid in the construction of the maps. 0022—2461  1997 Chapman & Hall 1409