Produce Nanocrystalline Structures Research Articles

Modelling of High Pressure Torsion using FEM

Abstract High Pressure Torsion (HPT) is historically the first and is currently most common process of the severe plastic deformation (SPD). HPT is used for producing nanocrystalline structures in metals and alloys, as well as for cold welding of powders. The result of the HPT treatment is determined by the flow kinematics and the stress state of a material under deformation. P. Bridgman proposed a simple model for the initial assessment of shear strain in the sample during HPT. A number of recent investigations show that the true plastic flow during HPT can differ significantly from the simple model. We present in our report results of the FEM investigation of different types of flow during HPT. We propose a hypothesis that the vortex formation during the SPD process is caused by the local blocking of simple shear in a material under deformation.

Tailoring Calcium Carbonate to Serve as Optical Functional Material: Examples from Biology and Materials Science

In nature and technology there are many examples of micron sized optical systems based on minerals. The thermodynamically most stable polymorph of calcium carbonate (CaCO3) –calcite– exhibits birefringence. However, nature uses biomineralization approaches to produce CaCO3 based optical functional materials, for example calcite mircrolens arrays as part of the photoreceptor system of brittlestars, amorphous CaCO3 (ACC) plant cystoliths as light scatterers and calcite trilobite eyes. In this short review, we illustrate basic strategies to produce optical materials from CaCO3 by manipulating the material structure. These strategies are driven by the aim to eliminate or reduce the birefringent properties of calcite, or even, to turn this property into an advantage. We report on the formation, structure, and functions of micron sized (hemi‐)spherical objects made from CaCO3 with biological origin but also in bio‐inspired synthetic examples. We highlight aspects, which pave the way to learn from nature and how to characterize natural and artificial systems. In conclusion, we identify three main possibilities to make optical materials based on CaCO3: (i) orienting the optical axis along the desired light propagation direction, (ii) stabilizing metastable phases, and (iii) producing nanocrystalline structures, which reduce birefringent properties.

Formation of nanocrystalline structure in tantalum by sliding friction treatment

We have investigated the microstructural evolution as a function of depth from the surface of a pure tantalum plate subjected to sliding friction treatment (SFT), which is suggested to be a low-cost, simple and efficient severe plastic deformation technique to produce nanocrystalline structure on the surface of metals. A differential scanning calorimetry was used to examine the thermal stability of the microstructure. A transmission electron microscope was used to analyze the different microstructure features as a function of the depth from the surface. Details of the microstructural evolution were discussed and mechanisms were suggested. With an increasing depth from the treated surface, a progressive change in the microstructure from nano-grains to micro-grains was found, which is proposed to be dominated by dislocation activities.

Hydrogen storage properties of cold rolled magnesium hydrides with oxides catalysts

In this work, we compared ball milling and cold rolling as a mean to add transition metal oxides to magnesium hydride. We found that irrespective to the mixing technique the oxides NiO and Nb2O5 gave the fastest desorption kinetics. In general, sorption kinetics were slightly slower for cold rolled samples compared to their ball milled counterparts. Ball milling 30min is more effective to get a nanocrystalline structure than rolling 5 times. However, as rolling was performed in air for a limited number of times, it could be expected that rolling under inert atmosphere for a larger number of times will be as effective as ball milling to produce nanocrystalline structure and enhance hydrogen storage properties.

Magnetic properties of nano-sized 5 at.%Fe–Al systems

High energy ball milling is a promising materials processing technique that is widely used to produce nanocrystalline structures. However, when stainless steel or hardened steel containers and balls are used for milling, contamination from the milling medium can influence the material properties of the final nanostructured products due to intercalation of iron (Fe) as an impurity. This study reports the effect of iron contamination on nanocrystalline aluminum (Al) powder. 57Fe Mossbauer spectroscopy and bulk magnetization studies using a vibrating sample magnetometer show that pure Al powder milled in hard steel media is strongly ferromagnetic at room temperature due to Fe contamination of about 5 at.% from the milling medium. TEM studies indicate that the system consists mainly of nano-sized Fe interspersed in Al with average crystallite sizes of ~2 and ~5 nm for Fe and Al, respectively. A comparative study of this system made with a mechanically alloyed Fe–Al system with the same percentage of Fe mixed with pure Al and mechanically alloyed using tungsten carbide vials and balls shows that the saturation magnetization, coercivity, Curie temperature, and low temperature behavior (field cooled–zero field cooled) are very different in the two cases. The different magnetic properties of the two systems can be attributed to the presence of magnetic and non-magnetic phases present.

Functional properties of nanocrystalline, submicrocrystalline and polygonized Ti–Ni alloys processed by cold rolling and post-deformation annealing

Thermomechanical processing consisting of cold rolling ( e = 0.3–2.0) and post-deformation annealing (300–450 °C, 1 h) was applied to binary Ti–Ni alloys to produce nanocrystalline structures (NS) or polygonized dislocation substructures (PDS), or their mixture. The evolution of the material structure and properties was studied using TEM, X-ray, microhardness, calorimetry and tensile testing techniques. Recovery stress and strain of the Ti–50.26 at.%Ni alloy and superelastic strain of the Ti–50.6 at.%Ni alloy were measured under static and fatigue conditions. It was found that higher true yield stress of NS alloys not only increases the recovery stress potential, but, since it is combined with a relatively low transformation yield stress; it increases the completely recoverable strain. NS alloys generate recovery stresses that are twice as high as those of PDS alloys (1200 MPa), completely recoverable strains that are 10% greater (up to 6% in tension), and they demonstrate a higher cyclic stability of shape memory and superelastic properties. This improvement comes with the cost of a lower NS alloy fatigue damage tolerance, aggravated by the presence of microcracks caused by cold working. Binary Ti–Ni alloys, processed by annealing of an intermediately cold-worked ( e = 0.75…1) alloy and containing mixed nanocrystalline structure and polygonized dislocation substructure, allow a high fatigue life combined with relatively high and cyclically stable functional properties.

Microstructure characterization of nanocrystalline Ni 3C synthesized by high-energy ball milling

Preparation of a metal carbide by a conventional route requires a huge instrumentation regarding melting the metal and graphite under vacuum at a very high temperature. However, mechanical alloying process is a very useful method for the fabrication of high melting point materials like metal carbides, which additionally produce nanocrystalline structure with improved properties. This article reports the microstructure characterization of nanocrystalline Ni 3C synthesized at room temperature by mechanical milling of Ni and graphite powders under inert atmosphere. The stoichiometric Ni 3C phase is formed after 5 h of milling. Microstructure of the prepared materials is characterized in terms of lattice imperfections, primarily by analyzing the X-ray powder diffraction data, employing both Warren–Averbach's method of line profile analysis and Rietveld's structure refinement method. The phase transformation kinetics studied by the Rietveld method reveals that the graphite powder becomes amorphous at the early stage of milling and then diffuses into Ni matrix and a Ni–C solid solution is formed within 1 h of milling. The high-resolution electron microscopy reveals that the Ni 3C phase is initiated as a thin shell on Ni–C core within 2 h of milling and becomes stoichiometric in composition within 5 h of milling. Electron microscopy observations agree well with the X-ray measurement of particle size (∼9 nm) of Ni 3C phase in 5 h milled sample and Ni–Ni 3C core shell structure in 3 h milled sample.

Influence of Severe Thermomechanical Treatment on Formation of Nanocrystalline Structure in Ni 718 and Ni 718Plus Alloys and their Mechanical Properties

The influence of severe thermomechanical treatment via multiple forging on the formation of a nanocrystalline (NC) structure in bulk samples of Alloy 718 and ATI 718Plus has been investigated. It was observed that a step-wise decrease of processing temperature from 950 down to 575°C allowed the refinement of the initial coarse grain structure to a NC state. Investigations of structural changes in the deformed samples have shown that extending the temperature interval of dynamic recrystallization to low homologous temperatures resulted in the formation of a fine-grained recrystallized structure. The temperature of NC structure formation in ATI Alloy 718Plus was 50-100°С higher than that required for alloy 718. This was due to the presence of the additional γ′-phase, which increased the recrystallization temperature. This decreased the total strain required to produce NC structure, as compared with Alloy 718. It was observed that increasing the total strain and decreasing temperatures step-wise during deformation via multiple forging resulted in a uniform structure across the cross-section of the samples. The room temperature mechanical properties of the investigated alloys with various grain sizes are also compared.

Role of strain gradient on the formation of nanocrystalline structure produced by severe plastic deformation

In the present study, the formation of nanocrystalline structure by high speed drilling was investigated. Several micrometers thick nanocrystalline layer was observed at the top surface of drilled hole. Nanocrystalline layers showed high hardness and good thermal stability. The estimation of necessary dislocation density to produce nanocrystalline structure and the comparison between the estimated and measured strain gradient near the drilled hole surface was conducted. It is proposed that deformation with a large strain gradient is an important condition to produce nanocrystalline structure.

Nanocrystallization of Cu<SUB>50</SUB>Zr<SUB>45</SUB>Ti<SUB>5</SUB> Metallic Glass Induced by Electron Irradiation

The effect of electron irradiation on the structure of melt-spun Cu50Zr45Ti5 metallic glass was investigated. Microstructural evolution during electron irradiation was examined by means of conventional and high-resolution transmission electron microscopy (CTEM and HRTEM) equipped with X-ray energy dispersive spectroscopy (EDS). The glassy phase was not stable under electron irradiation and crystalline nanoparticles with the monoclinic CuZr structure were formed. The volume fraction of the nanocrystalline phase increased with the electron dose while the average grain size remained at about 7 nm. This result indicates that electron irradiation is effective in producing nanocrystalline structures from the metallic glass. [doi:10.2320/matertrans.47.1930]

Nanocrystalline Structure in Steels Produced by Various Severe Plastic Deformation Processes

The formation of nanocrystalline structure in steels by ball milling, shot peening and drilling were studied. In ball milling and shot peening, nanocrystalline layers form with sharp boundaries from deformed structure regions. Nanocrystalline layer showed extremely high hardness. By annealing, nanocrystalline layer showed substantially slow grain growth without recrystallization. The temperature of the specimen during deformation is low and deformation is done in ferrite state. In drilling, several μm thick nanocrystalline layers form at the top surface of a drill hole. Nanocrystalline layers showed high hardness and good thermal stability. The fresh martensite and retained austenite near a drill hole indicate that the temperature reached above Ac3 and nanocrystalline layers are produced in austenite condition. It is recognized that nanocrystalline layers produced in the processes studied in the present investigation has similar characteristics irrespective of the temperature it produced. It is proposed that deformation with a large strain gradient is an important condition to produce nanocrystalline structure.

Microstructural evolution and nanocrystal formation during deformation of Fe–C alloys

Abstract The microstructural evolution has been investigated under different plastic deformation processes, which produce nanocrystalline structures from coarse-grained crystalline materials. Fe–C alloys were selected due to their popular application in industry and submitted to the deformation induced nanocrystallization processes. The investigation revealed that grain refinement to submicron level during deformation is related to the production of dislocations and their transformation to subgrain boundaries at low strain levels. The further refinement to a nanocrystalline structure is realized with the aid of high strain rates, which lead to the production of high densities of dislocations in a short time period to form nanoscaled cells and subgrain boundaries. These transform into nanocrystalline grain boundaries via the possible dynamic recovery and/or recrystallization processes induced by adiabatic heating at localized deformed regions.

Formation of Nanocrystalline Structure at the Surface of Drill Hole in Steel

Microstructure of martensite steel near drill hole surface was investigated. The microstructure of drill hole surface can be divided into 3 layers with depth. From top surface, nanocrystalline layer, equiaxed submicron grain layer and deformed martensite structure layer. Nanograined layers have extremely high hardness, similar to those observed in the specimens treated by shot peening, ball milling, ball drop and particle impact deformation. The measured amount of shear strain produced by drilling was found to be an exponential function of depth from the hole surface. The amount of true strain at the hole surface region was estimated to exceed 7 which is considered to be the necessary amount of strain to produce nanocrystalline structure.

鉄鋼材料のボールミルによるナノ結晶化機構の解明とそのバルク材表面ナノ結晶化への応用

The mechanism of nanocrystallization in steels by ball milling was studied. The importance of severe plastic deformation at high strain rate rather than repeated cold-welding was recognized. This idea was confirmed by the nanocrystallization in other deformation methods such as a ball drop, a particle impact and shot peening deformations those can apply similar severe plastic deformation to specimens with that of ball milling. The nanocrystalline structure produced by these methods has essentially the same characteristics; sharp boundary with work-hardened region, dissolution of cementite, substantial high hardness (about 10 GPa) and without recrystallization and slow grain growth by annealing. It was proposed that a large strain (larger than 7.3 in true strain) and high strain rate (above about 104/s) are necessary deformation conditions to produce nanocrystalline structure in steels. It was suggested that shot peening is the most practical technique to produce nanocrystalline surface layer.

Experimental test of Néel's theory of the Rayleigh rule using gradually devitrified Co-based glass

It is shown that gradually devitrified Co-based nonmagnetostrictive metallic glass is an excellent model material to verify Louis Néel's theory of the Rayleigh rule. In the course of the calculations, Néel showed that the parameter p= bH c/ a (where H c is the coercivity, a and b are the coefficients of a quadratic polynomial expressing the Rayleigh rule) is expected to range between 0.6 (hard magnets) and 1.6 (soft). However, the experimental values of this parameter, reported in the literature for a number of mono- and poly-crystalline magnets, are much greater than those expected from the theory presented by Néel (in some cases even by two orders of magnitude). The measurements, performed for a series of Co-based metallic glass samples annealed at gradually increasing temperature to produce nanocrystalline structures with differentiated density and size of the crystallites, have shown that the calculated values of the parameter p fall within the range expected from Néel's theory.

Gradually Devitrified Co-Based Metallic Glass As a Model Material for Testing Néel's Theory of the Rayleigh Rule

It is shown in the present paper that gradually devitrified Co-based non-magnetostrictive metallic glass is an excellent model material to verify Louis Neel's theory of the Rayleigh rule. In the course of the calculations, Neel showed that the parameter p = bH c /a (where H, is the coercivity, a and b are the coefficients of a quadratic polynomial expressing the Rayleigh rule) is expected to range between 0.6 (hard magnets) and 1.6 (soft). However, the experimental values of this parameter, reported in the literature for a number of monoand poly-crystalline magnets, are much greater than those expected from the theory presented by Neel (in some cases even by two orders of magnitude). The measurements, performed for a series of Co-based metallic glass samples annealed at gradually increasing temperature to produce nanocrystalline structures with differentiated density and size of the crystallites, have shown that the calculated values of the parameter p fall within the range expected from Neel theory.

Ferromagnetic resonance studies of amorphous and nanocrystalline FeCuNbSiB alloys

Alloys with composition Fe73.5Cu1Nb3Si22.5−xBx, where x=6 and 9, have been studied from the as-cast state, through various stages of crystallization, in annealing range of 300–650 °C, by the technique of ferromagnetic resonance (FMR). The annealing was performed isothermally at preset temperatures to produce nanocrystalline structures in an amorphous matrix. Both the nanocrystalline structures and the surrounding amorphous matrix are ferromagnetic, and will therefore contribute to the FMR spectra. The spectral features, resonance field, intensity, and linewidth have been used to characterize the structurally related changes in the sample during the crystallization process. The major changes in the spectra are observed to occur in the region of the crystallization peak in the differential thermal analysis curves for these samples. The FMR spectra exhibit a complex in-plane angular variation, which is understood in terms of preferential orientation of the magnetization vector in the direction of the ribbon, and shape effects. The square cut samples give rise to multipeaked spectra when the external magnetic field is applied in an “off-square” direction. This is the first report of the appearance of a second resonance feature of this type. This is explained as arising from the magnetic confinement of the nanocrystallites in the amorphous matrix, producing spin waves localized at the interface of the two phases.

Magnetic properties of Fe 80B 20 alloy with nanocrystalline structures

The magnetic properties of devitrified amorphous Fe 80B 20 ribbons have been studied by Mössbauer spectroscopy and magnetization measurements. The mean magnetic hyperfine field of the interfacial structural components has been found to be higher in nanocrystalline samples than in microcrystalline ones: this effect is ascribed to the epitaxial growth of small clusters of bcc Fe, with topological disorder, on Fe 3B during annealing, producing nanocrystalline structures. The interfacial components present large topological disorder with three kinds of chemical short-range order. Magnetization measurements show larger hysteresis loops after heat treatment, indicating the formation of magnetic phases with higher anisotropy.

The positron annihilation and hall-petch relation in polycrystalline Fe 78B 13Si 9 alloys with ultrafine grains

Nanometer-sized crystalline materials, which are sometimes called nanocrystalline materials', have been reported to exhibit different properties from ordinary crystalline materials. The possibility of affecting the physical or mechanical properties of materials by producing nanocrystalline structures constitutes a major subject of interest in materials science. The positron annihilation and the Hall-Petch behaviors in the nanocrystalline FeBSi alloy prepared using the crystallization method were studied. The results indicate that because of the unusual nature of its grain boundaries (existing a great amount of defects) the nanocrystalline FeBSi alloy has higher values of microhardness than those of coarse grained crystalline and amorphous FeBSi alloys with the same composition. Hardness increases with the reduction of grain size in the nanocrystalline FeBSi alloys, i.e. the normal Hall-Petch relationship was obtained. The variation of interface characteristics with the grain size will strongly influence structure-sensitive properties, such as H[sub v]. From the present results, a very good overall agreement between the trends of the mean lifetime value [bar [tau]] and Vickers microhardness value H[sub v] can be obtained.

Nanocrystalline twinned structures produced in an aluminium-iron intermetallic compound by ball milling and hot deformation : M.A. Morris, D.G. Morris, (University of Neuchâtel, Switzerland), Materials Science and Engineering, Vol A136, 1991, 59–70

In order to obtain nanocrystalline structures, different techniques are available which may lead to certain modified properties of the final materials. Amongst these, mechanical alloying by ball milling offers the unique possibility of producing nanocrystalline structures of different alloy systems. Materials, such as intermetallics, where large areas of disordered grain boundaries may favourably affect the mechanical properties, can be expected to benefit most from such nanocrystalline structures.This study attempts to understand the effect of composition, milling time and consolidation temperature on compound formation and structural stability of the intermetallic Al3Fe. It has been found that nanocrystalline structures of Al3Fe can directly form by ball milling of powders with composition Al-20at.%Fe and Al-25at.%Fe whilst for the composition Al-12.5at,.%Fe the compound only forms after annealing the milled powders. The degree of long-range order of the latter seems to increase for increasing milling time prior to annealing. The high-temperature-compacted material has been found to have a lower degree of long-range order because of the presence of twinning faults produced by the hot-deformation process.