MECHANICAL BEHAVIOUR OF NANOCRYSTALLINE IRON AND NICKEL IN THE QUASI-STATIC AND LOW FREQUENCY ANELASTIC REGIME E. Bonetti, E.G. Campari, L. Del Bianco, L. Pasquini and E. Sampaolesi Dipartimento di Fisica, Universita ` di Bologna and Istituto Nazionale per la Fisica della Materia, viale Berti-Pichat 6/2, I-40127 Bologna, Italy (Received April 7, 1999) (Accepted July 14, 1999) Abstract—In this research, we made use of mechanical spectroscopy to study the anelastic behaviour of nanocrys- talline Fe and Ni in quasi-static, low-frequency (0.01–10 Hz) regime. The elastic energy dissipation coefficient (Q -1 ) and the stress relaxation have been measured as a function of frequency and temperature, in a range of temperatures where appreciable grain growth is not expected to occur. The use of such low frequency probes puts into evidence a very strong change in the material response, induced by low temperature annealing (T  200 °C). In spite of the fact that the grain size was nearly the same after the annealing treatment, the as-prepared samples displayed much higher Q -1 values and faster rates of stress relaxation than the annealed samples. The anelastic spectrum of annealed specimens has been analysed by a combination of quasi-static and dynamic measurements. The results of this study are discussed in terms of typical activation energies reported for lattice and grain boundary diffusion in coarse-grained polycrystalline and nanocrystalline metals. ©1999 Acta Metallurgica Inc. Introduction In recent years, the mechanical behaviour of metallic nanocrystalline (n-) materials has been the subject of many studies (1,2). The interest on this topic has been partly motivated by the potential use of these materials in a number of structural applications. This potential is based on the expected mechanical property improvements with decreasing grain size deduced from the mechanical behaviour of coarse- grained polycrystalline metallic materials. In fact, by extrapolating the phenomenological Hall-Petch relation, relating the strength to the inverse square root of grain size (3,4), down to nanoscale dimension, significantly enhanced strength and hardness are expected. Moreover, from the phenomenological equations describing diffusional creep, enhanced ductility is also expected (5). In addition, recent theoretical studies, based on molecular dynamics first-principles simulations, have confirmed that in the nanometer scale regime, pure metals display a plasticity component that is dependent on grain boundary viscosity controlled by self-diffusion processes (6). In most of the previous studies on nanocrystalline metals, the mechanical property results were obtained from microhardness measurements, stress-strain curves or from plastic creep studies (1). However, in many cases these results appear to have been strongly influenced by some extrinsic factors, such as nanoporosity, contamination products, and incomplete consolidation. These extrinsic factors, in turn, are strongly dependent on synthesis and processing methods used. These extrinsic factors are certainly a limiting factor for samples obtained from powder processing routes, such as those produced by inert gas condensation or by mechanical attrition. In order to get some valuable insight into the deformation mechanisms of nanocrystalline metals, at the microscopic level, we have employed an alternative approach. This approach, widely employed for Pergamon NanoStructured Materials, Vol. 11, No. 6, pp. 709 –720, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter PII S0965-9773(99)00359-1 709