Grain-size stabilization in nanocrystalline FeZr alloys

Darling, Kristopher A.; Chan, Ryan N.; Wong, P.Z.; Semones, Jonathan E.; Scattergood, R.O.; Koch, Carl C.

doi:10.1016/j.scriptamat.2008.04.045

Cited by 155 publications

(79 citation statements)

References 15 publications

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“…Significant grain coarsening occurs at higher temperatures, where the lattice strain is considerably reduced. Here, the average crystallite size increases sharply, reaching a value of about 160 nm at 773 K. Within the applied techniques, it appears that the grain size of the Mg-7.4%Al powder is retained up to high temperatures; Two basic mechanisms may inhibit grain growth: (i) the kinetic mechanism, which is based on grain boundary pinning through residual pores, impurities and solutes, as well as second phase particles [21]; and (ii) the thermodynamic mechanism, which is based on the reduction of the driving force for grain growth through the addition of solute atoms that segregate at the grain boundaries [21,22]. In the present Mg-7.4%Al powder, the role of the γ-Mg 17 Al 12 and MgO particles in kinetically stabilizing the structure may be quite important.…”

Section: Resultsmentioning

confidence: 99%

Synthesis and Characterization of NanocrystallineMg-7.4%Al Powders Produced by Mechanical Alloying

Chaubey

Scudino

Khoshkhoo

et al. 2013

Metals

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Nanocrystalline Mg-7.4%Al powder was prepared by mechanical alloying using a high-energy mill. The evolution of the various phases and their microstructure, including size and morphology of the powder particles in the course of milling and during subsequent annealing, were investigated in detail. Room temperature milling leads to a rather heterogeneous microstructure consisting of two distinct regions: Al-free Mg cores and Mg-Al intermixed areas. As a result, the material is mechanically heterogeneous with the Mg cores displaying low hardness (40-50 HV) and the Mg-Al intermixed regions showing high hardness of about 170 HV. The Mg cores disappear and the microstructure becomes (also mechanically) homogeneous after subsequent cryo-milling. Rietveld structure refinement reveals that the crystallite size of the milled powders decreases with increasing the milling time reaching a minimum value of about 30 nm. This is corroborated by transmission electron microscopy confirming an average grain size of ~25 nm.

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Section: Resultsmentioning

confidence: 99%

Synthesis and Characterization of NanocrystallineMg-7.4%Al Powders Produced by Mechanical Alloying

Chaubey

Scudino

Khoshkhoo

et al. 2013

Metals

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“…These suggest that equilibrium solute segregation lowers the grain boundary energy to varying degrees. Experimentally, a reduction in the propensity for grain growth in nanocrystalline materials has been observed in a variety of binary alloys [21][22][23][24][25][26][27][28][29][30][31]. There are many indications in experimental systems that there is a "preferred" grain size which emerges during processing which is closely linked to the solute content [2,24,30,32,33]; this is considered significant evidence for a thermodynamic contribution to stabilization.…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…Using experimental data from these systems as input, they predicted the distribution of solute, an increase in solute solubility with a decrease in grain size, and that the precipitation temperature of an ordered compound can be suppressed for these systems [37]. Darling and coworkers adapted a model of surface segregation energy to evaluate potential alloying elements with iron on the basis of their propensity to lower grain boundary energy [26,28,38].A more general, and generalizable, model is that of Trelewicz [39], who proposed a regular nanocrystalline solution (RNS) model for the free energy of mixing in binary alloys with both crystalline and intercrystalline atomic environments. The RNS model reduces properly to a regular solution model for the crystalline phase in the limit of infinite grain size, and to a standard grain boundary segregation isotherm in the dilute limit.…”

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Stability of binary nanocrystalline alloys against grain growth and phase separation

2013

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Grain boundary segregation has been established through both simulation and experiments as a successful approach to stabilize nanocrystalline materials against grain growth. However, relatively few alloy systems have been studied in this context; these vary in their efficacy, and in many cases the stabilization effect is compromised by second phase precipitation. Here we address the open-ended design problem of how to select alloy systems that may be stable in a nanocrystalline state. We continue the development of a general "regular nanocrystalline solution" model to identify the conditions under which binary nanocrystalline alloy systems with positive heats of mixing are stable with respect to both grain growth (segregation removes the grain boundary energy penalty) and phase separation (the free energy of the nanocrystalline system is lower than the common tangent defining the bulk miscibility gap). We calculate a "nanostructure stability map" in terms of alloy thermodynamic parameters. Three main regions are delineated in these maps: one where grain boundary segregation does not result in a stabilized nanocrystalline structure, one in which macroscopic phase separation would be preferential (despite the presence of a nanocrystalline state stable against grain growth), and one for which the nanocrystalline state is stable against both grain growth and phase separation. Additional details about the stabilized structures are also presented in the map, which can be regarded as a tool for the design of stable nanocrystalline alloys. Atomistic simulations on nanocrystalline alloys show that structural stabilization is contingent upon the distribution and character of the solute atom. A certain minimum concentration of solute is often found to be necessary for grain size stabilization, as for various solute species in simulated copper [11][12][13][14][15][16][17]. The efficacy of different solute species is variable, and in some studies has been related to the size difference between solute and solvent atoms [11][12][13]. However, in these studies, the grain boundaries are manually decorated with solute atoms, which may represent artificial segregation states. There have been fewer simulation works on systems where segregation is thermodynamic (by, e.g., Monte Carlo methods) [16][17][18][19][20]. These suggest that equilibrium solute segregation lowers the grain boundary energy to varying degrees. Experimentally, a reduction in the propensity for grain growth in nanocrystalline materials has been observed in a variety of binary alloys [21][22][23][24][25][26][27][28][29][30][31]. There are many indications in experimental systems that there is a "preferred" grain size which emerges during processing which is closely linked to the solute content [2,24,30,32,33]; this is considered significant evidence for a thermodynamic contribution to stabilization. The grain size that is stable against coarsening is correlated to the solute concentration in these systems, but the system also often exhibits instabilities w...

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“…Extremely fine nanocrystalline grain sizes have been realized in a variety of binary alloy systems, for example Y-Fe, 32 Ni-P, 33 Pd-Zr, 27,34 and Fe-Zr. 35 Because the elements composing these alloys are highly immiscible with a large positive heat of mixing, these systems are classified as strongly segregating, with high assumed values of H seg ͑Ն0.5 eV͒ that can substantially reduce the grain boundary energy via Eq. ͑2͒.…”

Section: Introductionmentioning

confidence: 99%

Grain boundary segregation and thermodynamically stable binary nanocrystalline alloys

2009

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A free-energy function for binary polycrystalline solid solutions is developed based on pairwise nearestneighbor interactions. The model permits intergranular regions to exhibit unique energetics and compositions from grain interiors, under the assumption of random site occupation in each region. For a given composition, there is an equilibrium grain size, and the alloy configuration in equilibrium generally involves solute segregation. The present approach reduces to a standard model of grain boundary segregation in the limit of infinite grain size, but substantially generalizes prior thermodynamic models for nanoscale alloy systems. In particular, the present model allows consideration of weakly segregating systems, systems away from the dilute limit, and is derived for structures of arbitrary dimensionality. A series of solutions for the equilibrium alloy configuration and grain size are also presented as a function of simple input parameters, including temperature, alloy interaction energies, and component grain boundary energies.

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Grain-size stabilization in nanocrystalline FeZr alloys

Cited by 155 publications

References 15 publications

Synthesis and Characterization of NanocrystallineMg-7.4%Al Powders Produced by Mechanical Alloying

Synthesis and Characterization of NanocrystallineMg-7.4%Al Powders Produced by Mechanical Alloying

Stability of binary nanocrystalline alloys against grain growth and phase separation

Grain boundary segregation and thermodynamically stable binary nanocrystalline alloys

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