It discusses microstructure and properties of titanium matrix composites.

1. Modern Composite Materials
Presentation
By
Shahid Ali

2. Microstructure and Properties of Titanium
Matrix Composites Synergistically Reinforced by
Graphene Oxide and Alloying Elements
Jiashi Yu,* Yongqing Zhao,* Qinyang Zhao,* Wei Zhang, Wangtu Huo,
and Yusheng Zhang

4. RESEARCH QUESTION:
• In this work, the microstructure and mechanical properties of titanium
matrix composites were tuned by introducing graphene and alloying
elements.
• The titanium matrix composites reinforced by graphene oxide (GO) and
cobalt elements (GO@ Co/TA1) are synthesized by the spark plasma
sintering (SPS) technique. Results show that the precipitated Ti2Co phase is
almost reticulate distributed in the tissue, while the in situ TiC particles are
discontinuously distributed at the grain boundaries.
• The ultimate tensile strength and yield strength of GO@ Co/TA1
composites are 160% and 126% higher than those of pure TA1,
respectively. In addition, composites show excellent ductility where the
elongation of GO@Co/TA1 is around 10.95%. Therefore, such enhanced
mechanical properties can be attributed to the synergistic strengthening
effect of interfacial-distributed TiC particles and intragranular-distributed
intermetallic compounds (Ti2Co), which would provide a new idea for the
design of metal matrix composite.

5. Introduction/Literature Review:
• Titanium and titanium alloys are widely used as structural materials in
aviation, automobiles, biomedicine, and other fields because of their high
specific strength, high-temperature mechanical properties, and good
biocompatibility properties.
• Titanium and its alloys are fast becoming a part of significant research
interest for wide range of applications such as automotive and aerospace
industries.
• These materials are light weight and generally have attractive properties
such as high specific strength, excellent chemical resistance and excellent
biocompatibility. The combination of such properties makes them an ideal
candidate for structural, chemical, petrochemical, marine and biomedical
applications

6. • However, Young’s modulus, wear resistance and heat resistance of
titanium materials are inferior to those of steel and Ni-based alloys.
• To further improve the properties of titanium alloy, the most direct
and effective method is to use alloying or compounding methods to
introduce the second phase for strengthening.
• The physical and mechanical properties of titanium alloys can be
adjusted by adding alloying elements. The strengthening mechanisms
of titanium alloys are mainly solid solution strengthening and second
phase precipitation strengthening caused by alloying elements.
• However, when the content of alloying elements is too high, the
movement of dislocations inside the material becomes difficult due to
the effect of lattice deformation and stress field, and the
strengthening efficiency decreases.

7. • Compared with titanium alloy obtained by alloying, titanium matrix
composites (TMCs) have high specific strength, high specific modulus,
excellent thermal and electrical conductivity, good wear resistance,
low thermal expansion coefficient, etc.
• Replacing traditional titanium alloys with titanium matrix composites
for primary and secondary loadbearing structural components is of
great significance to improve the economy, reliability, and other
performance indexes of aviation products.
• However, the high-strength titanium matrix composites are often
accompanied by a sharp decline in ductility.
• For TMCs, the titanium matrix is the primary carrier to ensure the
plasticity of the material, and the reinforcement provides an increase
in strength.

8. • Generally speaking, the alloying elements of titanium can be divided
into α-stabilizing elements, β-stabilizing elements, and neutral
elements.
• By alloying with β-stabilizing elements in titanium alloys, it is possible
to decrease the temperature of the liquid and lessen the reactivity of
the titanium, thus improving their physical and chemical properties β-
stabilizing elements will make titanium alloys maintain good
mechanical properties.
• Currently, the added β-stabilizing elements are mainly copper and
nickel in graphene-titanium matrix composites,on the one hand,
because the introduction of copper and nickel helps to improve the
strength of the composites, on the other hand, copper and nickel do
not react with graphene and can be effectively used as a binder
between titanium and graphene.

9. • Cobalt (Co) is well-known for its strong precipitation hardening effect
on β-titanium alloys because of the formation of dispersive Ti2Co
hard precipitates during the aging process. Compared to copper and
nickel, Co is also a eutectoid β-stable element and has a similar
density. It has the highest solubility (17%) in β titanium and can retain
β-titanium at room temperature.
• In addition, the intermetallic compound Ti2Co has a CF96 crystal
structure, good strength and ductility, and compatibility and matching
with titanium alloy.
• Furthermore, the intermetallic compound Ti2Co has good high-
temperature oxidation resistance and high wear resistance, which
improves the high-temperature performance and wear resistance of
the composite.

10. • Graphene has been widely used as a reinforcement in the metal matrix due
to its excellent mechanical, thermal, and electrical properties.
• As a reinforcement, it is possible to achieve a significant improvement in
the overall performance of the composite at a lower content.
• The large specific surface area makes the contact area with the metal
better than other carbon nanomaterials such as carbon nanotubes, which
is beneficial to improve the interface bonding strength, which can hinder
the growth of the matrix grains and facilitate load transfer.
• The interface reaction between graphene and titanium matrix is conducive
to the connection between matrix and graphene, and plays a good role in
load transfer. In addition, the surface has a special fold structure, and there
is a flattening stage during the stress process, which is expected to obtain a
good match of strength and ductility.

11. Method:
• To further optimize the match between the strength and ductility of TMCs,
in this work, a two-step ball-milling method was adopted. First, the β-
stabilized elements (Co) were coated on the surface of the TA1 matrix
(Co/TA1) by ball milling.
• then Co/TA1 composite powders were mixed with graphene oxide (GO) by
a short-time ball milling method.
• After that, GO@ Co/TA1 composites were prepared by spark plasma
sintering.
• The microstructure and mechanical properties were characterized and the
microstructure evolution was studied in detail to investigate the effect of
Co elements and GO on the evolution of the microstructure and the
strengthening mechanisms of titanium matrix composites.

12. Experimental:
• Material and Preparation:
The primary materials were commercial pure titanium (TA1, purity >
99%), pure cobalt (purity > 99.9 wt%), and GO powders (GO with a
diameter of 0.5–5 um and a thickness of 1–3 nm was purchased from
XFNANO Technology Co., Ltd., China).
 The schematic illustration of fabricating GO@ Co/TA1 composites is
presented in Figure 1, which is mainly divided into three steps, as
follows:

14. • Step 1: Fabricating Co/TA1 mixture powders.
• In this work, ball milling process was utilized for powder distribution.
As shown in Figure 1a, Co powder (3 g), 96.7 g spherical TA1 powder,
and stainless-steel ball were sealed in a stainless-steel jar. The milling
process was conducted at a rotation speed of 300 rpm, and the ball
milling time was set to 5 h. After the impact of the stainless-steel ball,
most of the Co powders were well attached to the surface of TA1
powders.
• Step 2: Fabricating GO@ Co/TA1mixture powders.
• Based on the previous study 0.3 wt% GO (0.3 g) was selected to be
added into the Co/TA1 mixture powders using a low-energy ball
milling speed of 200 rpm for 2 h. The aim is to evenly attach GO to
the surface of the Co/TA1 mixture powders to obtain a good
dispersion effect.

15. • Step 3: Fabricating GO@ Co/TA1 composites.
• Finally, GO@ Co/TA1 composite powders were loaded into a graphite
die with an internal diameter of 50 mm and sintered into the bulk
samples in an spark plasma sintering (SPS) furnace (SPS-80T-20) at
1000◦C for 5 min of holding time under an axial pressure of 45 MPa.
• As a comparison, TA1, 0.3 wt% GO/TA1, and 3 wt% Co/TA1 composite
powders were also sintered at 1000◦C for 5 min holding time under
the same conditions.

16. Characterization:
• For microstructural analysis, the surfaces of the samples were
mechanically polished using a standard metallographic procedure and
then etched in a solution of water, nitric acid, and hydrofluoric acid
(with the volume ratio of 7:3:1).
• Microstructural characterizations were performed using a field-
emission scanning electron microscope (FESEM, JEOL, JSM-6700F,
Japan) equipped with an energy-dispersive spectrometer (EDS), and a
transmission electron microscope (TEM, JEOL, JEM-2100).
• X-ray diffraction (XRD, D8, ADVANCE) analysis for the phase
composition was operated at 30 kV and 30 mA with CuKα radiation.

17. • The tensile tests were carried out at room temperature using an
MTS810 universal testing machine with a tensile rate is 1 mm/S.
• At least three measurements were performed to ensure the
consistency and repeatability of experimental results. After the tensile
test, the fracture surfaces were observed using the SEM equipped
with EDS.

18. Results:
• Composite Powders:
Figure 2 demonstrates the SEM micrographs of GO@ Co/TA1 mixture
powders. As shown in Figure 2a, TA1 powder is spherical with an
average particle size of about 100 μm. Figure 2a1 shows the GOs/TA1
composite powders, the shape of TA1 powders has changed after ball
milling. Moreover, GO is well dispersed on the surface of the TA1
powder in Figure 2a2.
 Figure 2b,b2 shows the SEM images of the GO@ Co/TA1 composite
powders. Spherical TA1 powders have obvious slight deformation, and
GO and Co powders are adhered on the surface of TA1 powders.
 As observed in Figure 2b 2, EDS mapping images display that GO and
Co elements can be homogeneously distributed on the surface of TA1
powders.

20. Microstructure of Composites:
• Figure 3 displays XRD patterns of SPS-fabricated TA1 and GO@ Co/TA1
composites. It can be seen that all samples have α-Ti peaks.
• When the Co is introduced, a small amount of Ti2Co phase is
generated, and the intensity of its characteristic peaks is low. At the
same time, a small number of characteristic peaks of the β-Ti phase
also appeared, which is due to Co as a β-stable element can retain
some β-Ti in the composite. In addition, no peaks of GO and TiC are
detected due to the low content of GO.

23. • Figure 4d,g,j shows the morphology of the GO@ Co/TA1 composite.
With the introduction of Co elements, a lamellar microstructure can
be observed in Figure 4d, and some new phases are formed in the
lamellar microstructure. EDS mapping (Figure 4f ) confirmed that the
new phase in the lamellar microstructure is Ti2Co phase.
• In addition, discontinuous network structures can be clearly seen in
the composite, as shown in Figure 4d. By magnifying the boundary of
the network structure, it can be found that TiC particles are
distributed at the grain boundaries to construct a discontinuous
network structure (Figure 4e). Finally, the TiC particle and
intermetallic Ti2Co phase formed in the composite. Compared with
the GO@TA1 composite, the intermetallic Ti2Co phase can further
enhance the strength of composites.

25. • Take GOs@ Co/TA1 composite as an example, its typical interface
microstructure is presented in Figure 5.
• Some nanoparticles can be clearly observed in the intracrystalline, as
shown in Figure 5a.
• The nanoparticles can be confirmed by the selected area electron
diffraction (SAED) analysis as Ti2Co phase (Figure 5b).
• In addition, it can be found that dislocations occur around the
nanoparticles (Figure 5c), suggesting that the nanoparticles effectively
block dislocation movement.
• The morphology of the TEM sample (Figure 5d) exhibits that some
nanoparticles that can be observed at grain boundaries are TiC, which
is confirmed by the SAD pattern (Figure 5e).

26. • Actually, the TiC phase is easily generated due to the low
Gibbs free energy of the reaction between graphene and the
Ti matrix.
• The high-resolution TEM (HRTEM) image of the interface
between the TiC phase and Ti matrix is carried out in Figure
5f, it can be observed that the TiC phase is well combined
with the Ti matrix without apparent gaps and impurities. The
good interfacial bonding between the TiC phase and Ti
matrix facilitates load transfer and thus enhances the
mechanical properties of the composite.

27. Mechanical Properties:
• Figure 6 shows the tensile properties of fabricated TA1 and composites at room
temperature. Figure 6a exhibits the engineering stress–strain curve of fabricated
TA1 and composites. Detailed data are presented in Figure 6b. It can be observed
that the yield stress (YS) and ultimate tensile stress (UTS) are significantly
increased with the introduction of GO and Co elements.
• Compared to the TA1, the YS of the GO/TA1 composite is 379 MPa, which is
39.3% higher than that of the TA1; the YS of the Co/TA1 composite is 572 MPa,
which is 110% higher than that of the TA1.
• With the co-introduction of Co elements and GOs, the strength of GO@ Co/TA1
composites has been dramatically improved. The UTS and YS of the GO@ Co/TA1
composites are increased to 824.5 and 616.5 MPa, which is 160% and 126%
higher than those of TA1 (UTS: 316.5 MPa and YS:272 MPa). Additionally, GO@
Co/TA1 composites had excellent ductility (the elongation of GO@ Co/TA1 is
about 10.95%).

29. • Figure 7 displays the fracture morphology of the tensile samples (TA1,
GO/TA1, and GO@ Co/TA1 composite). Figure 7a shows the fracture
morphology of the TA1 alloy. It can be seen that lots of fine dimples
distribute on the fracture surface which reflects the excellent ductility
of the TA1 alloy.
• As shown in Figure 7b,b1, dimples and micro-cracks are observed on
the fracture surface of the GO/TA1 composite, presenting the ductile
fracture mode. It can be observed in Figure 7b1 that the crack is
derived from the interface between TiC and TA1 matrix, indicating
that cracks are caused by the interface debonding. The fractured TiC
indicates that the stress exerted by the TA1 matrix is effectively
transferred to the reinforcements during the tensile process.

30. • Figure 7c,c2 shows the fracture morphology of the GO@Co/TA1
composite. A mixed fracture mode involving both cleavage and
dimple fracture is present in Figure 7c, indicating that the fracture
mode of composites has been changed. It is well known that the
introduction of Co element exists in the composite as the
intermetallic compound (Ti2Co), which also helps to retain β-Ti in the
matrix.
• The uniform distribution of Ti2Co has a certain strengthening effect
due to the strong hindering effect of Ti2Co on dislocation. Moreover,
the retention of β-Ti results in the formation of the α/β phase
interface in the matrix.

32. • Dislocations are difficult to cross the α/β phase interface, resulting in
stress concentration at the interface and decreasing the denaturation
coordination ability. In addition, the precipitation of Ti2Co in the
composite makes the deformation of the matrix more difficult, which
leads to the formation of microcracks and reduces the plasticity of the
composite as well. Moreover, crushed GO is found in the crack (Figure
7c1), which means that the GO could absorb great energy during the
tensile process.
• Generally speaking, cracks preferentially develop along with the
interface between the reinforcement and Ti matrix which agrees well
with the reference. Therefore, the deterioration of ductility can be
attributed to crack formation at the interface and the brittleness of
the precipitated intermetallic compounds.

33. The Evolution of Microstructure:
• Corresponding to the microstructure in Figure 4 and 5 of the GO@
Co/TA1 composite, it can be seen that GO and Co elements effectively
tune the microstructure of GO@ Co/TA1 composites. Related studies
in graphene titanium matrix composites have reported that graphene
and titanium matrix can easily react to form carbides during sintering.
In addition, the eutectoid reaction between alloying elements and
titaniuim matrix will inevitably occur and form eutectoid products
during the slow cooling process. The microstructure evolution
schematic of the GO@ Co/TA1 composite is illustrated in Figure 8.

35. • After ball milling, the Co powders and GO are successively coated on the
surface of TA1 powders, as shown in Figure 8a. In this work, the ΔG of TiC
formation at 1000 ◦C is 174.61 kJ /mol , [6] indicating that TiC particles are
easily formed spontaneously during the sintering process (Figure 8b).
Therefore, most of GO are distributed at grain boundaries as TiC particles
(Figure 8c).
• It is known that Co is considered as β-stable element in titanium alloys. As
the composite cools rapidly from the β phase region, the alloying elements
(Co) and titanium matrix are easy to undergo eutectoid transformation,
and the furnace-cooled sample exhibits a eutectoid structure with a
lamellar α-Ti þ intermetallic compounds (Figure 8c).
• Therefore, the microstructure of the GO@ Co/TA1 composite is mainly
composed of the α-Ti phase, interfacial-distributed TiC particles, and
intragranulardistributed intermetallic compounds (Figure 8d), which finally
forms a two-scale network structure.

36. Strengthening Mechanisms:
• It is well known that the properties of metal matrix composites are closely
related to the reinforcement and metal matrix.
• Generally, the analysis of strength for the composites is related to the interaction
between dislocations and hard particles.
• In this work, TiC and intermetallic Ti2Co particles formed in the composites.
• Consequently, it can be considered that the TMCs exhibited a superior strength
due to the strengthening effect of the TiC particles and intermetallic Ti2Co
compounds. The specific strengthening mechanisms include solution
strengthening, grain-boundary strengthening, dislocation strengthening, and
precipitate strengthening For GO@ Co/TA1 composites, GO and in situ TiC are
mainly distributed at grain boundaries which limit the grain growth.
• Moreover, most Co atoms are dissolved in the matrix, and then precipitated and
distributed in the intracrystalline during cooling.

37. • Therefore, according to previous studies, the addition of graphene
can improve the strength of composites through fine-grain
strengthening. However, Co elements mainly bring a strengthening
effect to the composite through solid solution strengthening. In
addition, the intermetallic compound (Ti2Co) achieves precipitation
strengthening as the second phase.
• These precipitates limit dislocation motion and thus improve the
strength of the composite. Therefore, the strength difference
between the GO/TA1 composite and the TA1 can be considered as the
sum of the strength brought by TiC particles and the strength brought
by fine-grain strengthening,
• while the strength difference between Co/TA1 composite and TA1
alloy can be considered as the strength contribution brought by the
solid solution strengthening of Co element, formation of α/β interface
and precipitation of Ti2Co particles.

38. • Hence, according to the mixture rule of composites, the strengthening
effects (Δσ) of the GO@ Co/TA1 composite can be expressed as Δσ = Δσ
intermetallic compounds + Δσ Go+TiC+Grain size -------(1).
• The values of each strengthening mechanism in composites are listed in
Table 1. It can be found that the strength contribution brought by GO is 107
MPa, the strength brought by Co element reaches 300 MPa. However, the
strength brought by the synergistic strengthening of the two is 344.5 MPa.
• Therefore, alloying elements play a dominant role in improving the
strength, which indicates that matrix micro-alloying effectively improves
the strength of the composites. Besides, it can be found that the actual
strength contribution is about 85% of the strength calculated according to
the mixture rule of composites. It can be considered that GO and alloying
elements (Co) have achieved a synergistic strengthening effect on
composite materials. Therefore, micro-alloying titanium matrix may be an
effective way to improve the mechanical properties of graphene titanium
matrix composites.

40. Conclusion:
• In this work, GO@ Co/TA1 composites were successfully synthesized by
the SPS technique. The microstructure is characterized by the precipitation
of intermetallic compound particles within the intragranular and interfacial
TiC particles. Compared to TA1, the YS of GO@ Co/TA1 composite were
increased by 126% and possessed a ductility of more than 10.95% at the
same time.
• The strength enhancement can be attributed to the synergistic
strengthening of the interfacial TiC particles and the intragranular
intermetallic compounds (Ti2Co). Compared with the theoretical strength
obtained by the mixture rule, the synergistic strengthening conversion rate
of GO and Co elements have reached to 85%. As a result, this work proved
that microalloying the titanium matrix can further improve the mechanical
properties of graphene titanium matrix composites.

41. Thanks