CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Hone
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
THE EFFECT OF MICROSTRUCTURE ON THE SHOCK
BEHAVIOUR OF y-TITANIUM ALUMINIDES.
J.C.F. Millett, I.P. Jones*, N.K. Bourne, G.T. Gray III**
Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK.
*Dept. of Metallurgy and Materials Science, University of Birmingham, Elms Rd, Edgbaston,
Birmingham, B15 2TT, UK.
**Los Alamos National Laboratory, Los Alamos, NM.
Abstract. Plate impact experiments have been performed on two alloys based on the intermetallic
compound TiAl. Previously, workers have shown that microstructural features (such as grain size and
phase distribution) significantly affect the mechanical response both at quasi-static and intermediate
strain-rates (Hopkinson bar). In this paper, the effects of such microstructural features are extended into
the shock-loading regime. In particular, the microstructural effects upon the Hugoniot, the Hugoniot
Elastic Limit and the shock induced shear strength are explored. Results show that the differences in the
properties of these two alloys under shock loading can be explained in terms of the microstructural
differences observed at lower strain-rates.
Hugoniots of two alloys, Ti-46.5Al-2cr-2Nb (5) and
Ti48Al-2Cr-2Nb-lB (6) were shown to be
significantly steeper than that of the engineering
alloy Ti-6Al-4V. The shear strength in both
materials was also seen to increase rapidly with
increasing shock stress (7, 8), as might be expected
from materials with high rates of work hardening.
In the latter alloy, elastic precursor decay was also
demonstrated (6). In this paper, we compare the
shock behaviour of these two alloys, and attempt to
explain them in terms of the microstructural
differences between them.
INTRODUCTION
In the past few decades, alloys based on the L10,
face-centred-tetragonal phase, TiAl have received
increasing attention as light-weight materials for jet
turbine applications. As such, there exists a great
deal of literature concerning their quasi-static
mechanical properties and microstructure (1-3).
However, despite that fact that materials in these
environments can experience high loading rates
such as bird strike, foreign object damage and blade
containment, their exists a scarcity of high-strainrate data in the literature. With regard to shock
loading, much of the early work on the behaviour of
bulk specimens was performed by Gray (4), who
showed that dislocations operated on the
a/2<110>{lll} slip system, with additional
deformation occurring via twinning on the
{112}(111) system. Post-shock strengths were also
shown (4) to be unchanged from the unshocked
state when equivalent strains were considered. Very
recently, the mechanical response of TiAl alloys
during shock loading has been investigated. The
EXPERIMENTAL.
Plate impact experiments were performed on 75
mm and 50 mm diameter single stage gas guns. The
alloy Ti-46.5Al-2Cr-2Nb (referred henceforth as Ti46.5-2-2) was supplied in the form of 50 mm x 50
mm x 5 mm plates. The alloy Ti-48Al-2Cr-2Nb-lB
(Ti-48-2-2-1) was in the form of a 100 mm diameter
cast billet, from which samples 45 mm x 45 mm of
varying thicknesses were cut, such that the impact
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axis was parallel to the long axis of the ingot.
Longitudinal stress measurements were made in
two ways. In the Ti-46.5-2-2 material, manganin
stress gauges (MicroMeasurements type LM-SS125CH-048) were supported on the back of the
targets with 12 mm of polymethylmethacrylate
(PMMA). The same method was used with the Ti48-2-2-1 alloy to determine the Hugoniot Elastic
Limit (HEL) and precursor decay, where target
thicknesses of 2, 5, 8 and 12 mm were investigated.
Internal stresses (<JY) were calculated according to,
2Z n
MATERIALS DATA
Ti-46.5Al-2Cr-2Nb
This material has a longitudinal wave speed (CL)
of 7.36±0.03 mm jis"1, shear wave speed (cs) of
4.12±0.03 mm ^is"1, density (p0) of 3.99 g cm"3, and
a Poisson's ratio (v) of 0.26. The microstructure had
a duplex nature, with a grain size of ca. 125 um.
Ti-48Al-2Cr-2Nb-lB.
The properties of this alloy are, CL= 7.29±0.03
mm jas"1, cs= 4.25±0.03 mm us"1, pQ= 3.99 g cm"3
and v= 0.26. The grain size was ca. 250 um, where
the grains were lamellar in nature, consisting of
both y and a2 lathes.
The microstructures of are shown in Fig. 1.
(1)
where Zg and Zp were the shock impedances in the
samples and the PMMA backing plates, and crp was
the stress measured in the gauge. In practice, the
shock impedance of the samples was unknown,
hence the acoustic impedance was used instead. In
practice, this had the effect of introducing a small
(ca. 5%) error into the measurement.
The second method used the same gauge was
placed between two 5 mm thick plates of Ti-48-2-21. In this embedded configuration, the gauge now
experiences the internal longitudinal stress
generated by the impact conditions. In addition,
another gauge was supported on the front of the
target assembly with a 1 mm coverplate, of the
same material as the flyer plate. In this way, that
gauge would also experience the internal stress, as
well as providing timing information that could be
used to calculate the shock velocity. In both cases,
the voltage-time data was converted to stress using
the methodology of Rosenberg et al. (9) To
determine the lateral stress, 5 mm thick plates of
both alloys were sectioned in half, and manganin
stress gauges introduced 2 mm from the impact
surface. The samples were them reassembled using
a low viscosity epoxy adhesive with a curing time
of approximately 12 hours. Stress data from these
gauges (MicroMeasurements type J2M-SS-580SF025) were calculated according to the analysis of
Rosenberg and Partom (10) using a modified
analysis that does not require prior knowledge of
the impact stress (11).
a. Ti-46.5Al-2Cr-2Nb
b.'.
FIGURE 1. Optical micrographs of the alloys in this
investigation.
(0
D-
o
0.8 •
0.7
1 0.6
(X
-
4
• Ti-48AI-2Cr-2Nb-1B •'.
D Ti-46.5AI-2Cr-2Nb :
T
9
0.5 •
*
.y 0.4
(0
111 0.3
;
*
02
6
8
10
12
Specimen Thickness (mm)
FIGURE 2. Elastic precursor decay in y-titanium aluminides.
635
;
14
Even though the Hugoniot of Ti-48-2-2-1 has been
measured over a much extended stress range
compared to Ti-46.5-2-2, it is clear that the latter
lies significantly above the former. This seems
surprising when one considers the similarity in
composition of these materials. It has been shown in
other metallic systems, for example tungsten base
(12) materials that the Hugoniot (in stress-particle
velocity space at least) is relatively insensitive to
composition. The differences in the Hugoniots of
these materials are consistent with the observations
of the HELs and the quasi-static work of Kim (2). A
possible explanation presents itself when the
differences between the hydrostat (P) and the
Hugoniot stress, as defined by the well-known
relation,
RESULTS AND DISCUSSION
In Fig. 2, we present the elastic precursor
amplitude as a function of specimen thickness for
the alloy, Ti-48-2-2-1. As a comparison, we have
also included the HEL for Ti-46.5-2-2, in this case,
taken from a specimen thickness of 5 mm. Even
though we were not able to measure precursor
amplitude as a function of thickness in Ti-46.5-2-2,
it is still possible to make a comparison between the
two materials from specimen thicknesses of 5 mm.
Here it can be seen that Ti-46.5-2-2 is stronger than
Ti-48-2-2-1. As can be seen from Fig. 1, the Ti46.5-2-2 alloy has a duplex microstructure with a
grain size of ca. 120 jim, whilst the Ti-48-2-2-1
material has a larger grain size of ca. 250 jam in a
fully lamellar microstructure. Whilst it should be
borne in mind that these materials have different
compositions, the differences in HEL follow the
trends in quasi-static properties (yield strength etc.)
shown by Kim (2). Here it was demonstrated that
duplex microstructures are stronger than those of a
lamellar nature. Strength has also been shown to
increase with decreasing grain size. Thus, it would
appear that the factors that govern the mechanical
properties of y-based titanium aluminides at quasistatic strain rates also hold true at the extreme
strain-rates imposed by shock loading.
In Fig. 3, we present the shock Hugoniots of
both alloys in stress-particle velocity space.
(2)
where r is the shear strength. If the hydrostat of
both materials is similar (which from the similarity
of the acoustic properties would seem possible),
then the differences in the Hugoniots must be due to
variations in the shear strengths between these two
alloys. This issue is examined further in Fig. 4.
Here we plot the variation of shear strength with
longitudinal stress in both alloys.
2.5
I "T
12 i * Ti-46.5AI-2Cr-2Nb
I 0 Ti-48AI-2Cr-2Nb-1B
^ 10 : l Typical error
s.
o
-
CO
(0
'-_
£
6
° :
'•
:
o
f 2
CL
CD,
CM
^
j
+ Ti-46.5AI-2Cr-2Nb
O Ti-48AI-2Cr-2Nb-1 B
0.5
-!_•
CO
4 :
i
2
:
O/
0
o
:
:
:
°
0.1
0.2
0.3
0.4
'&
2
4
6
8
10
12
14
Longitudinal Stress (GPa)
FIGURE 4. Shear strength versus shock stress for Ti-46.5-2-2
and Ti-48-2-2-1.
0.5
Partide Velocity (mm ijs"1)
In both materials, it can be seen that shear
strength increases rapidly with increasing
longitudinal stress. This has been explained
FIGURE 3. Shock Hugoniot of Ti-46.5-2-2 and Ti-48-2-2-1.
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has a lamellar microstructure with a grain size of
ca. 250 jim. These trends have proven to be
consistent with observations in the variations of
mechanical properties of these materials at quasistatic strain-rates.
previously (7, 8) in terms of the high workhardening that these materials display. Meyers and
Murr (13) have also shown that in two phase
materials, if the elastic moduli of each phase are
different, it is possible for dislocations to be
'punched out', thus increasing the overall
dislocation density, and hence the shear strength.
Schafrik (14) has measured the temperature
dependence of the elastic moduli of the y and oc2
phases, with the Young's and shear moduli at room
temperature for each phase being 173 GPa and 70
GPa (Y) and 146 GPa and 57 GPa (a2) respectively.
It can also be observed that the Ti-46.5-2-2 alloy
displays a higher shear strength than the Ti-48-2-21 material. With a higher proportion of a2 phase, it
would seem possible that this would lead to greater
dislocation generation. This is consistent with the
hypothesis made above that the differences in the
Hugoniots could be explained in terms of the
differences in the shear strengths. These in turn can
also be explained in terms of differences in the
grain size, where strength increases with decreasing
grain size. They are also consistent with the trends
in other shock properties discussed in this
investigation and in the quasi-static properties.
Another possibility presents itself when the shear
moduli are considered. Other metallic systems have
been shown to display similar increases in shear
strength, including copper (15) and tungsten (12).
This has been shown to correlate with an increase in
shear modulus with pressure of both materials,
where Steinberg (16) has quoted values of the
pressure dependence of the shear moduli of copper
and tungsten at 0.0283 GPa'1 and 0.0098 GPa"1
respectively. Whilst such data for titanium
aluminides does not appear to be available from the
literature, this possibility should be born in mind.
REFERENCES
1. Kim, Y.-W. Recent advances in gamma titanium aluminide
alloys Mat, Res. Soc. Symp. Proc. 213 (1991) 777-794.
2. Kim, Y.-W. Microstrucural evolution and mechanical
properties of a forged gamma titanium alloy Acta Met 40 (1992)
1121-2234.
3. Kim, Y.-W. ordered Intermetallic alloys 3. Gamma titanium
aluminides Journal of Metals. 46 (1994) 30-39.
4. Gray, G.T. Influence of shock loading on the structure /
property response of Ti-48Al-2Cr-2Nb and Ti-24Al-llNb
Journal de Physique /FColloque C8 (1994) 373-378.
5. Millett, J.C.F., Gray, G.T. and Bourne, N.K. The shock
Hugoniot of the intermetallic alloy, Ti-46.5Al-2Nb-2Cr J. Appl
Phys. 88 (2000) 3290-3294.
6. Millett, J.C.F., Bourne, N.K. and Jones, I.P. Shock induced
mechanical response of a g-TiAl alloy J. Appl Phys. 89 (2001)
2566-2570.
7. Millett, J.C.F., Gray, G.T. and Bourne, N.K. Lateral stress
measurements and shear strength in a shock -loaded g-TiAl alloy
J. Phys. IV10 (2000) 781-785.
8. Millett, J.C.F., Bourne, N.K. and Jones, I.P. Shear strength
measurements in the TiAl-based alloy, Ti-48Al-2Nb-2Cr-lB
during shock loading J. Appl. Phys. (2001) In press.
9. Rosenberg, Z., Yaziv, D. and Partom, Y. Calibration of foillike manganin gauges in planar shock wave experiments. J. Appl.
Phys. 51 (1980) 3702-3705.
10. Rosenberg, Z. and Partom, Y. Lateral stress measurement in
shock-loaded targets with transverse peizoresistive gauges. J.
Appl. Phys. 58 (1985) 3072-3076.
11. Millett, J.C.F., Bourne, N.K. and Rosenberg, Z. On the
analysis of transverse stresses during shock loading experiments
J. Phys. D. Applied Physics 29 (1996) 2466-2472.
12. Millett, J.C.F., Bourne, N.K., Rosenberg, Z. and Field, J.E.
Shear strength measurements in a shock-loaded tungsten alloy J.
Appl. Phys. 86 (1999) 6707-6709.
13. Meyers, M.A. and Murr, L.E., Defect generation in ShockWave Deformation, in Shock Waves and High Strain-RatePhenomena in Metals, M.A. Meyer and L.E. Murr, Editors.
1981, Plenum Press: New York. p. 487-530.
14. Schafrik, R.E. Dynamic elastic moduli of the titanium
aluminides Met. Trans. 8A (1977) 1003-1007.
15. Millett, J.C.F., Bourne, N.K. and Rosenberg, Z. Shear stress
measurements in copper, iron and mild steel under shock loading
conditions J.Appl. Phys 81 (1997) 2579-2583.
16. Steinberg, D.J., (1996), Equation of state and strength
properties of selected materials, Lawrence Livermore National
Laboratory, UCRL-MA-106439.
CONCLUSIONS.
The shock properties of two Y-based titanium
aluminide alloys, Ti-46.5Al-2Cr-2Nb and Ti-48Al2Cr-2Nb-lB have been investigated. Comparison of
the shock properties, in particular the HEL,
Hugoniot and shear strength show that the former
material is significantly stronger than the latter. The
first alloy has a duplex microstructure with a grain
size of ca. 120 jim, compared to the second which
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