Mechanical properties of nanocrystalline materials

Hall-Petch relation and boundary strengthening

Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect

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dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid.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

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

Nanocrystalline materials and coatings

High Temperature Aluminides and Intermetallics

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

Binary and ternary L10 type titanium alloys were rapidly quenched into thin foils by the hammer-anvil quenching technique. The nominal alloy compositions include Ti1-xAlx and (Ti1-xAlx)100-@#@ yMy, wherex = 0.5 to 0.6; y= 1 to 4; and M is V, Cr, Mn, Fe, Co, Ni, and Ge. Lattice parameters of Ll0 phases in these alloys were studied by X-ray diffractometer using the foils and their powders. Simultaneously, long-range order parameters of these Ll0 phases in the as-quenched foils and the annealed powders were determined from the superlattice intensity measured by X-ray diffractometer. Rapid quenching suppresses long-range ordering (S) during solidification and, thereby, reduces lattice distortion (c/a) in Ll0 titanium alloys. A small amount of alloying element has a large effect on thec/a ratio. The atomic size effect and probably substitutional site preference play an important role in such ac/a reduction. From the density measurement, it is found that antisite defects are a predominant mode in Al-rich off-stoichiometric TiAl compound alloys. Determined long-range order value is significantly low in as-quenched binary TiAl due to rapid quenching, and the highest ordering occurs at as-quenched Ti45Al55 alloy. The relationship betweenc/a andS in Ll0 Ti45Al55 may be written asc/a = 1 + 0.025 ·S.

Effect of rapid solidification and alloying addition on lattice distortion and atomic ordering in L1 0 tial alloys and their ternary alloys

The six independent elastic constants of a γ -TiAl (Ti 44 Al 56 ) single crystal have been measured for the first time from room temperature to 750 K using a resonant ultrasound spectroscopy technique. The values of C 12 and C 13 are virtually temperature independent, whereas C 11 , C 33 , C 44 , and C 66 decrease between 6 and 9% from 300 to 750 K. The two independent linear thermal expansion coefficients α 1 (along [100] direction) and α 2 (along [001] direction) of single crystal γ -TiAl have been measured from 4 to 750 K using two different apparatus. The elastic constants have been corrected for the effect of thermal expansion, and the corrected C ij have been used to calculate the Young's and shear moduli along various crystal directions. The elastic constants and moduli values have also been extrapolated to 1273 K. Finally, the elastic anisotropy has also been calculated as a function of temperature.

S.H. Whang

Papers

High Temperature Aluminides and Intermetallics

Tensile behavior and fracture in nickel and carbon doped nanocrystalline nickel

Creep behavior of nanocrystalline nickel at 290 and 373 K

Effect of rapid solidification and alloying addition on lattice distortion and atomic ordering in L1 0 tial alloys and their ternary alloys

Elastic constants and thermal expansion of single crystal γ-TiAl from 300 to 750 K