The Hall–Petch breakdown in nanocrystalline metals: A crossover to glass-like deformation

The deformation behavior of nanocrystalline Ni–W alloys is evaluated by nanoindentation techniques for grain sizes of 3–150 nm, spanning both the range of classical Hall–Petch behavior as well as the regime where deviations from the Hall–Petch trend are observed. The breakdown in strength scaling, observed at a grain size of 10–20 nm, is accompanied by a marked transition to inhomogeneous, glass-like flow (i.e. shear banding) at the finest grain sizes approaching the amorphous limit. As a consequence of this mechanistic crossover, additional inflections arise in the mechanical properties; maxima are observed in both the rate and pressure dependence of deformation at approximately the same grain size as the onset of the Hall–Petch breakdown. These data experimentally connect the mechanical properties of nanocrystalline alloys to the well-known behavior of amorphous metals.     

Effect of solute segregation on the strength of nanocrystalline alloys: Inverse Hall–Petch relation

We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe₉₀Ni₁₀, Fe₈₅Al₄Si₁₁, Ni₉₉Fe₁ and Ni₉₀Fe₁₀ powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T < 0.4 T_m) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0 at.% impurities).     

Microstructural changes of nanocrystalline nickel during cold rolling

The microstructural changes of electrodeposited nanocrystalline Ni with an initial grain size of about ∼30–40 nm during cold rolling up to 76% thickness reduction have been studied using X-ray diffraction and transmission electron microscopy. In response to the cold deformation processing we observed significant changes in the scale and morphology of grains, defect content, as well as of crystallographic texture. Our experimental findings for nanocrystalline Ni are directly compared to the behavior of coarse-grained Ni. The role of the grain scale reduction to the nanometer regime is discussed with respect to the microstructural changes.     

High performance tungsten synthesized by microwave sintering method

The W–Ni alloys with varying amount of Ni (0.1, 0.25, 0.5 and 1.0 wt.%) were microwave sintered at 1450 °C for different holding time 5, 15 and 30 min, and their microstructure, grain size, relative density, thermal conductivity, and Vickers microhardness were characterized. Comparing to the addition of Fe and Cr, the Ni addition can greatly improve the relative density and maintain the high thermal conductivity of W at the same time. It was shown that the addition of 1.0 wt.% Ni into W and microwave sintering at 1450 °C for 5 min would be the best conditions to obtain W–Ni alloys with a relative density close to 100% and an average grain size as small as 15 μm. The Vickers microhardness and thermal conductivity of the sintered W–Ni samples range from 370 to 440 and from 90 to 130 W/m K, respectively.     

A comparative study of spark plasma sintering and hybrid spark plasma sintering of 93W–4.9Ni–2.1Fe heavy alloy

Mixed 93W–4.9Ni–2.1Fe powders were sintered via the spark plasma sintering (SPS) and hybrid spark plasma sintering (HSPS) techniques with 30 mm and 60 mm samples in both conditions. After SPS and HSPS, the 30 mm and 60 mm alloys (except 60 mm-SPS) had a relative density (> 99.2%) close to the theoretical density. Phase, microstructure and mechanical properties evolution of W–Ni–Fe alloy during SPS and HSPS were studied. The microstructural evolution of the 60 mm alloys varied from the edge of the sample to the core of the sample. Results show that the grain size and the hardness vary considerable from the edge to the core of sintered sample of 60 mm sintered using conventional SPS compared to hybrid SPS. Similarly, the hardness also increased from the edge to the core. Furthermore, the 60 mm-HSPS alloy exhibited improved bending strength of 1115 MPa when compared to that of 60 mm-SPS, 920 MPa. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–Ni–Fe, however in the 60 mm-SPS alloy peeling of the grains was also observed which diminished the properties. The mechanical properties of SPS and HSPS 93W–4.9Ni–2.1Fe heavy alloys are dependent on the microstructural parameters such as tungsten grain size and overall homogeneity.     

Plasma nitridation of a novel Ni–10.8 wt.%Cr nanocomposite

A Ni–10.8Cr nanocomposite (by wt.%), consisting of nanocrystalline Ni matrix (mean grain size: 60 nm) and dispersed Cr nanoparticles (mean particle size: 42 nm), has been synthesized by nanocomposite electrodeposition. The unique structure causes the nanocomposite to form a double-layered nitrided zone during plasma nitridation at 560 °C for 10 h. The outer layer (∼50 μm thick) precipitates nanometer-sized CrN (<100 nm), which increased in size but decreased in number with increasing nitridation depth (following Böhm–Kahlweit’s mode). The inner layer (∼5 μm thick) exhibits larger-coarsened nitride precipitates (100–200 nm) which almost link together. The greatly enhanced nitriding kinetics in the nanocomposite compared to a compositionally similar but microstructurally different Ni–10Cr alloy (mean grain size: 30 μm) is mainly associated with the fact that the numerous grain boundaries dramatically increase the nitrogen permeability, according to the treatment using a classical Wagner’s approach. The nanohardness profile in relation to the microstructure of the nitrided zone in the nanocomposite has also been investigated.     

Nanocrystalline Fe–C alloys produced by ball milling of iron and graphite

A series of nanocrystalline Fe–C alloys with different carbon concentrations (x_tot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing x_tot. Eventually the size reaches a lower limit of about 6 nm in alloys with x_tot > 6.19 at.% (1.40 wt.%); a further increase in x_tot leads to the precipitation of carbon as Fe₃C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe₃C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy.     

Sliding wear of nanocrystalline Ni–W: Structural evolution and the apparent breakdown of Archard scaling

Sliding wear of nanocrystalline Ni–W alloys with grain sizes of 3–47 nm, a range which spans the transition in deformation mechanisms from intra- to inter-granular, has been studied through pin-on-disk wear testing. The extreme conditions produced during sliding wear are found to result in structural evolution and a deviation from Archard scaling for the finest grain sizes; in the finest nanocrystalline materials wear resistance is higher than would be expected based on hardness alone. The repetitive sliding load is found to lead to a modest amount of grain growth and grain boundary relaxation, which in turn leads to local hardening in the wear track. Analysis of the dynamic microstructure suggests that it is produced primarily as a result of local plasticity and is not principally due to frictional heating.     

Effect of interstitial carbon on the mechanical properties of electrodeposited bulk nanocrystalline Ni

Solid solution strengthening by carbon and sulfur in bulk nanocrystalline Ni was studied by electrodeposition and first-principles calculations. Bulk nanocrystalline Ni with a carbon content of 30–1600 ppm and a sulfur content of 140–1200 ppm was prepared using a sulfamate bath with different complexing agents and gloss agents. The hardness values of the bulk nanocrystalline Ni were scattered as the grain size decreased to ～12 nm with increasing carbon and sulfur content. It was found that the scatter could be explained by considering the effect of impurities such as solute atoms on the hardness of electrodeposited Ni, in addition to the Hall–Petch relationship. Thus, to determine the structure of Ni–C and Ni–S solid solutions and estimate the contribution of impurities to hardness, the enthalpy of solution and misfit strain were calculated by first-principles calculations. The results indicate that carbon exists as an interstitial solute atom in the Ni matrix, producing large misfit strains, and sulfur exists as a substitutional solute atom, inducing no significant changes. A model of solid solution strengthening due to interstitial solute atoms was developed by considering the interaction between mobile dislocations and solute atoms. This study has effectively divided the observed solid solution effect from the grain refinement effect in electrodeposited nanocrystalline Ni. The results of this study point to the origin of high-strength electrodeposited bulk nanocrystalline Ni.     

Processing and microstructural characterization of liquid phase sintered tungsten–nickel–cobalt heavy alloys

In this study, the effects of composition and sintering temperature on the microstructural characteristics of liquid phase sintered 90W–Ni–Co alloys were investigated. 90W–Ni–Co alloys having Ni/Co ratios of 3/1, 4/1 and 6/1 were examined. It was found that the alloys studied have reached almost to full density when sintered at and above 1475 °C. The microstructures of the alloys were typical of liquid phase sintered alloys, which consisted of rounded, nearly pure W grains embedded in a ternary Ni–Co–W binder matrix phase. The binder matrix phase in these alloys was observed to dissolve up to 42 wt.% W. The relative amount of the binder matrix phase and the average size of the W grains were found to increase with increasing sintering temperature. The activation energies for grain coarsening are determined for the investigated alloys by assuming that the coarsening process is mainly governed by Ostwald ripening mechanisms in the liquid state. The calculated activation energies, which were within 113–162 kJ/mol range, were found to be in rather close agreement to the literature data given for W–Ni–Fe alloys. This indicates that grain coarsening in W–Ni–Co and W–Ni–Fe alloys most probably takes place through similar diffusional processes.     

What is the theoretical density of a nanocrystalline material?

We prepared nanocrystalline Ni by a severe deformation method – high-energy ball milling – and collected neutron diffraction patterns during the annealing of nanocrystalline Ni. Analyzing the neutron diffraction patterns provides the lattice parameter, dislocation density and grain size of nanocrystalline Ni. We found that a low-temperature (T < 260 °C) anneal annihilates the statistically stored dislocations whereas a high-temperature (T > 260 °C) anneal grows the nanograins. For T < 260 °C, where nanocrystalline Ni has a constant grain size, the excess volume is proportional to the density of statistically stored dislocations. For T > 260 °C, where the statistically stored dislocations are completely annealed out, the excess volume is inversely proportional to the grain size. However, 80% of the excess volume in our severely deformed nanocrystalline Ni is due to the statistically stored dislocations. We finally used our experimental data to derive the grain size dependence of the theoretical density of a nanocrystalline material free from excess dislocations. The derived theoretical density agrees well with the experimentally measured density of nanocrystalline metallic materials that are relatively free from deformation-induced defects.     

Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations

Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.     

Densification behavior of nanocrystalline W–Ni–Fe composite powders prepared by sol-spray drying and hydrogen reduction process

This paper studied the densification behavior of nanocrystalline composite powders of 93W–4.9Ni–2.1Fe (wt.%) and 93W–4.9Ni–2.1Fe–0.03Y synthesized by sol-spray drying and hydrogen reduction process. The X-ray diffraction (XRD) analysis showed that γ-(Ni, Fe) phase was formed in the final obtained powders. Powders morphology characterized by scanning electron microscope (SEM) showed that the 93W–4.9Ni–2.1Fe nanocrystalline composite powders exhibited larger agglomeration and grain size compared with the 93W–4.9Ni–2.1Fe–0.03Y nanocrystalline composite powders. Both kinds of green compacts can obtain full density if sintered at 1410 °C for 1 h. When sintering temperature was above 1410 °C, the sintering density for both compacts decreased rapidly. In addition, the sintering density, densification rate and grain coarsening rate of 93W–4.9Ni–2.1Fe compacts were higher than those of 93W–4.9Ni–2.1Fe–0.03Y. The effect of trace yttrium on the densification behavior of nanocrystalline composite powders was also discussed.     

Effects of Ni addition and cyclic sintering on microstructure and mechanical properties of coarse grained WC–10Co cemented carbides

Coarse grained WC–10(Co, Ni) cemented carbides with different Ni contents were fabricated by sintering-HIP and cyclic sintering at 1450 °C. The effects of Ni addition and cyclic sintering on the microstructures, magnetic behavior and mechanical properties of coarse grained WC–10(Co, Ni) cemented carbides have been investigated using scanning electron microscope (SEM), magnetic performances tests and mechanical properties tests, respectively. The results showed that the mean grain size of hardmetals increases from 3.8 μm to 5.78 μm, and the shape factor Pwc decreases from 0.72 to 0.54, with the Ni content increases from 0 to 6 wt.%. Moreover, the W solubility reaches the highest value of 10.33 wt.% when the Ni content is 2 wt.%. The hardness and transverse rupture strength of WC–8Co–2Ni are 1105 HV₃₀ and 2778 MPa, respectively. The cyclic sintering is conducive to increase the WC grain size of WC–10(Co, Ni) and improves the transverse rupture strength of WC–10Co without compromising the hardness of alloys.     

Peculiar size effect in nanocrystalline BaTiO3

Dense BaTiO₃ ceramics with grain sizes of 35 nm to 5.6 μm were prepared, and the electrical properties investigated in the temperature range 500–700 °C by means of impedance spectroscopy. Charge carriers (oxygen vacancies and holes) are depleted in the space charge regions at the BaTiO₃ grain boundaries. When the grain size is ?250 nm, the width of the space charge region was determined to be ～40 nm. Therefore, the depletion regions were expected to overlap when the grain size decreases to 35 nm; in such a situation, charge carriers would be depleted over the entire grain, resulting in depressed conductivity. However, the conductivity of the 35 nm grain size sample was measured to be one to two orders of magnitude higher than those of the microcrystalline samples, and the activation energy markedly lower. Moreover, we determined a width of ～7 nm for the space charge regions in the 35 nm grain size sample; therefore, the space charge regions do not overlap. The enhanced conductivity is ascribed to a reduced oxidation enthalpy in nanocrystalline BaTiO₃, and the distorted grain boundaries in nanocrystalline BaTiO₃ are believed to be the atomic level origin of the reduced oxidation enthalpy.     

Synthesis and characterization of mechanically alloyed and shock-consolidated nanocrystalline NiAl intermetallic

The synthesis, microstructural characterization and microhardness of nanocrystalline B2-phase NiAl intermetallic are discussed in this paper. Nanophase NiAl powders were prepared by mechanical alloying of elemental Ni and Al powders under an argon atmosphere for different times (0–48 h). The alloyed nanocrystalline powders were then consolidated by shock compaction at a peak pressure of 4–6 GPa, to 83% dense compacts. Characterization by transmission electron microscopy (TEM) revealed that the microstructure of the shock-consolidated sample was retained at the nanoscale. The average crystallite size measurements revealed that mechanically alloyed NiAl grain size decreased from 48±27 to 9±3 nm with increasing mechanical alloying time from 8 to 48 h. The long-range-order parameters of powders mechanically alloyed for different times were determined, and were observed to vary between 0.82 for 5 h and 0.63 for 48 h of milling time. Following shock compaction, the long-range-order parameter was determined to be 0.76, 0.69 and 0.66, respectively, for the 16, 24 and 48 h alloyed specimens. Both the mechanically alloyed nanocrystalline NiAl powder and the shock-consolidated bulk specimen showed evidence of grain boundary dislocations, subgrains, and distorted regions. A large number of grain boundaries and defects were observed via high resolution TEM (HRTEM). Shear bands were also observed in the mechanically alloyed NiAl intermetallic powders and in the shock-consolidated compacts. Microhardness measurements of shock-consolidated material showed increasing microhardness with increasing crystallite size refinement, following Hall–Petch behavior.     

Properties of nanocrystalline and nanocomposite WxZr1−x thin films deposited by co-sputtering

W_xZr_1?x thin films were deposited at room temperature on glass substrates by co-sputtering tungsten and zirconium targets in argon. The composition was found in the range 0 ≤ x ≤ 0.81. The grain size deduced from X-ray diffraction analysis ranged from 1.3 nm to 16 nm depending on the composition. The events in the resistivity, optical reflectivity and thickness evolutions were correlated with the X-ray diffraction analysis. Depending on the composition, the local organization can be attributed to a nanocrystalline solid solution of W in Zr, to a nanocomposite structure involving ZrW₂ nanograins embedded in an amorphous matrix, to ZrW₂ Laves phase nanograins and to a nanocrystalline solid solution of Zr in W. For 0 < x ≤ 0.72, the equivalent grain size is very small (less than 2 nm) and the evolution of the resistivity can be fitted by the estimated volume of the material perturbed by the grain boundaries.     

Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 °C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution but instead a composite of bcc structured Ni–Al-, Cr–Fe- and Fe–Cr-based regions and of fcc Cu–Zn-based regions. The Cu–Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr–Fe phase that was inherited from the hot compacted state. The Ni–Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20–30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept.     

Size distribution of precipitated Ni clusters on the surface of an alkaline-treated LaNi5-based alloy

The size distributions of precipitated Ni clusters on the surface of a LaNi₅-based alloy immersed in alkaline solution (alkaline treatment) at 383 K for 0–110 min were precisely determined by combining superparamagnetic analysis and transmission electron microscopy (TEM) observations. The superparamagnetic analysis indicated that the diameters of the Ni clusters were smaller than ∼25 nm in all samples, while their average values increased approximately from 5 to 9 nm with increasing alkaline treatment time. The spatial distribution of the Ni clusters was successively observed by TEM, which agreed fairly well with the estimated size distribution by superparamagnetic analysis. Therefore, estimation of the actual size distribution of Ni clusters by superparamagnetic analysis was proved to be feasible. Based on the above results, a precipitation process for Ni clusters by alkaline treatment is proposed.     

On the latest stage of transformation from nanocrystalline to amorphous phases during ARB: Simulation and experiment

By applying a simple room temperature accumulative roll bonding (ARB) process, the multilayered Zr and Ni elemental foils can be mixed and transformed into nanocrystalline and eventually amorphous bulk materials. The latest stage of the transformation from the nanocrystalline elemental phase measuring ∼2 nm to the amorphous alloying phase is examined, using high resolution transmission electron microscopy and molecular dynamics simulation. The gradual evolutions of atom mixing of the binary Zr–Ni alloy, as well as the final radial distribution function and the nearest neighboring distance, are traced experimentally and numerically.