Thermoelectroc Power for Phase Transformation Study in Metallic Alloys

Abstract

Recent experiments have proved the efficiency of T.E.P. measurements in phase transformation studies. The main features of the technique are presented and two particular examples of application are given (influence of plastic deformation on short range ordering (S.R.O.) and study of dissolution of the equilibrium 0 phase in Al-Cu alloys).Since the first studies [1] only very few attempts have been made to use thermoelectrical power measurements (T.E.P.) to characterize the evolution of an alloy during phase transformation. However, in spite of the complexity of the theoretical interpretations, T.E.P. measurements can give valuable informations on the microstructure of the material, sometimes with more easiness or more precision than with other techniques or even informations which would be impossible to obtain by another way.EXPERIMENTAL METHOD AND MAIN FEATURES OF T.E.P. Fig. 1 gives the principle of T.E.P. measurements. Two metallic reference blocks a and b with a good thermal conductivity (Al or Cu) are respectively maintained at temperatures T and T + AT. The sample is pressed on these blocks so as to ensure a good thermal and electrical contact. The junction with a manovoltmeter is realized with wires of the same metal as the reference blocks. If the voltage measured is Av, the relative thermoelectric power LS of the sample respective to the metal constitutive of the reference blocks is AS AV (1) AT and S = S + AS (2) s r if S and S are the absolute thermopower of the reference metal and of the sampfe. s The absolute thermopower depends on temperature [2]. Owing to this influence, the smallest temperature gradient AT compatible with a sufficient accuracy onAv is necessary to obtain a value of AS representative of the isothermal value at T T AT/2. In our case present measurements have been performed at a mean temperature of 20 0 C with AT = 10 0 C. For the study of phase transformations it is necessary to know the influence of some microstructural parameters on T.E.P. a) Influence of elements in solid solution : elements in solid solution modify the electric properties of the alloy and so the T.E.P. The contribution of elements in solid solution can be expressed at high temperature (when the diffuslonal component Sd of thermopower is predominant [2])) and under some simplifying assumptions L3] by p AS = p (s-s°) = a c. C. (s.-S) (3) 0 j J1 with P = P + Z i j. C. (4) 0 3 J J Mat. Res. Soc. Symp. Proc. Vol. 21 (1984) Q Elsevier Science Publishing Co., Inc.