Literature Review Chapter Literature Review 2.1 Strengthening of crystalline materials By restricting dislocation motion, crystalline solids can be strengthened. Other dislocations, internal boundaries (such as grain, subgrain, or cell boundaries), solute atoms and second-phase particles are commonly employed as obstacles to the motion of dislocations. Additional dislocations generated during plastic deformation give rise to work or strain hardening. Material strength increases owing to the decrease in dislocation mobility concurrent with an increase in dislocation population. Strain hardening is caused by dislocations interacting with each other and with barriers which impede their motion through the crystal lattice. Dislocation multiplication can arise from condensation of vacancies, by regeneration under applied stress from existing dislocation by Frank-Read mechanism or multiple cross-slip mechanism or by emission of dislocations from a high-angle grain boundary. Generally, the rate of strain hardening is lower for hcp metal than for cubic metal. Solid solution impurity atoms are generally considered weak hardener whereas second-phase particles sometimes provide exceptional strengthening. Solute atoms have more influence on the frictional resistance to dislocation motion than on the static locking of dislocations. Generally, the strength of such precipitation or dispersion hardened alloys are limited by the fineness of the particle dispersion in the matrix. Most high-strength alloys are hardened by more than one mechanisms and the total Literature Review hardening can be approximated as the sum of the strength contributions resulting from the separate hardening mechanisms such as work hardening, solid solution strengthening, precipitation strengthening, grain and subgrain strengthening, dislocation strengthening, load transfer between matrix and reinforcement in metal matrix composites (MMCs). It is common to strengthen an alloy by dispersion hardening in which small second phase particles such as oxides, carbides, nitrides, borides, etc. are introduced into a ductile matrix. These finely dispersed second phase particles are more effective in resistance to recrystallization and grain growth than those in precipitation-hardening system. Second phase particles act in two distinct ways to retard the motion of dislocations: particles either may be cut by the dislocations or the particles resist cutting and the dislocations are forced to bypass them. The degree of strengthening resulting from second phase particles depends on the distribution of particles in the ductile matrix. In addition to shape, the second-phase dispersion can be described by specifying the interrelated factors such as volume fraction, average particle diameter, and mean interparticle spacing. A moving dislocation is unable to penetrate a grain boundary and hence grain boundary is a particularly effective strengthening agent. High density of grain boundary can be obtained by reducing the grain size to usually àm or less. When the grain size is reduced to nanometre range, nc materials exhibit a variety of properties that are different and often considerably improved in comparison with those of conventional coarse grain polycrystalline materials. Therefore, grain refinement is one of the most effective strengthening methods in polycrystalline materials. Literature Review The well-known effect of mean grain size on low-temperature mechanical properties is described by the Hall-Patch (H-P) empirical relationship: [1,2] y i k HP d / or H H k HP d / (2.1) where y is the yield strength, i the lattice frictional stress, d the grain size, H the hardness and kHP a constant often referred to as the Hall-Petch slope and is material dependent. This equation is based on the concept that grain boundaries act as barriers to dislocation motion. It is assumed that a dislocation source at the centre of a grain d sends out dislocations to pile-up at the grain boundary. The stress at the tip of this pileup must exceed some critical shear stress to continue slip past the grain-boundary barrier thus initiating slip in the next grain as illustrated in Fig. 2.1. S2 S1 grain grain Figure 2.1 Schematic illustration of a pile-up formed in grain under an applied resolved shear stress . S2 is a source in grain 2. The trace of the preferred slip plane in each grain is marked by a dashed line. At a critical stress the yielding process rapidly spreads across the specimen. As the grain size reduces, the increase in grain boundaries constitutes more pile-ups to act as barriers to dislocation motion causing higher stress concentrations in the neighbouring grains resulting in increase in yield stress. The mechanical behavior of a 10 Literature Review polycrystalline pure solid varies with grain size and can be schematically summarized in Fig. 2.2 [3]. The figure is divided into four regions. Region I (d>1 m), where materials have been widely studied, is characterized by a relatively strong work hardening (caused by dislocation interactions), relatively low strength, and high ductility. Plasticity is controlled primarily by dislocation motion within the grains. Material strength in this region follows the classical HP relationship, namely, yield strength increases with decreasing grain size. Tensile failure initiates at macroscopic necking and the fracture mode is intragranular. III II I IV ~1 0-20 nm ~1 m G rain size Figure 2.2 Yield strength as a function of grain size. According to HallPetch relationship, properties are classified into four regions. In Region II (1 m>d>20 nm), the HP relationship still prevails and the strength of a material continues to increase as a result of reducing grain size. However, both the strain hardening rate and the tensile ductility decrease. There is also a gradual transition of fracture mode from intragranular to intergranular. Another important observation was that shear deformation becomes localized [4]. 11 Literature Review As the grain size further reduces, one enters into Region IIIa region where only limited reliable experimental data are available [5]. However, recent computer simulations indicate that materials in this region are characterized by an inverse HP relationship, i.e. strength decreases as grain size decreases [6]. Materials exhibit negligible strain hardening in this region. Plasticity occurs primarily within the grain boundary region in which sliding of atomic planes is the dominant mode. Region IV (marked by an arrow) corresponds to amorphous materials (also known as metallic glasses), which have been extensively explored in recent years [7] and [8]. Experimental results showed that a metallic glass in compression [8] exhibits no strain hardening and behaves like a perfectly plastic material. In tension, on the other hand, the material is highly elastic and essentially brittle [7]. The fracture of metallic glasses occurs by highly localized shear banding. The mechanical characteristics in the four regions can be conveniently summarized in Table 2.1. Table 2.1 Mechanical characteristics in different grain size regions [3] Reg. Grain size Strength Ductility Strain hardening Fracture Dislocation activity Grainboundary activity I >1 m Low High Strong Transgranular, ductile fracture High Negligible II 20 nm Decreases with grain size Moderate Low Transition from transgranular to intergranular Moderate Moderate III 1 m. These include increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient in comparison with conventional coarse grain materials. All of these properties are being extensively investigated to explore possible applications. Figure 2.3 Schematic arrangement of atoms in an equiaxed nc metal distinguishing atoms associated with the individual grains ( ) and those constituting grain boundary network () [19]. One of the specific features of deformation processes in nc materials manifests itself in deviation from the well-known classical Hall-Petch relationship which well behaves 13 Literature Review for grains larger than about a micron. Most of the investigators try to explain such unique behaviour based on the effect of large volume fraction of grain boundary and structural defects induced during material processing. From investigation of nc iron materials using Mửssbauer spectroscopy, nc materials consist of two components of comparable volume fractions: a crystalline component and an interfacial component formed by atoms located either in the crystals or in the interfacial regions between them [20]. Within the large volume fraction of grain boundaries and interfaces, highly disordered lower atomic density state with vacancy-size free volume is verified with positron lifetime spectroscopy [21]. Significantly larger component of grain boundary relative to coarse-grained counterparts suggests the unique mechanical properties different from coarse-grain polycrystalline materials. Different from coarse grained structure, at the smallest grain sizes, new phenomena have to be used to explain the controlling deformation behaviour. It has been suggested that such phenomena may involve GBS and/or grain rotation accompanied by short-range diffusion assisted healing events [22]. The properties of nc materials are sensitive to their processing history which influences the types of microstructures and the processing flaws such as contaminants, porosity, etc. depending on the processing techniques. Such structural defects generated during processing play an important role to alter the properties of bulk nc materials. 2.3 Mechanical behaviours of nanocrystalline materials For understanding of the mechanical properties of nanophase materials in general, a quantitative framework as shown in Fig. 2.4 is useful. It appears that with decreasing grain sizes into nanophase regime, the frequency of dislocation activity decreases and 14 Literature Review that of GBS increases. Which of these effects dominates depends upon the grain size regime, the specific type of material and most importantly, the nature of its interatomic frequency (arbitrary units) bonding. Dislocation activity metals Grain boundary sliding intermetallics ceramics decreasing grain size (arbitrary units) Figure 2.4 Schematic framework for grain size dependence of dislocation activity and GBS contributions to the deformation behavior of various classes of nanophase materials. The nature of its interatomic bonding determines the appropriate location for a particular material [17]. 2.3.1 Hardness/Strength and ductility Most experimental results on the mechanical behaviour of nanophase metals are from measurement of hardness, which is like strength typically derived from the difficulty in creating dislocations and the impedance of their motion by the development of barriers [23]. It has long been observed experimentally in conventional metallic materials that hardness/strength varies with the grain size through the empirical Hall-Petch relation (equation 2.1). Hardness typically increases with decreasing grain size and pure nc metal can be to times harder than large-grain metal (>1 m). 15 Literature Review In nanograin-size regime, conventional Hall-Petch hardening from the introduction of increasing number of grain boundaries as barriers against dislocation motion seems to play an insignificant role. The paucity of mobile dislocations in nanophase grains has been well documented experimentally [24] and is simply a result of the long known and well understood image forces that act on dislocations near surfaces and hence in confined media [25]. The difficulty in creating new dislocations within the spatial confinements of ultra fine crystallites has also long been evident [26, 27] from earlier research on single crystal whiskers and wear debris. Since the minimum stresses required to activate common dislocation sources (such as Frank-Read source) are inversely proportional to the distance between dislocation pinning points, these stresses will increase dramatically with decreasing grain sizes into the nanophase regime owing to the limitation of the maximum distance between such pinning points. Thus, it appears that the increasing hardness and strength observed in pure nanophase metals with decreasing grain size is simply a result of diminishing dislocation activity. While it is clear that the hardness of pure metals increases as their grain sizes are reduced into the nano size regime, the full extent to which this hardening occurs is not clear. Elastic modulus changes can be expected as materials enter the nanophase regime. The apparent elastic moduli measured to date on nanophase materials [23, 28] have decreased in value relative to those in their coarse-grained counterparts, probably because of porosity and flaws resulting from processing [29]. Both grain boundaries and triple junctions have some contribution to the decrease in the Youngs modulus due to the increased volume in the interfacial region. However, the steep increase in 16 Literature Review the triple junction volume fraction largely accounts for the sharp drop in the Youngs modulus at the smallest grain size [30]. Ductility of nc materials is sensitive to flaws and porosity, surface finish and method of testing (e.g. tension or compression test). The limited ductility of nc materials is attributed to difficulties associated with the generation, movement and multiplication of dislocations inside the nanograins and/or the presence of significant flaw populations [23, 31, 32]. Since nc materials are very hard (and strong), it is doubtful whether the formability can be substantially improved (at least in tension), especially in non-cubic intermetallic compounds. However, room temperature or low homologous temperature superplasticity in nc materials has been reported by many investigators [30, 33-36]. Mohamed et al. [36] summarized the experimental results for both creep and superplasticity in nc materials as shown in Table 2.2. Recently, high tensile ductility of 45% at room temperature with softening behaviour indicating inverse Hall-Petch relationship has been reported in bulk nanostructured Mg-5wt.% Al alloy synthesized via MA with a grain size of ~45 nm [37]. This enhancement of superplastic behaviour was attributed to enhanced grain boundary diffusional creep providing the plasticity at ambient or low homologous temperatures. In other words, Coble creep may be enhanced when the grain size is reduced to nanoscale region. It has been reported that as grain size decreases, superplasticity occurs at lower temperature and higher strain rate. PM alloys containing reinforcement and MMCs often exhibit high-strain-rate superplasticity (HSRS). This will result in economically 17 Literature Review results in decrease in the yield stress and in the tension/compression asymmetry [102, 103]. At room temperature, the yield stress of Mg-Y2O3 composites produced by powder metallurgy depends strongly on the intensity of the { 10 } fiber texture which decreases with yttria volume fraction [103]. At low temperature (below 300C), substantial influence of texture on mechanical properties is observed in extruded Mg [104, 105] exhibiting higher yield stress of unreinforced extruded Mg with respect to Mg-Y2O3 [106]. As the testing temperature increases, the activation of non-basal slip system would result in the disappearance of the texture contribution to the material strengthening and the composite would present higher mechanical strength compared with the unreinforced magnesium. It is generally accepted that grain refinement enhances the GBS mechanism and GBS progressively removes the texture. It is also known that texture weakens under superplastic deformation. The texture effect is connected with changes in the grain boundary structure and Kaibyshev [107] claimed that high angle misorientations enhance GBS. However, there are controversial reports claiming that texture has a negligible effect on GBS (or superplasticity) [108,109]. Wide range of reports on grain size (up to nanometer range) and texture (for micrometer range) dependence of mechanical properties in Mg can be found in literature. However, the influence of texture on the mechanical properties of nanograined Mg has not been clarified so far. 2.6 Processing of bulk nanocrystalline materials via mechanical milling/alloying Nc materials have been synthesized by a number of techniques starting from vapour phase (e.g., inert gas condensation, sputtering, plasma processing, physical/chemical vapour deposition), liquid phase (e.g., electrodeposition, rapid solidification), and solid 30 Literature Review state (e.g., MA/MM, sliding wear, spark erosion) [110]. The advantage of using MM for the synthesis of nc materials lies in its ability to produce bulk quantities of materials in the solid state at room temperature using simple equipment. Besides grain size, chemical composition, and structure, additional parameters such as the structure and the thickness of the boundary regions are equally important parameters that control the mechanical properties of a nanostructured material such as strength and ductility. Nc Ni with the same composition and the same grain size of about 10 nm exhibited low ductility (100%) [9]. In this case, depending on the processing history, the difference in ductility behavior seems to reflect the difference in the energy stored in the interfacial regions of both grades of Ni. Different processing procedures may lead to significant differences in the initial microstructures of a particular nc material that in turn may influence the trend of experimental data. Figure 2.9 Schematic drawing of ball-powder-ball collision. MA/MM is usually carried out in high-energy mills such as vibratory mills (Spex 8000 mixer/mill), planetary mills (Fritsch and Retsch mills), and attritor mills (Szegvari attritor) [110]. The kinetics of alloying and other phase transformations induced during 31 Literature Review MA/MM depend on the energy transferred to the powder from the balls during milling (Fig. 2.9). This process is governed by many parameters such as the type of mill, milling speed, type, size and size distribution of the balls, ball/powder weight ratio, extent of filling of the vial, dry or wet milling, temperature of milling, atmosphere in the mill and finally, the duration of milling. The kinetic energy of the balls changes with speed of milling and mass of balls [111-114]. The higher the ball-to-powder ratio, the shorter the required milling time due to the increase in the number of collision per unit time. Higher collision frequency leads to the increase in milling temperature, which in turn favours the diffusivity and defect concentration thus influencing the phase transformation induced by milling. However, high energy and high frequency collision of balls can introduce contamination from milling tools. Ball-to-powder weight ratio is normally in the rage of 10:1 and 20:1 [115, 116] and a ratio of about 20:1 is often used particularly for planetary ball mills [112]. One of the sources of contamination is process control agent (PCA) which is added to minimise excessive cold welding. However, the PCAs are usually added in very small quantities (about to wt %) and are not a cause for much concern. Though finer particle size can be obtained with higher content of PCA, excessive PCA may lead to inhibition of cold welding and hence prevents the formation of new materials [112]. Furthermore, the addition of PCA results in appreciable pick-up of carbon, oxygen or nitrogen in the final powder. 32 Literature Review Another potential source of contamination during high-energy milling is the milling atmosphere. Oxygen contamination is most severe for reactive metals such as Al, Ti, Mg and Zr and contamination to the extent of 10% is possible in Zr- and Ti- base alloys. Milling in high purity argon or in vacuum or in a reducing medium such as toluene and performing powder handling in an argon-filled glove box can minimize the atmospheric contamination [117]. The reduction in grain size in powder samples to a few nanometers during heavy mechanical deformation enhances the strength of MMed materials. However, the amount of impurity increases with milling time, leading to degradation of overall combination of mechanical properties. Consolidation of the fine powders and the thermal stability of the nanometre-sized grains is another concern for material scientists. Powder consolidation to produce bulk shapes through secondary processes results in loss of metastable effects. Defects induced during milling have a significant role to play in the alloying process. However, the nature and concentration of these defects remain among the unanswered questions. Porosity has almost always been associated with nc-materials produced through powder preparation routines, and the size of the pores is often comparable to the grain size. Besides the processing flaws and porosity, poor bonding usually are resulted from consolidation of MMed/MAed nc powders at lower temperature to exclude the effects of significant grain growth. Experimental results from Sander et al. [39] and Youngdahl [118] indicated that bulk nc materials consolidated from nc powders showed premature failure resulting in low strength and ductility due to possibility of porosity. 33 Literature Review The evolution of microstructure during MM of magnesium powder in a modified SPEX 8000 shaker mill in an inert atmosphere has been studied by Hwang et al. [119]. In the early stage of milling, deformation by twinning and re-twinning within the grains develops sub-grain boundaries, which eventually defined nanometre-sized grains. A rapid decrease followed by saturation of the grain size at approximately 42 nm was reported. Compared to other MMed metals, due to the high recovery rate of magnesium, a relatively large final grain size was obtained. Enhanced recovery during milling was confirmed by the corresponding low internal strain. The internal strain during milling showed inverse grain size dependence. Absence of dislocations within the grains of as milled magnesium powder was confirmed by Moirộ fringe patterns in TEM micrographs. During milling, mechanical cold-worked powder resulted in dislocation generation, multiplication and congealing that produced grain refinement. As the grain size approached nano-dimensions, dislocations are no longer sustained within the grain and once generated rapidly diffuse to the grain boundary. MA of substitutional aluminium atoms into iron powder resulted in the aluminium atoms substituting for iron atoms in the grain boundary cells and providing a grain boundary structure similar to that of the iron powder processed in argon. The aluminium did not become mechanically infused throughout the iron grain, but became part of the nano-grain boundary [120]. 2.7 Metal matrix composite via mechanochemical route Magnesium alloys and magnesium MMCs especially particulate MMCs have increased the scope of application as structural materials, in automotive and aerospace applications, where high strength and stiffness combined with low density is required. 34 Literature Review Due to the needs of higher specific modulus, stiffness, strength, wear resistance, isotropic and high temperature mechanical properties, particulate reinforced MMCs have attracted considerable attention among various types of MMCs. The reinforcement particles are generally cheap and readily available [121]. SiC [122], Al2O3 [123], TiB2 [124] and B4C [125] have been recognized as potential reinforcements for metal matrices. The properties of MMCs are controlled by the size and volume fraction of the reinforcements as well as the nature of the matrixreinforcement interfaces. The interfacial reactions between the reinforcements and the matrix, and the poor wettability between the reinforcements and the matrix have to be overcome. Uniform and homogeneous dispersion of fine and thermally stable ceramic particulates in the metal matrix can produce optimum mechanical properties. To overcome all the drawbacks, a processing method of in-situ MMCs has been developed. In in-situ MMC, the reinforcements are synthesized in a metallic matrix by chemical reactions between elements or between elements and compounds during the composite fabrication. In-situ MMCs have the following advantages compared to the conventional MMCs produced by ex-situ methods: in-situ formed reinforcements are thermodynamically stable at the matrix, leading to less degradation in elevated-temperature services; the reinforcementmatrix interfaces are clean, resulting in strong interfacial bonding; 35 Literature Review in-situ formed reinforcing particles are finer in size and their distribution in the matrix is more uniform, yielding better mechanical properties. In-situ MMCs processing methods can be classified into four categories [126]: (a) solidliquid reaction process, i. Self-propagating high-temperature synthesis (SHS) ii. Exothermic dispersion (XD) iii. Reactive hot pressing (RHP) (b) vaporliquidsolid reaction process, (c) solidsolid reaction process, i. Mechanical alloying (MA) ii. Reactive hot pressing (RHP) iii. Isothermal heat treatment (IHT) (d) liquidliquid reaction process. The efficient mixing of the matrix and reinforcement particles is vital in establishing a homogeneous composite structure and hence obtaining optimum MMCs properties. MM technique or ball mill process assists greatly in improving the homogeneity of particle distribution. In comparison with those fabricated by conventional methods, the MMed materials generally show improved mechanical properties. The grain refinement associated with high dislocation density of the metal matrix can contribute to the strengthening and enhancement of ductility in addition to the strengthening of uniformly dispersed fine particles. Since the MM process is entirely in the solid state, fabrication of new alloys 36 Literature Review from virtually immiscible components is possible [127]. MM shows higher solid solubility extensions when compared to rapid solidification [128]. It was reported that solid solubility extension of aluminium in magnesium reached 3.7 at. % by MM [129]. MM also fractures reinforcement particle with internal flaws thus eliminating preexistent defects which often act as crack initiations and limit the tensile strength of the consolidated MMCs [112,130]. Recent research papers [131-135] reported Al/AlN composites as a new class of materials which exhibit interesting properties. Nitrides possess unique properties such as high hardness, high stability at elevated temperatures, good electrical and optical properties. Active gas (N2 or NH3) nitridation of metal powder at elevated temperature is the basic preparation method of nitrides. More recently, materials such as nitrides [136] and carbides [137] are synthesized by room temperature mechanochemical alloying technique in which solid-gas chemical reaction is accomplished by high energy ball milling. It was found that this solid-gas mechanochemical nitridation reactions could be extended to solid-solid mechanochemical reactions milling iron with the amine compounds piperazine [138-140], pyrazine [138,141] as well as pyrazole [139,141]. MM process is a unique technique for preparing several metal nitrides [142-145] using a method known as reactive ball milling (RBM) [143]. The nitridation process taking place on particle surfaces during milling can enhance particle size reduction [139]. MM leads to both enhancement of the thermodynamic driving force through reduction in length scale and possible breaking of the bonds and the kinetics of mass transport by 37 Literature Review introducing structural defects [146]. One of the major mechanisms for the enhancement of the thermodynamic driving force is refinement of grain size. The increase in the surface area leads to a positive contribution to free energies through an additional surface energy component. In general, the kinetics of reactions in the solid state through a mechanochemical route is significantly slower compared to that observed in solution. However, since mechanical energy can influence the reaction interface, opportunity exists in designing and controlling interface reaction in such a way to significantly enhance the rate of reaction. In case where the driving force is low the mechanical energy should in principle be able to alter the reaction path under favourable conditions. 2.8 Recycling of metal scraps In recent years, a wide range of changes in environmental policy towards integrated pollution prevention and control draws the attention of the process industry on effective environmental management and the treatment of wastes has become one of the most important concerns of modern society to protect the environment [147]. Environmental regulations and quality demands require clean and efficient processes for recovery of materials from scraps and wastes. With an increase in demand and due to magnesiums excellent machinability, production of a large amount of magnesium machine chips is expected. Magnesium is a highly recyclable material, consuming only 5% of the energy required to manufacture the primary metal. Currently only high quality magnesium scrap can be recycled easily into high purity alloys. As Mg becomes more abundant in its applications, the recycling of second-generation components from the recovery of end 38 Literature Review of life products is likely to play an increasingly important role in the supply of Mg in the long term. To realize this prediction, effective recycling technologies must be developed in the industry. The effort for recovery of Mg depends on the chemical and physical quality of the scrap. Preliminary treatments for efficient recycling can include shredding, sorting, briquetting and deliquescing. The additional steps required for effective Mg melting determine the economical attractiveness of recycling [148]. Currently, magnesium machined chips are remelted or burnt with sands. However, due to the susceptibility to oxidation, these treatments are expensive [149]. The conventional recovery method, melting or recasting as a fundamental step, generates new scraps due to post-melting processes such as casting, cutting and rolling or extrusion. It also consumes high energy, loses substantial amount of material due to oxidation, and requires large number of operations, hence produces high pollution and bears higher costs [150]. The scraps produced during post-melting processing results in 25 wt.% of metal losses [151]. Zapata et al. [152] and Gronostajski et al. [153] presented a process that converts the chips into powder through high energy ball milling with or without reinforcement particles such as ceramic particulates, tungsten or iron-chromium powder, followed by cold pressing and hot extrusion to produce composite materials. It has been demonstrated that extrusion processed from machined chips showed not only good combination of high strength and moderate elongations-to-failure of 5-12% at room temperature but also superplasticity at elevated temperature [154]. In processing of chip-extruded material, it is important to ensure bonding between the machined chips. Powder consolidation by hot pressing utilizing superplastic flow is useful to enhance 39 Literature Review densification [155] and bonding by breaking and dispersing the oxide layer on the surface of the chips [149]. Synthesis of nc material with suitably large size remains a major challenge for material researchers and one of the feasible methods to overcome this challenge is consolidation of MMed/MAed powders. MM is the direct conversion method of Mg scraps without any harmful effects on the environment and with minimum wastage of material. Other secondary processes such as extrusion, forging and rolling can be employed to further enhance the properties of final product. MM offers a nonequilibrium method at a relatively low cost compared to other methods since it can be carried out at room temperature using simple equipment to produce large quantity of nanograined powders which is not feasible for other synthesis techniques. However, chemical and physical quality of the Mg scraps is the main criteria to determine the economical attractiveness of recycling and the additional steps such as initial sorting and cleaning of the scraps have to be taken into consideration for costing. Recycling of Mg by melting with flux is the most common industrial practice which is expensive and harmful to the environment. The recycling of slag and dross, which are by-products of remelting and refining, is also an unsolved problem [156]. To ensure the competitiveness of magnesium application, alternative methods of Mg recycling have to be established. Although MM method has pros and cons for recycling as mentioned in section 2.6, its attractive advantages over other processing methods definitely overshadow its drawbacks. MM will become a preferred choice at least for tailoring the properties of final product to meet different engineering requirements. Therefore, it has great potential contribution to closure of the utilization cycle of Mg in a sustainable manner which is essential to preserve resources for future generations. 40 Literature Review Some commercial ball mill machines which can produce milled powder up to 90 tons per hour are available in the market for large scale recycling of Mg scraps. However, productivity will be varied according to the required powder size and selection of milling parameters. Theoretically, it is feasible to produce nanograined composite on commercial scale. However, it is inevitable to perform trail and error experiments to optimize the machine design and milling parameters. 2.9 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. EO Hall, Proc Phys Soc London B 64 (1951) 747-753. NJ Petch, J Iron Steel Inst 174 (1953) 25-28. TG Nieh, JG Wang, Intermetallics 13 (2005) 377-385. 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G Hanko, H Antrekowitsch, P Ebner, JOM 54 (2002) 51-54. 45 [...]... during milling was confirmed by the corresponding low internal strain The internal strain during milling showed inverse grain size dependence Absence of dislocations within the grains of as milled magnesium powder was confirmed by Moiré fringe patterns in TEM micrographs During milling, mechanical cold-worked powder resulted in dislocation generation, multiplication and congealing that produced grain... reinforcements are thermodynamically stable at the matrix, leading to less degradation in elevated-temperature services;  the reinforcement–matrix interfaces are clean, resulting in strong interfacial bonding; 35 Literature Review  in- situ formed reinforcing particles are finer in size and their distribution in the matrix is more uniform, yielding better mechanical properties In- situ MMCs processing. .. grain size is less than 10 nm [90] 27 Literature Review 2. 5 Texture and mechanical property relationship for hexagonal materials The deformation texture of hexagonal Mg develops in accordance with the relative contribution mainly from three distinct slip systems and one twinning system: basal {0001} < 1 120 >, Prismatic { 10 1 0 } < 1 120 >, Pyramidal { 10 1 1 } < 1 120 >, { 11 2 2 } < 1 123 > and twinning... extent of filling of the vial, dry or wet milling, temperature of milling, atmosphere in the mill and finally, the duration of milling The kinetic energy of the balls changes with speed of milling and mass of balls [111-114] The higher the ball-to-powder ratio, the shorter the required milling time due to the increase in the number of collision per unit time Higher collision frequency leads to the increase... evidence of deformation mechanisms in nc materials [ 72- 76] However, it is questionable that deformation processes in a thin, electron-transparent foil can represent those in the bulk material Mechanical thinning during TEM sample preparation followed by ion thinning or twin jet polishing, thinning the foil to 22 Literature Review perforation in most cases, will inevitably result in some relaxation of a... mechanism and a weaker dependence on grain size The study of the creep of nc (28 nm) Ni80P20 at temperatures ranging from 27 0- 320 °C [ 42] also suggested that the governing factor of creep deformation under the experimental condition was grain (and/ or phase) boundary diffusion Superplastic behaviour of Al alloys with grain size in the sub-micrometer region indicated that the grain boundaries in ultrafine-grained... Review of life products is likely to play an increasingly important role in the supply of Mg in the long term To realize this prediction, effective recycling technologies must be developed in the industry The effort for recovery of Mg depends on the chemical and physical quality of the scrap Preliminary treatments for efficient recycling can include shredding, sorting, briquetting and deliquescing The... quantity of nanograined powders which is not feasible for other synthesis techniques However, chemical and physical quality of the Mg scraps is the main criteria to determine the economical attractiveness of recycling and the additional steps such as initial sorting and cleaning of the scraps have to be taken into consideration for costing Recycling of Mg by melting with flux is the most common industrial... leads to the increase in milling temperature, which in turn favours the diffusivity and defect concentration thus influencing the phase transformation induced by milling However, high energy and high frequency collision of balls can introduce contamination from milling tools Ball-to-powder weight ratio is normally in the rage of 10:1 and 20 :1 [115, 116] and a ratio of about 20 :1 is often used particularly... [1 12] Furthermore, the addition of PCA results in appreciable pick-up of carbon, oxygen or nitrogen in the final powder 32 Literature Review Another potential source of contamination during high-energy milling is the milling atmosphere Oxygen contamination is most severe for reactive metals such as Al, Ti, Mg and Zr and contamination to the extent of 10% is possible in Zr- and Ti- base alloys Milling . illustrated in Fig. 2. 1.  grain 1 grain 2 S 1 S 2  Figure 2. 1 Schematic illustration of a pile-up formed in grain 1 under an applied resolved shear stress  . S 2 is a source in grain 2. The. Fraction 100010010 Grain Size (nm) 0.6 0.8 0.4 0 .2 1.0 0.0 1 Crystalline Triple line Grain boundary Quadruple node Intercrystalline Figure 2. 6 Plot of volume fractions of crystalline and intercrystalline components. materials [23 , 28 ] have decreased in value relative to those in their coarse-grained counterparts, probably because of porosity and flaws resulting from processing [29 ]. Both grain boundaries and