Mechanical properties of nanocrystalline materials

Grain refinement is known to lead to improvements in strength and wear resistance. Inherent processing involved in grain refinement alter both the bulk and the surface of a material, leading to changes in grain boundary density, orientation, and residual stress. Ultimately, these surface changes can have an impact on electrochemical behavior and, consequently, corrosion susceptibility as evidenced by the large number of studies on the effect of grain size on corrosion, which span a range of materials and test environments. However, there has been limited work on developing a fundamental understanding of how grain refinement and more generally how grain size affects the corrosion resistance of an alloy. Existing literature is often contradictory, even within the same alloy class, and a coherent understanding of how grain size influences corrosion response is largely lacking. A survey of previous work related to the relationship between grain size and corrosion resistance for a number of light meta...

Effect of Grain Size on Corrosion: A Review

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respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid.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

The effect of solid solution W additions on the mechanical properties of nanocrystalline Ni

The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Effect of grain size on mechanical properties of nanocrystalline materials

Nanocrystalline materials, having a crystal size less than ~10 rim, have been shown to possess unusual properties [1-3]. These properties are primarily the result of a substantial fraction of the atoms (20 50%) lying in intercrystalline regions [3]. On the basis of x-ray scattering [4], EXAFS [5], hydrogen solubility [6], small angle neutron scattering [7], and self diffusivity measurements [8] conducted on bulk nanocrystalline materials, it has been suggested [3] that the grain boundaries in these materials are more disordered than those in conventional polycrystals. Recent studies[9,10] involving direct observation of nanocrystalline interfaces by HREM, provide contradictory results. Wunderlich and co-workers[9] have shown that interfaces in nanocrystalline Pd show an 'extended' structure not typically observed in conventional systems. However, Thomas et al [10] observed that the interfacial structure of nanocrystalline Pd is consistent with that typically observed in coarse-grained materials.

S.J. Thorpe

Papers

On the contribution of triple junctions to the structure and properties of nanocrystalline materials