Nondestructive Characterization for Metal-Matrix Composite Fabrication.pdf

Peter K. Liaw, 1 Robert E. Shannon, 1 William G. Clark, Jr., 1 and
William C. Harrigan, Jr. 2
Nondestructive Characterization for
Metal-Matrix Composite Fabrication
REFERENCE: Liaw, P. K., Shannon, R. E., Clark, W. G., Jr., and Harrigan, W. C., Jr.,
"Nondestructive Characterization for Metal-Matrix Composite Fabrication," Cyclic Defor-
mation, Fracture, and Nondestructive Evaluation of Advanced Materials, ASTM STP 1157,
M. R. Mitchell and O. Buck, Eds., American Societyfor Testing and Materials, Philadelphia,
1992, pp. 251-277.
ABSTRACT: Nondestructive characterization has been performed on composite products at
different stages of fabrication processes including raw powders, powder mixtures, billets, and
final product extrusions. Eddy current was found to be effective in identifying matrix powder
alloy chemistry and particle size, and in determining the mix ratio of silicon carbide (SIC)
reinforcement particlesin aluminum matrix alloypowders. Ultrasonic techniques were capable
of identifying SiC clusters in large-scale, consolidated powder metallurgy (P/M) metal-matrix
composite (MMC) billets,while eddy-currentmethods could be used to determine near-surface
density variations in the billets. Multiple nondestructive evaluation (NDE) techniques (eddy
current, ultrasonics, and resistivity) could be employed to quantify microstructural character-
istics of composite extrusions. These results indicate that NDE methods can be integrated into
manufacturing processes to provide on-line, closed-loop control of fabrication parameters.
KEY WORDS: metal-matrixcomposites, nondestructive evaluation, aluminum, silicon carbide,
ultrasonics, eddycurrent, resistivity,powders, billets,extrusion, microstructure, intermetallics,
fabrication,materialprocessing,powder metallurgy,matrix, density, mixtures, alloychemistry,
particle size, mix ratio, clusters, manufacturing, linearsuperpositionmodel, ultrasonicvelocity,
compounds, fatigue (materials), advanced materials
Metal-matrix composites (MMCs) are becoming commercially viable engineering materials
that offer significant potential over conventional monolithic alloys. The engineering
applications of the state-of-the-art composites are often limited, however, by the lack of
reliable material qualification methods. In particular, MMCs offer a significant challenge
for conventional nondestructive evaluation (NDE) techniques because of their complex
microstructures.
A particularly important aspect of NDE considerations for metal-matrix composite ap-
plications involves material processing and the option for process-interactive control. Be-
cause of the large number of processing parameters associated with the manufacture of
MMCs, the likelihood of the presence of detrimental discontinuities is high, and in-process
NDE can be a promising cost-effective option. The detection of potential defects early in
the processing cycle would enhance the overall system yield and material quality. However,
very little work has been performed in support of process-interactive NDE of MMCs [1-
8]. In the present investigation, NDE was conducted at various stages of MMCs fabrication
~Fellowengineer, senior engineer, and manager, respectively,Westinghouse Science and Technology
Center, Pittsburgh, PA 15235.
2General manager, DWA Composite Specialties, Inc., Chatsworth, CA 91311-4393.
251
Copyright91992by ASTMlntcrnational www.astm.org

252 EVALUATION
OFADVANCED
MATERIALS
processes. The NDE responses were correlated with microstructural features of composite
products at each step of the manufacturing process. These correlations between NDE sig-
natures and microstructures can be used for controlling fabrication parameters.
Metal-Matrix Composite System
MMCs are usually fabricated by powder metallurgy (P/M) or casting techniques. The
composite system investigated was a silicon-carbide particulate (SiCp) reinforced aluminum
metal-matrix composite. Figure 1 shows a schematic of the P/M processing of aluminum/
silicon-carbide particular (A1/SiCp) composites. First, the aluminum alloy matrix powder
and the SiCp reinforcement powder are blended to form a powder mixture. The powder
mixture is then hot-pressed and consolidated into a composite billet. Following consolidation,
the composite billet undergoes thermal-mechanical treatments to develop final extrusion
products. For instance, the composites can be extruded into bars or plates.
To assure the quality of composite fabrication, composite products were nondestructively
characterized at each stage of the manufacturing process, as shown by the numbers from
one to four in Fig. 1. These composite products included aluminum alloy powders, SiCp
powders, powder mixtures, consolidated billets, and final extrusions. The developed NDE
results were correlated with microstructural features, providing useful information for proc-
ess control as well as quality control.
NDE of Raw Powders and Mixtures
Test Materials
During powder processing, a large number of material variables have to be considered,
including matrix alloy chemistry, powder size, and reinforcement concentration. In the
,,k
RAPIDLYSOLIDIFIED
ALUMINUMPOWDERS
+
POWDER
MIXTURE Q
SILICONCARBIDE
PARTICLES
HOT-PRESS
CONSOUDATION i I
THERMAL& BILLET
MECHANICAL ~
TREATMENTS ~(~) ~ / /
(Heat Treatment,
Extrusion, Rolling, MPOSITE
Forging, etc.) ~ EXTRUSION
COMPOSITE
EXTRUSION
/|
FIG. 1--Powder metallurgyprocessingof aluminum~silicon-carbidecomposites.

LIAW ET AL. ON METAL-MATRIX COMPOSITE FABRICATION 253
present investigation, the aluminum matrix alloy powders included 2124 A1, 6061 A1, and
7091 A1. The reinforcement powder was SiCw The size of aluminum alloy powder equaled
approximately 17 Ixm, while that of SiCp ranged from about 5 to 20 txm. Furthermore, the
powder mixtures of pure aluminum and SiCp were prepared with various concentrations of
SiCp.
Test Techniques
Figure 2 shows a schematic of nondestructive evaluation of composite powders. Aluminum
matrix alloy powders, silicon-carbide powders, and selected mixtures of aluminum and
silicon-carbide powders were placed in glass vials. Eddy-current responses were developed
from powders in the tap density condition using either an encircling coil or a surface-riding
pancake coil (Fig. 2). Eddy-current data were obtained through the glass vials, and the
results were evaluated against powder condition and composition. The specific variables
examined with the powder samples included aluminum matrix alloy composition, SiCp size,
and various mixtures of aluminum matrices plus silicon-carbide powders.
During eddy-current testing, all of the data were gathered using conventional eddy-current
flaw detection instrumentation (NORTEC NDT-25L). In all cases, the instrumentation was
balanced to reflect a variation in the eddy-current response as a relative change in the signal
amplitude (vertical deflection).
Results and Discussion
Figure 3 shows the relationship between eddy-current signature and aluminum alloy pow-
der. Note that these powders had a particle size of approximately 17 Ixm and were in a tap
density condition. The encircling coil probe was used with the test frequency of 1 MHz. A
Sarr
Vi
Encircling
Coil
ke"
FIG. 2--Schematic of eddy-current measurement on powders.

254 EVALUATION OF ADVANCED MATERIALS
Eddy Current Response (Volts)
1.0
0.8
0.6
0.4
0.2
0.0
2124 AI
7091 AI
Encircling Coil Probe
Test Freq. 1 MHz
Average Particle Size
17 Microns
Density - Tap
6061 AI
FIG. 3--Eddy-current response versus prealloyed aluminum powder.
significant variation in the eddy-current response with alloy type was found because of the
difference in the conductivity of the alloy powder [9].
The relationship between eddy-current signature and average SiCpsize is presented in
Fig. 4, In this case, the particle sizes were varied from 5 to 20 txm. The encircling coil probe
was used with the test frequency of 1 MHz. A positive correlation was observed, where an
increased SiCpsize resulted in an increased eddy-current response. In particular, the eddy
signature was quite sensitive to SiCp size in the range of 5 to 10 Ixm.
Figure 5 presents the relationship between eddy-current signature and SiCp volume fraction
in the A1/SiCo powder mixture. The percent by volume of SiCp was varied from 0 to 100%.
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02 ] I I I I I I I
4 6 8 10 12 14 16 18 20 22
Average Particle Size, (Microns)
FIG. 4--Eddy-current response versus average particle size for silicon-carbide powders.

LIAWETAL.ON METAL-MATRIXCOMPOSITEFABRICATION
3.5
255
3.0
2.5
2.0
1.5
1.0
0.5
0.0
m
_ /" Freq: 5 MHz
/ Density - Tap
i I I I I I I I I I
0 10 20 30 40 50 60 70 80 90 100
SiC Volume Fraction, Percent
FIG. 5--Eddy-current response versus silicon-carbide volume fraction in A1/SiC powder mixtures.
The mixture was a pure aluminum powder blended with 0 to 100% SiCp.The powder mixtures
were in the tap density condition. Eddy-current testing was conducted at a frequency of 5
MHz. The eddy-current response was found to be related to the percent by volume of SiCp
in the AI/SiCp powder mixture. Note that in Figs. 3 through 5, both encircling and pancake
coils resulted in similar eddy-current responses.
The present results show that eddy-current testing can be used to differentiate matrix
powder alloy, monitor average SiCp size, and measure AI/SiCp powder blend ratio. These
results rely on the sensitivity of the eddy-current technique to changes in the material's
effective electrical conductivity [9]. Providing care is taken to control sample geometry, the
eddy-current technique can be used easily in powder processing applications.
NDE of Consolidated MMC Billets
Test Materials
A large-scale, 335-mm-long by 349-mm-diameter MMC billet was specially fabricated as
a test sample. This billet was processed with near-optimum parameters, with adjustments
made to accentuate any powder processing anomalies. The billet material consisted of a
6090 aluminum alloy matrix reinforced with 25% SiCv using P/M techniques. Following
compaction of the blended composite powder, a series of fabrication discontinuities was
implanted into the green-state billet, including 6.4-mm-long SiCp clusters with diameters of
1.6 mm, 3.2 mm and 6.4 mm, and 6.4-mm-long aluminum particle clusters with a diameter
of 3.2 mm. Two identical sets of clusters were positioned at two elevations (planes) to
investigate the ability of NDE techniques for resolving defects at different depths. The two
implanted planes were located at the distances of 165 and 229 mm from the bottom of the
billet, respectively. On each plane, nine SiCp clusters and three aluminum clusters were
implanted. Figure 6 shows the general arrangement of the implanted discontinuities within
the billet. Following the implantation of the target discontinuities, the billet was consolidated
by vacuum hot pressing. This specially fabricated billet, with implanted SiCpand aluminum

6 E E
E E E
~ 6
i
o
E
C
I i !1
~ ~ EE Ee EE EE ~
I: ~ EE EE E~ EE
I
E '=
E d
~ N
E
_= _= _=.~

clusters, provided a unique reference standard for assessing the detectability limits of ul-
trasonic testing in critical MMC applications.
Test Techniques
Ultrasonics--Ultrasonic tests of the consolidated MMC billet were conducted in an im-
mersion tank equipped with an automated X-Y-Z transducer positioner and a rotating
turntable. A personal computer (PC)-based data acquisition system was used throughout
the tests. A Krautkramer USIP-12 was employed as a pulser-receiver, and the radio-
frequency signal output was digitized using Sonix 16-bit A/D conversion hardware and
software installed in the PC. Signal amplitude and time-of-flight (TOF) data were recorded
for processing and display using a Sonotek C-VUE software package.
Ultrasonic scanning was conducted with a 5.0-MHz, 19-mm-diameter focused transducer.
The ultrasonic beam was directed normal to the billet surface, producing a 0~ longitudinal
wave directed parallel to the billet axis. System gain for the axial scan was calibrated by
setting the amplitude of the signal developed from the 6.4-mm SiCp cluster near the center
of the billet to the 100% screen height (digitizer amplitude equaled 255). The transducer
was scanned in the X direction at a speed of 25 mm/s with the data sampled at intervals of
1 mm. At the end of each scan pass, the transducer was indexed at 1-mm intervals in the
Y direction until a complete C-scan map was developed.
Eddy Current--Eddy-current tests were performed on the circumferential surface of the
bottom section of the billet. This surface area corresponded to ultrasonic data indicating
that the adjusted processing parameters resulted in material with less than a 100% theoretical
density. A PC-based data acquisition system, as used in ultrasonic testing, was employed
for the eddy-current tests. The Nortec NDT-25L eddy-current instrument was used as the
frequency generator/impedance detector, and the output was digitized by a Data Translation
A/D converter installed in the PC system. The eddy-current sensor consisted of a 25.4-mm-
diameter air-core, pancake-style coil at a test frequency of 30 kHg. The system was calibrated
to provide a horizontal deflection (0~ phase) on the instrument's impedance plane display
for the probe lift-off signal from a reference standard (a 6090/SiC/25p composite) with a
100% theoretical density (2.74 g/cm3). Scanning was conducted by placing the probe in
contact with the circumference of the billet along the top edge and rotating the billet at
approximately 10 rpm, while the out-of-phase signals (vertical deflections) were recorded.
At the end of each revolution, the probe was indexed axially at intervals of 0.64 mm until
the entire circumferential surface was tested.
Microstructural Characterization--Samples containing each of the ultrasonic indication
areas were machined from the billet and prepared for examination using optical and scanning
electron microscopy. Initial polishing was conducted using 240- to 600-grit emery papers.
Fine-diamond pastes were then employed to polish the specimens further to a 1-1~mfinish.
Final polishing was accomplished using cerium oxides with a particle size of approximately
0.05 txm. An attempt was made to correlate microstructures with NDE responses.
Ultrasonics--The primary objective of the ultrasonic tests was to demonstrate the ca-
pability of detecting and characterizing internal structural discontinuities in the MMC billet
induced by the manufacturing process. The results of the ultrasonic tests were evaluated to
determine the optimum ultrasonic system parameters using the SiCp clusters and aluminum
powder clusters as performance targets (Fig. 6). Figure 7 shows an ultrasonic C-scan map
developed when axially scanning the billet from the top surface, using the optimum trans-

FIG. 7--Axial ultrasonic C-scan using a 5-MHz transducer from the top of a 6090/SiC/25p billet
(P3390).
ducer design configuration (a 5-MHz focused transducer). Six of the nine implanted SiCp
clusters were clearly characterized, as illustrated by the rectangular indication pattern in the
center area of the map. The largest ultrasonic indications, B3 and C3, resulted from the
largest diameter (6.4-mm) SiCp clusters lying at the top elevation of implants. Ultrasonic
Indications B4 and C4, from the 3.2-mm-diameter SiCpclusters, were found to be propor-
tionally smaller than B3 and C3. Similarly, Ultrasonic Indications B2 and C2 were found
to be representative of the smallest (1.6-mm) SiCp clusters. A careful analysis of the C-scan
data, using standard half-amplitude (-6 dB) sizing techniques, confirms the relative di-
ameters of the SiCp cluster implants.
The three remaining implanted SiCp clusters (A2, A3, and A4), positioned near the billet
circumference, were detected at significantly lower amplitudes (- 12 dB), compared with
those near the billet center. Additional tests, where the reflected signals from the entire
bottom surface were compared to the bottom surface signal amplitude along the axial
centerline, were conducted to assist in identifying a cause for this reduced signal response.
The results indicated that the signal amplitudes decreased as a function of distance from
the billet centerline, possibly due to attenuation and scattering from microstructural
variations.
As presented in Fig. 7, no clear indications were obtained from the implanted aluminum
clusters (A1, B1, and C1 in Fig. 6). Very low-amplitude signals were found in the appropriate

X-Y locations. However, a detailed review of the corresponding TOF data revealed the
signals as originating from elevations not representative of the aluminum implants.
A number of ultrasonic indications of significant amplitude were observed, that could not
be attributed to reflections from the intentionally implanted microstructural discontinuities.
For example, Ultrasonic Indications D, E, F, and G exhibited similar signal amplitudes,
when compared to the 1.6-ram-diameter implanted SiCv clusters at approximately equivalent
distances from the billet centerline. The axial distances from the scanning surface were
measured ultrasonically to be 109.7 mm, 101.3 mm, 82.8 mm, and 66.3 mm for Indications
D, E, F, and G, respectively. These distances represent shorter beam travel and less ultra-
sonic attenuation, relative to the 132.3-mm axial distance measured for the implanted clus-
ters. This trend suggests that Ultrasonic Indications D, E, F, and G result from defects
smaller than 1.6 mm in diameter, assuming the reflection sources were similar in nature to
the implanted clusters. In addition to the implanted Clusters B1, B2, B3, and B4, Areas E
and G were selected for a destructive analysis to determine the nature of the microstructural
discontinuities that caused these ultrasonic indications.
Correlation of Ultrasonic Indications to Microstructures--An example of the microstruc-
tures associated with the implanted SiCp clusters (B3) is presented in Fig. 8. The scanning
electron micrograph exhibited the degree to which the SiCv reinforcement particles are
clustered at the 6.4-mm-diameter indication, B3. As shown at the top of the micrograph,
the clustering is sufficiently dense to prevent the aluminum alloy matrix from consolidating.
The resulting porosity, combined with the SiCp clustering, was readily sufficient to cause a
FIG. 8--Microstructure of Ultrasonic Indication B2.

260 EVALUATION
OF ADVANCED MATERIALS
reflected ultrasonic signal. In comparison, Fig. 9 shows the typical appearance of the SiCp-
reinforced aluminum composite microstructure in the consolidated billet away from any
known defects or ultrasonic indication areas. Note that the SiCp particles remained uniformly
distributed around the aluminum matrix alloy as a result of the billet consolidation process.
An example of the microstructure found at the ultrasonic indication areas that are not
associated with the intentionally implanted defects is presented in Fig. 10. The scanning
electron micrograph shows an irregularly shaped SiCp cluster (Fig. 10a), approximately 1.0
mm by 1.0 ram, that was the reflection source for Ultrasonic Indication G. A high-
magnification micrograph (Fig. 10b) clearly exhibits that the silicon-carbide particulates were
closely packed as clusters. These results indicated that SiCp clusters, as small as 1 ram, were
efficient ultrasonic reflectors to be reliably detected using the present ultrasonic techniques.
The SiCp clusters are undesirable constituents in the AI/SiCp composites because they
typically act as crack initiation sites and they degrade material properties [1,10,11]. The
present ultrasonic method can identify the presence of SiCp clusters on the order of 1 mm
in the large-scale composite billets, which provides an effective means to evaluate the quality
of the billets during composite fabrication.
Eddy Current--Compaction density, or the presence of porosity, should affect the com-
posite material's effective electrical conductivity [9]. Thus, it was expected that an eddy-
current test could be developed to nondestructively monitor the as-consolidated density of
the MMC billet. Figure 11 shows a map of the eddy-current data resulting from the billet
circumference scan. The vertical axis of the map corresponds to the axial position along the
FIG. 9--General microstructure.

FIG. lO--Microstructural features of Ultrasonic Indication G: (a) general feature of Ultrasonic In-
dication G and (b) microstructure in Area A.

262 EVALUATION
OF ADVANCED MATERIALS
FIG. ll--Eddy-current map of circumference surface of a 6090/SiC/25p billet (P3390).
length of the billet, and the horizontal axis corresponds to the billet circumference. In this
case, the top of the map represents the bottom of the billet. An eddy-current signal amplitude
of 0 V was established for a 25% SiCp reinforced 6061-A1 composite at a 100% theoretical
density. An increased eddy-current signal amplitude corresponds to a decreased MMC
density. The results showed a relatively uniform decrease in the composite near-surface
density as the bottom of the billet was approached. Note that high-amplitude readings at
the very ends of the billet were caused by edge effects, and did not represent accurate
density assessment. Following completion of the nondestructive mapping, a series of samples
was machined from the billet for a comparison of physical density measurements with eddy-
current readings. Figure 12 presents a plot of physical density measurements versus eddy-
current amplitudes. An increased eddy-current amplitude was observed to be directly related
to a decreased MMC density. Using this relationship, the present eddy-current method can
be calibrated for directly monitoring the near-surface density of aluminum matrix MMC
billets at the consolidation process stage.3 This kind of technology can provide useful in-
formation for the process and quality control of composite billet fabrication.
NDE of Extruded Products
Test Materials
The SiCp-reinforced aluminum MMC extrusion plates were used for the present investi-
gation. The matrix alloys were 2124 A1, 6061 A1, and 7091 A1, and the percentages by
volume of SiCp reinforcement were 0, 10, 20, 25, and 30%.
3Note that the penetration depth of eddy-current measurements at the test frequency of 30 kHz
equaled approximately 1 mm in the present investigation [6}.

LIAW ET AL. ON METAL-MATRIXCOMPOSITEFABRICATION
4.0
3.5
3.0 O
2.5
2.0
1.5
1.0
0.5
0.0
263
2.2 2.3 2.4 2.5 2.6 2.7 2.8
Density, gm/cm 3
FIG. 12--Eddy-current assessment of billet density.
The composite extrusions were received in an extruded-plate form with the final extrusion
ratios ranging from 11:1 to 39:1. Following extrusion, the 2124 A1 composite materials were
heat-treated to the T4 condition, while the 6061 A1 and 7091 A1 materials were heat-treated
to the T6 condition. Table 1 presents the detailed information regarding matrix alloy, billet
number, heat treatment, percent by volume of SiCp, extrusion ratio, plate dimensions, tensile
properties, and density of the 13 composite plates investigated.
In some cases, there were two composite billets at a given percent by volume of SiCp.
For example, two 2124 AI composite samples (manufactured from Billets PE-2404 and PE-
2229, respectively) at 25% of SiCp were available for this investigation. Figure 13 presents
a photograph of the 6061 A1 composite extruded plates. The thickness of the extrusions was
12 mm, the width ranged from approximately 50 to 100 mm, and the length from 600 to
3000 mm.
Test Techniques
Ultrasonics--Using the pulse-echo method, ultrasonic velocities of the composite extru-
sions were measured. The ultrasonic travel time through the thickness direction of the as-
received composite plate was determined. Ultrasonic velocity measurements were deter-
mined using a 10-MHz, 12.7-mm-diameter, longitudinal-mode contact transducer and a 5-
MHz, 12.7-mm-diameter, shear-mode contact transducer. The transducers were pulsed and
the resulting echoes were received using the Krautkramer ultrasonic instrument (Model
USIP-12), and time-base readings were recorded using a calibrated Tektronix Model 2430
oscilloscope.
The thickness of each composite plate was measured to the nearest 0.0025 mm using a
micrometer. The ultrasonic longitudinal velocity, V1, and the shear velocity, Vs, were ob-
tained by measuring the two-way travel time between identical radio-frequency, wave-pulse

[
<
O0 O0 O0 O~
X X X X X X X X X X X X X
MMMNMMMM~MMdM
O~ O~ O~
~d
~<<<<<<<<<<<<
0
,.4
9
.~ ,--.
=1 -,.~ 9 0

FIG. 13--1"hotograph of 0001 AI metal-matr& composite extrusions in the as-received condition.
peaks in the second and third back-reflection multiples. The following relationships
V1 = 2T/tl
V~ = 2T/t,
were used to calculate the velocities, where T is the measured plate thickness, t1 is the
longitudinal-wave (two-way travel time), and ts is the shear-wave (two-way travel time).
To facilitate the analysis of the ultrasonic test results, the bulk density of the composite
material was determined using the water displacement method. The results of the density
measurements are included in Table 1.
Electromagnetics--Eddy-current and direct-current resistivity test techniques were used
to characterize the electromagnetic properties of the composite materials. Eddy-current
testing was performed at a frequency of 700 kHz using the Nortec NDT-25L eddy-current
test instrument.
A 19-mm-diameter, air-core, surface-contacting probe was used by incorporating a pan-
cake coil designed for optimum applications. The eddy-current system was calibrated to
provide for a horizontal deflection (0~phase) corresponding to the probe lift-off signal, and
to record the out-of-phase signals relative to an unloaded (0% SiCp) 2124 A1 alloy reference
plate.
Direct-current resistivity measurements were conducted using an AT&T Microhmeter
(Model 100). A 6.4-mm-diameter, four-point contact probe was used for measuring the
resistivity of the present composite plates. A direct reading in micro-ohm-centimetre (ixohm-
cm) was obtained for each composite plate.
Microstructural Characterization--Metallographic specimens were machined from the as-
received plates for microstructural characterization using scanning electron microscopy. The

same procedure described previously was employed to polish the specimens for the evaluation
of microstruetures.
The microstructural features investigated included the measurements of percentages of
SiCp, intermetallic compounds, and porosity. The SiCpwas readily visible on secondary-
electron-image photographs. Back-scattered-electron-image photographs were used to re-
veal the presence of intermetallic compound and porosity. Moreover, an energy-dispersive
X-ray (EDS) spectroscopy analysis was performed to identify the chemical compositions of
intermetallic compounds.
The percentages of SiCp, intermetallic compounds, and porosity were quantitatively meas-
ured by the point counting method [12]. Several SEM photographs (secondary-electron-
images and back-scattered-electron images) were taken of each polished metallographic
specimen. A fine mesh of lines was then placed on the photograph to perform the point
counting analysis [12]. The particle size and the aspect ratio of SiCp were also measured.
Ultrasonics--The results of ultrasonic velocity measurements are shown in Fig. 14. Figure
14a shows the relationship between ultrasonic longitudinal velocity and percentage of SiCp.
For the three composite systems examined, an approximately linear relationship was found
between longitudinal velocity and percentage of SiCp. Since SiCphas a greater ultrasonic
velocity than the aluminum base alloy [13],increasing percentage of SiCpincreased ultrasonic
velocity (Fig. 14a). Figure 14b presents the relationship between ultrasonic shear velocity
and percentage of SiCp. Similar to the results shown in Figure 14a, there appeared an
approximately linear relationship between shear velocity and percentage of SiCp. Further-
more, increasing percentage of SiCpincreased shear velocity.
Based on the acoustic wave theory [1,14], moduli can be represented as a function of
ultrasonic velocities, as presented in the following equations
E = 4pV~
!-
= pVs2
where
E = Young's modulus,
p = density, and
= shear modulus.
Using the measured ultrasonic longitudinal and shear velocities, the values of Young's and
shear moduli can be determined. Figure 15 compares Young's moduli measured by ultra-
sonics and tension testing. For the three composite systems, there was a good agreement
between Young's moduli determined by ultrasonics and tension testing. Thus, the ultrasonic
method can be used to measure the moduli of the A1/SiCp composite systems. A detailed
discussion regarding the theoretical prediction and the anisotropy of elastic moduli of the
present composites can be found in Refs 5, 15 through 17.
Electromagnetics--The relationship between eddy current and percentage of SiCpis shown
in Fig. 16. Increasing percentage of SiCp generally increased eddy-current response. This

FIG. 14--(a) Ultrasonic longitudinal velocity versus SiCp loading and (b) ultrasonic shear velocity
versus SiCp loading.
behavior is particularly true for the 7091 A1 composite system. However, there were some
variations in the eddy-current responses for the 2124 AI and 6061 A1 composites. In the
2124 A1 composite system, the two 25% SiCp samples showed different eddy-current re-
sponses. Also, for the 6061 A1 composite system, the 20% SiCp sample exhibited a much
greater eddy-current signature than the 25% and 30% samples.
Figure 17 shows resistivity as a function of the percentage of SiCp. Increasing the per-
centage of SiCp was found to generally increase resistivity, since SiCp has greater resistivity
than the aluminum base alloy [18-20]. This trend also suggests the reason for the eddy-

268 EVALUATION
OF ADVANCEDMATERIALS
Young's Modulus by Ultrasonics (GPa)
200
150
100 - ~ B =
O~lI I
50 9 2124 AI/SiC_
~606
1 AI/SiC_
v
7091 AI/SiC~
I I I
0 50 100 150 200
Young's Modulus by Tensile Tests (GPa)
FIG. 15--Comparison of Young's moduli determined by ultrasonics and tension tests.
current responses, as found in Fig. 16. Some variations in the resistivity for the 2124 A1 and
6061 A1 composites were observed, however. In the 2124 A1 composite system, the two
25% SiCp samples had different resistivity values. In addition, for the 6061 A1 composite
system, the 20% SiCp sample showed greater resistivity than the 25% and 30% SiCp samples.
These variations in the resistivity data exhibit the same trend found in the eddy-current
results (Fig. 16).
Correlation of NDE to Microstructures--To understand the variations in both eddy-
current and resistivity results, the microstructural characteristics of composite materials were
10
9 2124 AI/SiC_
AB6061 AI/SiC-
v
7091 AI/SiC~
1 /I- I I I
o
0 10 20 30 40
SiCp Loading (v/o)
FIG. 16--Eddy-current versus SiCp loading at 700 kHz.

Resistivity(# ohm-cm)
15
13
- ,~ 9
7 O ~ nn ~A
~ A
s,. f
1 I
9 2124 AI/SiC_
A 6061 AI/SiC_
~
7091 AI/SiC~
I I
0 10 20 30 40
SiCp Loading (v/o)
FIG. ]7--Direct-current (d-c) resistivity versus SiCp loading.
analyzed carefully. Figure 18 shows an example microstructure of the 2124 A1 composite
with 30% SiCp using both secondary-electron imaging and back-scattered-electron imaging
techniques. While each secondary-electron-image photograph clearly exhibited the mor-
phology of SiCp (Fig. 18a), the companion back-scattered-electron-image photograph was
particularly effective in identifying intermetallic compounds and porosity (Fig. 18b). The
average size and the aspect ratio of SiCp are included in Table 2 for the composite materials
examined. The average SiCpsize ranged from approximately 2 to 4 ~m, and the aspect ratio
ranged from about 2 to 3. All of the composites including the 0% SiCp (unloaded) samples
exhibited the presence of intermetallic compounds (Table 2). These intermetallic compounds
are shown as white on the back-scattered-electron-image photographs (Fig. 18b), and are
relatively fuzzy in shape on the secondary-electron-image photograph (Fig. 18a). An EDS
analysis using the scanning electron microscope showed that the intermetallic compounds
consisted of various combinations of elements, such as aluminum, silicon, manganese, iron,
copper, chromium, magnesium, cobalt, zinc, titanium, and zirconium, depending upon the
base alloy. Table 3 shows the elements contained in the intermetallic compounds for each
composite system. The percentage of intermetallic compounds for the composite materials
is included in Table 2.
Porosity was found often in the composites with 30% SiCp (Fig. 18b and Table 2). The
porosity was shown as black on the back-scattered-electron-image photograph. The porosity
was found generally at the interface between the SiCp and the base alloy (Fig. 18b).
As presented in Table 2, for the 2124 A1 composite at 25% SiCp, one sample had ap-
proximately 2.5 times greater percent by volume of intermetallic compounds than the other
sample. Consistently, the composite containing a greater amount of intermetallic compounds
exhibited greater eddy-current signature and resistivity. This trend correlated with the result
that intermetallic compounds typically showed greater resistivity than aluminum base alloys
[21]. For the 6061 A1 composite system, the 20% SiCp sample had about five times greater
percent by volume of intermetallic compounds than the 25% sample. Moreover, the 20%
SiCp sample showed approximately 12 times greater percent by volume of intermetallic

FIG. 18--Microstructure of 2124/SiC/30p composite (PE2488): (a) secondary electron image and
(b) back-scatterelectronimage.
compounds than the 30% sample. Correspondingly, the 20% SiCpsample showed much
greater eddy-current and resistivity than the 25% or 30% sample (Figs. 16 and 17).
These correlations between NDE signatures and the presence of intermetallic compounds
indicate that whenever the composite materials exhibit a greater amount of intermetallic
compounds, they will show higher values of eddy-current response and resistivity. Inter-
metallic compounds are undesirable constituents in composites because they generally serve

TABLE 2--Microstructural characteristicsof SiCp reinforced aluminum metal-matrix composites.
SiCp SiCp SiCp Intermetallic
Base Billet Loading, Size, Aspect Compound, Porosity,
Alloy Number %" p~m Ratio %" %"
2124 AI PE-2600 0 0 0 7.4 • 2.1 0
2124 AI PE-2404 25 2.5 • 1.8 2.3 • 1.7 4.4 • 2.8 0
2124 AI PE-2229 25 2.4 • 1.5 2.2 • 1.7 10.0 • 3.9 0
2124 A1 PE-2488 30 3.9 • 2.9 2.1 _ 1.3 6.7 • 3.7 1.4 • 1.8
6061 A1 PE-2045 0 0 0 5.2 • 2.2 0
6061 A1 PE-2047 20 2.3 • 1.8 2.1 --_ 1.4 15.5 • 4.8 0
6061 A1 PE-2099 25 2.6 • 1.7 2.2 • 1.5 2.9 • 2.2 0
6061 A1 PE-2731 30 2.8 • 1.7 2.3 • 1.5 1.2 • 2.1 2.6 • 2.3
7091 A1 PE-2730 0 0 0 6.9 • 2.6 0
7091 A1 PE-2711 10 2.4 • 1.2 2.8 • 1.3 6.9 • 2.8 0.5 • 0.9
7091 A1 PE-2712 20 2.3 • 1.5 2.5 • 1.7 4.4 • 2.6 0
7091 A1 PE-2713 30 3.5 • 2.8 2.2 • 1.5 3.2 • 1.1 4.2 • 2.8
7091 A1 PE-2665 30 3.7 • 2.1 2.5 • 1.7 6.9 • 2.8 1.6 • 1.4
"Percent by volume.
as crack initiation sites and, thus, degrade material properties [1,10,11]. The present results
suggest that both eddy-current and resistivity techniques can provide an effective means to
identify the composite materials that contain high percentages of intermetallic compounds.
This kind of information will be quite useful for quality control of composite materials.
Model Development--The eddy-current results shown in Fig. 16 were further analyzed,
as presented in Fig. 19, where eddy-current response is represented as a function of total
loading. Total loading included the percentages of SiCp, intermetallic compound, and po-
rosity. In Fig. 19, an increased total loading corresponded to an increased eddy-current
response regardless of the composite materials examined. The variations in the plot of eddy-
current response versus percentage of SiCp (Fig. 16) disappeared for the 2124 A1 and 6061
A1 composite systems by plotting eddy-current versus total loading (Fig. 19). This behavior
indicates that besides SiCp, the presence of intermetallic compounds also contributes to the
eddy-current signatures of composite materials.
In Fig. 20, the same analysis was performed on the resistivity results, where resistivity
was represented as a function of total loading. Similar to the behavior observed in the eddy-
current results, increasing total loading increased resistivity for each composite system. The
variations in the plot of resistivity versus SiCp (Fig. 17) disappeared by plotting resistivity
versus total loading (Fig. 20). This trend suggests that the presence of intermetallic com-
pounds and porosity also contributes to the resistivity of composite materials.
A semiempirical linear superposition equation was employed to model the relationship
between NDE results and microstructures
AX + BY + CZ + DW = NDE signature
TABLE 3--Chemical elements of intermetallic compounds.
Composite System Chemical Composition
2124 AI AI, Si, Mn, Fe, Cu, Cr
6061 AI AI, Si, Mn, Fe, Cr
7091 AI AI, Si, Mg, Fe, Co, Cu, Zn, Ti, Zr

272 EVALUATION
OFADVANCEDMATERIALS
10 /
t-- Total Loading = SiCp + Intermetallics + Porosity
9
/
8
[- tA ..u
~t f
3 i~.~ ~ 9 2124AI/SiC
i ~ - A 6061AI/SiCp
21~ ''A~ 9 7091AI/siCPp
0 ~ I I I
0 10 20 30 40
Total Loading (v/o)
FIG. 1Q--Eddy-current at 700 kHz versus total loading.
where X, Y, Z, and W are the percentages by volume of SiCp, intermetallic compound,
porosity, and aluminum alloy, respectively; and A, B, C, and D are the NDE coefficients
of SiCv, intermetallic compound, porosity, and aluminum alloy, respectively. NDE signature
equals the magnitude of each NDE measurement, such as ultrasonic velocity, eddy current,
or resistivity.
Prior to applying the linear superposition model to characterize microstructural features,
the coefficients, A, B, C, and D, have to be empirically evaluated. For the ultrasonic velocity,
Resistivity (~ohm-cm)
14
12
10
8
6
4
2
0
Total Loading = SiCp + Intermetallics + Porosity
- __tA/,~B i==
9 2124 AI/SiC_
A6061 AI/SiC_
~
7091 AI/SiC~
i I I
10 20 30 40
Total Loading (v/o)
FIG. 20--Direct-current (d-c) resistivity versus total loadmg.

the superposition model equation can be simplified as
AX + DW = ultrasonic velocity
since the ultrasonic velocity is a strong function of SiCpand, therefore, the coefficients of
intermetallic compound and porosity can be set at zero. These two values of coefficients,
A and D, have to be empirically evaluated. In each composite system, there are at least
two composites that have different percentages of SiCp. Therefore, using the ultrasonic
results of only one composite system, the values of A and D can be estimated, Table 4. The
results of eddy-current and resistivity measurements are sensitive to SiCp, intermetallic
compound, and porosity. Thus, the whole model equation has to be employed, and the four
coefficients, A, B, C, and D, have to be determined for eddy current and resistivity, re-
spectively. In each composite system, there are four composites. Using the NDE results of
only one composite system, there are four linear superposition equations and four coeffi-
cients. Therefore, the values of these coefficients can be estimated for eddy current and
resistivity, as presented in Table 4.
Using these coefficients, the microstructural features of composite materials can be char-
acterized as follows. The ultrasonic velocity results can be used to estimate the percentage
of SiCp since ultrasonic velocity is a strong function of the percentage of SiCp. Following
the prediction of the percentage of SiCp,the results of eddy current and resistivity can be
used to determine the percent by volume of the intermetallic compound and porosity. There
are two unknown values to be determined, that is, the percent by volume of intermetallic
compound and the percent by volume of porosity; and there are two linear superposition
equations, that is, one equation from the eddy-current measurement and the other equation
from the resistivity measurement. Thus, there are two unknown variables and two equations,
and the percentages of intermetallic compound and porosity can be estimated. This procedure
simply demonstrates the fact that using multiple NDE procedures, the percentages of various
constituents in the composite can be determined.
Figure 21 presents the predicted versus the measured percentage of SiCp based on the
ultrasonic longitudinal velocity. There was a good agreement between the predicted and the
measured percentages of SiCpfor the three composite systems examined. Figure 22 shows
the predicted versus the measured percentage of intermetallic compound and porosity based
on the eddy-current and resistivity measurements. A good agreement was found between
the predicted and the measured percentages of intermetallic compound and porosity. There-
fore, multiple NDE method can be employed to determine microstructural features of metal-
matrix composite materials. Furthermore, these NDE methods can be interfaced readily
with computers to offer a speedy assessment of the microstructures of composite extrusions.
Process-Interactive NDE
The overall objective of the present work was the development of nondestructive eval-
uation (NDE) methods useful for process-interactive control during the manufacturing of
P/M-based metal-matrix composites. The approach taken is to establish the correlations of
NDE signatures from multiple test methods to microstructural features of the MMC products
corresponding to the critical manufacturing stages. In particular, investigations included the
NDE assessment of matrix alloy powders and reinforcement particles, the detection of
powder blending ratio, the complete volumetric examination of consolidated billets, and
monitoring the quality of final extrusion products.
The greatest cost saving can be derived from the proper process control at Step 1 (see
Fig. 1) of the manufacturing process, the selection and handling of the powder materials.

.,ff
o
d
I
t~
.2
t",l
<
Z
X
X
X
,d
o,~ ('-4 oo
I t
X X X X X X
ss~
.~ ~ -
I
9
.~ ..~ ,.~ .~
X X X X X
X X X X X "~
~C
X X X X X ~
~.~
c x x x x ~
r
~S
",3
8
,.-t,
0
p,,-,
,,,,o
I

LIAW ET AL. ON METAL-MATRIXCOMPOSITEFABRICATION 275
Predicted SiCp Loading (v/o)
40
30 - ~
25
15
Ideal -~
10 -- "~/ e 2124 A!!S!Cp I
0 I I I I I I
0 5 10 15 20 25 30 35 40
Measured SiCp Loading (v/o)
FIG, 21--Predicted versus measured SiCp loading based on longitudinal velocity.
Eddy-current methods can be employed to assure that the proper aluminum powder alloy
has been selected. Eddy current can also find applications in the nondestructive, on-line
determination of average particle size. The Step 2 process (blending and mixing) is of
particular importance to the eventual quality of the manufactured MMC. The proper blend-
ing ratio of matrix alloy and reinforcement particle can be measured by eddy-current meth-
ods. Eddy-current techniques also allow for the application of encircling coils that can be
employed in the process stream, and are adaptable to on-line, closed-looped feedback control
applications.
Predicted Intermetallic Compound + Porosity Loading (v/o)
25
20 - 9 J
15 -
10 -
9 ,4 "m-~ 9 9 9 2124 AI/SiC
5 - /0,.. 9 ~ 9 6061 AI/SiCp
,t [] 7091 AI/SiC~
0 ~ I ........ I I I
0 5 10 15 20 25
Measured Intermetallic Compound + Porosity Loading (v/o)
FIG. 22--Predicted versus measured percent by volume of intermetallics and porosity based on eddy-
current and resistivity results.

At Step 3 of the process, multiple nondestructive evaluation methods were shown to be
effective in assessing the quality of the large-scale, consolidated billets. Specifically, ultra-
sonic methods were found to be capable of detecting SiCp clustered structures as small as
1 mm, while eddy-current measurements were identified as an effective tool for directly
measuring and mapping the near-surface consolidation density. By monitoring the presence
and the extent of these internal structural nonuniformities, manufacturing engineers can
make intelligent decisions regarding subsequent thermal and mechanical treatment
processes.
At Step 4, the final stage of MMC product manufacturing, quality control tools are required
to assure the delivery of quality materials to potential users. Ultrasonic and electromagnetic
evaluation methods were sensitive to the overall microstructural composition of A1/SiCp
extrusions. Methods for determining the percentages of SiCv reinforcement by ultrasonics,
as well as identifying the presence of intermetallic contaminants by eddy-current and resis-
tivity techniques, are readily available for a wide range of matrix alloy systems and rein-
forcement percentages by volume.
Obviously, multiple NDE techniques are required to assess microstructural features that
are important to the quality and long-term serviceability of MMCs. The strong correlations
between the NDE responses and the microstructures of composite products at different
manufacturing stages indicate that NDE methods are available to effectively identify im-
portant microstructural characteristics that affect material properties of composites. Through
further development, these NDE methods can be readily integrated into specific fabrication
processes to provide on-line, closed-loop control of manufacturing parameters.
Conclusions
1. Aluminum powder alloy chemistry and SiCp size altered eddy-current response. Eddy-
current techniques were effective in measuring the mix ratio of SiCv reinforcement particles
in aluminum powders.
2. Ultrasonic methods could be used to detect SiCp clusters, as small as 1 mm, in large-
scale, powder metallurgy (P/M) consolidated MMC billets. Furthermore, eddy-current meth-
ods were effective in determining near-surface density variations in the billets.
3. Ultrasonic velocity measurements can be used to nondestructively predict the percent
by volume of SiCp reinforcement in the final extrusions of MMC.
4. Electromagnetic measurements, eddy current or resistivity, could be correlated with
the presence of intermetallic compounds in composite extrusions.
5. Multiple NDE techniques are available for assessing material quality during the critical
stages of MMC fabrication. The NDE techniques demonstrated are compatible with
microprocessor-based data analyses and control systems, suggesting that a closed-loop, feed-
back process control can be applied readily for the manufacturing of P/M-based metal-matrix
composites.
Acknowledgments
The authors wish to thank J. N. Iyer for her involvement in the initial phase of this
investigation, and to W. R. Junker for his guidance in the application of NDE methods.
We also would like to acknowledge R. Hovan, T. Mullen, J. P. Prohaska, W. Hughes, and
P. Yuzawich for conducting microstructural characterization, and L. W. Burtner, B. J.
Sauka, and M. F. Fair for performing NDE tests. This project was supported jointly by the
Westinghouse Electric Corporation and by the U. S. Air Force Systems Command, Industrial
Materials Division, under Contract No. F33733-89-C-1011. This paper is based on a pres-
entation given in the Morris E. Fine Symposium [22].

References
[1] Mott, G. and Liaw, P. K., Metallurgical Transactions A, 1988, Vol. 19A, p. 2233.
[2] Liaw, P. K., ljiri, Y., Taszarek, B. J., Frohlich, S., Gungor, M. N., and Logsdon, W. A.,
Metallurgical Transactions A, Vol. 21A, 1990, p. 529.
[3] Ijiri, Y., Liaw, P. K., Taszarek, B. J., Frohlich, S., and Gungor, M. N., Metallurgical Transactions
A, Vol. 19A, 1988, p. 2215.
[4] Liaw, P. K., Shannon, R. E., and Clark, W. G., Jr., Proceedings, Symposium on "Fundamental
Relationships between Microstructures and Mechanical Properties of Metal Matrix Composites,
M. N. Gungor and P. K. Liaw, Eds., The Metallurgical Society of the American Institute of
Mining, Metallurgical, and Petroleum Engineers, Warrendale, PA, 1990, p. 581.
[5] Jeong, H., Hsu, D. K., Shannon, R. E., and Liaw, P. K. in Review of Progress in Quantitative
Nondestructive Evaluation, D. O. Thompson and D. E. Chimenti, Eds., Plenum, New York, Vol.
9B, 1990, pp. 1395-1402.
[6] Shannon, R. E., Liaw, P. K., and Harrigan, W. C., Jr., "Nondestructive Evaluation for Large
Scale Metal-Matrix Composite Billet Processing," Metallurgical TransactionsA, May 1992, in press.
[7] Clark, W. G., Jr., and Iyer, J. N., Materials Evaluation, Vol. 47, April 1989, p. 460.
[8] Clark, W. G., Jr., and Shannon, R. E., Advanced Materials and Processing, Vol. 137, 1990, p.
59.
[9] Libby, H. L., Introduction to Electromagnetic Nondestructive Test Methods, R. E. Krieger Pub-
lishing Company, Huntington, NY, 1979.
[10] Williams, D. R. and Fine, M. E., Proceedings, 5th International Conference on Composite Ma-
terials (ICCM/V), The Metallurgical Society of the American Institute of Mining, Metallurgical,
and Petroleum Engineers, Warrendale, PA, 1985, p. 639.
[11] Lewandowski, J. J., Liu, C., and Hunt, W. H., Jr., Materials Science and Engineering, Vol. 107A,
1989, p. 241.
[12] Hilliard, J. E. and Cahn, J. W., Transactions, The Metallurgical Society, American Institute of
Mining, Metallurgical, and Petroleum Engineers, Vol. 221, 1961, p. 344.
[13] Generazio, E. R., Roth, D. J., and Baaklini, G. Y., Materials Evaluation, Vol. 46, 1988, p. 1338.
[14] Krautkramer, J. and Krautkramer, H., Ultrasonic Testing of Materials, 2nd ed., Springer-Verlag,
Berlin, 1977.
[15] Jeong, H., Hsu, D. K., Shannon, R. E., and Liaw, P. K., "Nondestructive Characterization of
Elastic Moduli of AI/SiCp Metal Matrix Composites," Proceedings, Morris E. Fine Symposium,
P. K. Liaw, J. R. Weertman, H. L. Marcus, and J. S. Santner, Eds., The Metallurgical Society
of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Warrendale, PA,
1990, p. 209.
[16] Jeong, H., Hsu, D. K., Shannon, R. E., and Liaw, P. K., "Characterization of Anisotropic Elastic
Constants of Particulate Reinforced Composites: Part I. Experiment," Metallurgical Transactions
A, submitted for publication.
[17] Jeong, H., Hsu, D. K., Shannon, R. E., and Liaw P. K., "Characterization of Anisotropic Elastic
Constants of Particulate Reinforced Composites: Part II. Theory," Metallurgical Transactions A,
submitted for publication.
[18] Nelson, W. E., "Beta-Silicon Carbide and its Potential for Devices," Stanford Research Institute,
Final Report No. 21, Contract No. 87235, 31 Dec. 1963.
[19] Proceedings, International Conference on Silicon Carbide, Materials Research Bulletin, H. K.
Henisch and R. Roy, Eds., Pergamon Press, New York, 1968.
[20] Silicon Carbide-1973, R. C. Marshall, J. W. Faust, Jr., and C. E. Ryan, Eds., University of South
Carolina Press, Columbia, SC, 1974.
[21] Teixeira, S. R., Dionisio, P. H., Vasconcellos, M. A. Z., DaSilveira, E. F., Schreiner, W. H.,
and Baumvol, I. J. R., Materials Science and Engineering, Vol. 96, 1987, p. 279.
[22] Proceedings, Morris E. Fine Symposium, P. K. Liaw, J. R. Weertman, H. L. Marcus, and J. S.
Santner, Eds., The Metallurgical Society of the American Institute of Mining, Metallurgical, and
Petroleum Engineers, Warrendale, PA, 1990.

Nondestructive Characterization for Metal-Matrix Composite Fabrication.pdf

Recommended

Recommended

More Related Content

Similar to Nondestructive Characterization for Metal-Matrix Composite Fabrication.pdf

Similar to Nondestructive Characterization for Metal-Matrix Composite Fabrication.pdf (20)

More from mahmoodkhan77

More from mahmoodkhan77 (20)

Recently uploaded

Recently uploaded (20)

Nondestructive Characterization for Metal-Matrix Composite Fabrication.pdf