EDDY CURRENT AND MICROWAVE CHARACTERIZATION OF (FE 65 CO 35 ) 70 AL 30 NANOCRYSTALLINE ALLOY SYNTHESIZED BY MECHANICAL ALLOYING PROCESS

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1 The 10 th International Conference of the Slovenian Society for Non-Destructive Testing»Application of Contemporary Non-Destructive Testing in Engineering«September 1-3, 2009, Ljubljana, Slovenia, EDDY CURRENT AND MICROWAVE CHARACTERIZATION OF (FE 65 CO 35 ) 70 AL 30 NANOCRYSTALLINE ALLOY SYNTHESIZED BY MECHANICAL ALLOYING PROCESS A.Haddad 1,a, M.Zergoug 1,b, S.Bergheul 2, M.Azzaz 3 1 Centre de Recherche Scientifique et Technique en soudage et Contrôle, Route Dely- Ibrahim, BP 64, Chéraga, Algérie a haddad_ah@hotmail.com, b mzergoug@yahoo.fr 2 Université de Blida,Algeria Département d Aéronautique, Université de Blida, BP 270, Route de Soumaa, Blida, Algéria. sabergheul@yahoo.fr 3 U.S.T.H.B, Algeria. Laboratoire de Science et Génie des Matériaux, BP 32, Bab-Ezzouar, Algérie, azzaz@wissal.dz ABSTRACT An investigation was conducted to explore the applicability of eddy current and microwave techniques to characterize grains size variation during mechanical alloying. A series of Nanocrystalline (Fe 65 Co 35 ) 70 Al 30 samples have been prepared, these structures are prepared using mechanical alloying based on planetary ball mill under several milling conditions. Mechanical alloying is a non-equilibrium process for materials synthesis. The structural effects of mechanical alloying of powders were investigated by X - Ray diffraction analysis, microwaves and eddy current technique. Consequently, alloy powder with an average grain size about of 8 nm was obtained. Experimental results show that fine nanocrystalline alloy powders prepared by mechanical milling are very promising for microwave applications. It is suggested that eddy current measurement technique is a useful tool for the characterization of Nanocrystalline materials. Keywords: Fe-Co powder; Mechanical alloying; Magnetic properties; Microwave. 1. Introduction Mechanical alloying (MA) of metallic powders has become popular in recent years for the synthesis of nanostructured alloys through solid-state reactions. This technique modifies the structure and solid solubility limits of alloys and solid solutions and induces lattice strains and phase transformations due to collisions with balls and is repeatedly deformed, cold welded and fractured for producing materials alloying gives rise to interesting mechanical and magnetic properties. 531

2 The mechanism of phase formation has been explained by an interdiffusion reaction of the components occurring during the milling process. The formation of metastable phases and disordering of the lattice through alloying gives rise to interesting mechanical and magnetic properties (Michael.E et al1999). Fe-Co alloys have been, for some time the ideal materials, the applications of a high magnetic saturation is a design parameter, particularly in the aerospace industries where volume and mass need to be minimized (Bormann, R. et al,1993). There were some methods to improve the material properties, such as composites, addition of other elements, heat treatment, control of grain size ( Bormann, R et al, 1993). Especially, the control of grain size at nanometer and addition of other elements will effectively improve the properties of Fe-Co. There have been several studies on the magnetic properties and ordering behavior of mechanically alloying Fe Co alloys (Sourmail, T et al, 2005), However, less has been reported on MA of Fe Co-Al alloys, especially the behavior of these alloys at high and low frequency. Microwave measurements have shown that small particle size tends to a low coefficient of reflection (Bergheul et al., 2006). Our choice was made on the influence of Al addition with the system Fe-Co. Does this addition affect the structures and properties in the Fe Co system? How do the microstructures and magnetic properties change with Al addition? Can the microstructure be related to magnetic properties? These are questions we intend to address here. In the present study, we formed FeCoAl based alloys by MA and investigated their magnetic properties; especially the eddy current dependence at low frequency and the coefficient of reflection studied at high a frequency. Eddy current nondestructive testing of conductive materials is important in many domains of industry: energy production (nuclear plants), transportation (aeronautic) ( Ma.X et al, 2006), workpiece manufacturing etc. This technique, based on the analysis of impedance of one or more coils placed near the sample, is used to detect and characterize possible flaw or anomalies in the workpiece. Typical testing configurations may consist of ferrite or air core bobbin probes which are placed above a sample which are operated in the time-harmonic regime (Uchimoto.T et al, 2003). The probe was operating in absolute mode with subtractive flux and ferrite conic core. In this study the probe performance which is usually assessed via the normalized impedance diagram. Microwave absorption material is an essential part of a stealth defense system for all military platforms and wireless communication tools such as mobile telephones and computer local area networks (LAN) (Mohammad. H.S et al,2008). Wave absorbing materials are required to have a large electric and magnetic loss in the frequency range of interest. The usual method for designing wave-absorbing materials is to find a zero reflection condition (Deng. L.W et al,2003).magnetic powder materials with low values of the coefficient of reflection have been promising in the application of microwave absorption. The effective microwave of the composite depends on both intrinsic characteristics of the particles and their microstructural, electrical and magnetic parameters: such as particle size, saturation magnetization and magnetic anisotropy field (Miura.K et al,2006), (Ghasemi.A et al2008). 532

3 2. Experimental details Initial powders, having an average particle size of 70 µm and a mean crystallite size of 35 nm produced by CERAC, were introduced into a cylindrical tempered steel vial of a 25 ml capacity. The materials were sealed under high purity argon atmosphere with ball to mass ratio 32:1. The milling was performed up to 54 hours with a planetary rotation speed at about 380 tr/mn. About 2 g of nanocrystalline powders of(fe 65 Co 35 ) 70 Al 30 were compacted at ambient temperature at a 2 Gpa pressure during 2 hours in a cold uniaxial press (13 mm inner diameter, 25 mm outer diameter and mm height). The powder mixtures were characterized by a Siemens D500 diffractometer using Cu K radiation in the 2 range from 30 to 120. Magnetic measurements were carried out with a Teslameter and an electronic oscilloscope TDS The eddy current impedance was carried out with Hp 4192ALF impedance analyzers were used in the test, which can supply a sinusoidal signal variable frequency output to the coil in a frequencyscanning manner. The voltage and current across the coil give the real and imaginary parts of the coil impedance. During the test, the analyzer was programmed to provide scale ranging from 50Hz to 500KHz. The impedance values measured when a sample was present below the coil. The normalized impedance was done by ratio to values when the coil was empty. 3. Results and discussion The X-ray diffraction patterns for the initial powder and the milled (Fe 65 Co 35 ) 70 Al 30 samples are shown in figure 1. The peaks related to pure Co and Al powder disappear completely for milling time longer than 4 hours. This shows the substitution of Co and Al into the Fe f.c.c structures and the formation of the disordered Fe-Co-Al solid solution with a f.c.c structure. At the same time, the widths of the peak related to f.c.c phase gradually with the milling time due to the decreases of grain sizes. Fig. 1: The XRD results (Cu-K ) for powder (Fe 65 Co 35 ) 70 Al 30 milled at room temperature 533

4 The estimate of the crystallite size was obtained from the refinement of X-ray patterns with Williamson-hall method were represented in Figure 2, during the early stage of milling, the crystallite size decreases rapidly and slows down afterwards and becomes gradually smaller about 8nm. Fig. 2: Evolution of crystallite size of (Fe 65 Co 35 ) 70 Al 30 on several milling times. 4. Magnetic study The evolution of the coercivity Hc and remanence induction Br as function of the milling time was presented in figure 3, the curves obtained show a regular and a similar diminution of the two parameters when the milling time is increased. After 8 h, Hc and Br remained nearly unchanged. However, a slight increase of Br and Hc decrease after 36 h suggests that the cobalt alloy formation proceeds simultaneously with the transformation Co (hcp) into Co (fcc) (Sourmail, T et al, 2005) Fig. 3: Evolution of remanence Induction Bs and coercivity Hc for MA (Fe 65 Co 35 ) 70 Al 30 powders for several milling times. 534

5 The measured impedance curves of the samples ranging 50Hz to 500KHz figure 4. As can be seen from this curve, their imaginary parts virtually decrease but a signeficatif increasing at 54h of milling time in the same as variation of Br curve, due to a relative permeability of sample (Ma.X et al, 2006). However, the real parts increase with the milling time and frequencies variation. We can see that curve for the samples milling more than 2hours. The normalizes impedance does not close again, is due to the diminutions of ratio grains size to boundaries. Fig. 4: Normalized impedance diagram for MA (Fe 65 Co 35 ) 70 Al 30 powders for several milling times. The measured impedance of coil corresponding of milling times with frequencie 5KHz Figure 5, when the electromagnetique skin depth is ensured from an initial depth of 0h sample. The curve have the same variation of remanece induction field figure 3. The represents variation of the coefficient of reflection according to the milling times at 9GHz frequencies was showing in figure 6. We can notice that the coefficient of reflection decrease when the times milling increase or grains size decrease. Furthermore, it is noticeable that the smallest 535

6 value of the coefficient of reflection is obtained with 54 h milling but more than FeCo nanostructure alloy (Bergheul et al., 2006). These results have, without any link with the remanence induction field Br or coercivity field Hc. At this stage, it is clear enough that the (Fe 65 Co 35 ) 70 Al 30 alloy becomes less absorbing than Fe 65 Co 35 (Bergheul et al., 2006). Fig. 5: Impedance of coil corresponding to different times Fig. 6: The coefficient of reflection for MA (Fe 65 Co 35 ) 70 Al 30 powders for several milling times. 4. Conclusion The Fe-Co-Al alloys powders have been prepared by a mechanical alloying process using highenergy ball milling. On the basis of the XRD results we conclude that the (Fe-CO-Al) solid solution is formed by substitution the solution of Co and Al into bcc Fe. Intensive milling results in a non-equilibrium microstructure composed of refined grains ranging from 10 to 8 nm.. FeCoAl become a soft magnetic alloy than FeCo with varying milling times. 536

7 This paper has presented the capability to apply the eddy current nondestructive testing to characterize nanostructure FeCoAl alloy. Our microwave measurements have shown that small particle size leads to a low reflection coefficients. It was found that the fine (Fe 65 Co 35 ) 70 Al 30 powder has the bigger coefficient than Fe 65 Co 35 at 9 GHz. These results are a clear indicate that Al addition affect the structures and properties in the Fe Co system. Eddy current can be used to characterize nanostructure. 5. References [1] Bergheul. S,Haddad. A, Taffat, H. and Azzaz, M (2006), Magnetic microwave and absorbing properties of Fe-Co alloy synthesised by mechanical alloying process, Int. J. Microstructure and Materials Properties, Vol. 1, Nos. 3/4, 2006 p [2] Bergheul, S., Taffat, H. and Azzaz, M. (2005) Ball-milling of Fe-Co compound, Proceedings of EUROMAT 2005, Prague, Czech Republic, September, pp.74, 75. [3] Bormann, R. (1993), Mechanical alloying fundamental mechanism and application, International Conference on Materials by Powder Technology, Dresden, Germany, p.247. [4] Buckleyu. R.A(1998), Some aspects of rapid solidification processing of Fe-Al-X alloys, Materials Science and Engineering A258 (1998).p [5] Deng. L.W (2003). GHz microwave permeability of CoFeZr amorphous materials synthesized by two-stepmechanical alloying, Journal of Magnetism and Magnetic Materials 264 (2003) p [6] Ghasemi.A(2008), Investigation of the microwave absorptive behavior of doped barium ferrites, Materials and Design 29 (2008) p [7] Ma.X (2006), Eddy current measurements of electrical conductivity and magnetic permeability of porous metals,ndt&e International 39(2006)p [8] Miura.K (2006), Microwave absorption properties of the nano-composite powders recovered from Nd Fe B bonded magnet scraps, Journal of Alloys and Compounds (2006) p [9] Michael.E(1999), Amorphous and nanocrystalline materials for applications as soft magnets, Progress in Materials Science 44 (1999) p [10] Mohammad. H.S(2008), Electromagnetic wave absorption characteristics of Mg Ti substituted Ba-hexaferrite, Materials Letters 62 (2008) p [11] Moumeni, H., Alleg, S., Djebbari, C. and Bentayeb, F.Z. (2004) Synthesis and characterisation of nanostructured Fe-Co alloys, Journal of Material Science, Vol. 39, Nos , p [12] Moumeni, H., Alleg, S. and Greneche, J.M. (2005) Structural properties of Fe-Co nanostructured powder prepared by mechanical alloying, Journal of Alloys and Compounds, Vol. 386,Nos. 1 2, p.12. [13 Sourmail.T(2005), Near equiatomic FeCo alloys: Constitution, mechanical and magnetic properties, Progress in Materials Science 50 (2005) [14] Uchimoto.T (2003), Eddy current evaluation of cast irons for material characterization, Journal of Magnetism and Magnetic Materials (2003) p [15] Zeng. Q, Baker.I(2006) Magnetic properties and thermal ordering of mechanically alloyed Fe 40 at% Al, Intermetallics 14 (2006) p [16] Zeng.Q,(2007), Soft ferromagnetism in nanostructured mechanical alloying FeCo-based powders Journal of magnetism and magnetic material 318(2007)p