ELSEVIER Materials Science and Engineering A191 (1995) 85-89 Analogy between creep cycles and stress relaxation series for activation volume measurement A. Orlovtia, J. Bonnevilleb, P. Sp2tigb aInstitute of Physical Metallurgy, Academy of Sciences of the Czech Republic, 616 62 Bmo, Czech Republic ‘Ecole Poly technique Fid&-ale de Lausanne, D.P.-I.G.A., CH 101.5 Lausanne, Switzerland Received 2 1 April 1994 Abstract Two techniques that are commonly used to measure the activation volume in the course of constant strain rate deformation experi- ments consists of performing either strain rate jump or stress relaxation tests. In this paper, a new method based on repeated creep tests is presented. It is demonstrated that, as for the technique of stress relaxation series, this new procedure allows the measurement of the apparent activation volume and the corresponding strain hardening correction term, which yield the effective activation volume. Only this last parameter is relevant for characterizing the thermally activated processes that control plastic flow. Keywords: Creep; Stress; Activation volume 1. Introduction The description of plastic flow of metals in terms of thermally activated mechanisms requires the use of appropriate experimental techniques that allow rigo- rous measurement of various thermodynamic vari- ables, such as the activation energy and activation volume. This second parameter is an important quan- tity, since it characterizes the stress dependence of the dislocation mobility, and it has a typical value and stress dependence for each atomic process. Unfortu- nately, direct measurements of dislocation velocities are restricted to very few metals and considerable effort has been devoted to developing indirect measurement techniques. The two indirect techniques, which are commonly used during tensile or compression tests, are the strain rate change and stress relaxation experiments. For both indirect techniques, it can be shown, in the framework of the thermal activation theory, that an activation volume I/” is measured. However, I/” only represents an apparent activation volume and it must be corrected to obtain the ‘true’ or ‘effective’ activation volume Pff, by a correction term that accounts for strain hardening and machine stiffness [l-.5]. Recently, Spgtig et al. [l] developed a new method for measuring I/‘” during constant strain rate deforma- tion tests. This method consists of a series of stress relaxations with a constant duration; these are per- formed at a given applied stress level t, of the stress-strain curve (z,, yJ. The basic idea of perform- ing repeated stress relaxations to measure veff was originally suggested by Kubin [2]. In the method of Kubin, the stress relaxation series is performed at a constant stress drop. Spgtig et al. [l] have shown that both techniques lead to similar results, but that the new method yields the following: (i) a more accurate Pff value, since it can be deter- mined from a single series, while the Kubin’s technique requires at least two tests to be per- formed on different specimens; (ii) an experiment with a shorter time duration and which is less dependent on the temperature stability of the deformation device. The two previous techniques [1,2] that have been proposed to measure the strain hardening correction term I/h are based on stress relaxation tests. A stress relaxation test represents a deviation from a structural state (t,, ya) that was formed under constant strain rate 0921-5093/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSLII 0921-5093(94)09630-F