CORRELATION BETWEEN LOCAL ULTRASONIC PROPERTIES
AND GRAIN SIZE WITHIN JET-ENGINE NICKEL ALLOY BILLETS
P. Haldipur, F. J. Margetanand R. B. Thompson
Center for Nondestructive Evaluation, Iowa State University, Ames, IA 50011, USA
ABSTRACT. Ultrasonic velocity, attenuation, and back-scattered grain noise have been measured in
rectangular coupons cut from representative 10"-diameter billets of IN718 and Waspaloy. Ultrasonic
attenuation and backscattered noise were found to vary significantly with position within a billet,
principally with radial depth. However, at a given measurement site there was little dependence of
ultrasonic properties on inspection direction, suggesting an approximately equiaxed, untextured
microstructure. Subsequent metallographic examinations revealed equiaxed grain structures in which
the average grain diameter varied with position, tending to be largest at sites with large attenuations and
large grain noise levels. The manner in which attenuation or backscattered-noise capacity (FOM)
grows with increasing average grain diameter is similar to that expected for Pure-Ni microstructures.
However, the rise rates are somewhat smaller for the jet-engine alloys, likely due to differences between
the single-crystal elastic constants of the alloys and those of pure Ni. This paper reviews the methods
used for ultrasonic measurements and metallographic analyses, and summarizes the interrelationships
between attenuation, backscattered noise capacity and average grain diameter.
INTRODUCTION
As part of an effort to quantify and improve inspection practices, the FAAsponsored Engine Titanium Consortium is investigating the ultrasonic properties of
cylindrical billets used in the fabrication of rotating jet-engine components. Last year we
reported on initial work to survey the UT properties of typical 10"-diameter Ni-alloy billets,
and to correlate those properties with the local billet microstructures [1]. The present paper
provides an update of this ongoing effort. We begin by briefly reviewing specimen
selection, cut-up procedures, and UT measurement methods. Selected results of the
measurements are then presented to illustrate the manner in which UT properties vary with
position within billets. We then discuss, in detail, the two complementary techniques used
to estimate average grain diameters at the UT measurement sites. The correlations between
mean grain diameter, ultrasonic attenuation, and ultrasonic backscattering are then
discussed. Experimental results are compared to model predictions for simple, pure-Nickel
microstructures.
COUPONS SELECTION AND ULTRASONIC MEASUREMENTS
Billet selection, cut-up procedures, and UT methods were discussed at some length
in Ref. 1, and will be only briefly summarized here. The objective was to build up a
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/$20.00
1355
0"
r
2"
T
4"
Poshed
FIGURE 1. (a) Geometries of "strip" and "slice" coupons from Ni-alloy billets, (b) UT measurement
sites, relative to the billet center, (c) Locations and designations of the 12 small metallography coupons
associated with each strip coupon.
coherent picture of how ultrasonic velocity, attenuation, and backscattered noise vary with
position and sound propagation direction in typical 10"-diameter Nickel-alloy billets.
Candidate IN718 and Waspaloy billets were first inspected in the radial direction using a
multi-zone inspection system [2] containing several transducers each focused at a different
depth. Prominent bands of high and low backscattered grain noise were observed in each
case. Based on the noise-banding patterns, rectangular "strip coupons" with approximate
dimensions of 2"x2"xlO" were cut along billet diameters, as illustrated in Figure la, and
used for UT measurements in the axial and hoop directions. The strip coupons were later
cut into 1 "-thick slices for UT measurements in the radial direction. In all, four strip
coupons were studied, denoted: WASP (from a high noise band of a Waspaloy billet);
GFM-A (from a high noise band of a GFM-forged IN718 billet); and V-Dffi-A and V-DIEB (from high and low noise bands, respectively, of a V-die-forged IN718 billet). The
emphasis was placed on high noise sites since flaw detection tends to be more difficult at
those locations. Back-wall amplitude and backscattered grain noise C-scans were performed
through the 2" x 10" faces of each strip coupon. For a given strip coupon, inspections in the
axial and hoop directions yielded similar results, and a strong dependence of attenuation
and backscattered noise on radial position was observed. Based on these findings, it was
decided that detailed measurements would be made at five radial sites on each coupon, as
indicated in Figure Ib, with the sites denoted by the distance (in inches) from the billet
center. At each site, longitudinal wave speed, attenuation, and
1356
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FIGURE 2. Measured attenuation values (left) and grain noise FOM values (right) at 7.5 MHz at
selected sites on the four strip coupons. Results are shown for three orthogonal sound propagation
directions.
1357
4
backscattered noise were measured for sound propagation in the axial, hoop, and radial
directions. The backscattered noise data was analyzed to deduce the so-called Figure-ofMerit (FOM), a measure of the noise generating capacity of the microstructure (equal to the
square root of the standard backscattered power coefficient) [3]. Attenuation and FOM
were found to depend strongly on frequency, and were typically measured in the range from
about 5 to 15 MHz. The measurement techniques are described in Ref. 1, and typical
attenuation-versus-frequency and FOM-versus-frequency curves are shown there.
Sound speeds were found to be very uniform throughout each billet. Attenuation
and FOM, however, varied systematically with radial position within each strip coupon, as
illustrated in Figure 2 using the measured values at 7.5 MHz. Note in Figure 2 that at a
given site in a given billet, the UT properties are approximately independent of inspection
direction. Thus, the UT results are consistent with an equiaxed, untextured billet
microstructure in which the average grain diameter varies systematically with position.
METALLOGRAPHY COUPONS AND GRAIN SIZE ESTIMATION
One goal of our study was to correlate the measured ultrasonic properties with the local
billet microstructure. As shown in Figure 1, small metallographic coupons were cut from
locations adjacent to the UT measurement sites. For each strip studied there were 12
metallography coupons, three for each of four inspection sites (0", 2", 3" & 4"). The 1" site
was skipped as it typically showed very similar attenuation and noise values as the central
(0") site. Selected faces of the metallography coupons were polished and etched, as shown
in Figure Ic, to provide axial, radial and hoop views of the microstructure at each
inspection site. A small section of one of the resulting micrographs is shown in Figure 3,
together with an associated image obtained by tracing the grain boundaries on a transparent
sheet placed over the micrograph, and then digitizing and "skeletonizing" the tracing. When
FIGURE 3. (a) Photograph of the microstructure at one site within the Waspaloy billet, (b) Grain
boundaries are traced and digitized, (c) In the Mean Chord Length technique for estimating grain
size, the average of the chord lengths (Sj) is calculated, (d) In the P(L) technique, line segments of
length L are randomly placed on the image, and the number not crossing grain boundaries is tallied .
1358
such tracings are made, so-called "twin boundaries" are intentionally excluded, since it is
believed that the angles between the atomic planes along the twin boundaries are very
small. Consequently the reflection coefficients are not significant enough to bring about any
changes in the velocities and hence they do not contribute to attenuation and backscattered
noise.Two different methods from reference [4], are illustrated in Figures 3c and 3d,
respectively, were used to deduce average grain diameters from a given micrograph image.
The first, known as the Mean Chord Length (MCL) method is equivalent to: (1) drawing a
straight line through the micrograph; (2) noting where the line crosses grain boundaries; (3)
calculating the average length of the straight segments which lie within a single grain; and
(4) multiplying this average chord length by 1.5 to obtain a grain diameter estimate. A
computer program operating on a digital image of the grain boundaries is used to "draw" all
possible lines in either the horizontal (X) or vertical (Y) direction. The conversion factor of
1.5, which translates from 2D images to 3D diameters, is believed to be appropriate for
equiaxed grains of the kind seen in these billets. In an alternative approach, known as the
"P(L) Method", one determines the probability P that a line segment of length L, randomly
placed on the micrograph, lies entirely within a single grain . A computer program
operating on a digital image of the grain boundaries is used to "draw" all possible lines in
either the horizontal (X) or vertical (Y) direction. The conversion factor of 1.5, which
translates from 2D images to 3D diameters, is believed to be appropriate for equiaxed
grains of the kind seen in these billets. In an alternative
1.2 -i
GRAIN DIAMETER ESTIMATES
USING THE MCL METHOD
V-DIE-A COUPON "D" (AXIAL VIEW)
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USING THE MCL METHOD
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120
140
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Estimate along X-direction
FIGURE 4. (a) Average grain diameters measured along X and Y directions by the MCL technique, (b)
Exponential fits to the P(L) data for one case. (c) and (d) Selected comparisons of average grain
diameters determined by the two methods.
1359
Correlation Between Measured Ateruation and
Measured Average Grain Diameter Nckel Superalloys
0.060
Correlation Between Measured Noise FOM and
Measured Average Grain Dfameter Nickel Superalloys
@ 7.5 MHz
• WASPALOY
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Estimated Grain Diameter (microns)
Correlation Between Measured Attenuation and
Measured Noise FOM for Nickel Superalloys
.A.
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FIGURE 5. Interrelationships between measured attenuation, noise FOM .and grain diameter values, (a)
Dependence of attenuation on grain diameter (b) Dependence of FOM on grain diameter, (c) Dependence
of FOM on attenuation.
approach, known as the "P(L) Method", one determines the probability P that a line
segment of length L, randomly placed on the micrograph, lies entirely within a single grain.
Again a computer program is used to generate the line segments and to decide whether a
given segment crosses a grain boundary. Again the method can be applied separately in the
X and Y directions of Figure 3. The X or Y analysis of a micrograph yields a probabilityversus-length curve, P(L). This curve is then fit to an exponential function P(L) = exp(-L/b),
and 2b is used as an estimate of the mean grain diameter [5]. Typical measured P(L) curves
do not exactly follow an exponential function, leading to some uncertainty in the fitting
parameter "b" and the associated grain diameter estimate. To document this uncertainty, we
perform separate fits to different regions of the measured P(L) curve . This is illustrated in
Figure 4b, where fits to the "low L" and "high L" halves of the data are shown for one
case. In practice we perform separate fits to 10 overlapping regions of the measured P(L)
curve, and tabulate the minimum, maximum, and mean values of "b" thus obtained. We
note that P(L) curves, sometimes referred to as two-point correlation functions, are of
interest because they appear in a direct manner in certain models of ultrasonic attenuation
and backscattered grain noise [6,7]. In all of the cases examined, the grain cross-sections
seen in the micrographs have roughly similar average dimensions in the X and Y directions,
indicating an equiaxed (near spherical) grain structure. This is demonstrated in Figure 4a
1360
for the MCL method. The P(L) analysis similarly finds nearly equiaxed grain diameters. As
shown in Figures 4c and 4d, the two methods generally yield similar average grain
diameters. When a given micrograph is analyzed, the MCL estimate usually falls in
between the minimum and maximum estimates from the P(L) method. As shown in Figure
4c, the average grain diameter estimate from the P(L) method tends to be about 5% below
that of the MCL method. Since the grains appear to be equiaxed and the two analysis
methods yield similar diameter estimates, we have averaged over the two analysis
directions [X and Y] and over the two methods [MCL and average P(L)] to arrive at an
overall grain diameter estimate for each micrograph. Measured average grain diameters
varied from 15 to 35 microns for the Inconel specimens, and from 35 to 120 microns for
Waspaloy.
CORRELATION BETWEEN UT PROPERTIES AND GRAIN DIAMETERS
Measured attenuation, FOM, and mean grain diameter values are plotted versus one
another in the three panels of Figure 5. Each plotted point represents measurements made
at one site for one particular inspection direction. Thus in each panel there are 12 points
shown for each strip coupon: (4 inspection sites per specimen)(3 inspection directions per
site). For comparison, Figure 5 also displays predicted curves for randomly-oriented,
equiaxed, single-phase (cubic), pure Nickel microstructures. Attenuation and FOM values,
as functions of the mean grain diameter, were predicted using the formalisms of References
[6] and [7], respectively, assuming an exponential P(L) curve. As expected for a simple
equiaxed microstructure, attenuation and FOM tend to increase with increasing grain
diameter. The observed relationship between FOM and attenuation is seen in Figure 5c to
have a similar form to that predicted for pure Nickel. The manners in which the measured
attenuation and FOM values tend increase with grain diameter are also similar to those
predicted by the models. However, the measured rise rates, which are slightly different for
the IN718 and Waspaloy cases, are somewhat lower than those predicted for pure Nickel.
This may indicate that the single-crystal elastic constants of the two alloys are significantly
different from those of pure Nickel. The scatter of the plotted experimental points about
their general trend is much larger in Figure 5a-b than in Figure 5c. This is likely because
absolute errors in grain diameter (D) estimates are significantly larger than absolute errors
in measured attenuation and FOM values. Grain sizing errors are believed to mainly arise
from two factors: (1) the process of identifying "non-twin" grain boundaries is somewhat
subjective; and (2) a given micrograph containing a few hundred grains represents only a
very minute fraction of the specimen volume that is insonified in a given ultrasonic
measurement. We also note that one of the strip specimens (V-Die-A) had a much larger
scatter of measured attenuation-versus-D and FOM-verus-D values about the general trend.
It is possible that some of the metallography coupons for V-Die-A were mislabeled during
cutting. Additional V-Die-A metallography work is planned to verify the current results.
SUMMARY AND FUTURE WORK
Coupons were cut from representative 10"-diameter Inconel and Waspaloy billets
for UT and metallographic studies. Longitudinal sound velocity, attenuation, and
backscattered grain noise capacity (FOM) were measured in three orthogonal directions in
the 5-15 MHz range. Sound velocity was found to be quite uniform; attenuation and FOM
were found to be locally isotropic, but varied with position (chiefly radial depth).
1361
Metallographic studies found the grains to be equiaxed, with the average grain size varying
with position. Two methods were used for determining the mean grain diameter at a given
measurement site, and both yielded similar results. Sites with large (small) mean grain
diameters tended to have large (small) UT attenuation and noise FOM. The UT and
metallographic results are consistent with having equiaxed, randomly-oriented crystallites,
whose mean grain diameter varies with position in the billet. The manner in which
attenuation and FOM increase with mean grain diameter is similar in form to that predicted
for pure-Nickel microstructures, although the rise rates are different. It may be possible to
use the available UT data to estimate the single-crystal elastic constants of IN718 and
Waspaloy. For example, one could vary the elastic constants input to the models to
optimize the agreement between measured and predicted velocities, attenuations and FOM
values (as functions of grain diameters). Work along these lines is planned.
Finally, we note that the grain-noise FOM, being a property of the microstructure
alone, should be independent of the details of its measurement. In our work, FOM values
are deduced by analyzing spectral components of the backscattered noise using a simple
model that neglects multiple scattering events [3]. We found that in some circumstances
the deduced FOM values depended on the choice of time gate used in the analysis. This
phenomenon and the relative importance of multiple scattering in grain noise are discussed
in a companion article [8].
ACKNOWLEDGEMENT
This material is based upon work supported by the Federal Aviation Administration
under Contract #DTFA03-98-D-00008, Delivery Order IA029 and performed at the Iowa
State Univ. Center for NDE as part of the Center for Aviation Systems Reliability program.
REFERENCES
1. P. Haldipur, F. J. Margetan, L. Yu and R. B. Thompson, "A Study of Ultrasonic
Property Variations Within Jet-Engine Nickel Alloy Billets" Review of Progress in
QNDE, Vol. 22, p. 1502.
2. E. J. Neiters, et al., "A Multizone Technique for Billet Inspection", Review of Progress
in QNDE, Vol. 14, op. cit. (1995), p. 2113.
3. F. J. Margetan, R. B. Thompson, and I. Yalda, "Modeling Ultrasonic Microstructural
Noise in Titanium Alloys", Review of Progress in QNDE, Vol. 12, (1993), p. 1775.
4. Fred E. Stanke, " Spatial autocorrelation functions for calculation of effective
propagation constants in polycrystalline materials", J. Acoust. Soc. Am 80, 1479-1485.
5. Fred E. Stanke and G.S. Kino, " A unified theory for elastic wave propagation in
polycrystalline materials", /. Acoust. Soc. Am 22, 665-681 (1984).
6. J. A. Turner and R. L. Weaver, Radiative Transfer and Multiple Scattering of Diffuse
Ultrasound in Polycrystalline Material", /. Acoust. Soc. Am 96, 3675-3683 (1994).
7. J. H. Rose, "Ultrasonic Backscatter from Microstructure", Review of Progress in
QNDE, Vol. 11, eds. , op. cit. (1992), p. 1677.
8. A. Li, R. Roberts, P. Haldipur, F. J. Margetan and R. B. Thompson, "Computational
Study of Grain Scattering Effects in Ultrasonic Measurements", these proceedings.
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