Vysoká škola báňská Technická univerzita Ostrava NANOMATERIALS II. Selected chapters. Miroslav Greger

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1 Vysoká škola báňská Technická univerzita Ostrava NANOMATERIALS II. Selected chapters Miroslav Greger Ostrava 2015

2 CONTENT 1. PREFACE: SPD NANOMATERIALS AND NANOTECHNOLOGY 3 2. PROPERTIES OF FINE GRAINED AND NANOSTRUCTURED MATERIALS Mechanical properties of fine grained materials Modulus of elasticity in tension Yield strength, hardness, strength Hall-Petch relation Diffusion creep Grain boundary sliding Yield strength at very small grains 7 3. PLASTIC PROPERTIES OF ULTRA-FINE GRAINED AND NANOSTRUCTURED MATERIALS Ductility, plasticity Superplasticity 9 4. GRAIN GROWTH AND TEMPERATURE STABILITY OF ULTRA-FINE GRAINED AND NANOSTRUCTURED MATERIALS Grain growth Temperature stability METHODS OF PREPARATION OF ULTRA-FINE GRAINED MATERIALS Batch processing ECAP Equal Channel Angular Pressing HPT High Pressure Torsion HPTT - High-Pressure Tube Twisting RCS Repetitive Corrugation Straightening TE - Twist Extrusion CONTINUOUS METHODS OF PREPARATION OF ULTRA-FINE GRAINED MATERIALS Continuous processes ARB Accumulative Roll Bonding Conshearing Process C2S2 - Continuous Confined Strip Shearing ECAP Conform Continuous RCS (Repetitive Corrugation and Straightening) Comparison of the severe plastic deformation methods Research of steels with ultra-fine grained and nanocrystalline structure The most important research workplaces CONCLUSION 23 LITERATURE 24 Miroslav Greger 2

3 1. PREFACE: SPD NANOMATERIALS AND NANOTECHNOLOGY In recent 20 years the attention was focused on the research of nanostructured materials, which promise to achieve new mechanical and other physical properties in comparison with their coarse-grained equivalents. There are various shapes and forms of nanostructured materials featuring extraordinary chemical, physical and mechanical properties. If the grain size is in the critical size region, i.e nm, then 50 vol.% of atoms are connected with grain boundaries or interphase interface. On the basis of this fact the grain boundary regions influence the complex properties of nanomaterials. In this case dislocation chains do not originate and the Hall-Petch relation for formation of coarse-grained material is not applied. Nevertheless, grain boundaries play an important part for deformation of nanomaterials, the most frequently superelasticity occurs here at lower temperatures than in conventional materials. Structures of nanocrystalline materials can be divided into four groups: dimensionless atomic clusters, one-dimensional modulated layers, two-dimensional fine-grained layers, three-dimensional nanostructures. Nanocrystalline materials can comprise crystals, quasicrystals and amorphous phases, which may occur in metals, intermetallics, ceramics and composites. Gleiter divided nanocrystalline matters into twelve groups relating to their shape and chemical composition of basic structural elements. The first dividing of nanomaterials relates to shapes of crystallites: layered crystals, rod crystals (the thickness of which is in order-ofmagnitude of several nanometers) and equilateral crystals. The second dividing categorizes the above mentioned three groups into four subgroups according to the difference in chemical composition of crystals. The atomic structure of all crystals is identical. A single difference between them is their crystallographic orientation. However, this does not apply in regions of boundaries, where crystals meet. In the regions of crystal boundaries the average atom density and ordering between the nearest neighbours of atoms differ. The presence of two structural components of a comparable volume with a typical crystal size of several nanometers is decisive for properties of nanostructural materials. An important group of nanomaterials are nanocomposite layers, the thickness of which is lower than 100 nm. Many times the layers consist of a two-phase structure and are formed as a result of segregation of the second phase on the boundaries of the first phase. The essential property of these layers is that the number of atoms in the boundary regions is comparable to or higher than the number of atoms in the volume, which are surrounded by the segregated atoms. This means that the material properties are not determined only and solely by atoms in grains, as in polycrystalline metals, but also by behavior of boundary atoms and their mutual interaction. This fact dramatically differs properties of highly fine-grained layers from properties of polycrystalline layers consisting of large grains (> 100 nm). Nanocrystalline materials are perspective materials with many unique properties. Their greatest asset is a possibility to enhance usable properties substantially in comparison with conventional coarse-grained materials. Some ceramic and metal nanocrystalline materials have been used in operation already. While the material grain sizes decrease, the grain surface sizes increase, which in a case of metal materials leads to enhancement of strength and hardness, electrical resistance, an increase in the specific thermal capacity, a decrease in the thermal conductivity and enhancement of magnetic properties. Nanocrystalline metal materials can also be applied as catalyzers with an active area many-fold higher in comparison with coarse-grained materials. In preparation of nanocrystalline metal structural materials, the preparation of bulk nanostructured materials has been and still is a problem. Over time, various methods for the preparation of these materials have been developed, whereas the main problem, which is being solved now, is the inner homogeneity of semiproducts (in the preparation technologies based on compaction of powder nanomaterials), a size of the obtained nanostructured materials (in technologies using grain refinement by severe plastic deformations) and also plastic properties of materials manufactured this way. Another remaining problem is enhancement of resistance of fine-grained materials against grain growth during its processing at higher temperatures or heating, which is many times necessary for performing the forming process and for achieving required properties. Usually, nanostructured materials are considered those the structure of which consists of components having at least one dimension between nm. These components may be atom clusters (to c. 10 nm), larger particles ( nm), subgrains, grains, lamellae, layers, filaments, tubes, etc. For instance, lamellar pearlite can be considered a nanocomposite (it consists of ferrite and cementite lamellae of widths usually below 100 nm). Miroslav Greger 3

4 Many authors use a term nanostructured material for the materials comprising particles of sizes below 1 micrometer. For materials with components ranging between nm, the name ultra-fine grained materials is also used. The research of preparation of ultra-fine grained and nanostructured materials and investigation of their properties and possibilities of their practical application has been focused on a large quantity of chemical elements, particularly metals, their alloys, but also e.g. on semiconductors. From the point of view of practical operation, the highest attention is nowadays focused on aluminum, copper and titanium alloys and a number of intermetallic compounds. The research of preparation technologies and properties of ultra-fine grained structured steels is essential, too. These lecture notes are a follow-up to the part Nanomaterials I and deal particularly with preparation of ultra-fine grained materials through the extreme plastic deformation methods known under the abbreviation SPD (Severe Plastic Deformation). This is the topic the main attention is focused on in these lecture notes. 2. PROPERTIES OF FINE GRAINED AND NANOSTRUCTURED MATERIALS Time needed for studying: 90 minutes Aim: After studying this chapter you will Define ultra-fine grained and nanostructured materials Describe the dependency of mechanical properties of nanostructured materials on a grain size Calculate strength properties of nanostructured materials using the Hall-Petch relation Explication 2.1 Mechanical properties of fine grained materials It has been known that strength and hardness of a material increase, while a grain size in its structure decreases, which has been known since the beginning of the fifties of the last century, when N.J. Petch and E.O. Hall independently of each other defined the well-known Hall-Petch relation: σ y = σ o + k d -1/2, (1) where σ y stands for yield strength, σ o stands for a certain stress required to overcome Peierls-Nabarro friction stress, resistance of dissolved foreign atoms, resistance of precipitates form the solid solution and defects of a lattice, k stands for a constant and it is a factor for the shear stress required to release accumulated dislocations and d stands for a grain dimension. Equation (1) implies that the material yield strength increases, while the grain dimensions decrease. This applies also for hardness of a polycrystalline material. This phenomenon became a driving force for the research and development of high strength structural materials, especially in steels. Later it was shown that refinement may lead also to the increased ductility of metallic materials. A preparation of sufficiently large samples with small dimensions of structural components was the main problem for investigation of strength properties in particular. Attention to this topic was turned in the early eighties of the last century especially by Gleiter et al., who started to deal with preparation and properties of materials with ultra-fine grained structure, which were called nanocrystalline materials i by them. Plain enough was a consideration that based on the same hardening mechanism, reducing the grain sizes down to the nanometer level may result in a great increase in the material strength. It can be calculated that for grain sizes of nm the yield strength value gets close to the theoretical strength of the material. Over time Miroslav Greger 4

5 the validity of relation (1) has proved, with the exception of its validity for large grains and very fine grains (c. below 10 nm). Step by step, the results of the original investigation of ultra-fine grained materials were revised, since the results were not reliable due to faulty preparation of the samples. At those times the materials were prepared from nanoparticles (nanocrystals) by imperfect compacting processes (sintering etc.), leading in the early stages of the research to structures containing many pores and other defects (e.g. cracks), which distorted the results. Nanocrystalline materials contain high density of grain boundaries and other interfaces, which has led to an idea of a possibility to extend the effect of high-temperature deformation mechanisms respecting the role of grain boundaries to lower temperature regions. For example, there was an idea that a nanocrystalline material would be deformable by processes controlled by the grain boundary diffusion at significantly lower homologous temperatures. This indicated a possibility of production of plastic ceramics, effect of diffusion creep of pure copper at room temperature, a possibility of superplastic behavior of metals and ceramics at low temperatures etc. In the following part the most important mechanical properties of materials with ultra-fine grained and nanocrystalline structures and mechanisms of processes that determine them are discussed Modulus of elasticity in tension Although former works pointed out low values of the elasticity modulus of nanocrystalline materials compared to coarse-grained materials, it has been judged nowadays that modulus of elasticity in tension E is the same for polycrystalline as well as for nanocrystalline materials Yield strength, hardness, strength The basis of every deformation behavior is kinetics of generation of defects (lattice defects, on interphases), their motion and annihilation. Micro-mechanisms respecting lattice dislocations, dislocations on grain boundaries and vacancies are particularly important. These defects may contribute to the total plasticity, independently or in a combination, but a dominating mechanism can be identified by evaluation of strain rate, grain size and temperature dependence. Above all, three ideas of the mechanical behavior of nanocrystalline materials have attracted the attention: 1. The Hall-Petch relation, in which the deformation stress dependency on the grain size at low temperatures results from the process of blocking the move of dislocations on grain boundaries. 2. Diffusion creep mechanism which involves the movement of vacancies at the gradient of the applied stress. 3. The mechanism of grain boundary sliding, which involves a movement of all three above mentioned defects depending on specific micro-mechanisms Hall-Petch relation In the above mentioned Hall-Petch relation (1), σ o changes along with chemical composition of material, structure and technological processing. k constant is thermally independent, but σ o increases noticeably, while temperature decreases. Lots of mechanisms were suggested for explanation of the Hall-Petch relation, three of which are schematically depicted in Fig. 1. Dislocation pile-up before the grain boundary, which at a specific stress activate the Frank-Read source in a neighbouring grain and deformation propagates through the grain and through the entire material afterwards. Generation of dislocations on ledges of grain boundaries formed during the deformation. Generation of dislocations on grain boundaries forming a hardening layer on them. In this case equation (1) includes also the expression d 1, which is significant at small grain sizes and reduces a value of k constant. Other mechanisms were proposed by Conrad, Ashby and others; they are based on activity of dislocations in grains or near their boundaries. Miroslav Greger 5

6 Fig. 1 Basic models proposed to explain the Hall-Petch relation It was found out that the Hall-Petch relation (1) applies for various materials approximately to the grain dimension of 30 nm, then the strength stops to increase or even decreases. This is schematically depicted in Fig. 2. A region below the critical grain size d c (regime II - G.B. sliding) is sometimes called the inverse Hall-Petch relation and dislocation mechanisms stop to have effect here. Fig. 2 Hardness (strength) to the grain size dependency in nanometer field Diffusion creep Coble has proposed a modification of the known Nabarro-Herring diffusion creep mechanism, involving into their relation the faster grain boundary diffusion at medium temperatures. He presented the relation έ = 47.7 (Ω δ D g σ/k T d 3 ), (2) where έ stands for the strain rate, Ω stands for the atomic volume, δ stands for the grain boundary thickness, D g stands for the grain boundary diffusivity, σ stands for the strain stress, k stands for the Boltzmann constant, T stands for the absolute temperature and d stands for the grain size. The grain boundary thickness value is typically 1 nm. According to relation (2), if the grain size decreases from 10 μm to 10 nm (i.e. by three orders of magnitude), the strain rate at creep will increase by nine orders of magnitude. At the same time, it is known that the grain boundary diffusivity in nanocrystalline materials is higher than in materials with larger grains. A combination of these phenomena has led to the expectation of a possibility of plastic deformation of ceramic materials and intermetallic compounds at room temperature. The theory of Coble creep effect in a region below d c was supported e.g. by Masumura et al. In his model the dependency of the H-P relation should be sensitive to temperature, which has not been observed so far Grain boundary sliding A constitutive relation for grain boundary sliding can be written as: έ = (D g G b/k T) (b/d) 3 (σ/g) 2 (3) where G stands for shear modulus, b stands for the Burgers vector, the other symbols have the same meaning as in relation (2). The grain boundary sliding is a dominating deformation mechanism of the structural superplasticity of fine-grained materials. A prerequisite for the relation (3) validity for nanocrystalline materials is the grain boundary sliding micro-mechanism effect in this dimension area. Miroslav Greger 6

7 The validity of relations (2) and (3) was tested experimentally. Fig. 3 shows the dependencies of the strain stress on the strain rate according to relation (2) a dashed line and relation (3) a dotted line for 673 K temperature. Both the relations are compared with the results of tests of pure iron with grain dimensions of 100 nm, made by severe plastic deformation, without pores and cracks that can be found in samples manufactured by powder metallurgy. A high discordance between the experimental results and theoretical assumptions can be seen. The measured values of the strain stress are much higher. The diffusion creep model will need to be made more accurate then Nanocrystalline Fe, 673 K, 100 nm Experimental values Diffusion creep (Coble) Grain boundary sliding (Sherry and Wadsworth) Stress (MPa) Strain rate (s 1 ) Fig. 3 Comparison of experimentally found values of strain stress with a theoretical prediction according to relations (2) and (3) for nanocrystalline iron with grain dimensions of 100 nm Yield strength at very small grains Unsuitability of the Hall-Petch relation (1) for a calculation of yield strength of nanocrystalline materials (see Fig. 2) at the grain size values below c. 30 nm and the evident inapplicability of relations (2) and (3) led to an attempt to explain this phenomenon. This co-called inverse Hall-Petch relation was observed in many materials and a relationship is assumed. k 1 d. (4) y / To explain the limited validity of the Hall-Petch relation, the following models are used: An influence of the volume occupied by triple points. As the grain dimension decreases to nanometers, the volume proportion of the triple point area, to which softening of material may be ascribed, becomes more significant. This idea was for the first time presented by V.B. Rjabuchin and supported experimentally e.g. by Palumbo et al. on samples of nanocrystalline Ni obtained by electro-deposition. A composite model. This model takes into account both the influence of the crystal matrix and the intercrystalline layers. Grjaznov et al. formulated a dependency of yield strength on the grain size of nanocrystalline materials as a sum of a contribution of a matrix and an intercrystalline contribution. Both of the contributions are connected linearly with the corresponding shear moduli and they are the grain size function. This model seems not to be adequately theoretically supported. Conclusion While the material grain size decreases, its strength (hardness, yield strength) increases as high as to the nanocrystalline material region. The Hall-Petch relation describes this phenomenon well enough (1). This fact is a driving force for the research of manufacturing technologies of massive nanocrystalline materials for structural purposes. In the grain critical size area d c (below c nm) the dislocation activity diminishes and yield strength (very high) becomes independent of the grain size, or even decreases in some materials. Miroslav Greger 7

8 3. PLASTIC PROPERTIES OF ULTRA-FINE GRAINED AND NANOSTRUCTURED MATERIALS Time needed for studying: 90 minutes Aim: After studying this chapter you will: Define plastic properties of ultra-fine grained and nanostructured materials Calculate the basic plastic properties Determine an influence of strain rate, homologous temperature and grain size on the superplastic behavior of metal materials Explication 3.1 Ductility, plasticity Plastic deformation in conventional methods of forming, such as rolling, drawing or extruding, can significantly increase strength of metals. Nevertheless, this increase is usually accompanied by a loss of ductility. This phenomenon is shown in Fig. 4 in the dependency of yield strength on elongation at break for a number of coarse-grained pure metals. Fig. 4 also shows two curves obtained by cold forming of Cu and Al samples. The percentage data at the particular points refer to the values of the sample thickness reduction. The values for Cu and Ti with the structure in nanodimensions are shown here, too. The Cu sample was formed by the ECAP method and the Ti sample by HPT. The forming was performed at room temperature. The grain size was c. 150 nm for both the materials. For copper forming 16 passes were needed to increase ductility. For Ti, 5 turns of the tool were used to increase ductility. Application of the high plastic strain in both the cases has resulted in an increase in strength as well as ductility. This was the first observation of the phenomenon. It implies the change in the deformation mechanism during forming through the severe plastic deformation methods using very high strain. Fig. 4 General dependency of strength and plastic properties for metal materials There are three ductility limiting factors in ultra-fine grained and nanostructured materials: 1. Artefacts formed during the preparation of material. If the material was manufactured by powder compaction (the so-called two-step method), porousness of material is particularly important. 2. Instability when subjected to a tensile load. During tensile testing, necking begins at maximum loading. Uniform extension in the cylindrical sample depends on strain hardening, thus the real deformation = n (n stands for the strain hardening coefficient). For the ideal plastic material, such as amorphous alloys, n = 0 and the necking (instability) begins as soon as a localized plastic deformation occurs. 3. Nucleation of cracks or instability during their propagation may also influence the resulting ductility of material significantly. Ductility, which is an ability of material to change its shape without failure, is in a homogenous material depending on the magnitude of strain hardening and the sensitivity to strain rate. High values of these parameters help to suppress the initiation of the localized deformation (necking) at the tensile stress and thus to Miroslav Greger 8

9 increase ductility. The strain hardening results from the accumulation of crystal lattice defects, such as dislocations, which makes further deformation more difficult. However, in nanostructured metals the accumulation of dislocations is impossible, since grains are too small ii. Dislocations are emitted from one grain boundary segment and vanish in another, thus the inside of the grains is dislocationless. In many nanostructured metals a zero strain hardening has been really observed. Nevertheless, a distinctive relationship between a certain mechanism of plastic deformation and ductility has not been investigated yet. The endeavor to enhance low values of nanocrystalline materials ductility is a subject of continuing research, the results have been optimistic so far. 3.2 Superplasticity The superplasticity phenomenon was observed in many fine-grained crystalline materials. For this phenomenon to occur, two essential conditions must be met: 1. The material grain size must be very small (typically below 10 μm) and grain boundaries must be large-angled. 2. Deformation in tension must be applied at relatively high temperatures, so that diffusion-controlled mechanisms can occur. This is usually at temperatures above 0.5 T m. During superplastic deformation, grains preserve an equiaxed shape in principle, even if elongated very much. Necking at fracture has not been found. Under optimal conditions, grain boundary sliding is the control mechanism. However, this sliding cannot occur without an initiation process inside grains and theoretical models of superplasticity assume that the sliding is initiated by a motion of dislocations in grains and the sliding speed is determined by the rate of the dislocations climb to the opposite grain boundaries. The strain rate at the superplastic behavior depends both on the stress and the grain size Fig. 5. Superplasticity occurs at medium values of the strain rate. At higher stresses a transition to the dislocation climb mechanism with a stress exponent ~ 5 can be found. At lower stresses a transition to the impurities-controlled mechanism on grain boundaries can be found, again with a stress coefficient ~ 5. The strain rate in the superplastic region changes along with 1/d 2 (d stands for the grain size). In the dislocation creep region the grain size influence does not occur, because the control mechanisms are processes inside grains. A grain size decrease moves the superplastic region towards higher strain rates and simultaneously the impuritiescontrolled region moves towards that direction, too. A grain size decrease from 2 m to c. 200 nm reduces the total deformation time from c minutes to c s. It was also found out that while the grain size decreases at high strain rates, the elongation at break increases (insufficient time for cavities to form and grow on grain boundaries). Fig. 5 Schematic depiction of the strain rate stress dependency referring to superplasticity in the central area of the strain rate and a change in the curve location towards higher strain rates at the decreased grain size The above mentioned findings lead to the idea of superplasticity at high strain rates even at lower temperatures, namely in ceramic materials, intermetallic compounds and alloys, which has brought about a considerable interest in study of superplasticity of ultra-fine grained and nanostructured materials. Some of the achieved outcomes: Lu et al. studied superplasticity of nanocrystalline copper ( %) with grain dimensions c nm. The material was prepared by electrodeposition. Plastic deformation was initiated by rolling at room temperature. Strain rate ranged between s -1. Compared to polycrystalline copper, where ductility up to 500 % can be achieved, elongation up to 10x higher was achieved Fig. 6. Miroslav Greger 9

10 Fig. 6 Shapes of copper samples after superplastic deformation After annealing of samples at 500 C temperature for a period of 48 hours, the grains grew to 100 m and ductility of c. 700 % was achieved, but the material was significantly harder and tore on the edges of the strips the same way as observed in conventional materials. Mishra, et al. dealt with deformation mechanisms and superplasticity in tension of nanocrystalline materials for a lot of alloys, compounds and Ni. For grain sizes between nm, at various temperatures, the elongation between % was achieved. Xu, et al. investigated the ability to superplastic behavior for three aluminum alloys (AlMgSc, AlCuMg and AlZnMgCaZr) after the severe plastic deformation by the ECAP method. The grain sizes after the deformation ranged between nm. All the alloys exhibited a good to exceptional ability to superplastic forming (elongation of 200, 450 and 1100 %). Information on superplastic behavior of steel with ultra-fine grained structure was not found out. Conclusion Plastic deformation in the conventional forming can increase strength of metals significantly,this increase is usually accompanied by a loss of ductility,a grain size decrease from 2 μm to c. 200 nm reduces the total deformation time from c minutes to c s, while the grain size decreases at high strain rates, the elongation at break increases. Miroslav Greger 10

11 4. GRAIN GROWTH AND TEMPERATURE STABILITY OF ULTRA-FINE GRAINED AND NANOSTRUCTURED MATERIALS Time needed for studying: 90 minutes Aim: After studying this chapter you will: Describe the grain growth in conventional and nanocrystalline materials Define the activation energy for the grain growth in nanocrystalline materials Describe the grain growth kinetics and mechanism Determine the phases preventing or decelerating migration of grain boundaries Explication 4.1 Grain growth Regarding to small grain dimensions and ensuing large surface area, nanocrystalline materials tend to the grain growth. Knowledge of thermal stability of nanocrystalline materials is important both for scientific and technological reasons. From the technological point of view, the thermal stability is important for e.g. nanopowders consolidation with elimination of structure coarsening or for superplastic processing of ceramics and metals. From the scientific point of view, it is important whether the grain growth mechanism in nanocrystalline materials differs from the grain growth in coarse-grained materials. The grain growth in conventional materials is described by the equation d n d o n = K o exp (-Q/R T) t, (4) where d stands for the grain size after annealing the sample at temperature T for a period t, d o stands for the initial grain size, n stands for the grain growth exponent, K o stands for the constant, Q stands for the activation energy of the grain growth and R stands for the gas constant. Q and n are important parameters characterizing the grain growth kinetics and mechanism. The exponent n in an ideal case equals 2, which assumes a parabolic course of the grain growth. However, in nanocrystalline materials the exponent n values from 2 to 10 were observed. The value 2 is only reached when annealing is performed at relatively high values of the temperature proportion T/T m (T m is melting point of the investigated material). Factors, which could explain higher values of the exponent n, include segregation of dissolved substances on grain boundaries and grain boundary blocking. The activation energy for the grain growth in nanocrystalline materials (Q n ) is usually compared to the activation energy of volume diffusion (Q v ) or grain boundary diffusion (Q gb ) in coarse-grained materials. Usually Q n can be compared to Q gb rather than to Q v, although some exceptions were found. It was also observed that Q n value of nanocrystalline Fe at temperatures above 500 C got close to Q v for coarse-grained Fe and at temperatures below 500 C got nearer to Q gb, which indicates different mechanisms of the grain growth. A large grain growth was observed e.g. in annealing nanocrystalline materials prepared by high plastic strain or prepared by condensation of clusters and their consolidation. 4.2 Temperature stability In order to use the advantageous properties of a nanostructure is important to maintain the dimensions of nanocrystals during heat treatment, forming, in case of need during processing at higher temperatures, the same or only a little increased. One of the methods to achieve this is to locate a fine dispersed phase on the grain boundary to avoid or moderate their migration. This strategy was used e.g. by Kim et al. for development of a composite ceramic material, which sustained high strain rates during superplastic forming. The material was a nanocomposite containing ZnO, alumino-magnesium spinel and α-al 2 O 3. The nanocomposite Miroslav Greger 11

12 was superplastic even at strain rate 1 s -1. Ductility exceeding 1000 % was achieved. Another method is a segregation of impurities on grain boundaries and achieving the grain boundary stabilized structure. An example of this method can be found in the work by Liu et al with fine-grained RuAl containing impurities. Another method to prevent the grain growth is also residual porosity, which was observed in nanocrystalline ceramics and nanocrystalline Pd prepared by electrodeposition. Conclusion Nanocrystal size increases during forming and processing at higher temperatures, the grain growth in conventional materials is described by equation (4), the grain growth depends on temperature T, exposure time t, grain growth exponent n, constant K o and the activation energy of the grain growth Q, exponent n for conventional materials (coarse-grained) is about the value 2, for nanostructured and ultra-fine grained materials, n ranges between Miroslav Greger 12

13 5. METHODS OF PREPARATION OF ULTRA-FINE GRAINED MATERIALS Time needed for studying: 90 minutes Aim: After studying this chapter you will: Define the basic methods of preparation of ultra-fine grained and nanostructured materials. Describe the preparation of nanostructured and ultra-fine grained materials by the SPD processes. Select an applicable method of preparation of an ultra-fine grained structure through application of ECAP. Calculate a value of shear and real strain in the particular SPD technologies. Explication In order to achieve an ultra-fine grained or nanocrystalline structure, the real deformation c. 6 8 is needed and forming must be performed at low homologous temperatures. This chapter is focused only on the preparation of ultra-fine grained materials by Severe Plastic Deformation SPD. The advantage of application of the severe plastic deformation is, in comparison e.g. with compaction of powders, a possibility to obtain almost homogenous porousless material, nowadays even of larger dimensions. Over the years, a number of technologies using SPD were developed to form structures with a grain size between 10 to 1000 nm. Fig. 7 shows some methods used for the preparation of nanostructured and ultrafine grained materials. Fig. 7 Schematic depiction of the selected SPD methods: (a) ECAP, (b) Conshearing Process, (c) C2S2, (d) Conform The above mentioned methods serve above all for preparation of materials for research purposes. They are useful, because they allow various strain rates. They were used experimentally for the research of nanostructures of 4 types of steels. The High Pressure Torsion HPT method, which is described below, is used the most frequently of the methods shown in Fig. 7 for the research of various aspects of SPD effect on properties of materials. At present many SPD methods are used and developed for research and pilot plant experiments. These are both methods and technologies through which individual semi-products are made (Batch Processing) and Continuous SPD technologies. Following is a summary of these methods, which is not complete, however, it presents an adequate overview of the present state of the development. Miroslav Greger 13

14 5.1 Batch processing These technologies include: ECAP, HPT, MCF, RCS and TE ECAP Equal Channel Angular Pressing A typical flowchart of this technology is shown in Fig. 8. ECAP allows deformation of a metal sample by shear without changing its dimensions. A die comprises two channels with the same cross section, which intersect each other and create a bend. When extruding a sample through the die, the shear strain occurs in this place. The process can be repeated to achieve intensive plastic strain and a finer grain. The real deformation in one pass depends on the angle between axes of both the channels and to a less degree also on the bending radius. A sample can be rotated along its longitudinal axis between the particular passes, thus creating various technological routes of ECAP. The technological route influences the process of refinement of grains and their shapes significantly. To find the most effective technological route for grain refinement of individual materials is a subject of research. Fig. 8 Equal Channel Angular Pressing (ECAP) Four technological routes for the development of the formed material microstructure are studied systematically Fig. 9. Fig. 9 Technological routes of the microstructure development during the ECAP method application: Route A - material orientation between the individual passes does not change. Route B C - material is rotated clockwise by 90 after each pass. Route B A material is rotated by 90 alternatively clockwise and counterclockwise after each pass. Route C material is rotated by 180 after each pass. Another factor affecting significantly the development of the microstructure is the channel bending radius angle Ф. This angle determines the shear strain magnitude during the particular passes. The shear strain can be expressed by a relation: = 2 cotg( /2). (5) A less angle results in higher shear strain in each pass and then this arrangement is more effective for the grain refinement. Nakashima, et al. studied the influence of angle value between 90 o o for aluminum using the technological route B C. He found out that at the same number of passes the grain refinement is the most effective at 90 o angle. This is a result of 60 o angle, which is contained by two shear planes in the deformed sample in this case. For materials with low formability an angle of 120 o and a higher extrusion temperature are usually used. The amount of the real accumulated shear strain can be calculated from the relation: where N stands for the number of cycles (passes in a die). = 2N/ 3 cotg( /2), (6) Miroslav Greger 14

15 The microstructure development during the ECAP method was to the great extent studied only for materials with a face centered cubic lattice (Al, Cu) and with a hexagonal close packed lattice (Ti, Mg, Zn). A lattice of these metals contains few slip systems and the strain is influenced by a value of stacking fault energy. Metals with a medium to high value of the stacking fault energy (Al, Cu) are deformed above all by slip mechanisms, while metals with low values of the stacking fault energy (Ag) are deformed mainly by twinning. Metals with a body centered cubic lattice (Ni, Fe ) have more slip systems, such as {110}<111>, {112}<111> and {123}<111>, therefore they are deformed by slip mechanisms. In the meanwhile, the research of the ECAP method for forming metals with the body centered cubic lattice is of a limited extent, above all due to low formability of these metals at low and medium temperatures. The ECAP method has been still developed. Improved conditions of friction between the deformed sample and inner surface of the die resulted in obtaining an ultra-fine grained structure in materials with low formability, such as W and Ti. For example, ultra-fine grained bars with a diameter of 30 mm and length of 150 mm were manufactured from Ti. During the rod forming, the back-pressure on the formed material was also used, which increased formability of the used material significantly. Also, a rotational die was developed, the use of which eliminates a need to perform the extrusion of the entire sample and to re-insert it into the die between the particular passes. A device which is a modification of the ECAP method has been installed in the RMSTC laboratory (VŠB Technical University of Ostrava). The equipment is used for forming of long rods (the Conform technology) HPT High Pressure Torsion A flowchart of this technology and the photo of the real equipment are shown in Fig. 10. Fig. 10 Metal forming by the High Pressure Torsion (HPT) method Unconstrained HPT Constrained HPT The HPT method belongs among favourite severe plastic deformation processes. The effect of high pressure (several GPa) on a sample can be unlimited (Fig. 10) or limited by a die. Grains with sizes to 10 nm, or also smaller, can be obtained by this method. High-angle grain boundaries form during the deformation. Samples are of a disc shape, typically with a diameter of mm and thickness from 2 to 5 mm. They are placed between two tools, the one of which is rotating and the second one is fixed. Friction during rotation between the tool and a sample surface enables to increase shear strain gradually. Imposed compression stress during shear strain effectively eliminates a possibility of a sample failure, in spite of very large deformation. Significant refinement of the structure was observed just after half of a turn or a whole turn of the tool. However, more turns are usually needed in order to achieve a homogenous structure. The amount of the real shear strain can be calculated according to the relation: = 2 N r/d, (7) where N stands for a number of turns, r stands for a sample radius and t stands for its thickness. Miroslav Greger 15

16 The HPT method was used successfully for refinement of a microstructure of non-ferrous metals and alloys, steels, composites and semiconductors. At present, HPT is only used for processing of small samples, it is limited to a laboratory research HPTT - High-Pressure Tube Twisting This technology is used for processing of tubes without dimension changes. The deformation process is achieved by high hydrostatic pressure in the axial direction, initiated by a cylindrical mandrel. A tube is inserted into a mould and compressed by a mandrel. The extreme plastic deformation is achieved by friction force, external torque and hydrostatic pressure. HPTT appears to be very promising for future industrial applications. The flowchart of this method is shown in Fig. 11. Fig. 11 Method High Pressure Tube Twisting (HPTT) RCS Repetitive Corrugation Straightening The flowchart of this method is shown in Fig. 12. Two more used names and links are given here. This method principle was used for continuous process of high plastic strain on flat materials. Fig. 12 Repetitive Corrugation and Straightening (RCS) TE - Twist Extrusion This technology flowchart and an image of a workpiece are shown in Fig. 13. Fig. 13 Twist Extrusion method The twist extrusion principle consists in initiating intensive shear deformation by extruding a billet with rectangular cross section through a die with a twist channel. The channel shape and cross section does not change along the axis of extrusion, while the channel is twisted along this axis (Fig. 13a). The work-piece shape and cross section does not change as well, which allows repeated extrusion and thus an accumulation of plastic deformation. There are several possibilities of application of the pressure onto the extruded billet. One example is depicted in Fig. 13b. The billet is extruded through the die using a plunger. Twist extrusion can be used for processing of metallic materials (Ti). This technology is at the beginning of its development. Miroslav Greger 16

17 Conclusion Batch processing technology: ECAP Equal Channel Angular Pressing HPT High Pressure Torsion HPTT - High-Pressure Tube Twisting RCS Repetitive Corrugation Straghtening TE - Twist Extrusion Miroslav Greger 17

18 6. CONTINUOUS METHODS OF PREPARATION OF ULTRA-FINE GRAINED MATERIALS Time needed for studying: 90 minutes Aim: After studying this chapter you will: Define the ARB process principle. Describe the particular technologies of preparation of nanocrystalline materials. Compare the processes of severe plastic deformation, get to know the most important SPD research workplaces. Explication 6.1 Continuous processes These technologies include above all: ARB, Conshearing, C2S2, ECAP-Conform ARB Accumulative Roll Bonding A flowchart of this technology is shown in Fig. 14. The principle of this method is that two sheets with the same thickness stacked on top of each other are rolled simultaneously. In one pass, the thickness of the two sheets is reduced down to the thickness of one initial sheet. Then the sheet is cut into two, stacked together and this operation is repeated several times. This process is accompanied by microstructure refinement. For instance, when processing IF steel with % carbon content, the initial grain was reduced from 27 m to 420 nm after five cycles at the rolling temperature of 600 o C. The grain in the aluminum sheet (Al1100 grade) with a size of 37 m was reduced down to the size of 670 nm in 7 cycles at temperature of 200 o C. In spite of a significant increase in strength of the sheets, plastic instability at a decrease of the medium size of the grain below 1 m was found. The causes of this phenomenon have not been known yet. Fig. 14 Flowchart of the process Conshearing Process A flowchart of this technology is shown in Fig. 15. In principle, this is the ECAP method adapted for continuous forming. This method is intended above all for thin strip forming. The method was recently used for forming of Al1100 alloy and low-carbon steel. The continuous extrusion occurs, when friction force acting in gaps between four rolls is higher than the extrusion force. In order to initiate this phenomenon, the central roll surface is rough. It is important to find an optimal angle for forming various materials or samples of various thicknesses (see Fig. 15). For example, for 2 mm thick sample of Al alloy the optimal angle is c. 65 o and for a steel sample with the same thickness 55 o. Miroslav Greger 18

19 Fig. 15 Conshearing Process C2S2 - Continuous Confined Strip Shearing Thin strip Guide roll Upper die A flowchart of this technology is depicted in Fig. 16. In principle, this method is similar to the preceding one. It is applicable for forming of sheets (strips) with long lengths and large widths mm Feeding roll 1.45 mm 1.55 mm Lower die Fig. 16 Continuous Confined Strip Shearing (C2S2) ECAP - Conform A flowchart of this technology is shown in Fig. 17. This process is intended for continuous forming of rod-shaped ultra-fine grained materials. Fig. 17 Method ECAP Conform Continuous RCS (Repetitive Corrugation and Straightening) A flowchart of this technology is depicted in Fig. 18. The principle of this method is repeated rolling and straightening of a work-piece in a system of rolls with a cross section similar to a toothed wheel. This technology, applicable for sheet forming, is nowadays developed in the Los Alamos National Laboratory for industrial usage. The method involves a combination of shear and bending stress developed by compression on a special treated surface of rolls. The advantage of this method is its easy application on existing rolling mills. Its drawback lies in a structure inhomogeneity along the rolled piece, however, this can be eliminated by a higher number of passes. Fig. 18 Method Continuous RCS Miroslav Greger 19

20 6.2 Comparison of the severe plastic deformation methods Severe plastic deformation can be achieved through many methods, which can be divided according to processes of deformation: 1. Continuous deformation without changing a deformation route (compression, extrusion, HPT) 2. Accumulated deformation without changing a deformation route (rolling, drawing, ECAP route A) 3. Accumulated deformation with changing a deformation route (CEC cyclic extrusion and compression, ECAP route C) 4. Accumulated deformation with a variable deformation route (die forging, ECAP routes different than A and C, ARB) During all of these methods, material is formed at high hydrostatic pressure. ECAP remains the most favourite process used for formation of ultra-fine grained structures. It has a great potential for commercialization considering that the formed billets are expected to be of larger dimensions and regarding the development of continuous forming technologies (Conshearing, C2S2, ECAP- Conform). The research of optimization of dies and optimal modes and deformation routes for forming of specific materials is in progress. Another progressive technology (for strip forming) appears to be ARB (Accumulative Roll Bonding), which is a semi-continuous process. Practical application of ARB in commercial production is highly presumable. The essential advantage of this method is its potential to be used in conventional rolling mills. A temporary drawback is a structure inhomogeneity along the rolled product cross section and occurrence of cracks on edges in forming by large deformations. Possibilities to enhance ductility of formed materials need to be found, too. A microstructure formed during ARB is different from that formed during ECAP, since grains are elongated along the rolling direction. As in other methods, during ARB a mixture of low- and high-angle grain boundaries occurs. All the above mentioned methods need further development in deformation routes and reproducibility of properties of formed materials. It is also necessary for the methods to allow further grain refinement. Nowadays, the grains of sizes below 100 nm can be only achieved by the HPT method, which is not applicable for commercialization. In ECAP, an average grain size between nm and higher can be achieved so far. On principle, the grain refinement through ECAP is possible by increasing the total accumulated deformation. Technically this can be performed through forming by higher imposed pressures iii. The main technical problems of the development of the equipment for imposing high plastic deformations are similar to those that we face up when developing conventional forming technologies. The first of them is to maintain the integrity of the formed material. Lightweight ductile materials can be relatively easily formed at 20 C without fracture. More brittle materials require higher temperature at forming. This is limited by processes of recovering and structure recrystallization, which may negate a favourable influence of the finegrained structure. A certain solution is application of high strain compression, such as in the HPT or backpressure ECAP methods. However, the higher pressure demands a need to solve another problem a service life of forming tools. Prestrained dies were developed and tool materials were improved. The maximum compression strength of sintered carbides is approximately 3.5 GPa, which is not satisfactory e.g. for the present-day version of the HPT method. High stresses in tools along with high working temperature create difficult conditions for a solution. Friction also relates to the high pressure imposed on forming tools. This increases along with a forming force, hampers filling the die with material, causes wear of the inner surface of the die and in the worst case results in jamming of the material. Good lubrication is a good solution. This depends on the used material, technology parameters (above all on temperature), a way of application of a lubricant etc. Along with good lubrication, a tool surface finishing using hard coating with a low friction coefficient may help the situation. Laboratory devices for severe plastic deformations are no problem today. The use of standard tension testing machines is enough many times. The development of equipment for industrial use changes the situation dramatically. Basically, one of two possibilities may be chosen: either the use of existing presses and rolling stands or the development of brand new forming equipment. The first approach is cheaper, but more limited in term of results. It needs to be considered that operational equipment for severe plastic deformation is a system involving the preparation of billets, transportation and handling, lubrication, the forming equipment itself and other necessary equipment for following operations (heat treatment, final operations etc.). The technology needs to be monitored and controlled, too. In the meanwhile, no equipment for severe plastic Miroslav Greger 20