A simple, mixtures-based model for the grain size dependence of strength in nanophase metals

The Inverse Hall-Petch Effect—Fact or Artifact?

This paper critically reviews the data in the literature which gives softening—the inverse Hall-Petch effect—at the finest nanoscale grain sizes. The difficulties with obtaining artifactfree samples of nanocrystalline materials will be discussed along with the problems of measurement of the average grain size distribution. Computer simulations which predict the inverse Hall-Petch effect are also noted as well as the models which have been proposed for the effect. It is concluded that while only a few of the experiments which have reported the inverse Hall-Petch effect are free from obvious or possible artifacts, these few along with the predictions of computer simulations suggest it is real. However, it seems that it should only be observed for grain sizes less than about 10 nm.

Percolation Threshold in Superhard Nanocrystalline Transition Metal-Amorphous Silicon Nitride Composites: The Control and Understanding of the Superhardness

The hardness of the recently developed novel superhard nanocrystalline composites exceeds 5000 kg/mm2 (50 GPa) and the elastic modulus 550 GPa. This is due to a special microstructure which is formed when the fraction of the amorphous component reaches the percolation threshold. Experimental data are presented and discussed.

Electrochemical Deposition of Nanostructured Metals and Alloys from Ionic Liquids

Metals like aluminium, magnesium, tungsten or their alloys cannot be electrodeposited from aqueous electrolytes. We have developed a procedure using AlCl3-based ionic liquids for the deposition of nanostructured metals and alloys. The ionic liquids (IL) employed for these studies consist of mixtures of an inorganic (e.g., AlCl3) and an organic component (e.g., 1-butyl-3-methyl-imidazoliumchloride or [BMIm]Cl). In our contribution we describe the electrochemical deposition of less noble metals like Al or Fe and alloys like Al x Mn1−x with a controlled nanostructure. The variation of physical and chemical process parameters allows the deposition of samples with crystallite sizes from 10 up to several hundred nm. Deposits prepared from IL's show remarkable properties. Based on nanoindentation measurements we observe crystallite size dependent microhardness for nanostructured aluminium (from 1.44 GPa (100 nm) to 3.40 GPa (14 nm)). The thermal stability of the deposits was measured by high temperature X-ray diffraction. The deposits show a thermal stability up to 350 °C resulting from oxide impurities in the grain boundaries. An activiation energy of 41 kJ/mol can be determined for the crystallite growth process. The magnetization curves of nanostructurd iron exhibit soft magnetic behavior; the coercivity is inverse proportional to the crystallite size.

Nanocrystalline Metals Prepared by Electrodeposition

An overview is presented about the preparation of nanocrystalline metals by pulsed electrodeposition out of aqueous electrolytes and ionic liquids. Appropriate selection of the bath chemistry and the plating parameters provide the flexibility to control the crystallite size. For selected examples we demonstrate how these electrochemical methods can be used for the preparation of nanostructured bulk metals and nanoparticle for fuel cell applications. Electrochemical analytical methods like cyclic voltammetry and chronoamperometry are used to fundamentally understand the deposition mechanisms, the effect of bath additives on the micro- and nanostructure of the deposits and to characterize the catalytic activity of the electrocatalysts. Some improved physical properties of electrodeposited nanocrystalline metals are discussed, namely thermal stability of n-Fe, magnetisation of n-Ni and hardness of n-Al.

Modeling the dependence of strength on grain sizes in nanocrystalline materials

A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall-Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall-Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ-1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.

HighP-TNano-Mechanics of Polycrystalline Nickel

We have conducted highP-Tsynchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume-pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify "micro/local" yield due to high stress concentration at the grain-to-grain contacts and "macro/bulk" yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under highP-Tconditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 Tm) hardens nanocrystalline Ni, leading to an inverse Hall-Petch relationship. We explain this abnormal Hall-Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni.

A maximum in the strength of nanocrystalline copper

We used molecular dynamics simulations with system sizes up to 100 million atoms to simulate plastic deformation of nanocrystalline copper. By varying the grain size between 5 and 50 nanometers, we show that the flow stress and thus the strength exhibit a maximum at a grain size of 10 to 15 nanometers. This maximum is because of a shift in the microscopic deformation mechanism from dislocation-mediated plasticity in the coarse-grained material to grain boundary sliding in the nanocrystalline region. The simulations allow us to observe the mechanisms behind the grain-size dependence of the strength of polycrystalline metals.