Volumen II - SAM

Congreso SAM/CONAMET 2009 Buenos Aires, 19 al 23 de Octubre de 2009
BACKGROUND SUBTRACTION METHOD FOR RELAXATION SPECTRA
C. L. Matteo (1,2) , G. I. Zelada-Lambri (3) , P. A. Sorichetti (1) , P. B. Bozzano (4) ,
O. A. Lambri (2,3) and J. A. García (5)
(1) Departamento de Física, Facultad de Ingeniería,
Universidad de Buenos Aires, Avda. Paseo Colón 850, 1063 Buenos Aires, Argentina.
(2) Member of the CONICET’s Research Staff, Argentina.
(3) Instituto de Física Rosario, Facultad de Ciencias Exactas, Ingeniería y Agrimensura, Laboratorio de Materiales,
Escuela de Ing. Eléctrica,
Universidad Nacional de Rosario, Avda. Pellegrini 250, (2000) Rosario, Argentina.
(4) Laboratorio de Microscopia Electrónica, Unidad de Actividad Materiales, Centro Atómico Constituyentes,
Comisión Nacional de Energía Atómica, Avenida General Paz 1499, 1650 San Martín, Argentina.
(5) Departamento de Física Aplicada II, Facultad de Ciencias y Tecnología,
Universidad del País Vasco, Apdo. 644, 48080 Bilbao, País Vasco, Spain.
E-mail (autor de contacto): cmatteo@fi.uba.ar
ABSTRACT
Mechanical spectroscopy is a non-destructive technique which is a very useful method for studying the
movement of dislocations and their interaction with point defects. However, spectra obtained at higher
temperatures often show a noticeable background that obscure the interpretation of viscoelastic parameters.
In this work a systematic procedure for background subtraction is presented, based on the mathematical
properties of the complex elastic modulus. As an application, internal friction peaks of irradiated and nonirradiated
molybdenum were studied in a wide range of temperatures. Molybdenum, a group VI transition
metal has a melting point of 2883 K, high specific heat, and good corrosion and creep resistance. Also, it 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, making it attractive for use in the nuclear industry.
The only hypotheses about the background are that its general shape corresponds to the low temperature
side of a relaxation peak and that it changes very little during measurements at different temperature cycles.
Experimental results validate very satisfactorily the background subtraction method and the hypothesis on
which it is based. The new procedure is very important for the study of the relaxation processes in refractory
metals, but it can be also suitable to other kinds of materials and relaxation processes.
Keywords: mechanical spectroscopy, background subtraction, molybdenum, relaxation spectra.
1. INTRODUCTION
Measurement of viscoelastic properties at high temperatures by mechanical spectroscopy is a useful
technique for studying the relaxation of lattice imperfections in solids, particularly the movement of
dislocations and their interaction with point defects. However, at higher temperatures spectra show a
noticeable background, due to different physical causes, that obscures the interpretation of viscoelastic
parameters. Since the analysis requires background-free data, several of ad-hoc procedures for background
subtraction have been used by different authors, and are even included in commercial software packages.
However, these procedures are often applied without a careful analysis of their validity and therefore may
give unsatisfactory results at higher background levels.
In this work, a systematic approach to background subtraction is presented, based on the properties of the
complex modulus function and two reasonable physical hypotesis: first, that the general shape of the
background corresponds to the low temperature side of a relaxation peak and second, that it changes very
little between different measurements (at the same temperature). Following these general assumptions,
measured values of the complex elastic modulus are numerically fitted to the sum of two relaxation
processes described by the Havriliak-Negami (HN) function. One of the HN peaks describes the relaxation
of the lattice imperfection and the other models the background.
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