Thermodynamic and structural properties of zinc/copper alloys

Downie, David B (1963) Thermodynamic and structural properties of zinc/copper alloys. PhD thesis, University of Glasgow.

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The thesis, entitled as above, is concerned with the measurement of thermodynamic properties of zinc copper alloys in the liquid and solid states, and with the interpretation of these in terms of alloy structure. A conventional dew-point technique was used to determine, initially, the vapour pressure of the zinc component at a number of temperatures and, from these values, partial and integral thermodynamic functions were calculated. For evaluation of the vapour pressures, the equations of Kubaschewski and Evans, relating the vapour pressure of pure zinc, in solid and liquid otates,to temperature were used as a basis. The results of other workers were recalculated, wherever possible, to the same basis for comparison. In general, the present results show fair agreement with previous work but with certain differences. For liquid alloys, for instance, a change from negative to positive deviation +0. from ideality is shown by the thermodynamic activity of the zinc as high zinc contents are approached. Previously, the results of other workers have indicated negative deviation over the whole compositional range. Also, characteristic minima and maxima were obtained in the curves when Deltas-x zn values from the present work on liquid and solid alloys were plotted against composition. One of these minima, viz. that occurring at 0.43 NZn in the solid alloys, is considered difficult to explain and probably spurious. Otherwise, the minima occur at compositions corresponding approximately to the centre of the beta and r-phase fields and are associated with the critical electron/atom ratios (1.48 and 1.615 respectively) required, by electron theory for the formation of the appropriate electron compounds. The peak positive value (i.e. highest maximum) in the DeltaS-x zn curve for liquid alloys, which occurs at 0.68 Nzn, has analogies in at least two other binary systems, viz. Zn/Sb and Cd/Sb. This positive value of excess entropy is considered to be associated with an increased vibrational contribution to entropy, and linked with the previously reported defect structure of the delta-phase. Quantitative application of quasi-chemical theory to the partial and integral thermodynamic functions was attempted but, after comparison with other systems, reasons are advanced for the limited applicability. However, qualitative application, particularly when integral properties are considered, is possible. The partial thermodynamic functions of the second component, i.e. copper, were calculated from the Gibbs-Duhem relationship, and thus integral properties obtained. For the liquid alloys, both of the partial and the integral functions are compared and the conclusion drawn that, in certain compositional regions there is build up of "excess" enthalpy (stored probably as vibrational energy) which is released again to allow the "minimum" value to he attained at fixed compositions. Two of these fixed compositions co-incide approximately with the critical electron/atom ratios necessary, according to electron theory, for the formation of the beta and r-phases in solid alloys, but it is postulated that the true "critical" compositions are those corresponding to the maximum build-up of excess enthalpy. In addition, the variation of the partial properties in the region of those critical compositions suggests that the release of the excess enthalpy is accompanied by a redistribution of the vibrational energy between the two components of the solution. Because of the close approximation of the compositions, at which the minimal values occur, to the centres of the solid beta, -r and -phase fields, a fourth minimum, corresponding to the unexplored delta-phase field region is predicted. Also, the existence of these minima, in plots referring to liquid alloys, is taken as an indication of the extent to which "solid" structures persist above liquidus temperatures. The above postulations and the link with electron theory are less clearly applicable to the solid alloys, but sufficient similarity exists to suggest that complete analogy may be established by further study.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: E C Ellwood
Keywords: Materials science, Thermodynamics
Date of Award: 1963
Depositing User: Enlighten Team
Unique ID: glathesis:1963-73882
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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