Bulk nanostructured materials from severe plastic deformation

Principles of equal-channel angular pressing as a processing tool for grain refinement

Nanocrystalline solids, in which the grain size is in the nanometre range, often have technologically interesting properties such as increased hardness and ductility. Nanocrystalline metals can be produced in several ways, among the most common of which are high-pressure compaction of nanometre-sized clusters and high-energy ball-milling1,2,3,4. The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress ofthe material typically increase with decreasing grain size, a phenomenon known as the Hall–Petch effect5,6. Here we present computer simulations of the deformation of nanocrystalline copper, which show a softening with grain size (a reverse Hall–Petch effect3,7) for the smallest sizes. Most of the plastic deformation is due to a large number of small ‘sliding’ events of atomic planes at the grain boundaries, with only a minor part being caused by dislocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening will ultimately impose a limit on how strong nanocrystalline metals may become.

Softening of nanocrystalline metals at very small grain sizes

Despite rosy prospects, the use of nanostructured metals and alloys as advanced structural and functional materials has remained controversial until recently. Only in recent years has a breakthrough been outlined in this area, associated both with development of new routes for the fabrication of bulk nanostructured materials and with investigation of the fundamental mechanisms that lead to the new properties of these materials. Although a deep understanding of these mechanisms is still a topic of basic research, pilot commercial products for medicine and microdevices are coming within reach of the market. This progress article discusses new concepts and principles of using severe plastic deformation (SPD) to fabricate bulk nanostructured metals with advanced properties. Special emphasis is laid on the relationship between microstructural features and properties, as well as the first applications of SPD-produced nanomaterials.

Nanostructuring of metals by severe plastic deformation for advanced properties.

The process of grain refinement in equal-channel angular pressing

Mechanical behaviour and structural changes, such as the evolution of grain and dislocation structures and the formation of slip lines and grain-boundary-sliding traces, of a submicron-grained (SMG) copper during room-temperature compression have been studied. It is suggested that the absorption of dislocations into grain boundaries (GBs) is due to the migration and sliding of some highly non-equilibrium GBs during the deformation process and is influenced by high level internal stresses. From this point of view, the unusual behaviour of SMG copper, in particular, the high yielding and flow stresses, the absence of strain hardening, high plasticity and low strain rate sensitivity, are explained. Analogies of the mechanical behaviour of SMG copper with mechanical properties of metallic materials at large plastic strains in stage IV are discussed.

Deformation behaviour of ultra-fine-grained copper

Plastic behaviour and stretchability of sheet metals. Part II: Effect of yield surface shape on sheet forming limit

The static grain growth has been investigated in ultrafine grained copper with an initial grain size of 160 nm. It has been revealed that its kinetics follows to normal grain growth behaviour, but a grain growth starts at a relatively low temperature (0.32 T m ). Good fits with experimental data for several ultrafine grained metals have been obtained if the activation energy for grain boundary diffusion is assumed to be lower than for coarse grained materials, but increases during grain growth. It is suggested that this unusual behaviour of the activation energy is caused by the presence of non-equilibrium grain boundaries in ultrafine grained materials and their recovery during heating.

On the enhanced grain growth in ultrafine grained metals

The Hall-Petch relation in submicro-grained Al-1.5% Mg alloy

A model is proposed in the present paper to account for the mechanical behaviour of nanocrystalline materials. In this model, the distribution of the grain size in nanocrystals is simulated with a logarithmic normal distribution, and one dislocation per grain is assumed. The plastic yielding for nanocrystalline materials is considered to be controlled by the stress required to attain dislocation loops (the Frank-Read source) in a set of larger grains with their critical semicircle configuration. The dislocations in the rest of the smaller grains are considered to be in the subcritical configuration, which produces a reversible deformation and only contributes to an inelastic deformation. The model presents a very good agreement with the σyvs. Dav−1/2 relationships for five nanocrystalline materials; of these, three metals exhibit a negative Hall-Petch slope and two a positive Hall-Petch slope. The model also predicts a decrease in Young's modulus with diminishing grain size, which is in agreement with experimental results for nanocrystalline copper and palladium.

B. Baudelet

Papers

Deformation behaviour of ultra-fine-grained copper

Plastic behaviour and stretchability of sheet metals. Part II: Effect of yield surface shape on sheet forming limit

On the enhanced grain growth in ultrafine grained metals

The Hall-Petch relation in submicro-grained Al-1.5% Mg alloy

Model for the prediction of the mechanical behaviour of nanocrystalline materials