Magnetic Susceptibility Studies of the Influences of Thermomechanical Processing and Mn Addition on Phase Stability and Precipitation in NbTi Alloys
The DC magnetic susceptibilities, χ, of series of NbTi and NbTiMn(0.5 at. %) alloys were measured using the Faraday (Curie) balance technique over the temperature range 77–300 K. The samples were measured in the conditions: (i) β-quenched (β-Q), (ii) cold worked (CW), and (iii) CW plus precipitation-heat-treated (CW+PHT). It was found that: (1) For the β-Q samples dχ/dT, which was positive within 77–300 K, increased as the Nb concentration decreased — effects that are attributable to the influences of temperature and Nb concentration, [Nb], on reversible ω-phase formation. (2) By the same token, due to its β-stabilizing effect, the addition of 0.5% Mn increased χ at 77 K. (3) The susceptibility results also showed evidence for the removal of ω phase by cold work. (4) As a result of α-phase precipitation, heat treatment substantially reduced the x of the CW+PHT samples below those of both the β-Q- and CW alloys. The [Nb] dependence of α-phase abundance, f α , was deduced from the χ data at RT where the ω-phase contribution was negligible; by comparing the [Nb] dependence of f α for the NbTi and NbTiMn alloys the β-stabilizing potency of Mn was estimated. Finally, the absence of any measurable Curie-Weiss component in the χα-phase(T) of a NbTiMn(0.5 at. %) alloy after CW+PHT indicated that the Mn content of the α-phase precipitate was negligibly small, i.e. less than about 250 ppm.
KeywordsMagnetic Susceptibility Cold Work Composition Dependence Niobium Concentration Molar Susceptibility
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