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Structure and Properties of Nano Crystalline TiC Full-Density Bulk Alloy Consolidated From Mechanically Reacted Powders | Sintering | Transmission Electron Microscopy

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TiC nanocrystalline materials
  L Journal of Alloys and Compounds 305 (2000) 225–238 www.elsevier.com/locate/jallcom Structure and properties of nanocrystalline TiC full-density bulk alloyconsolidated from mechanically reacted powders *M. Sherif El-Eskandarany  Mining , Metallurgy and Petroleum Engineering Department  , Faculty of Engineering , Al Azhar University , Nasr City , 1337  Cairo , Egypt  Received 30 November 1999; accepted 11 January 2000 Abstract High-energy ball milling has been successfully employed for synthesizing nanocrystalline powders of Ti C . The milling procedure 44 56 involves milling of elemental Ti and C powders at room temperature in an argon gas atmosphere. The progress of the mechanicallyinduced solid state reaction was monitored by means of X-ray diffraction, scanning electron microscopy and transmission electronmicroscopy and/or high resolution transmission electron microscopy at several stages of the milling time. A single phase of NaCl-typeTiC is obtained after 22 ks of milling time. No free Ti and/or C crystals could be detected at this stage of milling. Increasing the millingtime leads to a decrease in the grain size of the powders to , 50 nm in diameter after 40 ks of milling time. Towards the end of the millingprocessing time (720 ks) the powders possess excellent morphological characteristics, such as a homogeneous shape (spherical-likemorphology) with fine and smooth surface relief and uniform size ( , 1 m m in diameter). The lattice parameter of this end-product wascalculated to be 0.4326 nm. In addition, the powders at this final stage of milling consist of nanocrystalline grains ( , 5 nm in diameter)with cell-like morphology. In order to determine some physical and mechanical properties of the synthesized TiC material, differentsamples at different milling times were consolidated into bulk samples, using a plasma activated sintering method. The powders of the 3 final-product (720 ks) has a density of 5.21 g/cm , being nearly the theoretical density of TiC. In addition, this bulk sample maintains itsunique structure characteristics with nanocrystalline grains of  , 60 nm in diameter. On the basis of the results of the present study, theball-milling technique accompanied with plasma activated sintering can provide powerful tools for the fabrication of nanocrystalline TiCbulk alloys with unique and advanced properties. © 2000 Elsevier Science S.A. All rights reserved. Keywords :  Nanocrystalline materials; Carbides; Refractory materials; Ball milling; Powder metallurgy; Density consolidation; Nondestructive testing;Elastic modulii; Hardness 1. Introduction for preparing homogeneous composite particles with inti-mately dispersed uniform internal structure [10] andDue to their unique properties, nonequilibrium materials several amorphous alloys [11–17], has been considered a(amorphous, quasicrystals, solid solutions and nanocrystal- powerful technique for synthesizing numerous nanocrystal-line materials) have recently found a wide range of line materials of pure metals nitrides [18,19], carbidesindustrial applications. Nanocrystalline materials (also [20–22], composites [23,24], nanocomposites [25,26] andknown as nanostructured) [1] which are defined as materi- solid solutions [27,28] because it is simple and relativelyals with grain sizes , 100 nm, have received much inexpensive [29]. Carbides, especially those of the transi-attention as advanced engineering materials with improved tion metals of groups IV and V in the periodic table,physical and mechanical properties [2]. This class of possess unusual properties that make them desirable andunique materials has been prepared by several techniques, useful engineering materials for many industrial applica-including inert gas condensation [3], rapid solidification tions [30]. The cubic form of TiC (NaCl structure) with its[4], electrodeposition [5], sputtering [6], crystallization of extremely high melting point (3373 K) is a refractoryamorphous phases [7] and chemical processing [8]. Me- material having some of the characteristic properties of chanical alloying (MA) [9] which is a well known process metals (luster, metallic conductivity, etc.). In addition, ithas extraordinary hardness and toughness, excellent resist-ance to wear and abrasion and infusibility. TiC has *Fax: 1 20-2-260-1706. received much attention due to its usage as a hard coating  E  - mail address :  msherif@mst1.mist.com.eg (M. Sherif El-Eskan-darany) to protect the surface of cutting tools from wear and 0925-8388/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S0925-8388(00)00692-7  226 M  . Sherif El -  Eskandarany /  Journal of Alloys and Compounds 305 (2000) 225  – 238  Table 1 a Possible methods for fabrication of TiC via the technique of powder metallurgyMethod ReactionDirect reaction between metallic Ti or metallic Ti 1 C → TiCTi hydrides and graphite, under vacuum or inert gas TiH 1 C → TiC 1 H 2 2 Reduction of the Ti oxide by graphite, under TiO 1 3C → TiC 1 2CO 2 vacuum or inert gasReaction of the Ti with carburizing gas Ti 1 C H → TiC 1 H n 1 2 n 2 Precipitation from the gas phase by reacting TiCl 1 C H 1 H 4 x y 2 the metallic Ti halide or metallic Ti → TiC 1 HCl 1 (C H ) n m carbonyl in hydrogen Ti 1 carbonyl 1 H 2 → TiC 1 (CO, CO , H , H O) 2 2 2a After Kieffer and Schwarzkopf (see Ref. [32]). erosion, extending the tool life [31]. In the industrial scale tory Centrifuge Mill, P6, Germany). The ball-millingof production, TiC is prepared at a temperature ranging experiments were interrupted after selected milling timesbetween 2000 and 2300 K, using the methods that are and a small amount of the powder was taken from the viallisted in Table 1 [32]. Considerable attention has recently in the glove box. An individual MA experiment wasbeen paid to another technique for producing TiC using a carried out in order to determine the milling temperaturemethod called self-propagating high-temperature synthesis during the solid state reaction between the diffusion(SHS) [33]. For a successful SHS process, the reaction couples of Ti and C. This was achieved by fixing the endbetween Ti and C must take place at a temperature higher of a thermocouple at the outermost surface of the vial. Thethan the melting point of pure Ti (1843 K) because no ball mill was then operated without any interruptions andreaction can occur below this temperature [34]. The high the temperatures of the vial were recorded after selectedcost of preparation is a disadvantage of all the previous milling times. X-ray diffraction (XRD) with CuK a radia-methods. In this paper, the MA method is proposed for a tion, transmission electron microscopy (TEM) operated atmechanical carburization solid state reaction between 200 kV, were used to monitor the structural changes of theelemental Ti and C powders, which can be successfully powders after several ball milling periods. However, someachieved in a high-energy ball mill operated at room samples were characterized by high-resolution transmis-temperature. The feasibility of employing the MA process sion electron microscopy (HRTEM) operated at 200 kV.for preparing nanocrystalline fcc-TiC powders is demon- The samples of the TEM and/or HRTEM were preparedstrated. The end-product of the milled powders are then by mixing the powders with a small amount of pureconsolidated into a fully dense compact, using the plasma alcohol (2 ml in volume) and stirring for 30 s. Two oractivated sintering method. The choice of this consolida- three drops of the suspension were dropped on a Cu-tion technique comes from the fact that this method is microgrid and then well dried for about 1.8 ks beforeconsidered as one of the most suitable consolidation mounting the microgrid onto the TEM sample holder. Theprocesses for preparing nanocrystalline, bulk, fully-dense morphological (shape and size) changes of the powdermechanically reacted materials [18,35]. In spite of the after selected milling times were determined by scanningother techniques that are usually used for preparing electron microscopy (SEM) operated at 20–30 kV and annanocrystalline materials, the MA method can be easily optical microscope. In order to determine the Ti contentscaled up to produce larger quantities of several kilograms, and the contamination degree of Al (that might be intro-using larger types of mills, e.g. low energy ball or rod duced to the milled powders on using the sapphire millingmills [36,37]. tools), the powders of the final product (720 ks) wereanalyzed by the induction coupled plasma emission meth-od. On the other hand, the concentration of C, and the 2. Experimental contamination content of oxygen that is introduced into thepowders during the milling procedure and/or during thePure (99.9%) elemental Ti (30 m m) and carbon powders powder handling outside the glove box, were determined(5 m m) were used as the starting reactant materials and by the helium carrier fusion-thermal conductivity method.mixed to give the desired average composition of Ti C The mechanically reacted powders were then consoli- 44 56 and then sealed in a sapphire vial (80 ml in volume) dated in vacuum at 1963 K under pressure ranging fromtogether with sapphire balls (10 mm in diameter) in a 19.6 to 38.2 MPa for 0.3 ks, using a plasma activatedglove box under an Ar atmosphere. The ball-to-powder sintering (PAS) method. In order to avoid undesired grainweight ratio was 10:1. The milling process was carried out growth, the sintering process was applied for only 0.18 ks.at room temperature using high energy ball mill (Labora- No binding material was used in this consolidation pro-   M  . Sherif El -  Eskandarany /  Journal of Alloys and Compounds 305 (2000) 225  – 238  227Table 2 of the PAS. Nevertheless, the total contamination content Chemical analyses of as-milled and as-consolidated mechanically reacted of the bulk sample is 0.38 at.%, being acceptable for TiC after 720 ks of ball milling time several industrial applications. Sample Ti content C content Al content O content(at.%) (at.%) (at.%) (at.%)As-milled 44.80 54.93 0.03 0.24 3. Results As-consolidated 44.45 55.17 0.03 0.35 3.1. Formation of TiC powders by mechanical alloying cedure. The as-consolidated samples were also character-ized by means of XRD, TEM, SEM and chemical analyses, 3.1.1. Structural changes with milling time using the same experimental conditions as mentioned The XRD patterns of ball-milled Ti C powders are 44 56 above. Selected samples (the end-product) were investi- presented in Fig. 1 after selected MA times. After 2 ksgated by HRTEM. The density of the consolidated TiC (Fig. 1a), the powders are mixtures of the starting reactantwas determined by Archimedes’ principle using water materials, characterized by the sharp Bragg peaks of immersion. A Vickers indenter with a load of 50 kg was elemental Ti and C. Increasing the MA time (6–8 ks) leadsused to determine the hardness of the compacted samples to a remarkable decrease in the intensity of the Braggthat were milled for selected MA times. The hardness peaks for pure graphite crystals (Fig. 1b–c) that can bevalues reported below (Section 3) are averaged from ten hardly seen after 11 ks of the MA time (Fig. 1d). This isindentation results. In addition, some mechanical prop- attributed to a solid state diffusion of the C atoms that haveerties of the consolidated samples were determined by small atomic radii into the lattice of hcp-Ti. Moreover, thenon-destructive tests. Bragg peaks for pure Ti crystals shifted to the low angleTable 2 shows the chemical analyses of the as-milled side, suggesting the formation of an interstitial hcp-TiCand as-consolidated samples of TiC. Obviously, the as- solid solution (Fig. 1d). After 15 ks of MA time (Fig. 1e),sintered sample takes up 0.10 at.% oxygen during the a new phase corresponding to NaCl-TiC is formed, char-consolidation procedure, although, the sintering was acterized by the Bragg peaks of TiC (111), TiC (200), TiC 2 6 achieved under high vacuum, as high as 6.0 3 10 torr. (220), TiC (311) and TiC (222) reflections. After 22 ks of This may be attributed to some leaks in the vacuum system the MA time, the Bragg peaks of the reactant materials (Ti Fig. 1. XRD patterns of mechanically alloyed Ti C powders after selected MA times. 44 56  228 M  . Sherif El -  Eskandarany /  Journal of Alloys and Compounds 305 (2000) 225  – 238  and graphite) surprisingly disappeared and the reflectionsfrom the TiC crystals become sharp and pronounced,indicating the completion of the MA process (Fig. 1f).The lattice parameter ( a ) of the formed phase for TiC 0 was calculated after 40 ks (Fig. 1g) of the MA time andfound to be 0.4326 nm, in a good agreement with thereported value (0.4327 nm) [38]. It is worth noting that theintensity ratio of these Bragg peaks are almost in goodagreement with those of the TiC powder [38], suggestingthat the crystal structure of the obtained powders is of theNaCl type. Increasing the MA time to 80 ks (Fig. 1h) leadsto an increasing mechanical deformation that is generatedby the milling tools, causing a remarkable decrease in thegrain size of the powders, suggested by the broadening of the Bragg peaks at this stage of milling. Further milling(720 ks) leads to the formation of nanocrystalline TiC,indicated by very broad Bragg peaks, as shown in Fig. 1i.This phase of TiC does not transform to any other phase(s)even after longer milling times, as long as 1000 ks.Detailed TEM analyses were performed in order tounderstand the local structure changes of the mechanicallyreacted powders during the various stages of the MAprocess. The HRTEM micrograph of the powders that weremilled for 2 ks of the MA time is shown in Fig. 2. Thepowders are mixtures of pure graphite (extended thinveins) and the matrix of Ti (dark area). The lattice fringespacing in Fig. 2b is measured to be 0.335 nm, whichmatches with the plane (002) of carbon (0.338 nm). Theexistence of the TiC phase could not be detected at thisstage of milling, and the powders are typical Ti–Ccomposite particles, containing thin veins ( , 10 nm inthickness) of C layers embedded in the soft matrix of Ti. A Fig. 2. (a) Low magnification HRTEM image of the milled powders after mechanical solid state reaction takes place at the interfaces 2 ks of MA time, and (b) enlarged micrograph of region I which is of these fresh layers and a new phase of TiC results after marked in (a). The lattice fringe spacing in (b) is measured to be 0.335 further milling. nm, matching with the plane (002) of carbon (0.338 nm [38]). Fig. 3 shows the bright field image (BFI) of the powdersthat were milled for 15 ks of MA time. The structure of theparticle is fine. However, several faults and defects such astwins and nanotwins are observed. The presented BFI is with wide size distribution, ranging from 20 to 60 nm (Fig.classified into three zones, i.e., I, II and III and the 4a). Remarkably, a single phase of NaCl-type TiC iscorresponding selected area diffraction patterns (SADPs) detected, characterized by the Debye–Scherrer rings corre-(Fig. 3b–d) are shown in the inset of the micrograph. At sponding to TiC, as indicated in Fig. 4. No free Ti and/orthis stage of milling, the powders differ widely in the C crystals could be observed after this stage of milling.internal structure from region to region, displaying a After 80 ks of the MA time, the powders haveheterogeneous structure. The SADP presented in Fig. 3b nanocrystalline spherical grains of about 4 nm (or less)(corresponding to zone I), consists of sharp rings that (Fig. 5a) in diameter. The indexed SADP still shows clearcorrespond to the coarse fcc-TiC grains (the obtained fcc rings of the obtained TiC (Fig. 5b). The absence of theproduct), coexisting with unprocessed hcp-Ti (sharp spots). spots in the SADP indicates the formation of fine grainsZone II is a TiC-rich region, that coexisted with small TiC powders.mole fractions of unprocessed Ti crystals (sharp spot The HRTEM image of the final product for TiC that waspatterns), as shown in Fig. 3c. Zone III, however, shows ball-milled for 720 ks of the MA time, is presented in Fig.the existence of a single phase of fcc-TiC (Fig. 3d). 6. HRTEM observations show the lattice fringe image of The BFI and the corresponding SADP of the powders TiC alloy. This lattice fringe spacing was measured to bewhich were milled for 40 ks, are shown together in Fig. 4. about 0.250 nm, which matches the interplanar spacing of Overall, the sample consists of rather coarse lenses or cells TiC (111) [38].
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