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Title: Low temperature martensitic phase transformations in type 11 superconductors
Author: Pollock, John Thomas Anthony
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1967
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A review of the literature reveals that no optical metallographic examination of low temperature martensitic transformations in Type 11 superconducting alloys and compounds has previously been carried out. A variable temperature optical stage is described which enables continuous observation, using interference contrast techniques, of surfaces in the temperature range 300° - 6.5°K and measurement of the superconducting transition temperatures 'in situ' by the A.C. inductance method. This optical stage is used to investigate the F.C.C. -> F.C.T. martensitic transformation which occurs on cooling alloys in the indium rich-thallium system. A 'realisation' phase diagram is determined for this system in the temperature range 300° - 9.5°K and indicates that alloys containing more than 31 at.5T1 are F.C.C. at 0°K. There is a marked change in the nucleation and growth charactcistics of the nu.tensite bands at temperatures where M°K/ Tm.p. <0.5 which is interpreted in terms of the effect of the plastic transformation strain on the F.C.C. lattice. The superconducting transition temperaturee of the alloys are measured and the transition breadths and variation of Tc with composition within each phase region, explained by reference to the microstructures of the specimens. The basic effect of Tl concentration on transition temperature is considered to result from the influence on the electron phonon interactions in a manner analogous to the isotope effect observed in pure metals. Polycrystalline specimens of V3Si, Nb3Sn' Nb3A1 and Mo3A1, and single crystals of V3Si and Nb3Sn are not observed to transform martensitically at temperatures down to 6.5°K. However x-ray diffraction line-broadening, indicative of a structural change is observed below 35°K, in powder specimens of V3Si and Nb3Sn, made from the polycrystalline bulk specimens. These results are explained on the basis of nonstoichiometry of the bulk specimens. Lattice parameter measurements for V3 Si and Nb3Sn at 298°, 770 and 18°K indicate that these materials have a negative expansion coefficient at temperatures below about 77°K. This result is the first of its kind reported in a superconductor and is discussed in terms of the anisotropy of the elastic coefficients of materials with the Al5 crystal structure.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available