The origin of prestress in binary and ternary Nb3Sn multifilamentary wires

High resolution neutron powder diffractometry at temperatures between 10 K and 800 K was performed on commercially available Nb
3
Sn multifilamentary superconducting wires with additions of 1.6 Ti, 0.5 Ti and 7.5 Ta (wt.%), and with internal steel reinforcement as a continuation of our earlier work upon binary Nb
3
Sn conductors. In addition to still well characterized stress induced lattice distortion T
2
a volume compression of the unit cell caused by hydrostatic compression of the filaments by the surrounding bronze was observed for the first time. The volume compression (up to 0.4 %) was derived from a quantitative comparison with the results of high resolution X-ray measurements of etched filaments, where only thermal contraction is acting. Both lattice deformations are of the same order of magnitude and both stress induced effectS, the tetragonal distortion and the hydrostatically volume compressien were found to be reduced by ternary additions. Internal steel reinforcement seems to act dominantly by hydrostatic volume compression. Therefore the combination of this two effects is responsible for the prestress and the correlated J
c
degradation.