Hardening of an AgPdMg alloy and its interdiffusion with BiSrCaCuO compounds

Abstract

High-Tc superconducting tapes most often consist of a Bi compound and they include a flexible metal substrate or sheath in order to ensure their mechanical stability, in view of the fabrication of long lengths. Silver is universally used as a substrate owing to its excellent chemical compatibility with Bi superconducting compounds. Furthermore, it has been shown that its presence favours the crystallization and texturation of the Bi-2212 phase (Bi2 Sr2CaCu2O y). However, due to its low mechanical resistance, the ratio of the amounts of Ag and superconductor used is high, thus reducing the overall critical current densities. The use of oxide dispersion strengthened (ODS) Ag or Ag alloys [1– 3] could offer an appealing alternative to solve this problem. This letter presents preliminary results of a study on the hardening of several Ag alloys and their interaction with thick layers of Bi compounds prepared by a technique based on the sequential electrolytic deposition of the constituent metals (Bi, Sr, Ca and Cu) followed by very short heat treatments in the range 800–860 8C [4, 5]. The process is described schematically in Fig. 1. In general, the possibility of producing internal oxidation or a surface oxide layer on an alloy depends, among other factors, on the ratio of the diffusion of oxygen from the surface into the bulk of the alloy to that of solute towards the surface. In the case of our composite tapes, at least part of the oxygen diffusion takes place, during the thermal treatments applied to form the superconducting phase, through a 4 im thick layer of different Bi, Sr, Ca and Cu oxides, some of which are liquid at the annealing temperatures. Furthermore, the interdiffusion between the components of the alloy and the Bi compounds may lead to the formation of secondary phases or to partial substitution in the compounds, degrading the superconducting properties. In this letter we describe studies carried out on a AgPdMg (Ag ˆ 87:7 wt %, Pd ˆ 12 wt %, Mg ˆ 0:3 wt %) alloy supplied by ACCES e.V., Aachen, Germany. This alloy had been reported to present highly improved mechanical properties (as compared to pure Ag) [6, 7] after thermal treatments, which in the case of [6] involves a 10 h anneal at 800 8C in an oxygen flow. The hardening was ascribed both to the solid solution AgPd and to internal oxidation. We have studied the hardening of 50 im thick tapes of the alloy under different treatments by Vickers microhardness measurements, optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). These tapes were then used as substrates for composite Bi-2212 superconductors and the interdiffusion between the substrate and different phases of the Bi–Sr–Ca–Cu– O phase diagram was studied both on surfaces and on cross-sections. EPMA shows that the recrystallization anneals, usually performed as a first step vacuum or under an inert gas atmosphere, at temperatures of the order of 800–900 8C, provoke surface segregation of Mg. This type of annealing is thus to be avoided as it could give rise to surface oxidation during subsequent thermal treatments. Internal oxidation treatments carried out in air or in oxygen at temperatures of 800 8C or higher lead to the formation of nodules on the surface of the tapes as can be seen in Fig. 2. The size of the nodules is a function of the temperature and the duration of the annealing, and in our case ranges from 1 to 20 im and gives rise to appreciable surface roughness. Quantitative EPMA shows that the nodules are composed exclusively of Ag and Pd,