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Determination of properties of SiC Reinforced Aluminium metal matrix composites by Ultrasonic Techniques Esra ATAS, C. Hakan GÜR Middle East Technical University, Metallurgical and Materials Engineering Dept., 06531 Ankara Tel: 00-90-312-2105916, Fax :00-90-312-2101267, Email : chgur@metu.edu.tr Contact

ABSTRACT

Metal-matrix composites are candidate materials for special uses in various fields such as military and space applications. The major factors that control the properties are manufacturing route, reinforcement level, matrix alloy, size and distribution of the reinforcement. This study is mainly focused on the determination of the properties of SiC reinforced aluminium metal-matrix composite by ultrasonic techniques, considering various process parameters. The main trust of this study is to seach for the correlations between ultrasonic properties and the material properties in the light of metallographic examinations.

In the experiments, various specimens were prepared for different size combinations of SiC and Al powder. The powder mixtures were hot pressed in a single-end rectangular die at about 40 MPa under nitrogen atmosphere at a temperature between 590^oC and 600^oC. The ultrasonic velocities and sound attenuation were measured for different frequencies. It was concluded that the material properties of Al-SiC metal matrix composites can be determined by ultrasonic techiques. This will provide the possibility of controlling the process parameters, therefore the possibility of controlling the quality of the product.

Keywords : Ultrasonics, Metal-Matrix Composite, Hot Pressing, Sound Velocity, Attenuation

1. INTRODUCTION

1.1 Metal-Matrix Composites
A metal matrix composite (MMC) is normally fabricated using a ductile metal (e.g., Al, Ti or Ni) as the base material, which is reinforced by a ceramic (e.g., alumina, SiC or graphite). Combining the metallic properties such as good ductility and toughness of the matrix with ceramic properties such as high strength, hardness and elastic modulus of the reinforcement, the composites exhibiting high toughness, specific strength and stiffness and good wear resistance can be obtained. MMCs can also have low thermal and electrical conductivity and low sensitivity to temperature variation. Consequently, they have extensitive interest from defense, aerospace and automative industries and have become very promising materials for structural applications as well. The many factors related to the properties of MMCs include the properties of the base material, the type, shape, dimensions, geometric arrangement and volume fraction of the reinforcement and the wettability at the interface of the reinforcement and matrix and the presence/absence of voids [1].

Particulate reinforced MMCs are promising because of their homogeneous and isotropic material properties, low cost and ability to be formed using conventional metal processing techniques. Among the many ceramic reinforcements SiC has been found to have excellent compability with the Al-matrix. The incorporation of SiC particulates into the aluminium matrix results in increases strength and modulus, thus improving the specific properties of the material. Powder metalurgy and conventional ingot metallurgy, including infiltration techniques are the two commonly employed means by which these composites are fabricated. By powder metallurgy, it is possible to obtain a homogeneous distribution of the reinforcement in the matrix [2].

1.2 Hot Pressing
Among P/M methods, hot pressing has many advantages. Hot pressing would be substitute for sintering and accordingly, handling and losses due to breakage of the green compacts would be minimized. Closer size control is possible, since volumetric changes, caused by the shrinkage forces or by gas evolution, occur in the hot press die. Thus, subsequent sizing and coining operations become superfluous. Moreover, it can easily yield required heavy weights or high densities without repressing or subsequent working. Physical characteristics obtained are better and comparable in many cases to those of wrought materials [3].

Fully dense metal powder compacts with controlled microstructures can be produced by hot consolidation, in which pressure and heat are applied simultaneously rather than sequentially. Pressure is applied statically or dynamically to the heated powder in one or two opposing directions along a single axis or from all sides. Pressure, while molding the particles into a definite shape, has the important function of making this shape coherent by creating strong particle-contact bonds. Heat, on the other hand, while being allowed to act only in a moderate way to maintain the formed shape, has the equally important function of consolidating and really homogenising the structure by allowing for a sufficient atomic mobility and place interchange to transform the original pattern of agglomerated particles into a recrystallized, uniform grain structure. A controlled atmosphere is required to protect the hot powders or prepressed compacts from oxidation or nitridation by air [5].

If temperature and pressure are sufficiently high and a sufficient period of time is permitted for their action, increased plastic deformation of the powder particles results in completely consolidated compacts whose structures are recrystallized and may be stress-released by a simultaneous or subsequent annealing treatment. The properties of such compacts are dependent upon the amount and rate of diffusion which, in turn, depends on the number and area of the interparticle contacts as well as on the amount of plastic deformation to which the compact is subjected. Increasing temperature and time at temperature under sustained pressure tend to increase each of the above effects individually, as well as all of them collectively.

If, for example, the structure must be entirely free of voids, this may be achieved by the proper selection of any one of the variables in relation to the other two, such as high pressures at low temperatures for a certain period of time or lower pressures for a shorter time at very high temperatures. The best combination may depend on the plasticity of the metal, on its capacity for alloying with the die material, on the desired grain structure or on the die materials or lubricants available. On the other hand, a controlled porosity may be achieved by the use, at fairly high temperatures, of pressures.

1.3 Ultrasonic Measurements
The speed of wave propagation and energy loss by interactions with material microstructure are key factors in ultrasonic determination of material properties. Relatively small variations of velocity and attenuation can indicate significant property variations [4]. Ultrasonic methods can be used to determine microstructural differences in metals. The testing can be either the measurement of ultrasonic attenuation or the measurement of bulk sound velocity.

The attenuation method is based on the decay of multiple echoes from surfaces. Once a standard is established, other test pieces can be compared to it by comparing the decay of these echoes to an exponential curve. This test is especially suited for the microstructural control of production parts, in which all that is necessary is to determine whether or not the parts conform a standard. Attenuation means energy losses arising from all causes when ultrasonic waves are propagated through a solid medium. These total losses can be classed broadly as scattering and absorption arising from the intrinsic physical character of the solid as well as diffraction, geometrical and coupling losses [7]. The absorption depends markedly on the nature and structure of the material (grain size and grain orientation) and is also a function of the ultrasonic frequency. Many materials are anisotropic so that the absorption varies with the beam direction.

Mechanical properties of Al-SiC metal-matrix composites are correlated with metallographic observations. Moreover, the results of mechanical and ultrasonic tests can be associated with metallographic observations. The ultrasonic non-destructive evaluation techniques employed may prove a useful addition for qualifying the homogeneity of SiC distribution [4].

2. EXPERIMENTAL PROCEDURE

In the preparation of specimens, three different average sizes for Al-powder (25, 100, 180 mm) and two different average sizes for SiC powder (10, 40 mm) were used. For each size combination, specimens with four different percentages of SiC were prepared (0, 5, 10, 20%). Details are given in Table-1. 5% Cu powder with 50 mm size has been added to each mixture.

After hand mixing, Al and SiC powder mixtures were hot pressed in a single-end rectangular die made of hot-work tool steel. Prealloyed powder was hot pressed into fully dense specimens having dimensions of 40x30x20 mm by applying relatively low pressures (40 MPa) for short periods of time (2-3 minutes) under nitrogen atmosphere at a temperature range between 590^oC and 600^oC. However, this method is limited to simple geometric shapes, more complex shapes require hot isostatic pressing.

The pulse-echo system was used for the velocity and attenuation measurements using Krautkramer Branson-15, and 0.5 inch normal probes of 3.5, 5 and 10 MHz. The longitudinal velocity of sound (V_L) can be calculated using the following formula,

VL = d Vd / ( k T )

(1)

where d is the thickness of the specimen in mm, V_d is known longitudinal velocity in the calibrating block, k is the scale factor and T is the echo position read from the screen.

Specimen No.	Al (mm)	SiC (mm)	SiC %	Specimen No.	Al (mm)	SiC (mm)	SiC %
(25-10).1	25	-	0	(25-40).1	25	-	0
(25-10).2	25	10	5	(25-40).2	25	40	5
(25-10).3	25	10	10	(25-40).3	25	40	10
(25-10).4	25	10	20	(25-40).4	25	40	20
(100-10).1	100	-	0	(100-40).1	100	-	0
(100-10).2	100	10	5	(100-40).2	100	40	5
(100-10).3	100	10	10	(100-40).3	100	40	10
(100-10).4	100	10	20	(100-40).4	100	40	20
(180-10).1	180	-	0	(180-40).1	180	-	0
(180-10).2	180	10	5	(180-40).2	180	40	5
(180-10).3	180	10	10	(180-40).3	180	40	10
(180-10).4	180	10	20	(180-40).4	180	40	20
Table 1: The composition of specimens, and average sizes of the Al and SiC powder.

The attenuation coefficient can be measured by placing a transducer on a parallel-sided specimen and onserving the envelope of successive backwall echoes. The specimen must be accurately plane and parallel-sided and there must be enough multible echoes to detemine the shape of the envelop. Absolute measurements are very difficult because the echo amplitude depends not only on the attenuation, but also on a number of other influencing factors [6]. For absolute attenuation measurements, indicative of the intrinsic nature of the material, it is necessary to correct for specimen geometry, sound beam divergence, instrumentation and procedural effects. These results can be obtained with more specialized ultrasonic equipment and techniques. In this study, apparent attenuation was calculated using the two subsequent back reflections. Whereas true attenuation losses may be attributed to the basic mechanisms of absorption and scattering, the apparent attenuation observes energy loss. The apparent attenuation may also include losses attributable to instrumentation, specimen configuration, beam divergence, interface reflections and measurement procedure. The apparent attenuation is determined by the following relation (ASTM E 664-78),

(2)

whereA_m and A_n are amplitude of the mth and nth back reflections, T is the specimen thickness. 8 measurements for velocity and attenuation were done on each specimen. Then, average values were calculated neglecting the maximum and minimum values.

3. RESULTS AND DISCUSSION

The results of ultrasonic measurements are given at Table 2 and 3.

Specimen No.	3.5 MHz	5 MHz	10 MHz	Specimen No.	3.5 MHz	5 MHz	10 MHz
(25-10).1	6396	6359	6345	(25-40).1	6396	6359	6345
(25-10).2	6569	6543	6532	(25-40).2	6506	6568	6492
(25-10).3	6688	6652	6682	(25-40).3	6663	6667	6670
(25-10).4	6989	7006	7022	(25-40).4	7150	7133	7171
(100-10).1	6412	6370	6336	(100-40).1	6412	6310	6336
(100-10).2	6489	6512	6534	(100-40).2	6417	6515	6490
(100-10).3	6551	6631	6621	(100-40).3	6655	6662	6650
(100-10).4	7006	7010	7100	(100-40).4	6932	7007	7003
(180-10).1	6433	6403	6406	(180-40).1	6433	6403	6406
(180-10).2	6533	6501	6521	(180-40).2	6490	6453	6456
(180-10).3	6694	6685	6670	(180-40).3	6522	6607	6639
(180-10).4	6837	6889	6951	(180-40).4	6835	6793	6803
Table 2: Sound velocity values for different probe frequencies.

Specimen No.	3.5 MHz	5 MHz	10 MHz	Specimen No.	3.5 MHz	5 MHz	10 MHz
(25-10).1	0,2414	0,1454	0,0934	(25-40).1	0,2414	0,1454	0,0934
(25-10).2	0,2767	0,1729	0,1146	(25-40).2	0,3103	0,2257	0,2483
(25-10).3	0,2830	0,1918	0,1593	(25-40).3	0,2944	0,3090	0,1884
(25-10).4	0,2766	0,2308	0,2143	(25-40).4	0,3402	0,2324	0,2694
(100-10).1	0,0991	0,0902	0,1011	(100-40).1	0,0991	0,0902	0,1011
(100-10).2	0,2661	0,1470	0,1547	(100-40).2	0,1322	0,1369	0,0937
(100-10).3	0,2186	0,1860	0,1412	(100-40).3	0,1646	0,1669	0,1438
(100-10).4	0,2661	0,2204	0,2039	(100-40).4	0,3745	0,3157	0,2446
(180-10).1	0,2634	0,1679	0,1325	(180-40).1	0,2634	0,1679	0,1325
(180-10).2	0,2245	0,2282	0,1628	(180-40).2	0,2076	0,1757	0,1481
(180-10).3	0,2477	0,2678	0,1667	(180-40).3	0,1833	0,2250	0,2304
(180-10).4	0,2972	0,2154	0,2713	(180-40).4	0,2933	0,3142	0,2386
Table 3: Apparent attenuation values for different probe frequencies.

3.1 Effect of SiC % on Ultrasonic Properties
As seen in Figure 1, apparent attenuation increrases with increasing SiC % in the metal-matrix composites. The reason is the occurrence much more scattering due to higher amonut of SiC in the structure.

Fig 1: Effect of SiC % on apparent attenuation (f = 3,5 MHz)

The velocity of ultrasound that propagates in an inhomogeneous medium depends upon the overall effective stiffness and density of the medium. For SiC reinforced Al composites the reinforcement is much stiffer than the matrix material, while the density of the reinforcemnet is comparable to that of the matrix material. Thus, ultrasound propagates faster in SiC reinforced composites than in unreinforced ones. Consequently, when SiC % increases, stiffness so the elastic modulus of the composite increases. This will cause increase in the longitudinal wave velocity in accordance with the following equation (see Figure 2 ).

Fig 2: Effect of SiC % on ultrasonic velocity (f = 3,5 MHz)

V_L = { ( E/r ) (1-n ) / [(1+n ) (1-2n )]} ^1/2                         (3)

where r , E and n are density, Young's Modulus and Poisson's ratio, respectively [8].

3.2 Effect of SiC Powder Size on Ultrasonic Properties
For constant % SiC, as particle size decreases the number of particles increases, therefore stiffness of the composite increases. Thus, velocity increases in accordance with equation 3.

A general result could not be obtained for attenuation measurements. In the case of the specimens prepared from 25 and 100 mm Al powder, for a constant SiC %, when SiC powder size increases attenuation also increases because the possibility of scattering increases. However, for the specimens prepared from 180 mm Al powder, as the particle size of SiC increases, attenuation decreases. Since the difference between the matrix and the particle size is high, there are less number of coarse SiC particles around a big Al matrix particle. This will decrease the possibility of scattering.

As it seen in Figure.3, there are less number of SiC particles around coarse Al particles. This will cause decrease in apparent attenuation and velocity.

Al (25mm) + %10 SiC (10mm)	Al (100mm) + %10 SiC (10mm)	Al (180mm) + %10 SiC (40mm)
Al (25mm) + %20 SiC (10mm)	Al (100mm) + %20 SiC (10mm)	Al (180mm) + %20 SiC (40mm)
Fig 3: Micrographs of several specimens (´100 magnification)

It has also been observed that lowering the probe frequency resulted in an increase in attenuation, since increasing wavelength causes increase in scattering. However, frequency change does not affect velocity.

CONCLUSIONS

In this study, material properties of Al-SiC composites produced by hot pressing were investigated by ultrasonic methods. The main distinguishing point is the possibility of controlling the size and distribution of SiC particles during production. Since the process parameters are going to be under control, it will be also possible to control production quality. Consequently, without specimen preparation and metallographic examination or mechanical testing, material properties of metal-matrix composites can be examined ultrasonically during or at the end of any production route. The conclusions for the present study can be summarized as follows:

• For constant powder sizes, as SiC % in the composite increases, sound velocity and apparent attenuation also increase.
• For constant composition, while reinforcement particle size decreases, sound velocity increases. As matrix particle size decreases, sound velocity increases.
• For 25 and 100 mm Al powder, as the size of SiC powder increases, apparent attenuation increases; however, for 180 mm Al powder the behaviour is reversed. In order to analyse the effect of Al-powder size, further research is necessary.
• Although frequency does not affect velocity, the apparent attenuation increases with decreasing frequency.

ACKNOWLEDGMENT

The authors thank to Assoc.Prof.Dr. Bilgehan Ögel for his valuable opinions and recommendations.

REFERENCES

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2. V.V.Bhanuprasad, R.B.V. Bhat, The International Journal of Powder Metallurgy, vol 27, 1991, pp 227-235
3. C.G.Goetzel, Treatise on Powder Metallurgy, vol 1, Interscience Publishers, Inc., New York , 1949
4. R.S.Sharpe, Research Techniques in Nondestructive Testing, vol IV,1980
5. Powder Metallurgy Metals, Handbook Ninth Edition, vol 7
6. J. Krautkramer, H. Krautkramer, Ultrasonic Testing of Materials, 1977
7. R.Truell , C.Elbaum , B.Chick , Ultrasonic Methods in Solid State Physics,1969
8. L.P.Martin, D.Nagle, Materials Science and Engineering, 1998, pp 151-160

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