The stress induced cubic-tetragonal phase transformation and the strain behavior of Jsub(C) in Cu-Nb3Sn multifilamentary wires

The low temperature Nb3Sn phase diagram has been determined, as well under zero pressure as under the precompression conditions occurring in Cu-Nb3Sn multifilamentary wires prepared by different techniques. Low temperature X-ray and neutron diffraction data furnish the evidence for a stress induced cubic-tetragonal phase transformation in Nb3Sn wires and tapes. The NB3Sn phase of wires containing Ta or Hf+Ga additions did not transform. These observations serve as a basis for a model which explains the behavior of Jsub(C) under uniaxial stress by a gradual tetragonal-cubic-tetragonal phase transformation. This model is supported by a great number of literature data. As a consequence, the so-called 'prestress effects' on Jsub(C) of Cu-Nb3Sn wires can be practically avoided by choosing the appropriate additions to the Nb and the bronze, thus stabilizing the cubic phase. This fact is of great importance for large scale applications. (orig.)