In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

Twinnability is the property describing the ease with which a metal plastically deforms by twinning relative to deforming by dislocation-mediated slip. In this paper a theoretical measure for twinnability in face-centered-cubic (fcc) metals is obtained through homogenization of a recently introduced criterion for deformation twinning (DT) at a crack tip in a single crystal. The DT criterion quantifies the competition between slip and twinning at the crack tip as a function of crack orientation and applied loading. The twinnability of bulk material is obtained by constructing a representative volume element of the material as a polycrystal containing a distribution of microcracks and integrating the DT criterion over all possible grain and microcrack orientations. The resulting integral expression depends weakly on Poisson's ratio and significantly on three interfacial energies: the stacking-fault energy, the unstable-stacking energy and the unstable-twinning energy. All these four quantities can be computed from first principles. The weak dependence on Poisson's ratio is exploited to derive a simple and accurate closed-form approximation for twinnability which clarifies its dependence on the remaining material parameters. To validate the new measure, the twinnability of eight pure fcc metals is computed using parameters obtained from quantum-mechanical tight-binding calculations. The ranking of these materials according to their theoretical twinnability agrees with the available experimental evidence, including the low incidence of DT in Al, and predicts that Pd should twin as easily as Cu.

A first-principles measure for the twinnability of FCC metals

Discontinuous reactions are solid state moving boundary phase transitions characterised by a discontinuous or abrupt change in orientation and composition between the matrix phases in the reactant and product aggregate across the migrating boundary or reaction front that provides a short circuit path of diffusion. The reactions include discontinuous precipitation, discontinuous coarsening, discontinuous dissolution, and diffusion induced grain boundary migration. All these reactions may account for a substantial change in microstructure, composition, and material properties, and hence, deserve adequate scientific attention for a better understanding. The present review provides a comprehensive discussion on the current status of understanding about nucleation and growth mechanisms, genesis and driving force, product morphology and distribution, kinetic growth models, and related experimental techniques, and above all, the unresolved questions concerning these discontinuous reactions. In addition, ...

https://www.skpabi.com/documents/skp%20publications/67%29%20Internat%20Met%20Rev%20%282000%29.pdf

Discontinuous reactions in solids

We calculate the material properties needed to evaluate the tendency of a face-centered-cubic (fcc) metal to plastically deform by forming crystallographic twins as opposed to dislocation-mediated slip. We refer to this property as the twinnability of the metal. We use a formulation for twinnability derived from a coupling of continuum mechanics with an atomistic stress-slip relation. The essential quantities for evaluating the twinnability are elastic constants, which are measurable experimentally, and energies for various stacking sequences of the fcc (111) planes. These stacking sequences include the intrinsic stacking fault configuration as well as the unstable-stacking energy and unstable-twinning energy configurations which can only be determined computationally. We use a tight-binding model to evaluate the necessary stacking energies, as well as the extrinsic stacking fault energy and twin-boundary energy, for eight fcc metals. The accuracy of the tight-binding parameters is established by comparing them with first-principles values obtained through an extensive study of the literature. The results of the literature survey are included in the paper as a resource for the reader. We show that the ranking of these metals in order of twinnability agrees with available experimental results. We reproduce the low incidence of deformation twinning in Al, and explain it in terms of the material parameters using an approximation to the twinnability expression. We also predict that Pd, which has not been studied experimentally, should twin as easily as Cu.

Tight-binding calculations of stacking energies and twinnability in fcc metals

This paper investigates the behavior of carbon fibre reinforced plastics (CFRP) strengthened butt-welded very high strength (VHS) circular steel tubes. The VHS steel has a yield stress of 1350 MPa and an ultimate strength of 1500 MPa. Three types of epoxy resins with different lap shear strength were used. Tests were conducted to determine the lap shear strength between CFRP and VHS steel tubes. A total of 21 butt-welded VHS tubes strengthened with CFRP were tested in axial tension. Three kinds of failure modes, i.e. adhesive failure, fiber tear and mixed failure were observed. The suitable epoxy adhesive for strengthening VHS tubes was recommended. A significant strength increase was achieved using CFRP–epoxy strengthening technique. A theoretical model was developed to estimate the load carrying capacity of butt-welded VHS tubes strengthened using CFRP.

P. R. Howell

Papers

The Pearlite Reaction in Steels Mechanisms and Crystallography: Part I. From H. C. Sorby to R. F. Mehl

The application of metallographic techniques to the study of the tempering of HSLA-100 steel

On the origin of deformation twinning in electrodeposits and the phenomenon of cross-twinning

On the origin and nature of microcrystalline regions in an AlLiCuZr alloy

Two degrees of long-range order in the structures of the decagonal phase Al65Cu20Co15