Mechanical properties of nanocrystalline materials

Hall-Petch relation and boundary strengthening

http://nanomechanics.mit.edu/sites/default/files/documents/2007_Acta_nc_Overview.143.pdf

Toward a quantitative understanding of mechanical behavior of nanocrystalline metals

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dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid.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

Deformation of nanocrystalline materials by molecular-dynamics simulation: relationship to experiments?

The potential engineering applications of nanocrystalline materials need more detailed study on deformation and fracture mechanisms at room and elevated temperatures under tensile loading. This paper reports results of a series of experiments carried out on nickel and carbon doped nanocrystalline nickel with different carbon concentrations from 500 to 1000 ppm at room temperature to 300°C. Grain growth was observed in nanocrystalline nickels as the testing temperature increases. A fast grain growth was noticed at 300°C. Pure nanocrystalline nickel experienced an abnormal grain growth at 500°C and its tensile properties reduced to a very low level. The addition of carbon exerted a potential effect to enhance the stability of the microstructure in nanocrystalline nickel at intermediate temperatures. However, carbon doped nickels exhibited lower tensile properties. Nanocrystalline nickels displayed a conventional Hall–Petch relationship. The results are discussed in relation to microstructural characteristics by using TEM and SEM.

Tensile behavior and fracture in nickel and carbon doped nanocrystalline nickel

The uniaxial tensile creep behavior of porosity-free nanocrystalline nickel with 30 nm grains produced by an electrodeposition processing has been investigated under constant and step-load conditions at room temperature and 373 K in a load range 500–1050 MPa. The experimental results showed that significant creep deformation occurred even at room temperature at an initial applied stress of 600 MPa or higher. The creep resistance was very sensitive to test temperature. The grain size and microstructure of the as received and post-creep specimens have been characterized by conventional TEM techniques. An attempt has been made to explain the deformation behavior and creep mechanisms based on current findings.

Creep behavior of nanocrystalline nickel at 290 and 373 K

The fracture resistance curves of nanocrystalline nickel and carbon doped nanocrystalline nickel for different annealing temperatures have been generated and studied. The results indicate that crack growth resistance of pure nanocrystalline nickel is very sensitive to annealing temperatures. The crack growth resistance decreased with increasing annealing temperature for the nanocrystalline nickel. Carbon doping greatly reduces crack growth resistance of nanocrystalline nickel. However, the crack growth resistance of carbon-doped nanocrystalline shows improvement through annealing processing. A cluster model was used to explain the crack growth resistance behavior of nanocrystalline nickel.

R-Curve characterization of the fracture toughness of nanocrystalline nickel thin sheets


 There is an increasing demand for high quality Nitinol medical devices and implants to accommodate to the rapid growth in the biomedical market. Extensive studies have confirmed the significant impact of micro-cleanliness of Nitinol on fatigue life. Ultra-clean Nitinol material is required for most critical medical applications such as cardiovascular and neurovascular devices for which integrity and durability are critical. This poses challenges for upstream manufacturers to consistently produce ultra-low inclusion Nitinol mill products. This work is a comprehensive to evaluate hot rolled bars and coils of a new commercial scale ultra-clean Nitinol alloy. The robustness of the alloy production process and stability of product properties was confirmed by examining a large number of mill products manufactured in different campaigns. Extensive characterization and multiple approaches of inclusion analysis demonstrated the consistent ultra-high cleanliness of the products.

W.M Yin

Papers

Tensile behavior and fracture in nickel and carbon doped nanocrystalline nickel

Creep behavior of nanocrystalline nickel at 290 and 373 K

R-Curve characterization of the fracture toughness of nanocrystalline nickel thin sheets

The Assessment of Physical and Mechanical Property Variability in a New Generation of Low Inclusion NiTi Alloy