Mechanical properties of nanocrystalline materials

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

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

Two of the key parameters that are useful for understanding the deformation kinetics in metals with ultrafine grain (UFG) and/or nanocrystalline (nc) microstructure are the strain rate sensitivity (SRS) of the flow stress and the flow stress activation volume. In this paper, we present experimental results from our in-house tests on UFG Cu, Fe and Ta, along with those available in literature, regarding the behavior of SRS and activation volume of UFG/nc metals with face-centered cubic (fcc) and body-centered cubic (bcc) structures. The UFG metals were produced via severe plastic deformation (SPD), and confirmed using transmission electron microscopy. Our own results, as well as those in the literature, indicate that the SRS of the UFG/nc fcc metals is elevated, whereas that of UFG/nc bcc metals is much reduced compared to their coarse-grained counterparts. These trends appear to hold independent of the processing routes used to refine the grain size and of the testing method employed. The activation volumes in UFG fcc and bcc metals also exhibit different behavior. We present discussions to explain the observed trends. The implications of these findings for plastic instability are also discussed for the two lattice structures.

Effect of nanocrystalline and ultrafine grain sizes on the strain rate sensitivity and activation volume: fcc versus bcc metals

IN view of the interest attaching to the vaporisation and diffusion of solids, the following observations may be worthy of record.

The Diffusion of Solids

Mechanical behavior of high-density nanocrystalline gold prepared by gas deposition method

Self-diffusion in high-density nanocrystalline Fe

For melt-spun amorphous $(a\ensuremath{-}){\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ crystallization under electropulsing was studied by means of the discharge of a condenser with initial current density ${i}_{d0}$ of the order of ${10}^{9}{\mathrm{A}/\mathrm{m}}^{2}$ and decay time \ensuremath{\tau} in between 2 and 0.1 ms, where a specimen was sandwiched by AlN/BN substrates to minimize an effect of joule heating. The crystallization proceeds during electropulsing when ${i}_{d0}$ is higher than the threshold ${i}_{d0,c}.$ Here ${i}_{d0,c}$ is a function of \ensuremath{\tau} and shows a minimum value of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{A}/\mathrm{m}}^{2}$ at $\ensuremath{\tau}\ensuremath{\sim}2\mathrm{ms}$ for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and $2.6\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{A}/\mathrm{m}}^{2}$ at $\ensuremath{\tau}\ensuremath{\sim}0.9\mathrm{ms}$ for $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ where the maximum increase in temperature during electropulsing is about 120 K for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and 50 K for $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ respectively. One-half of the specimen volume crystallizes after a few repetitions of electropulsing with ${i}_{d0}$ beyond ${i}_{d0,c}$ for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and after several repetitions for $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20}.$ We surmise that for the density fluctuations existing in amorphous alloys, under electropulsing a high-density region undergoes a resonant collective motion as a whole, which induces migrational motions of atoms in the low-density matrix around it. For $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ it is observed that an unknown phase was formed in the early stage of the crystallization under electropulsing and disappeared after further electropulsing. It is also found for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20}$ that for electropulsing with high ${i}_{d0},$ the electrical resistivity of a specimen decreased at the early stage of the crystallization and then turned to increase for further electropulsing. These phenomena may be associated with changes in the thermodynamic free energy of phases under an electric current predicted by the theoretical works. We surmise that present electropulsing excites a resonant collective motion of many atoms and modifies the thermodynamic free energy of phases too.

/pdf/crystallization-under-electropulsing-suggesting-a-resonant-zcpa05oxgb.pdf

Crystallization under electropulsing suggesting a resonant collective motion of many atoms and modification of thermodynamic parameters in amorphous alloys

Hisanori Tanimoto

Papers

Mechanical behavior of high-density nanocrystalline gold prepared by gas deposition method

Self-diffusion in high-density nanocrystalline Fe

Crystallization under electropulsing suggesting a resonant collective motion of many atoms and modification of thermodynamic parameters in amorphous alloys

Young's modulus of silver films

Mechanical property of high density nanocrystalline gold prepared by gas deposition method