Mechanical properties of nanocrystalline materials

https://material.karlov.mff.cuni.cz/people/janecek/studenti/FyzMetNano/Estrin_AM61_2013_782.pdf

Extreme grain refinement by severe plastic deformation: A wealth of challenging science

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dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid.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

/pdf/nanocrystalline-materials-and-coatings-1pj6y7x45z.pdf

Nanocrystalline materials and coatings

Fatigue life improvement through surface nanostructuring of stainless steel by means of surface mechanical attrition treatment

Fatigue and microstructure of ultrafine-grained metals produced by severe plastic deformation

The fatigue performance of the fine-grain 5056 Al-Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is assessed in both stress- and plastic strain-controlled experiments. Compared to its conventional counterpart, the ECAP material exhibits a high tensile and low-cyclic fatigue strength under constant stress amplitude. However, its fatigue life under strain-controlled conditions is notably shorter than that of the O-temper specimens. Despite severe pre-straining of the specimen during ECA-pressing, cyclic softening was found to be rather small. It is shown that the mechanical characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperatures (150°C, 15 min) after fabrication. Such heat treatment is supposed to recover partially the grain boundary region, which has been most heavily distorted during processing. Mechanisms of fatigue in ECA-processed materials are discussed within a framework of a simple one-parametric model of dislocation kinetics.

Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing

The cyclic response of the fine-grain 5056 Al–Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is investigated in plastic strain controlled experiments with a plastic strain amplitudes e pl ranged from 5×10 −4 to 10 −2 . This material exhibits a fairly high yield stress of 400 MPa, however, its strain controlled properties appear worse than those of the O-temper specimens as is evidenced by the Coffin–Manson plot. The cyclic softening was found to be small with a visible tendency to increase with e pl . It is emphasized that the mechanical properties of the ECAP materials are strongly influenced by their susceptibility to strain localisation. It is shown that the engineering characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperature (150°C, 15 min) after fabrication. This annealing is supposed to recover the most heavily distorted grain boundary region, facilitating the dislocation mobility to carry out the imposed plastic strains.

K. Higashi

Papers

Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing

Cyclic response of fine grain 5056 Al–Mg alloy processed by equal-channel angular pressing