A process for the fabrication of high-quality and near-net-shaped superconducting specimens of (Y-123) with uniformly distributed fine (Y-211) is discussed. The process involves the fabrication of 211 preforms by conventional ceramic routes such as uniaxial and isostatic pressing, injection moulding and slip casting, and pressureless infiltration basically from a reservoir containing liquid phases. A compact of 123 or 123 rich in liquid phases(s), acting as a source of liquid phases, is placed in contact with the 211 preform, and heated above the peritectic temperature of 123. The liquid from the source compact infiltrates the 211 filler and the peritectic reaction occurring between the preform material (211) and the matrix (liquid phases) during slow cooling from the peritectic temperature results in the growth of 123 with uniformly distributed fine 211 particles. We have also demonstrated that the present process can be extended to the fabrication of other rare earth (RE) superconductors where a solid solution of the kind occurs due to comparable size of the RE and Ba atoms, by choosing the RE = Gd system as an example. Herein the advantages and bounds of the process are reviewed; the effect of various cooling rates on the growth kinetics of 123 and the resulting macro- and microstructures are discussed. Fabrication of composites containing fine and very uniformly distributed Ag is also demonstrated by the process. Comparisons are made between the characteristics of the samples produced in the present process and the existing melt texturing process. The utility of the process is demonstrated by the fabrication of a three-dimensional component - a hollow cylinder.

It has been argued in our recent papers that the heat of formation of intermetallic compounds is mostly concentrated in the nearest neighbor unlike atom-pair bonds, and that the positive term in Miedema's equation is associated with charge transfer on the bond to maintain electroneutrality. In this paper, taking examples of some well populated crystal-structure types such as MgCu(2), AsNa(3), AuCu(3), MoSi(2) and SiCr(3) types, the effect of such charge transfer on the crystal structures adopted by intermetallic compounds is examined. It is shown that the correlation between the observed size changes of atoms on alloying and their electronegativity differences is supportive of the idea of charge transfer between atoms. It is argued that the electronegativity and valence differences need to be of the required magnitude and direction to alter, through charge transfer, the elemental radius ratios R(A)/R(B) to the internal radius ratios r(A)/r(B) allowed by the structure types. Since the size change of atoms on alloying is highly correlated to how different R(A)/R(B) is from the ideal radius ratio for a structure type, the lattice parameters of intermetallic compounds can be predicted with excellent accuracy knowing R(A)/R(B). A practical application of the approach developed in our recent papers to superalloy design is presented.

An infiltration and growth (IG) process which enables the fabrication of three-dimensional (3D) components of REBa 2 Cu 3 O 7 (RE Y, Gd, Sm, Nd, etc.) (RE-123) superconductors with a highly textured microstructure is described. The advantages of the process in comparison with conventional melt processing are discussed. The process has been demonstrated to yield highly favorable microstructures in the case Y-123 processed in air, as well as in the case of Gd-123 processed in reduced oxygen partial pressure.

a b s t r a c tRajasekharan and Girgis reported that binary systems with intermetallic compounds of a particular crystal structure form a straight line on a map using Miedema's parameters. In this paper, the universality of that observation is examined. Observations from a study of 143 binary systems that crystallize in six different crystal structures at AB 3 composition are discussed. Prediction of concomitant and mutually exclusive structure types in binary metallic phase diagrams, and of phase transitions among different structure types, has been demonstrated. This behaviour is unexpected because Miedema's parameters are isotropic in nature and structural energies are generally assumed to be small. We argue in this paper that each point on the map stands for the energy of an unlike atom-pair (A-B) bond, with the bond energy remaining nearly the same at all compositions in the phase diagram. This argument is confirmed by comparing the nearest-neighbour (A-B) bond lengths for the compounds of the structure types CaCu 5 and CsCl, when concomitant with MgCu 2 structure type. This fact leads to an important conclusion that one can define bond energy for the metallic unlike-atom-pair bond as is usually done for a conventional chemical bond.

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