Interface controlled plasticity in metals: dispersion hardening and thin film deformation
Introduction
Plastic deformation and strengthening of metals is a classic subject of materials science, which has been strongly influenced by the pioneering ideas and insights of Michael F. Ashby. Many of his contributions have been invaluable for our own research over the last two decades; in particular his seminal papers on dispersion strengthening [1], [2], [3], [4], [5], deformation mechanisms [6], [7], [8], [9], and diffusional creep [10], [11], [12] provide an appropriate starting point for this paper. It is a particular pleasure to contribute, on the occasion of this conference in his honor, an overview of some recent results in this field.
Plastic deformation of crystalline solids is, at low temperatures, predominantly due to the motion of lattice dislocations (“glide”). At high temperatures, stress-driven diffusion adds to the deformation, either by aiding dislocation motion (“dislocation” or “power-law creep”) or by creating strain in its own right at grain boundaries (“diffusional creep”). The stress-temperature regimes in which these mechanisms dominate are conveniently displayed in deformation mechanism maps pioneered by Frost and Ashby [7]. Efficient strengthening strategies must address the dominant deformation mechanism, e.g. through pinning of lattice dislocations by particles in the dislocation creep regime or through grain size control in the diffusional creep field. The deformation of strengthened alloys is then largely controlled by “microstructural constraints” on dislocation or vacancy motion. A case in point is dispersion strengthening, by which high strength can be imparted to an otherwise soft metallic matrix.
When small metal volumes, such as thin films with typical thicknesses around 1 μm and below, are deformed, the deformation mechanisms are subject to size effects (for a recent review see [13]): dislocations may now be pinned by the interfaces to the adjacent materials and diffusional strains are limited by substrate effects. Compared to bulk materials, plastic deformation in thin films is therefore subject to a “dimensional constraint”, which can lead to high mechanical stresses and consequently affects the reliability of small-scale devices. The technological relevance of the topic of thin film strengthening has triggered a large amount of research work over the last decade. Although some of these phenomena have, at least to a first order, been clarified, many problems still persists, and in this new context the study of plasticity and strengthening is still very much at the heart of present-day materials research.
In this paper we first retrace the subject of dispersion hardening (Section 2), which has received much renewed attention over the last two decades. The concept of high-temperature creep controlled by dislocation detachment from dispersoid particles is reviewed; emphasis is placed on ordered intermetallic matrix materials in which cooperative dislocation mechanisms have been observed and modelled. Second, we will describe the current understanding of plastic deformation in thin films (Section 3). We present the results of recent insitu transmission electron microscopy (TEM) observations of dislocation activity and discuss them in the light of attempts to model dislocation-mediated and diffusional deformation mechanisms in small material volumes. The commonality of these two subject areas lies in the overriding influence of interfaces: dislocations interact with the particle/matrix interface, which controls the creep strength of most dispersion-strengthened alloys; and their interaction with the film/substrate interface, which also affects diffusional creep processes, determines the extent to which stresses can be relaxed in thin film systems. Both subjects are of great practical importance and will profit further from a deeper understanding of interface effects. For a more extensive treatment than is possible here the reader is referred to specific review papers (e.g. [14], [15], [16]).
Section snippets
Background — dislocation bowing and threshold stresses
Dispersion strengthening is an efficient mechanism for improving the hardness of metallic materials. Egon Orowan [17] was first to attribute this effect to the resistance of lattice dislocations against bowing between non-shearable obstacles. His expression for the resulting critical shear stress, later judiciously refined by Ashby [1], is given in its simplest form by the following equation:where Td is the line tension and b the Burgers vector of the dislocation, L the spacing
Thin film plasticity — some aspects of interface control in small-scale systems
In modern applications ranging from micro-electronic and micro-mechanical components to X-ray mirrors and corrosion protection, materials are increasingly used in thin-film form. There, the lattice defects may begin to “feel” the presence of the surface or an interface; as a result, a dimensional constraint can appear which superimposes on the microstructural constraint discussed above. With ever-continuing miniaturization, an understanding of these effects will be of increasing relevance, both
Concluding remarks
Small is strong — this, in a nutshell, is an essential lesson from the study of metal plasticity. It applies, within certain limits, to microstructural constraints on dislocation motion, which is most effectively impeded by small obstacles. In the case of dispersion strengthening considered here, an optimum in particle size was predicted, which however comes to lie at very small values. The lesson is also true — as we have shown — for artificially patterned materials, e.g. thin films, whose
Acknowledgements
The authors would like to thank H. Gao for fruitful discussions.
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