Characterization of solutionizing behavior in VT14 titanium alloy using ultrasonic velocity and attenuation measurements
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Characterization of solutionizing behavior in VT14 titanium alloyusing ultrasonic velocity and attenuation measurements
Anish Kumar a, T. Jayakumar a,*, Baldev Raj a, K.K. Ray b
a Metallurgy and Materials Group, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, Indiab Indian Institute of Technology, Kharagpur 721302, India
Received 11 December 2002; received in revised form 6 March 2003
Materials Science and Engineering A360 (2003) 58�/64
www.elsevier.com/locate/msea
Abstract
VT14 titanium alloy (Ti�/4.5Al�/3Mo�/1V) was subjected to a series of heat treatments consisting of solutionizing for 1 h at the
selected temperatures in range of 923�/1323 K at an interval of 50 K, followed by water quenching. Hardness and optical microscopy
results are correlated with ultrasonic longitudinal and shear wave velocities and attenuation in these specimens. Ultrasonic velocities
and hardness decrease with solution annealing temperature (SAT) in the 923�/1123 K range. Beyond 1123 up to 1223 K, they
increase slightly. Beyond 1223 K, ultrasonic velocities become constant, whereas hardness increases up to 1323 K. Ultrasonic
attenuation exhibits an opposite behavior to velocity and hardness. Further, for the first time, authors have shown that ultrasonic
velocity can be used to identify the b-transus temperature in this alloy. Because of non-monotonous variation of velocity and
attenuation with solutionizing temperature, it was not possible to identify the SAT using any one of these parameters. Hence, a new
parameter, ratio of normalized differential of ultrasonic attenuation to normalized differential of ultrasonic velocity (RNDAV) has
been used, which is found to increase monotonously with SAT and hence enabling unambiguous characterization of SAT in solution
annealed VT14 alloy.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Ultrasonic velocity; Ultrasonic attenuation; VT 14 titanium alloy; Microstructural characterization
1. Introduction
Titanium alloys, by virtue of their excellent specific
strength and modulus and better intermediate tempera-
ture strength, are the most preferable structural materi-
als for aerospace applications. Further, due to their
excellent corrosion resistance and good compatibility
with human organs, titanium alloys are also widely used
for human implants. Solution annealed and tempered
a�/b titanium alloys possess better mechanical proper-
ties, such as yield, tensile and fatigue strength, than a�/bannealed alloys [1]. The solution annealing temperature
(SAT) plays an important role, as it decides the volume
fraction of primary a and b phases and volume fraction
of the alloying elements in different phases [2]. The
amount of b stabilizing elements in b phase governs the
stability of the phase upon rapid cooling to room
temperature and hence decides the product phase.
Further, if solutionizing is carried out above b-transus
temperature, the alloy looses its ductility due to the
substantial increase in grain size. While elastic properties
of most of the structural materials differ very marginally
with heat treatments, titanium alloys can exhibit varia-
tions as much as 10% [2]. As propagation of ultrasonic
wave depends upon the elastic properties of the material,
ultrasonic velocity can be a very good parameter for
characterization of heat treatments and corresponding
microstructure in titanium alloys.
Ultrasonic parameters, such as velocity and attenua-
tion, have been correlated with the microstructural
features evolved during heat treatments in ferritic steels
[3,4], superalloy [5], aluminum alloy [6] and many other
materials. Ultrasonic techniques have been used for
determination of yield strength [7], fracture toughness
[8], grain size [3], volume fraction of second phases [3,9],
* Corresponding author.
E-mail addresses: anish@igcar.ernet.in (A. Kumar),
tjk@igcar.ernet.in (T. Jayakumar), dmg@igcar.ernet.in (B. Raj),
kkrmt@metal.iitkgp.ernet.in (K.K. Ray).
0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0921-5093(03)00495-7
etc. Recently, Anish Kumar et al. [3] have used the
ultrasonic velocity and attenuation measurements for
characterization of the microstructures obtained by
various solution annealing treatments in modified9Cr�/1Mo ferritic steel. They showed that ultrasonic
velocity is useful for the determination of volume
fraction of ferrite and martensite and for identifying
the Ac1 and Ac3 critical temperatures, whereas, ultra-
sonic attenuation could be well correlated with the prior
austenitic grain size.
In the present work, ultrasonic velocity and attenua-
tion measurements are correlated with the microstruc-ture generated by solutionizing treatment in VT14
titanium alloy. For the first time, authors have shown
that ultrasonic velocities can be used for identifying the
b transus temperature in this alloy. As it was not
possible to determine the SAT using either ultrasonic
velocity or attenuation alone due to their non-mono-
tonous variation with temperature, a new parameter has
been identified, which is a function of both velocity andattenuation. This parameter, defined as the ratio of
normalized differential of ultrasonic attenuation to
normalized differential of ultrasonic velocity (RNDAV),
is found to increase monotonously with SAT, hence can
be used for nondestructive identification of solution heat
treatment of VT14 alloy.
2. Experimental
2.1. Heat treatment and specimen preparation
Various specimens of VT14 titanium alloy (Ti�/4.5Al�/
3Mo�/1V) of dimensions 20�/20�/12 mm were pre-
pared from a b-heat-treated (1323 K/1 h) and water
quenched disk of diameter 100 and 12 mm thickness.
These specimens were subjected to a series of heattreatments consisting of solutionizing for 1 h at the
selected temperatures in the range of 923�/1323 K at an
interval of 50 K, followed by water quenching. Metallo-
graphic examination was carried out to reveal the
microstructures in different specimens. The etchant
used is Kroller’s reagent [10]. The hardness of these
specimens was also measured using Vicker’s hardness
tester at a test load of 10 kg. Surface grinding of thesespecimens was carried out to obtain the specimens of
10.59/0.3 mm thickness with plane parallelism to an
accuracy of better than 9/3 mm.
2.2. Ultrasonic measurements
Fig. 1 shows the schematic of the experimental setup
used for ultrasonic measurements. 100 MHz broad bandpulser-receiver (M/s. Accutron, USA) was used to give
the electrical pulse to the transducer for generating the
ultrasonic waves. The transducers consist of piezo-
electric crystals, which vibrate upon the application of
electrical pulse across the thickness and generate the
mechanical waves. For the longitudinal wave transdu-
cer, the piezoelectric crystal is cut perpendicular to the
electric axis (X-cut crystals) and hence when the
electrical pulse is applied in the thickness direction, the
crystal vibrates in the thickness direction (i.e. perpendi-
cular to the specimen surface) and generates the long-
itudinal wave in the specimen coupled to the transducer.
Whereas for the shear wave transducer, piezoelectric
crystal is cut perpendicular to the mechanical axis (Y-cut
crystal) and hence when the electrical pulse is applied in
the thickness direction, the crystal vibrates in the width
(i.e. parallel to the specimen surface) direction and thus
generates the shear wave in the specimen coupled to the
transducer. The schematic of the propagation of long-
itudinal and shear waves are as shown in Fig. 2. In the
case of longitudinal wave, the particle vibration is in the
direction of wave motion, whereas, in the case of shear
wave, the particle vibration is perpendicular to the
direction of wave motion.
A normal beam delay line piezoelectric transducer of
15 MHz frequency (PDG152), supplied by M/s. NDT
Systems Inc., California, was used for longitudinal wave
velocity and attenuation measurements and shear wave
velocity measurements were carried out using 5 MHz
Fig. 1. Schematic of the experimental setup for ultrasonic measure-
ments.
Fig. 2. Schematic of the particle vibration and wave motion during
propagation of (a) longitudinal and (b) shear waves.
A. Kumar et al. / Materials Science and Engineering A360 (2003) 58�/64 59
piezoelectric normal beam shear wave transducer
(V155), supplied by M/s. Panametrics Inc, USA. The
transducer was coupled with the specimen using ultra-
sonic couplant ZG-F (M/s. Krautkramer, Germany) for
longitudinal waves and honey (M/s. Panametrics, USA)
for shear waves. The reflected ultrasonic waves from the
specimen were picked up by the same transducer and
ultrasonic waves were converted to electrical signal. This
electrical signal was acquired by the receiver at optimum
gain and damping settings, and the rf signal was fed to
the 500 MHz digitizing oscilloscope (Tektronix
TDS524). The signal was digitized at 500 MHz and
averaged for about 50 signals. The gated backwall
echoes (2048 ns duration) from the oscilloscope were
transferred to the personal computer with the help of
General Purpose Interface Bus (GPIB) interfacing and
software developed in LabVIEW. Specific software was
developed in LabVIEW 3.1.1 for the acquisition of
ultrasonic signals and on-line determination of ultra-
sonic velocity and attenuation using cross correlation
and frequency spectrum based methodologies, respec-
tively. Fig. 3 shows the front panel of the software
developed for ultrasonic velocity and attenuation mea-
surements. Before starting the experiment, the thickness
of the specimen and the difference in the number of the
backwall echoes are entered in the front panel. If the
first and second backwall echoes are considered then
this difference in the number of the backwall echoes is 1.
Cross correlation technique has been used for the
precise velocity measurements. Various steps involved in
this methodology are as follows: (a) acquisition of two
digitized echoes from the oscilloscope, (b) cross correla-tion of the echoes to find the approximate time delay, (c)
application of an interpolation method for accurate time
delay and (d) calculation of velocity measurement using
time delay and thickness of the specimen along with the
following equation;
V�2 � Difference of Echo � Thickness
Time delay(1)
The accuracy in time of flight measurement is better
than 9/0.3 ns and in turn the accuracies in the
measurement of ultrasonic longitudinal and shearwave velocities for thickness of the specimens in range
of 10.59/0.3 mm are better than 9/2.5 and 9/1.5 m s�1,
respectively. The use of cross correlation may lead to
errors in the case of multiple peaks, pulse expansion and
jitters. To ensure that there is no error in the velocity
measurements, after the calculation of the time of flight,
the backwall echoes are given the calculated time delay
to check whether they exactly overlap or not (Fig. 3).However, in the present case there appears to be no
pulse expansion, jitter and appreciable change in the
signal, the backwall echoes overlapped well in all the
measurements.
For attenuation measurements, frequency spectrum
based ultrasonic attenuation measurement method was
Fig. 3. Front panel of the software for ultrasonic velocity and attenuation measurements.
A. Kumar et al. / Materials Science and Engineering A360 (2003) 58�/6460
used. The peak value in the autopower spectrum of the
first backwall echo was detected and the 6 dB drop point
was calculated. The frequency values corresponding to
the 6 dB drop level were calculated for both sides of the
peak value. All the data points lying in between these
two 6 dB drop points were summed for the first (S1) and
second (S2) backwall echoes. Ratio of S1 to S2 is the
ratio of the areas of autopower spectra of the two
echoes, which in fact gives the ratio of energy content of
the two backwall echoes in this frequency band. Then
the ultrasonic attenuation coefficient was calculated by
the following formula:
Attenuation Coefficient (dB mm�1)
�20 log
S1
S2
2 � thickness (mm)(2)
The maximum scatter in the attenuation coefficient
measurement was less than 9/0.02 dB mm�1.
3. Results and discussion
Fig. 4 shows micrographs of specimens solutionized at
1073, 1123, 1173 and 1223 K. The amount of primary adecreases with increase in the heat treatment tempera-
ture up to 1173 K. Whereas, the specimen solutionized
at 1223 K shows the presence of a? marteniste only,
indicating that the b transus (Ac3) temperature of VT-14
alloy lies between 1173 and 1223 K. In any a�/btitanium alloy, volume fraction of b phase increases
with increase in temperature up to b transus tempera-
ture. But as the amount of b stabilizing elements is fixed,
the volume fraction of b-stabilizing elements in b phase
decreases with increase in temperature. Such b phase
composition is unstable at room temperatures at which
it decomposes into a lesser volume fraction of b and
secondary a. The same occurs during cooling to room
temperature, if enough time is given for diffusion to take
place. If the alloy is quenched to room temperature
(diffusion is not allowed), the b phase becomes unstable
and either remains as unstable b, or transforms to soft
aƒ (orthorhombic) or hard a? (hexagonal closed packed)
martensite, depending upon the SAT [2]. The unstable bhas been reported to be a soft phase having the lowest
moduli and highest damping capacity among all the
phases present [2,3].
Figs. 5 and 6 show, respectively, the variation in the
longitudinal and shear ultrasonic velocities and in
attenuation with SAT for solution annealed and
quenched specimens. Fig. 6 also shows the variation in
hardness with SAT. Ultrasonic velocities and hardness
decrease with SAT upto about 1123 K followed by a
gradual increase upto 1223 K. Beyond 1223 K, the
ultrasonic velocities become almost constant, however,
hardness increases continuously. The initial decrease in
ultrasonic velocities and hardness with increase in SAT
is attributed to the increased amount of b phase having
lesser stabilizing elements [2]. This decreases the elastic
modulus of the b phase [2] and hence of the alloy soaked
Fig. 4. Micrographs of the specimens solution annealed at (a) 1073 K,
(b) 1123 K, (c) 1173 K and (d) 1223 K in air for 1 h followed by water
quenching.
Fig. 5. Variation in ultrasonic velocities with SAT in the specimens
that were solution heat treated followed by water quenched.
A. Kumar et al. / Materials Science and Engineering A360 (2003) 58�/64 61
at higher temperatures. The minimum in ultrasonic
velocities at about 1123 K is attributed to the formation
of maximum amount of soft unstable b phase having the
lowest modulus. The increase in the velocities and
hardness for SAT beyond 1123 K is attributed to the
formation of hard a? martensite during quenching
instead of unstable b phase. Beyond 1223 K, ultrasonic
velocities become constant due to formation of 100% a?martensite with similar modulus at all heat treatment
temperatures above 1223 K. This is in agreement with
the metallographic studies, which exhibited the presence
of only a? martensite in the specimen solutionized at
1223 K (Fig. 4d). The total variation in ultrasonic
longitudinal and shear wave velocities in the present
study are �/1.76 and �/6.9%, respectively. The larger
variation and better accuracy (due to lower velocity and
hence higher transit time) of shear wave velocity
compared with longitudinal wave velocity measure-
ments make shear wave velocity a more reliable para-
meter for microstructural characterization in VT14
alloy.
The variation in ultrasonic attenuation with SAT
exhibits opposite behavior to that of the velocities and
hardness. It increases with SAT to a peak at about 1123
K followed by continuous decrease up to about 1223 K.
Beyond 1223 K, ultrasonic attenuation remains almost
constant up to 1323 K. The maximum in the attenuation
at about 1123 K is again attributed to the formation of
maximum amount of soft unstable b phase having the
highest damping. The decrease in attenuation beyond
the maximum is attributed to the formation of hard a?martensite instead of unstable b phase.
The present study reveals that ultrasonic velocities
and attenuation in VT14 alloy are affected by the SAT.
However, because of non-monotonous variation of
ultrasonic velocity and attenuation with SAT (Figs. 5
and 6), any value of velocity or attenuation in an
unknown specimen may be correlated with solutionizing
at either a temperature below the peak or with a
temperature above the peak (about 1123 K). Identifica-
tion of SAT is difficult; when using either velocity or
attenuation parameter alone. For example, if a specimen
exhibits a shear wave velocity of 2940 m s�1 (line A�/B
in Fig. 5), it can be correlated with SAT of either 1200 or
1100 K (Fig. 5). Similarly, the attenuation plot (Fig. 6)
exhibits that the specimen is either solutionized at 1200
or at 1000 K. Hence for exact identification of SAT,
some other ultrasonic parameter is required to be
identified, whose variation is monotonous with tem-
perature.
Ultrasonic velocity is dependent upon the modulus
and density of the phases present, whereas ultrasonic
attenuation depends upon the damping and scattering of
the waves. Also as different phases affect the modulus,
density, scattering power, and damping power to
different extents, they will have different effects on
ultrasonic velocity and attenuation. Hence the correla-
tion between ultrasonic velocity and attenuation should
be different for different phases affecting these two
ultrasonic parameters differently. Based on this ap-
proach, Fig. 7 shows the variation in ultrasonic velo-
cities with ultrasonic attenuation. Examination of this
figure shows that ultrasonic velocities decrease more
rapidly upto the peak values (specimens solutionized
upto 1123 K), as compared with that beyond the peak
values. This means that a parameter, which accounts for
the variations in both ultrasonic attenuation and
velocity, can be used to identify whether the specimen
is solutionized at a temperature above or below the peak
(i.e. 1123 K). As the shear velocity is a better parameter
than longitudinal wave velocity, all further analyses
were carried out for shear wave velocity only. The ratio
of the normalized differential between ultrasonic at-
tenuation corresponding to the heat treatment tempera-
ture and the lowest treatment temperature (923 K) to the
normalized differential in ultrasonic shear wave velocity
relative to that at the lowest temperature (923 K) is used
as a new parameter named as RNDAV. RNDAV for
Fig. 6. Variation in ultrasonic attenuation and hardness with SAT in
the specimens that were solution heat treated followed by water
quenched.
Fig. 7. Variation in ultrasonic velocities with attenuation for speci-
mens solution annealed at different temperatures.
A. Kumar et al. / Materials Science and Engineering A360 (2003) 58�/6462
each specimen has been calculated as follows:
RNDAVu�(au � a923 K)=a923 K
(Vu � V923 K)=V923 K
where au and Vu are attenuation and shear wave
velocity for the specimen solutionized at temperature u
and a923 K (0.21 dB mm�1) and V923 K (3115 m s�1) are
attenuation and shear wave velocity for the specimen
solutionized at 923 K.
Table 1 and Fig. 8 show the variation in RNDAVwith SAT. The negative values of RNDAV are due to
the fact that the variations in ultrasonic velocity and
attenuation are always in opposite directions to each
other for all the specimens studied (Table 1). The
horizontal line XY at RNDAV:/�/43 corresponds
specimen solutionized at 1123 K (peak in attenuation
and velocity). Since the RNDAV increases monoto-
nously with SAT, it can be used to determine the SATunambiguously.
In the present study, the use of RNDAV has been
demonstrated for the identification of SAT for solution
annealed followed by water quenched VT14 alloys. The
use of RNDAV for other heat treatment conditions and
in different alloy systems is also being explored. The
specimen corresponding to the extreme values of velo-
city and attenuation can be taken as reference in thoseconditions.
4. Conclusion
The present study reveals that ultrasonic velocity and
attenuation measurements can be used for non-destruc-
tive characterization of solutionizing behavior in Ti�/
4.5Al�/3Mo�/1V (VT 14 alloy). For the first time, it
has been shown that ultrasonic velocities can be used in
identifying the b transus temperature in titanium alloys.
It is also shown that ultrasonic shear wave velocity is a
better parameter than longitudinal velocity for micro-structural characterization in VT14 alloy. Further, it is
found that neither of ultrasonic velocity nor attenuation
alone can be used for the determination of SAT due to
their non-monotonous variation with SAT. However,
the combined use of velocity and attenuation in terms of
a new parameter defined as RNDAV is found to vary
monotonously with SAT and hence is useful for
unambiguous identification of the SAT in the solutionannealed VT14 alloy.
Acknowledgements
We are thankful to Dr S.L. Mannan, Associate
Director, Materials Development Group and Mr P.
Kalyanasundaram, Head, Division for PIE and NDT
Development, Indira Gandhi Center for Atomic Re-search for their cooperation. Authors are also thankful
to Mr M. Narayana Rao, Nuclear Fuel Complex,
Hyderabad, for providing the Ti�/4.5Al�/3Mo�/1V alloy.
Table 1
Variation in ultrasonic shear wave velocity, ultrasonic attenuation and RNDAV with SAT
SAT (K) Ultrasonic shear wave velocity (V), (m s�1) Ultrasonic attenuation (a), (dB mm�1) /
au � a923 K
a923 K
/
Vu � V923 K
V923 K
RNDAV
923 3115 0.2068 0 0 �/
973 3096 0.2888 0.39652 �/0.0061 �/65.01
1023 3055 0.4043 0.95503 �/0.01926 �/49.58
1073 2998 0.56 1.70793 �/0.03756 �/45.47
1123 2900 0.835 3.03772 �/0.06902 �/44.01
1173 2923 0.46 1.22437 �/0.06164 �/19.86
1223 2954 0.3811 0.84284 �/0.05169 �/16.31
1273 2956 0.3607 0.7442 �/0.05104 �/14.58
1323 2957 0.308 0.48936 �/0.05072 �/9.65
Fig. 8. Variation in RNDAV with SAT.
A. Kumar et al. / Materials Science and Engineering A360 (2003) 58�/64 63
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