Mechanical Spectroscopy of Nanocrystalline Metals
- PDF / 395,999 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 14 Downloads / 225 Views
Mechanical Spectroscopy of Nanocrystalline Metals E. Bonetti, L. Pasquini, L. Savini Department of Physics, University of Bologna and INFM v. Berti-Pichat 6/2 40127 Bologna, Italy ABSTRACT The mechanical behavior of nanocrystalline iron and nickel prepared by mechanical attrition and inert gas condensation was investigated using mechanical spectroscopy techniques in the quasi static-and low frequency dynamic stress-strain regimes. The measures were performed on samples previously stabilized by thermal annealing at low homologous temperatures. The results of elastic energy dissipation, creep, and creep recovery measurements performed in the low strain regime (ε = 10-5-10-3) allowed to trace a phenomenological picture of the anelastic and viscoplastic behavior of nanocrystalline Ni and Fe in the 300-450 K range with different grain sizes and interfaces disorder degree. Activation energies of the thermally activated anelastic and plastic mechanisms responsible for the mechanical behavior have been evaluated. INTRODUCTION In the study of the mechanical properties of metals an important role is attributed to the grain size (d), which can play a hardening or softening role [1]. As is well known since many years, in coarse grained materials, a grain size decrease down to the micrometer range is accompanied by a hardness increase, phenomenologically described according to the well known Hall-Petch relation, predicting a d -½ dependence in the hardness or flow stress. Further, fine grained materials may exhibit diffusional creep or in some cases superplastic behavior at sufficiently high homologous temperatures, due to the increased volume fraction of the disordered interfaces, acting as a short circuit path for diffusion [2]. Entering the nanometer regime (10-102 nm) it is expected, as observed for other physical properties, that the approach of microstructural features, such as the grain size, to some physical length scale, may result in size effects on plasticity and creep behavior [2,3,4]. Recent theoretical predictions obtained by computer simulations [5,6] indicate that dislocation dynamics is strongly reduced below a critical grain size of about 10 nm. The volume fraction of intercrystalline regions increases considerably in nanocrystalline (n-) metals and therefore it is expected that the overall mechanical behavior, in particular the ductility and grain boundary sliding and migration, are modified compared to the coarse grained counterparts. Moreover also the structure of the interfaces in the n-regime may be different and thus modify the mechanical properties, as generally observed in polycrystalline metals where the grain boundary degree of order strongly affects the grain boundary anelastic relaxation. In n-metals the detailed structural configuration of the interfaces is still a matter of debate. Up to now, experimental and theoretical works have not yet provided a unified picture of the nature and degree of order of the interfaces [7,8,9]. In experimental investigations of the mechanical behavior of n-metals th
Data Loading...
