Full text of DOI:10.1557/JMR.2002.0338

Creep behavior of in situ dual-scale particles-TiB whisker
and TiC particulate-reinforced titanium composites
Z.Y. Maa)
Department of Metallurgical Engineering, University of Missouri, Rolla, Missouri 65409
S.C. Tjong and X.M. Meng
Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue,
Kowloon, Hong Kong, Peoples Republic of China
(Received 22 April 2002; accepted 6 June 2002)
A titanium composite reinforced by in situ dual-scale particle, high-aspect-ratio TiB
whiskers and fine TiC particulates was fabricated by a reactive hot pressing technique
from a B4C–Ti system. The composite was subjected to creep investigations in
compression at 873–923 K. This composite exhibited a stress exponent of 4.5–4.6 and
a creep activation energy of 298 kJ/mol. By comparison, unreinforced Ti exhibited a
stress exponent of 5.2–5.3 and a creep activation energy of 259 kJ/mol. No change in
the stress exponent with varying creep rates was observed in both composite and
unreinforced Ti under the investigated creep rates. The creep resistance of the
composite was more than one order of magnitude higher than that of the unreinforced
Ti. The load transfer mechanism accounted for this result. The creep of both composite
and unreinforced Ti was controlled by lattice diffusion in the titanium matrix.
I. INTRODUCTION
First, the addition of ceramic reinforcements into the ti-
Recent years have seen an increased interest in devel-
oping titanium matrix composites with discontinuous
reinforcements for high-temperature structural applica-
tions. Among the reinforcements, TiB and TiC are par-
ticularly attractive since they are completely compatible
with a titanium matrix. In recent years, novel processing
techniques, such as exothermic dispersion, rapid solidi-
fication processing, combustion-assisted synthesis, and
reactive hot pressing (RHP), have been developed in
which the reinforcements are grown in situ in the titanium
matrix.1–10 Such composites have good interfacial bonding
between the in situ reinforcement and the titanium matrix,
and a crystallographic orientation relationship has been ob-
served between them.11 Thus, the adverse effect of the sur-
face layer associated with the added reinforcements
is avoided. Therefore, the in situ titanium matrix
composites are expected to exhibit superior mechanical
properties.1,10
tanium matrix generally results in significant creep
strengthening.7,12–18 However, the strengthening mecha-
nisms are not yet well understood, though microstruc-
tural strengthening13,15 and increase in the modulus of
composite12,13 have been suggested as possible strength-
ening mechanisms. Secondly, a change in the stress ex-
ponent from n ס 2.3 under low creep rates to n ס 7.2
under high creep rates has been reported by Zhu et al.16
in the TiBw/Ti–6Al–4V composite. Similar behavior was
also observed in TiCp/Ti–6Al–4V composites.12,17
A
critical creep strain rate of 4 × 10−6s−1 to 1 × 10−5s−1, at
which a change in stress exponent occurs, was revealed
for these composites. However, these researchers did
not provide any mechanistic explanation for this
change.12,16,17 Thirdly, while Ma et al.18 and Ranganath
and Mishra13 reported that the stress exponent of 4.1–4.9
obtained at 873–923 K in TiBw/Ti, Ti2Cp/Ti, and
(TiBw + Ti2Cp)/Ti composites is close to that for the
lattice-diffusion-controlled dislocation climb process in
␣–Ti (n ס 4.3),19 high stress exponents of 6–7 were ob-
served at 823 K by Tsang et al.14 and Ranganath and
Mishra13 in TiBw/Ti, Ti2Cp/Ti, and (TiBw + Ti2Cp)/Ti
composites.
A fundamental knowledge of the mechanisms influ-
encing creep behavior of the titanium matrix composites
is required for their use in high-temperature applications.
Unfortunately, the creep studies on these composites are
limited, and no consistent experimental results and
mechanistic explanations have been obtained so far.12–18
Different mechanisms have been proposed to explain
the exceptionally high stress exponent observed in tita-
nium matrix composites.13,14 Ranganath and Mishra13
suggested that the high values of the stress exponent at
823 K were associated with the transition of creep
a)Address all correspondence to this author.
e-mail: zong@umr.edu
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Z.Y. Ma et al.: Creep behavior of in situ dual-scale particles-TiB whiskers and TiC-particulate-reinforced titanium composites
mechanism from lattice to pipe diffusion, and a kinetic
strengthening term involving volume fraction of rein-
forcement to the constitutive equation of power-law
creep was proposed to interpret the results that creep data
cannot merge even after compensation for threshold
stress. On the other hand, Tsang et al.14 reported that
after incorporation of the composite modulus-
compensated effective stress into the power-law creep
equation, the creep behavior of both unreinforced Ti and
composites containing various volume fractions of TiB
whiskers can be described by a modified creep equation.
It is important to point out that the creep data range
measured by these researchers is relatively narrow, cov-
ering only 2–3 orders of magnitude of the creep strain
rates. Thus, it is difficult to unambiguously elucidate the
operative creep mechanism in these titanium matrix
composites.
Westwood has proposed a concept of designer micro-
structures containing hard particle phases for strength,
resilient phases for toughness, and whiskers for creep
resistance.1 The titanium matrix composite fabricated
from the Ti–B4C system contains both rodlike TiB whis-
kers and equiaxed TiC particles. It is expected that the
(TiBw + TiCp)/Ti composite would exhibit high strength
and creep resistance. Indeed, the (TiBw + TiCp)/Ti com-
posite exhibited a higher compressive strength than the
TiBw/Ti composite at temperatures ranging from 623 to
923 K.9 In a previous work, we investigated the creep
behavior of the TiBw/Ti composite.18 It was shown that
the creep resistance of the TiBw/Ti composite was much
higher than that of unreinforced Ti. In this work, the
in situ (TiBw + TiCp)/Ti composite was subjected to
compressive creep investigations at 873–923 K, and four
orders of magnitude of creep strain rates were measured.
This work aims to (i) study the effect of the in situ dual-
scale hybrid reinforcements, i.e., TiB whisker and TiC
particulate on the creep properties of the composites;
(ii) verify whether there is a change in stress exponent
with varying creep rates in titanium matrix composites;
and (iii) elucidate the operative creep mechanism and
strengthening mechanism in discontinuously reinforced
titanium matrix composites.
occurred completely. The as-compacted green billets
were degassed and then reaction pressed at 1523 K for
0.5 h in a vacuum. Finally, the as-pressed billets were
extruded into rods with an extrusion ratio of 18:1 at
1373 K. Unreinforced Ti sample was also fabricated un-
der identical conditions. The resulting composite was
subjected to microstructural examination using optical
microscopy and transmission electron microscopy (TEM,
JEOL 2010, Tokyo, Japan). The thin foils for TEM were
prepared by the ion-milling technique.
The as-extruded composite and unreinforced Ti were
subjected to constant load compressive creep tests at 873,
898, and 923 K. Cylindrical compression specimens of
5-mm gauge diameter and 5-mm gauge height were ma-
chined from the extruded rods with the specimen axis
parallel to the extrusion direction. A universal testing
machine (Mayes, model ESM 100, United Kingdom)
with compression grip was used. The creep strain of the
sample was measured using two parallel linear variable
displacement transducers (LVDTs) mounted on the
ridges provided on the compression grips. The tempera-
ture of the sample was monitored using a thermocouple
tied to the center of the specimen. The specimen tem-
perature was controlled within 1 K.
III. RESULTS
Figures 1(a) and 1(b) show the optical micrographs of
the 15 vol% (TiBw + TiCp)/Ti composite in the direc-
tions parallel and vertical to the extrusion direction. The
microstructure of the composite is characterized by uni-
directional alignment of rodlike reinforcements along the
extrusion direction and uniform distribution of fine par-
ticulates. Previous x-ray diffraction (XRD) and TEM ex-
aminations have verified that the rodlike whiskers were
TiB and the fine particulates were TiC.9 The unidirec-
tional alignment of the TiB whiskers resulted from the
hot extrusion process. The TiB whiskers exhibited a
diameter of up to 10 ␮m, and a length of up to 250 ␮m.
Figure 2 shows a TEM micrograph of the (TiBw + TiCp)/
Ti composite. A clean interface was revealed between the
in situ reinforcements (TiB whisker and TiC particulate)
and titanium matrix. Furthermore, TEM examinations
showed the existence of a lot of nanometer TiC particu-
lates and a certain amount of submicrometer diameter
TiB whiskers.
Figure 3 shows a typical compressive creep curve for
the 15 vol% (TiBw + TiCp)/Ti composite at 898 K under
an applied stress of 100 MPa. A well-defined steady-
state creep stage is evidently visible from this plot. Simi-
lar behavior is also observed in unreinforced Ti. Figure 4
shows the variation of steady state creep rate with applied
stress for the 15 vol% (TiBw + TiCp)/Ti composite. It is
noted that the creep data measured for this composite
cover four orders of magnitude of creep strain rates
II. EXPERIMENTAL
TiB whisker and TiC particulate mixture-reinforced
titanium [(TiBw + TiCp)/Ti] composite was used in this
study. The composite was fabricated from a B4C–Ti sys-
tem by the RHP technique.9,10 In the process, 60-␮m Ti
powder (98% purity) and 3-␮m B4C powder (98% pu-
rity) were initially mixed in a biaxis rotary mixer and
subsequently cold compacted. During blending, the ratio
of the two powders was properly adjusted so that
12.1 vol% TiB whisker and 2.9 vol% TiC particulate
were generated—assuming that all the in situ reactions
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Z.Y. Ma et al.: Creep behavior of in situ dual-scale particles-TiB whiskers and TiC-particulate-reinforced titanium composites
(6.2 × 10−8 to 8.5 × 10−4s−1). Clearly, the data at each
temperature in Fig. 4 fit into a straight line with slopes
equal to the value of the stress exponent n. The observed
linearity in this plot indicates that the creep data follow a
power-law equation
n
AGbDo
kT
␴
Q
.
⑀ =
exp −
,
(1)
ͩ ͪ ͩ ͪ
G
RT
.
where ⑀ is the steady state creep rate, A is a structure
dependent parameter, G is the temperature-dependent
shear modulus, b is the Burgers vector, Do is the fre-
quency factor for diffusion, k is the Boltzmann’s con-
stant, T is the absolute temperature, ␴ is the applied
stress, n is the stress exponent, Q is the activation energy,
and R is the gas constant. The composite exhibited stress
exponents of 4.5, 4.6, and 4.6 at 873, 898, and 923 K,
respectively. The variation of steady-state creep rate with
applied stress for the unreinforced Ti is shown in Fig. 5.
Stress exponents of 5.3, 5.3, and 5.2 were observed for
the unreinforced Ti at 873, 898, and 923 K, respectively.
Figure 6 shows the effect of temperature on the steady-
state creep rate of both the (TiBw + TiCp)/Ti composite
FIG. 2. TEM micrograph showing clean interface between in situ
reinforcements and titanium matrix in 15 vol% (TiBw + TiCp)/Ti
composite.
FIG. 3. Typical compressive creep curve for 15 vol% (TiBw + TiCp)/
Ti composite at 898 K under an applied stress of 100 MPa.
FIG. 1. Optical micrographs showing the distribution of in situ rein-
forcements in 15 vol% (TiBw + TiCp)/Ti composite: (a) longitudinal
direction and (b) traverse direction.
FIG. 4. Variation of steady-state creep rate with applied stress for
15 vol% (TiBw + TiCp)/Ti composite.
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Z.Y. Ma et al.: Creep behavior of in situ dual-scale particles-TiB whiskers and TiC-particulate-reinforced titanium composites
and unreinforced Ti. The activation energy was deter-
mined to be 298 kJ/mol for the composite at a constant
stress of 215 MPa and 259 kJ/mol for the unreinforced
Ti at a constant stress of 65 MPa.
other researchers also reported a significant decrease in
the creep rate of the titanium matrix composites over the
unreinforced titanium matrix.7,12–16 It is important to
note that the creep resistance of the (TiBw + TiCp)/Ti
composite is much higher than that of the TiBw/Ti com-
posite in the range of investigated temperatures and creep
rates. This result is consistent with that of compressive
tests at high temperatures of 623–923 K.9 This indicates
that the (TiBw + TiCp)/Ti composite exhibits superior
high-temperature properties over the TiBw/Ti composite.
This is attributed to dual-scale hybrid reinforcements in
the (TiBw + TiCp)/Ti composite, i.e., TiB whiskers and
TiC particulates.
Microstructural design in metallic materials for high-
temperature applications has received increasing atten-
tion in recent years. Westwood proposed that a hybrid
microstructure containing both hard particles and whis-
kers is beneficial for increasing the strength and creep
resistance of the materials.1 Based on analyses of
creep data and microstructures of a series of aluminum
matrix composites, Mishra suggested an idealized hybrid
microstructure for high-temperature aluminum matrix
composites; i.e., the composites are reinforced by both
nanometer dispersoids and micrometer ceramic particu-
lates.20 Recently, considering the concept of thermally
activated dislocation detachment from dispersoids as a
rate-controlling mechanism in dispersion hardened ma-
trices, Ro¨sler and Ba¨ker analyzed theoretically the creep
behavior of dual-scale particle strengthened metals con-
taining nanometer size dispersoids and reinforcements
with typical dimensions in the micrometer to millimeter
range.21 They revealed that dual-scale particle strength-
ening can result in very high creep strength levels. Fur-
ther, they pointed out that rodlike reinforcement with
high aspect ratio is effective in strengthening a metal
IV. DISCUSSION
Figure 7 shows the comparison between creep rates of
15 vol% (TiBw + TiCp)/Ti composite and unreinforced
Ti at 898 K as a function of applied stress. For compari-
son, the creep data of 15 vol% TiBw/Ti composite fabri-
cated under identical conditions18 is also included in this
plot. Clearly, the creep rate of the (TiBw + TiCp)/Ti com-
posite is more than one order of magnitude lower than
that of unreinforced Ti. Similar results were also ob-
served at 873 and 923 K. In previous investigations,
FIG. 5. Variation of steady-state creep rate with applied stress for
unreinforced Ti.
FIG. 7. Comparison between creep rates of 15 vol% (TiBw + TiCp)/Ti
composite, unreinforced Ti, and 15 vol% TiBw/Ti composite at 898 K
as a function of applied stress.
FIG. 6. Effect of temperature on steady-state creep rate for 15 vol%
(TiBw + TiCp)/Ti composite and unreinforced Ti.
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Z.Y. Ma et al.: Creep behavior of in situ dual-scale particles-TiB whiskers and TiC-particulate-reinforced titanium composites
.
matrix, and a maximum strength is reached when a me-
tallic material is strengthened by the dual-scale particles
⑀ kT/DLGb versus ␴/G (Fig. 8), where DL [m2 s−1] ס 1.0 ×
10−4exp(−241/RT), G [MPa] ס 4.95 × 104 − 25T for ␣–Ti,
b ס 2.89 × 10−10 m,19 and k ס 1.38 × 10−23 J mol−1 K−1.
The creep data of the 15 vol% (TiBw + TiCp)/Ti compos-
ite merge onto a single straight line with a slope of 4.5.
This demonstrates that the creep of the (TiBw + TiCp)/Ti
composite is controlled by the lattice diffusion in the
titanium matrix. However, the normalized creep rate of
the composite is still much lower than that of unrein-
forced Ti, indicating significant creep strengthening of
the composite over the unreinforced titanium matrix.
Different mechanisms, e.g., microstructural strength-
ening13,15 and increase in the composite modulus,12,13
have been suggested as possible creep strengthening
mechanisms in titanium matrix composites. In discon-
tinuously reinforced aluminum matrix composites, load
transfer has been successfully used to explain the creep
strengthening by several investigators.22–24 In this case,
part of the external load is transferred to the reinforce-
ment with a corresponding reduction in the level of the
effective stress acting on the matrix.25,26 In the present
(TiBw + TiCp)/Ti composite, the main reinforcement is
TiB whisker with a high aspect ratio. It is well accepted
that high-aspect-ratio micrometer reinforcements
strengthen the metal matrix by means of load-transfer
mechanism.21 Furthermore, while the fine nanometer
TiC particulates retard the creep rate by an attractive
interaction between the fine particulates and dislocations,
a few large micrometer TiC particulates also contribute
to strengthen titanium matrix by load transfer in this
titanium matrix composite as in the micrometer particu-
late reinforced aluminum matrix composites.22,23 There-
fore, it is very likely that the load transfer contributes
mainly to the creep strengthening of the (TiBw + TiCp)/
with a volume fraction ratio of 3⁄
4
rodlike reinforcements
1
and
⁄
4
dispersoids. As mentioned above, the
(TiBw + TiCp)/Ti composite is reinforced by dual-scale
particles, i.e., high aspect ratio TiB whiskers and fine
equiaxed TiC particulates (Figs. 1 and 2). Clearly, the
microstructure in the (TiBw + TiCp)/Ti composite is in
good agreement with that suggested by Westwood1 and
Ro¨sler and Ba¨ker.21 Furthermore, the volume fraction
ratios of TiB whiskers and TiC particulates in the present
4
1
5
(TiBw + TiCp)/Ti composite are ⁄ and ⁄ , respectively.
5
This ratio is close to that suggested by Ro¨sler and
Ba¨ker.21 Therefore, the dual-scale hybrid reinforcements
can account for the higher creep resistance of the
(TiBw + TiCp)/Ti composite.
From Figs. 4 and 5, it is clear that the present creep
data showed no change in the stress exponent with vary-
ing creep rates for both 15 vol% (TiBw + TiCp)/Ti com-
posite and unreinforced Ti in the range of investigated
creep rates (6.2 × 10−8 to 8.5 × 10−4 s−1). Similar results
have been observed in the 15 vol% TiBw/Ti compos-
ite.18 The characteristic of constant stress exponent ob-
served in the (TiBw + TiCp)/Ti and TiBw/Ti composites
is quite different from that in TiBw/Ti–6Al–4V and TiCp/
Ti–6Al–4V composites in which a change in stress
exponent occurred at a critical creep strain rate of
4 × 10−6s−1 to 1 × 10−5s−1.12,16,17 This implies that the
composition of the titanium matrix exerts a great influ-
ence on the creep behavior of the titanium matrix com-
posites. Further research is needed to elucidate the effect
of matrix chemistry on the creep mechanism of the tita-
nium matrix composites.
Figure 4 shows that the 15 vol% (TiBw + TiCp)/Ti
composite exhibited a stress exponent of 4.5–4.6. These
values are very close to that for the lattice-diffusion-
controlled dislocation climb process in ␣-Ti (n ס 4.3).19
Similar stress exponent values of 4.1–4.9 were also re-
ported by other researchers in TiBw/Ti, Ti2Cp/Ti, and
(TiBw + Ti2Cp)/Ti composites.13,14 The unreinforced Ti
exhibits a stress exponent of 5.2–5.3. This value is higher
than that for ␣–Ti. Figure 6 shows that the activation
energy for creep of unreinforced Ti is 259 kJ/mol, which
is close to that for dislocation creep of ␣–Ti (241 kJ/
mol).19 For the 15 vol% (TiBw + TiCp)/Ti composite, the
creep activation energy is 298 kJ/mol. This value is
slightly larger than that for unreinforced Ti, and close to
that (284 kJ/mol) for 10 vol% (TiBw + Ti2Cp)/Ti as re-
ported by Ranganath and Mishra.13 Clearly, the values of
the stress exponent and activation energy for creep of the
present 15 vol% (TiBw + TiCp)/Ti composite suggest
that the creep of the composite is associated with the
lattice-diffusion-controlled dislocation climb and no
threshold stress exists for creep. Thus, the creep data of
the composite and unreinforced Ti can be rationalized as
.
FIG. 8. Variation of normalized creep rate, ⑀ kT/DLGb, with modulus-
compensated applied stress ␴/G for 15 vol% (TiBw + TiCp)/Ti com-
posite and unreinforced Ti.
J. Mater. Res., Vol. 17, No. 9, Sep 2002
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Z.Y. Ma et al.: Creep behavior of in situ dual-scale particles-TiB whiskers and TiC-particulate-reinforced titanium composites
15 vol% (TiBw + TiCp)/Ti composite exhibited a
stress exponent of 4.5–4.6 and an activation energy of
298 kJ/mol, whereas unreinforced Ti exhibited a stress
exponent of 5.2–5.3 and activation energy of 259 kJ/mol.
There is no change in stress exponent with varying
creep strain rate for both 15 vol% (TiBw + TiCp)/Ti com-
posite and unreinforced Ti in the range of the investi-
gated creep rate (6.2 × 10−8 to 8.4 × 10−4 s−1).
The creep of both 15 vol% (TiBw + TiCp)/Ti compos-
ite and unreinforced Ti is associated with the lattice
diffusion-controlled dislocation climb.
The creep resistance of the 15 vol% (TiBw + TiCp)/Ti
composite is more than one order of magnitude higher
than that of unreinforced Ti, which can be accounted for
by the load-transfer mechanism with a load-transfer co-
efficient of 0.46.
.
FIG. 9. Variation of normalized creep rate, ⑀ kT/DLGb, with modulus
and load transfer coefficient compensated applied stress, (1 − ␣)␴/G,
for 15 vol% (TiBw + TiCp)/Ti composite and unreinforced Ti.
ACKNOWLEDGMENTS
The authors would like to thank the State Key Labora-
Ti composite. In this case, a transfer load coefficient ␣ is
tory for Fatigue and Fracture of Materials (SKLFFM),
introduced into the analytical treatment. Thus, the rate-
Institute of Metal Research, Chinese Academy of Sci-
controlling equation is given by
ences for their support. Thanks are also due to Prof.
L. Geng and Mr. Z.Z. Zheng, National Key Laboratory
n
AGbDo 1 − ␣͒␴
Q
͑
.
for Precision Hot Processing of Metal, Harbin Institute
of Technology, for their assistance with fabrication of
the composite.
⑀ =
exp −
.
(2)
ͫ ͬ ͩ ͪ
kT
G
RT
The value of ␣ lies within a range from 0 (when load
transfer is absent) to a maximum value of 1 (when
load is transferred completely). For the present 15 vol%
(TiBw + TiCp)/Ti composite, incorporation of the load
transfer coefficient ␣ into the analytical treatment results
in a ␣ value of 0.46. This means that when 46% of the
external load is transferred to the reinforcement the creep
strengthening of the 15 vol% (TiBw + TiCp)/Ti compos-
ite can be explained by the load-transfer mechanism.
Thus the creep data of both (TiBw + TiCp)/Ti composite
and unreinforced Ti can be described by a united creep
equation [Eq. (2)]. Figure 9 shows the creep data nor-
malized by a load-transfer coefficient of 0.46. Clearly,
after introducing a load-transfer coefficient, the creep
data of both composite and unreinforced Ti merge onto a
single straight line with a slope of approximately 4.5.
This demonstrates that the load transfer can provide a
satisfactory explanation for the creep strengthening of
the (TiBw + TiCp)/Ti composite.
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