Evaluation of residual stress and texture in isotope based Mg11B2 superconductor using neutron diffraction.

Magnesium diboride (MgB2) superconducting wires have demonstrated commercial potential to replace niobium-titanium (NbTi) in terms of comparable critical current density. Its higher critical temperature makes MgB2 wire suitable for liquid-helium-free operation. We recently reported boron-11 isotope-based low-activation Mg11B2 superconducting wire with decent critical current density appropriate for low-cost superconducting fusion magnets. In this study, we have mainly focused on the neutron diffraction technique to measure the residual stress in Mg11B2 superconducting wire for the first time. The residual stress state was given qualitative and quantitative interpretation in terms of micro- and macrostress generation mechanisms based on the isotropic model confirmed by neutron texture measurements. The relationship between the stress/strain state in the wire and the transport critical current density is also discussed. This investigation could pave the way to further enhancement of the critical current density of low-activation Mg11B2 superconducting wires suitable for next-generation fusion grade magnets.

Microstructure Refinement in W-Y2O3 Alloy Fabricated by Wet Chemical Method with Surfactant Addition and Subsequent Spark Plasma Sintering.

With the aim of preparing high performance oxide-dispersion-strengthened tungsten based alloys by powder metallurgy, the W-Y2O3 composite nanopowder precursor was fabricated by an improved wet chemical method with anion surfactant sodium dodecyl sulfate (SDS) addition. It is found that the employment of SDS can dramatically decrease W grain size (about 40 nm) and improve the size uniformity. What's more, SDS addition can also remarkably improve the uniform dispersion of Y2O3 particles during the synthesis process. For the alloy whose powder precursor was fabricated by traditional wet chemical method without SDS addition, only a few Y2O3 dispersoids with size of approximate 10-50 nm distribute unevenly within tungsten grains. Nevertheless, for the sintered alloy whose powder precursor was produced by improved wet chemical method, the Y2O3 dispersoids (about 2-10 nm in size) with near spherical shape are dispersed well within tungsten grains. Additionally, compared with the former, the alloy possesses smaller grain size (approximate 700 nm) and higher relative density (99.00%). And a Vickers microhardness value up to 600 Hv was also obtained for this alloy. Based on these results, the employment of SDS in traditional wet chemical method is a feasible way to fabricate high performance yttria-dispersion-strengthened tungsten based alloys.