Influence of Manganese on the Upper Critical Field of Two Series of β-Quenched- and Precipitation-Heat-Treated NbTi Alloys
The superconducting transition temperature, Tc, and the 4.2-K upper critical field, Hc2, of a series of NbTi alloys both without and with the addition of about 0.5 at. % Mn has been measured resistively and inductively. The samples were in the form of: (1) β-quenched starting ingots and (2) precipitation heat treated (PHT) Cu-matrix mono-filamentary strands. It was noted that: (1) Although the addition of about 0.5 wt. % Mn produced no significant change in the Hc2 and Tc of the homogeneous β-quenched single-phase NbTi series it significantly modified those of the PHT set. The latter result is interpreted in terms of the α-phase-suppressing (i.e. β-stabilizing) propensity of Mn which turns out to be six times stronger than that of Nb. (2) The inductively measured superconducting-transition widths, ΔTc, of the PHT specimens were considerably larger than those of the β-quenched (single-phase) samples as a result of PHT-induced compositional inhomogeneity. (3) An anomalous composition dependence of Tc and ΔTc exhibited by the higher concentration PHT ternary alloys was interpreted in terms of proximity effect associated with a Mn-induced refinement of α-phase particle size.
KeywordsTernary Alloy Critical Field Superconducting Transition Temperature Niobium Concentration Electronic Specific Heat Coefficient
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