Microstructure characterization of nanocrystalline Fe3C synthesized by high-energy ball milling
Introduction
Metal carbides are generally referred as refractory materials due to their high melting temperatures and high hardness, and are being used as cutting tools, wear-resistance parts and surface coating. They deform plastically in a manner similar to fcc metals and become relatively soft and ductile at high temperatures. Because of their high chemical stability, high Young's modulus at high temperatures, good thermal shock resistance and thermal conductivity, metal carbides are also being used as structural materials [1]. Usually, metal carbides are prepared by conventional ceramic route, which requires a very high temperature as well as good vacuum condition or ultra pure inert gas atmosphere. However, nanocrystalline metal carbides with homogeneous composition can be prepared at room temperature by mechanical alloying (MA) the stoichiometric mixture of elemental powders under inert atmosphere.
Mechanical alloying process is a very useful solid-state technique for fabrication of high melting point compounds like metal carbides and nitrides, which additionally have nanocrystalline structure with improved properties [2], [3], [4], [5]. If the Fe–C system is composed of a very fine microstructure, which can be synthesized by MA, it is possible to produce an advanced material with high hardness, wear resistance, toughness and good magnetic properties. Four types of solid state reactions at the initial stage of MA of mixture Fe100−xCx with x ≤ 33% were suggested in the recent publications: (i) formation of martensite like interstitial solid solution of C in Fe [1], [6], [7], (ii) simultaneous formation of the Fe3C carbide and some hexagonal carbides [1], [8], [9], (iii) formation of Fe3C only [10], [11], [12], [13] and (iv) formation of the amorphous Fe–C phase prior to the formation of the Fe3C carbide [14], [15], [16], [17], [18]. Penetration and segregation of C atoms along the Fe grain boundaries and formation of Fe–C amorphous phase in the interface regions were also reported [19], [20], [21], [22] for the mixtures with x = 20–25%. But in all the above cases Fe3C phase was obtained after a long period of milling with significant amount of α-Fe as a un-reacted powder and in none of the cases microstructures of the ball-milled samples were characterized in terms of lattice imperfections related to physical properties of the prepared materials. The objectives of the present work are (i) to produce nanocrystalline Fe3C by high-energy ball milling the elemental α-Fe and C (graphite) powders at room temperature in a minimum time, (ii) to characterize the microstructure of the prepared materials in terms of lattice imperfections and (iii) finally to find the reason of Fe3C formation.
Heavy plastic deformation such as MA introduces a high density of lattice imperfections in prepared materials, which are responsible for the observed peak-broadening of their X-ray diffraction powder pattern [23], [24]. Besides peak-broadening, peaks may also become asymmetrical and/or shift with respect to their unmilled counterpart due to plastic deformation. All these effects on diffraction profile of a ball-milled sample (cold-worked) are related to several microstructure parameters like, change in lattice parameter, residual stress, density of stacking, twin/growth faults, coherently diffracting domain size (particle size), r.m.s. lattice strain, dislocation density and stacking fault energy. All these microstructure parameters can be estimated quantitatively by analyzing the XRD patterns of ball-milled samples employing either of the methods like Warren–Averbach's method of line profile analysis (WAMLPA) or Rietveld's structure refinement method. As the XRD pattern of Fe3C phase (orthorhombic) is composed of several overlapping reflections, the Rietveld's method of analysis based on structure and microstructure refinement is the best approach to characterize microstructure of ball-milled samples containing lattice imperfections of different kind. As the microstructure parameters are directly related to several physical properties of a material, a control over the microstructure leads one to prepare ‘tailor made’ materials having desirable properties.
Section snippets
Experimental
Pure α-Fe (Loba Chemie; purity 99.5%, 200 mesh) and graphite (Loba Chemie; purity 99.5%, particle size 50 μ) powders were used as starting ingredients and mixed in 3:1 molar ratio and then sealed in a chrome steel vial of 80 ml volume together with chrome steel balls of 10 mm diameter in a glove box under Ar atmosphere. The ball-to-powder mass ratio (BPMR) was 40:1. The milling was carried out in a planetary ball mill (model-P5, M/S FRITSCH, GmbH, Germany) at room temperature. The milling was
Method of analysis
In this study, we have adopted the Rietveld's powder structure refinement analysis [25], [26], [27], [28], [29] of X-ray powder diffraction data to obtain the refined structural parameters, such as atomic coordinates, occupancies, lattice parameters, thermal parameters, etc. and microstructure parameters, such as particle size and r.m.s. lattice strain. The Rietveld's software MAUD 2.06 [29] is specially designed to refine simultaneously both the structural and microstructure parameters through
Results and discussion
Fig. 1 shows the XRD patterns of the stoichiometric unmilled (0 h) mixture (3:1 mol) of α-Fe and graphite powders ball milled at room temperature under argon for different duration. It is clearly evident from the figure that the peaks of the both phases are quite sharp and resolved at relatively low scattering angle (∼55° 2θ). It indicates particle sizes of both elemental powders are quite large. The relative intensity (r.i.) ratios of α-Fe (bcc) reflections are in accordance with the JCPDF file
Conclusions
The nanocrystalline orthorhombic Fe3C phase is prepared by mechanosynthesis of stoichiometric mixture of α-Fe and graphite powders under argon in a high-energy ball mill. Formation of the thin layer of graphite phase from textured graphite layer is noticed within 30 min of milling. Nanocrystalline Fe3C phase is formed instantly by re-welding mechanism of nanocrystalline α-Fe and graphite layers within 2 h of milling. Mössbauer spectroscopy confirms that the stoichiometric Fe3C phase without any
Acknowledgements
A part of this work (Mössbauer spectroscopy) was performed at the UGC-DAE Consortium for Scientific Research, Kolkata Centre. The authors wish to thank the University Grant Commission (UGC) India, for granting DSA–III programme under the thrust area “Condensed Matter Physics including Laser applications” to the Department of Physics, The University of Burdwan under the financial assistance of which the work has been carried out.
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