Influence of sintering temperature on the densification of copper matrix composite reinforced with CoAl particle developed by SHS

Influence of sintering temperature on the

densification of copper matrix composite reinforced with CoAl particle developed by SHS

kahina kheloui, Said Azem, Mustapha Nechiche

Laboratoire Elaboration et Caractérisation des Matériaux et Modélisation (LEC2M) Université Mouloud MAMMERI de Tizi-Ouzou.

Emails:kahinakheloui@hotmail.fr,saidazem@mail.ummto.dz

Abstract—

In a first step, Co-50at.%Al mixture is sintered at 950°C to obtain the CoAl compound that will be milled for 24 hours and sintered in presence of copper in order to a preparation of a metal matrix composite. Then, the CoAl-50wt. %Cu mixtures are compressed under a pressure of 350MPa and sintered at two different temperatures (solid and liquid phase) under argon.

The SEM examination of the composite sintered in solid phase shows the ramifications of copper in CoAl particles, which highlights a diffusion of Cu through the nano pores particles.

Some particles present a phase contrast, which reveals the inhomogeneity of diffusion. The EDS-X analysis confirms this result since we find an important quantity of copper inside a particle. The Cobalt and aluminum are also found in the matrix.

Moreover, the discontinuity is observed with the interface CoAl–

Cu. It seems that liquid sintering phase would leads to a homogeneous diffusion, better wetting of particles and higher densification. A sphering of CoAl particles is also observed in the case of the composite sintered in liquid phase, which reflects the phenomenon of dissolution-precipitation at the edges of CoAl particles.

To a complementary results, X-ray diffraction analysis (XRD) was also performed. The diffractogram of CoAl-Cu composite sintered in liquid phase reveals the presence of Cu and CoAl with shift of the Cu peaks to the left, reflecting an increase in the parameter. This swelling of the mesh of copper is due to the substitution of copper atoms by those of aluminum and cobalt.

After the calculation of the open porosity in the composites sintered in liquid and solid phase, we found a better densification of the composite sintered in liquid phase with a rate porosity of 2%.

Keywords—composite materials, intermetallics, SHS, SEM- EDS X, XRD

I. INTRODUCTION

Intermetallic compounds and composite materials based on them are considered as prospective materials for various high temperature and structural applications. In many cases they offer a unique combination of high-temperature mechanical properties and low density, even superior to nickel-based superalloys. Transition metals aluminides exhibit fairly high melting points, good resistance to corrosion and oxidation at high temperatures, high mechanical strength, together with good chemical and thermal expansion compatibility with Al2O3 reinforcing fibers, that enable the fabrication of light weight and high creep strength composites [1-3]. Ni–Al is the most investigated system. Fewer studies have been devoted to the synthesis and characterization of Co-Al [4-5] systems.

Cobalt aluminides are under investigation for their potential applications as metallization layers in III–V semiconductors devices [6]. Moreover, information about the solid–solid and solid–liquid reactivity between Co and Al and about the kinetics of formation of the Co-Al intermetallics would be particularly helpful in understanding the performance of tunnel junctions based on Co/Al2O3 interfaces used in electronic devices and of Co-Al multilayers used in magnetic recording heads and magnetic random access memories [7].

An alternative method, known as combustion synthesis, represents an attractive technique for the preparation of intermetallics and a variety of other materials [8-11].

Combustion synthesis of intermetallic compounds can be conducted in either of two modes, the self-propagating high- temperature synthesis (SHS) and the thermal explosion. The former is possible only when the exothermic enthalpy of formation of the desired intermetallic is sufficiently large, which is usually not the case for most intermetallics.

The CoAl intermetallic compound is a promising material for many high-temperature applications because of its

melting temperature (1648°C), density (6.086 Mg m^-3), and moderate oxidation resistance and thermal conductivity that can be utilized for a number of applications. Its major limitation, however, is the intrinsic brittleness that limits its use in structural applications. One of the efforts is microstructural modifications by reinforcing this material with hard ceramics [12, 13].

A particular importance is also devoted to the use of these aluminides as reinforcement in composite materials to metal / ceramic matrix composite (MMC or CMC) [14, 15]. The choice of copper as the matrix of the composite particulate reinforcements CoAl is motivated by good adhesion ensured between these materials. Furthermore, the combination of interesting properties of this intermetallic and the matrix ductility and hardness of the metal reinforcement as well as its resistance to oxidation gives the composite a good impact resistance, good mechanical strength and use possible to these compounds in oxidizing atmospheres or aggressive environments.

The aim of the present study was to explorethe influence of the sintering temperature of a copper matrix composite and CoAl particles produced by SHS (Self Propagating High temperature Synthesis).

II. EXPERIMENTAL A. Materials

The raw materials used are elementals powders of cobalt, aluminium and copper whose characteristics are given in Table I.

B. Methods

In order to synthesize cobalt aluminides, Co-31.40% Al (by weight) mixtures were prepared and homogenized in a mixer for thirty minutes. The pellets of 4 mm in height and 13 mm in diameter were produced by uniaxial compression at 300 MPa and sintered in a vacuum tube furnace. The heating rate is set at 5 °C min−1 up to 950 °C during 1 h. In first time, the reaction product (CoAl) is manually grinded in agate mortar and passed through a 50 μm sieve.

Then, the CoAl-Cu mixtures were compacted to 450 MPa to undergo a sintering treatment (solid and liquid phase), under argon atmosphere, during 1 h. The heating rate is set at 15

°C.min^-1 up to sintering temperature and follow-up slowly cooling (~15 to 20 °C min^-1).

The materials products are analyzed by X-ray diffraction (XRD, Brucker D8 diffractometer, λ=1,54 Å equipped with second filter) with 1 h acquisition time, Scanning Electron Microscopy equipped with Energy Dispersive X-ray Spectroscopy (SEM-EDS-X, Philips XL30), density measurement (by geometrical method).

TABLE I. SOME CHARACTERISTICS OF RAW ELEMENTAL POWDERS USED.

Powder Purity (%) Average size

(µm) Specific area (cm²/ml)

Co 99,5 10,19 16,159

Cu 99,9 Bimodal : 13,6

et 120 1,579

Al 99,5 55,52 1,492

III. RESULTS A. Synthesis of CoAl compound

CoAl compound is synthesized by SHS reaction of pellet Co-31.40 wt.%Al. The XRD product analysis (Fig. 1) reveals the formation of CoAl compound and total consumption of the starting reactants. The SEM examination of powder obtained after crushing shows the micro porous particles (Fig. 2).

Fig. 1.X-Ray diffraction pattern of CoAl compound synthesized by SHS reaction.

Fig. 2. SEM examination of CoAl SHS reaction.

B. Sintering of CoAl-50%Cu in solid phase

The SEM micrograph of the composite CoAl-50% Cu (Wt

%) sintered in solid phase (750 ° C) is shown in Fig. 3.

This microstructure reveals two different contrast phases. A dark phase is the CoAl compound embedded in the copper matrix that appears in the clear. The presence of certain porosity is also highlighted in the matrix and the intermetallic compound.

Fig. 3. SEM examination of CoAl-Cu composite sintered in solid phase The X-ray diffraction analysis of this sample shows the presence of two unique phases are CoAl aluminide and Cu (Fig. 4,)

Fig. 4. X-Ray diffraction pattern of CoAl-Cu Sintered in solid phase C. Sintering of CoAl-50%Cu in liquid phase

The microstructural analysis of the CoAl-Cu composite sintered in liquid phase at 1120°C (Fig. 5) shows that the CoAl particles became well embedded in the matrix.

Moreover, continuity of the matter to the CoAl-Cu interface is observed, which reveals a good wetting at the sintering temperature. However, the zones located between the CoAl particles present pores owing to the diffusion of Cu in CoAl.

The uniformity of color in ramifications indicates the homogeneity of diffusion. CoAl particles of very low size are distributed in the matrix. Spherisation of particle-matrix interface confirms the dissolution-precipitation phenomenon during sintering. Moreover, the porosity observed in the

vicinity of the large particles suggests a capillary aspiration liquid.

The EDS-X analysis and the mapping (Fig. 6) confirms this result since we find an important quantity of copper inside a particle (23 wt.%). The cobalt and aluminum are also found in the matrix since their respective contents are 4.04 and 14.81 wt.%. Moreover, the discontinuity is observed with the interface CoAl-Cu. It seems that liquid sintering phase would leads to a homogeneous diffusion, better wetting of particles and higher densification reveals a good wetting at the sintering temperature. However, the zones located between the CoAl particles present pores owing to the diffusion of Cu in CoAl. The uniformity of color in ramifications indicates the homogeneity of diffusion. CoAl particles of very low size are distributed in the matrix.

Fig. 5. SEM examination of CoAl-Cu composite sintered in liquid phase

Fig. 6. Mapping of CoAl-Cu composite sintered in liquid phase

Co Kα1 Cu Kα1

Al Kα1

To a complementary result, an X-ray diffraction analysis was also performed on the sample. The diffraction pattern obtained is shown in Fig. 7.

After counting, we find that the peaks of Cu are shifted to smaller angles, reflecting an increase in the parameter. This swelling of the mesh of the copper is due to the substitution of the copper atoms by those of aluminum and cobalt in the aluminum system.

Fig.7.X-Ray diffraction pattern of CoAl-Cu composite sintered in liquid phase

For measuring the density of CoAl-Cu composite sintered into solid and liquid phase we used the hydrostatic method.

After the calculation of the open porosity in the composites sintered in liquid and solid phase we found 2% and 26%

respectively.

D. Discussion

The CoAl intermetallic compound was synthesized by exothermic reaction. The generated swell is mainly due to the phases transformation and to the rapidity of reaction.

The entire consumption of the Co and Al elemental powders means that the reaction ended. The XRD analysis of the synthesized material reveals that it is composed completely of the CoAl compound.

The structure of CoAl compound synthesized presents microporosity through which the copper diffuses when sintering CoAl-Cu composite.

When the copper is in a solid state (sintering at 750 °C), the diffusion occurs mainly through microporosity. This produces the inhomogeneous distribution of Cu in CoAl particles. At liquid phase sintering (1120°C), the diffusion is accentuated by capillary aspiration of liquid and generates the porosity in the matrix. These phenomena are accompanied by the interdiffusion of elements Co, Al and Cu which increase the hardness of different phases.

IV. CONCLUSION

The CoAl intermetallic compound was synthesized by sintering SHS of Co31, 40 wt.% Al at 950°C under vacuum.

The CoAl powder obtained by crushing was mixed with copper to develop a MMCp by sintering. It was noted that:

- With 50 wt. % Cu, the diffusion is inhomogeneous during sintering in solid phase. In liquid phase, it is accentuated by liquid aspiration and causes the appearance of porosity in the matrix.

- We found a better densification of the composite sintered in liquid phase with a rate porosity of 2%.

References

[1] N. Mungole, R. Balasubramaniam, A. Ghosh, Intermetallics 8 (2000) 717–720.

[2] K. Morsi, Mater. Sci. Eng. A299 (2001) 1–15.

[3] . Gauthier, F. Bernard, E. Gaffet, D. Vrel, M. Gailhanou, J.P. Larpin, Intermetallics 10 (2002) 377–389.

[4] C. C. Yeh, C. L. Yeh. Journal of alloys and compounds 388 (2005) 241-249

[5] C. Milanese, F. Maglia, A. Tacca, U. Anselmi Tamburim, C. Zanotti, P. Giuliani. J. Alloys compounds, 421 (2006) 156-162.

[6] M. Tanaka, N. Ikarashi, H. Sakakibara, K. Ishida, T. Nishinaga, Appl.

Phys. Lett. 60 (1992) 835.

[7] P. LeClair, H.J.M. Swagten, J.T. Kohlhepp, W.J.M. de Jonge, Appl.

Phys. Lett. 76 (2000) 3782.

[8] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277–

365.

[9] A.G. Merzhanov, Ceram. Int. 21 (1995) 371–379.

[10] J.J. Moore, H.J. Feng, Prog. Mater. Sci. 39 (1995) 243–273 [11] P. Mossino, Ceram. Int. 30 (2004) 311–332.

[12] S.N. Hosseini, T. Mousavi, F. Karimzadeh, M.H. Enayati : Thermodynamic Aspects of Nanostructured CoAl Intermetallic Compound during Mechanical Alloying. J. Mater.Science.Technol, 27 (2011), 601-606

[13] ] M. Rafiei, M.H. Enayati, F. Karimzadeh. J. Alloys and compound.

488 (2010) 144-147.

[14] M. Rosso, Ceramic and metal matrix composites: routes and properties, Journal of Materials Processing Technology 175 (2006) 364–375.

[15] Azem, M. Nechiche, K. Taibi. Powder Technology, 208 (2011) 515 520.

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