Dislocation Models for Strengthening in Nanostructured Metallic Multilayers
- PDF / 540,825 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 20 Downloads / 208 Views
Dislocation Models for Strengthening in Nanostructured Metallic Multilayers A. Misra, J. P. Hirth, H. Kung, R. G. Hoagland and J. D. Embury, MST Division, Los Alamos National Laboratory, MS K765, Los Alamos, NM 87545 ABSTRACT Ultra-high strength metallic multilayers are ideal for investigating the effects of length scales in plastic deformation of metallic materials. Experiments on model systems show that the strengths of these materials increase with decreasing bilayer period following the Hall-Petch model. However, as the layer thickness is reduced to the nm-scale, the number of dislocations in the pile-up approaches one and the pile-up based Hall-Petch model ceases to apply. For nm-scale semi-coherent multilayers, we hypothesize that plastic flow occurs by the motion of single dislocation loops, initially in the softer layer, that deposit misfit type dislocation arrays at the interface and transfer load to the harder phase. The stress concentration eventually leads to slip in the harder phase, overcoming the resistance from the misfit arrays at the interface. A model is developed within the framework of classical dislocation theory to estimate the strengthening from this mechanism. The model predictions are compared with experimentally measured strengths. INTRODUCTION Metal-metal composites, synthesized by co-deformation, electroplating or vapor deposition, possess strengths that approach 1/2 to 1/3 of the theoretical limit when the microstructural length scales are on the order of a few nanometers [1,2]. In some cases, the maximum strength of these composites may be an order of magnitude higher than the strength of the soft constituent phases. A fundamental understanding of the deformation mechanisms at nm-length scales is needed to allow optimum microstructural design of these nanostructured materials for desired applications, such as structural components in microelectromechanical systems (MEMS). For single-phase metals, the increase in yield strength (σ) with decreasing grain size (d) is interpreted by means of the Hall-Petch (H-P) model based on dislocation pile-ups: -1/2 σ = σ0 + kd (1) where σ0 represents the lattice friction stress and k (H-P slope) indicates the relative hardening contribution from grain boundaries. This model is also applicable if hardness data is used instead of σ. For lamellar composites, the increase in yield strength with decreasing interphase boundary spacing (h) is also described by the H-P model for the case where h replaces d in eq. (1). Several recent experimental [1,3] and theoretical [4-6] studies have, however, shown that the H-P model may break down at nm-length scales. At µm-length scales, dislocation pile-ups can be treated as a continuum distribution and eq. (1) holds. At the nm-length scales, pile-ups are discrete and the exponent in eq. (1) departs from 0.5. The assumption that the number of dislocations in the pileup (N) decreases in proportion with decreasing h provides one way to interpret the dependence of strength on h at nm-length scales [4,5]. In this approach,
Data Loading...
