Bulk Nanocrystalline Materials Research Articles

Percolating network of ultrafast transport channels in severely deformed nanocrystalline metals

Severe plastic deformation is nowadays used to produce sizable amounts of bulk nanocrystalline materials, which render them suitable for innovative applications ranging from biomedical implants to off-shore or aerospace structures, owing to favorable combinations of high mechanical strength and enhanced ductility they offer. Enhanced atom diffusion along internal interfaces is largely responsible for the resulting property combinations. Severe plastic deformation processing of metals is demonstrated to create bulk nanostructured materials with a hierarchy of internal interfaces. On top of that, specific diffusion channels providing pathways for ultrafast transport of atoms have been identified. The defects that represent the constituents of the fast diffusion network were visualized by means of the focused ion beam technique. Nonequilibrium grain boundaries, nonequilibrium triple junctions, and microvoids/microcracks compose the percolating network of ultrafast diffusion channels, which represent an important and newly recognized feature of severely deformed materials.

Structural-scaling transitions and thermodynamic and kinetic effects in submicro-(nano-)crystalline bulk materials

The size effects involved in the transition to a nanostructural bulk state is considered as a complex of phenomena associated with a change in material properties in response to a change in particle size and its attendant growth of grain-boundary defects in importance. Attention is focused on two fundamental problems: transition to submicro-(nano-)crystalline bulk states under severe plastic deformation and description of properties of nanocrystalline bulk materials. The key question to be resolved is whether a sharp distinction exists between the state of ordinary polycrystalline bulk materials and the state of nanostructured bulk materials. In qualitative terms, the transition to a nanostructural bulk state is related to new critical phenomena, namely structural-scaling transitions, occurring in systems with mesodefects. These transitions allow determination of the types of collective modes in defect ensembles for the characteristic solid states: quasibrittle, ductile and nanocrystalline.

New procedure to generate stable nanocrystallites by severe plastic deformation

A new method to produce bulk nanocrystalline metal matrix composites is presented. A metal powder is treated to obtain a relatively brittle layer with a defined thickness on the surface. The precompacted powder is deformed by high-pressure torsion until a dense bulk nanocrystalline material is formed. Due to fragmentation of the thin layer, it is possible to incorporate nanometre-sized dispersions into the nanocrystalline metal matrix. As an example, an oxidized nickel powder is investigated in which the thermal stability and microhardness are extraordinarily enhanced.

The evolution of porosity in bulk nanocrystalline materials during plastic deformation and its effect on the mechanical behavior

Uniaxial compression tests were carried out to completely understand the evolution of porosity in porous bulk nanocrystalline materials, and a new evolution law of porosity under uniaxial compression was proposed. Based on the energy principle, we built a mechanical model to calculate the overall mechanical properties of bulk nanocrystalline materials. The comparison between predicted results and the corresponding experimental data indicates that the established model is capable of describing the plastic mechanical behaviors of porous nanocrystalline materials.

Thermal properties and microstructure of bulk nanocrystalline Gd material

Bulk nanocrystalline gadolinium (Gd) material has been consolidated from Gd nanoparticles using spark plasma sintering (SPS). High density (>99.5%) bulk nanocrystalline material was achieved after sintering at a temperature of 280 °C with a pressure of 500 MPa. Microstructure analysis shows that the consolidated bulk material exhibits a single phase with hexagonal close packed structure and a fine grain microstructure with a mean grain size of about 15 nm. The structural transformation from hexagonal condensed packed to face centered cubic was not observed, and the second-order magnetic transition remained in the nanocrystalline Gd sample. The Curie temperature of the nanocrystalline Gd decreased by more than 10.7 K below that of the coarse-grained. The activation energies for the coarse-grained and the as-consolidated Gd materials are 2.7702 and 1.0130 eV, respectively.

A polycrystalline mechanical model for bulk nanocrystalline materials using the energy approach

To systematically understand the grain size, strain rate and defect development dependent mechanical behavior of bulk nanocrystalline materials, a new constitutive model is proposed to describe the deformation mechanism, microstructure evolution and mechanical response of bulk nanocrystalline materials using the energy approach. In this model, the grain interior and grain boundary were not taken as two independent phases with different volume fractions, but as an integral object sustained dislocation and accommodated grain boundary sliding mechanisms. Meanwhile, defect creation and evolution and their effects on the overall stress–strain relation as well as the failure process of bulk nanocrystalline materials were considered in the model. For experimental verification, we have prepared nanocrystalline Ni powder by the DC arc plasma evaporation method. Bulk nanocrystalline Ni samples were then made by compaction and hot sintering. Experimental measurements on the mechanical response of bulk nanocrystalline Ni were performed under different strain rates and grain sizes. Comparison between experimental data and model predictions show that the method developed appears to be capable of describing the mechanical response of bulk nanocrystalline materials. The model applications to nanocrystalline Mg and Cu have shown that it can reflect the asymmetric defect development between tension and compression under quasi-static conditions; this results in its good capacity to describe the dynamic strain rate sensitivity and strain hardening behavior over a relatively large strain range under both compression and tension conditions.

Enhancement of Thermoelectric Figure-of-Merit by a Nanostructure Approach

AbstractThe dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, the nanostructure approach has been demonstrated in a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient.

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.

Fabrication of Bulk Nanocrystalline Ceramic Materials

An alternative method to produce bulk nanocrystalline materials is to produce amorphous material by rapid solidification followed by controlled solid state crystallization. Four different materials studied in this work are based on near-eutectic mixtures of alumina, zirconia, and silica. Powder feedstock materials have been plasma sprayed using a water stabilized plasma torch (WSP®) and subsequently heat-treated to prepare materials with nanocrystallites of 12 nm diameter on average. The as-sprayed materials have very low porosity and are mostly amorphous. The as-sprayed amorphous materials crystallize at temperatures around 950 °C. The resulting structure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix. The role of silica on phase composition, microstructure, and mechanical properties of the as-sprayed and annealed materials is discussed. The as-sprayed amorphous materials exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated nanocrystalline samples.

Densification and grain growth during sintering of nanosized particles

The sintering of nanosized particles is a scientific and technological topic that affects the manufacture of bulk nanocrystalline materials and the understanding of the stability of nanoparticles. Owing to their extremely small size and the high surface to volume ratio, nanoparticles during sintering exhibit a number of distinctively unique phenomena compared to the sintering of coarse powders. Particularly, it is generally found that the sintering temperatures of nanosized particles are dramatically lower than that of their micrometre or submicrometre sized counterparts. Research has also shown that the grain growth during nanosintering consists of an initial dynamic grain growth stage that occurs during heating up and the normal grain growth stage that occurs mostly during isothermal holding. For nanoparticles, the effect of the initial grain growth cannot be ignored because that it is sufficient to cause the material to lose nanocrystalline characteristics. This review aims to bring to focus the understanding of the fundamental issues of nanosinteirng, including the thermodynamic driving force of nanosintering, non-linear diffusion and the kinetics of nanosintering, and the relationships between agglomeration, densification and grain growth. This review will also examine the effects of microstructure and processing variables.

Studies on the bulk nanocrystalline Ni–Fe–Co alloy prepared by mechanical alloying–sintering–hot rolling route

Nanocrystalline Ni–Fe alloys have emerged as a promising soft magnetic material for high frequency applications where low hysteresis loss is the prime requirement. Nanocrystalline 80Ni–20Fe (wt.%) alloys synthesized by mechanical alloying have shown improved magnetic properties as compared to conventional 80Ni Permalloy due to formation of nanocrystalline Ni 3Fe as major phase during mechanical alloying. However, retaining nanocrystallinity during consolidation to prepare bulk material is a key issue. The present paper describes preparation of bulk nanocrystalline 80Ni–15Fe–5Co (wt.%) alloy by a powder metallurgy route involving mechanical alloying of elemental powder mixture followed by sintering and hot rolling. The average grain size of the mechanically alloyed powder was approximately 10 nm after milling for 46 h. A multi-step sintering–hot rolling route was developed to prepare a crack-free strip having an average grain size 32 ± 2 nm and 99% theoretical density. The bulk nanocrystalline 80Ni–15Fe–5Co strip resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.

Studies on the Mechanical Alloying of Ni-Fe-Co Powders and Its Explosive Compaction

A bulk nanocrystalline 80Ni-15Fe-5Co (wt pct) soft magnetic material was successfully produced via “mechanical alloying-explosive compaction” route. A rapid grain refining was observed during initial 12 hours of milling in high-energy planetary ball mill, followed by a level-off trend. A comparison of crystallite size of 125-hour-milled mechanically-alloyed (MAed) powder (10 nm) and explosively-compacted material (15 nm) showed negligible grain growth during explosive compaction. It has been shown that no phase change was brought about by the explosive compaction of the milled powder, and Ni3Fe remained the predominant phase in both MAed and explosively-compacted material. A possible mechanism of densification of flaky MAed powder during explosive compaction has been proposed, which consisted of the plastic deformation of powder particles into elongated shape followed by joining and folding of elongated particles. This process produced a continuous network of elongated and folded particles in the compact. The bulk nanocrystalline material showed improved magnetic properties, such as high Curie temperature and negligible core loss, making it a promising soft magnetic material for applications involving high temperatures and changing magnetic environment.

Microstructure stabilization in bulk nanocrystalline materials: Analytical approach and numerical modeling: To the 60th anniversary of the Zener treatment of particle impact on grain growth

It has been shown that complete stability of microstructure, namely, invariability of the mean and maximum grain sizes, can be achieved under the influence of immobile disperse particles. The necessary drag force is inversely proportional to the most probable grain size in the initial microstructure. If particles of the mean radius 2 nm are attached to grain boundary junctions, microstructure stabilization in polycrystal with the most probable grain size 20 nm could be achieved at the particle volume fraction of the order of 0.001.

Synthesis of Bulk Nanocrystalline Materials and Bulk Metallic Glasses by Repeated Cold Rolling and Folding (RCR)

Repeated cold rolling with intermediate folding (RCR) represents a technique to obtain severe plastic deformation that avoids excessive heating at the internal interfaces and that proceeds without the simultaneous action of a high pressure in the range of several GPa. Aside from the opportunity to obtain amorphous bulk samples, the processing pathway also allows for synthesizing dense, bulk nanocrystalline materials. The sequential combination of different processing routes that drive a material to a different extent -, with different rates - and by different means from thermodynamic equilibrium present new and attractive processing opportunities to obtain bulk nanocrystalline or massive ultrafine grained materials that are widely unexplored. Here, an overview is presented concerning the sequential application of different deformation methods with largely different strain and pressure levels. The basic underlying mechanisms that can lead to ultrafine grained or nanocrystalline microstructures for pure metals or to two-phase nanocomposites or bulk metallic glasses for alloys are discussed and the current state of nanostructure control is highlighted by selected examples.

High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys

The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100 degrees C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250 degrees C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86 degrees , 106 degrees , and 119 degrees C with hot-side temperatures set at 50 degrees, 100 degrees, and 150 degrees C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

Extensive disordering in long-range-ordered Cu 3Au induced by severe plastic deformation studied by transmission electron microscopy

Bulk nanocrystalline materials can be made by severe plastic deformation. In L1 2 long-range-ordered alloys, this leads to extensive disordering which influences the highly improved properties of these nanocrystalline alloys. Transmission electron microscopy methods were applied to Cu 3Au; both diffraction contrast images and diffraction patterns reveal that disordering takes place locally. It is concluded that in addition to disordering by the refinement of the grown-in antiphase boundary domains, the formation of antiphase boundary tubes is a prominent process of disordering. The latter is facilitated by the fact that, unlike dislocations, antiphase boundary tubes can be stored at a very high density without causing long-range stresses. The local disordering indicates that the nanocrystalline structure nucleates inhomogeneously in the highly strained disordered regions.

Structural nanocrystalline materials: an overview

This paper presents a brief overview of the field of structural nanocrystalline materials. These are materials in either bulk, coating, or thin film form whose function is for structural applications. The major processing methods for production of bulk nanocrystalline materials are reviewed. These methods include inert gas condensation, chemical reaction methods, electrodeposition, mechanical attrition, and severe plastic deformation. The stability of the nanocrystalline microstructure is discussed in terms of strategies for retardation of grain growth. Selected mechanical properties of nanocrystalline materials are described; specifically strength and ductility. Corrosion resistance is briefly addressed. Examples of present or potential applications for structural nanocrystalline materials are given.

Preparation and mechanism study of bulk pure rare-earth metals with amorphous and nanocrystalline structures

The preparation and the mechanism study of bulk pure rare-earth metals with amorphous and nanocrystalline structures, which were produced by spark plasma sintering (SPS), were carried out in this paper. With different processing parameters, the amorphous, two phases of amorphous and nanocrystalline, and complete nanocrystalline microstructures have been obtained. The nano-grain sizes in the bulk nanocrystalline materials are found smaller than the original powder particles sizes, which may change the conventional viewpoint that the grains in the sintered bulk are generally coarser than the raw powder particles. The technique developed in the present work can be extended to the preparation of many other nano bulk metal materials, and thus enables the studies of the nano-size effects on the physical, chemical and mechanical properties of bulk nano materials.

Process Optimization for Nanostructured HVOF -Sprayed Al<sub>2</sub>O<sub>3</sub>-Based Ceramic Coatings

Special mechanical properties have widely been demonstrated with bulk nanocrystalline materials. An increasing effort has been made to transfer such improvements also into thermal sprayed ceramic coatings. This paper focuses on such efforts in alumina-based ceramic coatings. The optimization of process conditions and effect of different process parameters on the mechanical performance of high velocity oxy-fuel (HVOF) sprayed ceramic coatings is discussed.

The New SPD Processing Trends to Fabricate Bulk Nanostructured Materials

During the last decade severe plastic deformation (SPD) has become a widely known method of materials processing used for fabrication of ultrafine-grained materials with attractive properties. Nowadays SPD processing is rapidly developing and is on the verge of a transition from lab-scale research to commercial production. This paper focuses on several new trends in the development of SPD techniques for effective grain refinement, including those for commercial alloys and presents new SPD processing routes to produce bulk nanocrystalline materials.