Analysis of Internal Friction Peaks in High Purity Molybdenum by a Viscoelastic Procedure Independent of the Relaxation Strength

C. L. MATTEO; O. A. LAMBRI; G. I. ZELADA-LAMBRI; P. A. SORICHETTI; J. A. GARCÍA

Tula

Conferencia; IMPERFECTIONS INTERACTION AND ANELASTIC PHENOMENA IN SOLIDS, IIAPS XI; 2007

Molybdenum, a group VI of transition metal has a melting point of 2883K, a high specific heat, and good corrosion and creep resistance. The melting point of molybdenum is exceeded only by tungsten and tantalum, among the useful high temperature metals. Molybdenum is ductile at room temperature, with a brittle-ductile transition temperature significantly lower than that of tungsten. Molybdenum has also good strength at high temperatures, being lighter than tungsten and tantalum [1, 2]. These qualities make molybdenum attractive for the use in the nuclear industry [3 - 7]. Mechanical spectroscopy, referred also to as internal friction method is a non destructive technique and is a fundamental tool for studying the movement of dislocations and their interaction with point defects [8, 9]. It involves the simultaneous measurement of the damping or internal friction, Q-1, and the elastic modulus as a function of temperature. We have reported recently that molybdenum exhibits a damping peak at about 840K - 1050K, which is developed in deformed samples after annealing at temperatures above that of the vacancy migration [10, 11]. The intensity of the damping peak depended on the degree of plastic deformation at room temperature, but it was not affected by a bias stress. Moreover, the peak temperature and activation energy of this relaxation process increased with the temperature of the previous annealing of the sample; and it was independent of the crystal orientation. For instance, the activation energy increases from 1.6eV for peak temperature at around 840K to 2.7eV for a peak at 1000K. Besides, the shape of the peak when it appears at temperatures around 1000K is markedly asymmetrical. In addition, it has been proposed that the vacancy-dislocation interaction mechanism is controlling this peak [10, 11]. The Modified Relaxation Time (MRT) function, derived from a general linear viscoelastic formalism, and its applications are a very useful tool to discriminate if more than one relaxation process is occurring overlapped. In fact, it is well known that relaxation processes involving dislocations and non-dilute concentration of point defects originate damping peaks that are wider in comparison with the Debye model. Also, in most cases the observed peaks are asymmetrical. The MRT function clearly describes the breadth and symmetry characteristics of relaxation processes, as it will be later shown. In this work, the MRT is applied to the study of the relaxation damping peaks at high temperatures in deformed molybdenum. The dependence of experimental data from these relaxation processes with temperature are adequately described by a Havriliak-Negami (HN) function, and the MRT makes possible to find a relation between the parameters of the HN function and the activation energy of the process. The analysis allows us to relate the relaxation peak appearing at temperatures below 900K, to a physical mechanism involving vacancy-diffusion-controlled movement of dislocations. In contrast, when the peak appears at temperatures higher than 900K, the damping is controlled by a coupled mechanism of diffusion and creation plus diffusion of vacancies in the dislocation line.