Today, Discontinuously Reinforced Particulate Titanium Matrix Composites (DRPTMCs) have been the most popular and challenging in consideration with development and heat treatment due to their significant weight-saving capacity, high specific strength, stiffness and oxidising nature compared with other metals and alloys. Owing to their excellent capabilities, DRPTMCs are widely used in aerospace, automobiles, biomedical and other industries. However, regardless of the reinforcements, such as continuous fibres or discontinuous particulates, the unique properties of DRPTMCs have dealt with these composites for widespread research and progress around the domain.
1. Introduction
The application of composite materials began during the Egyptian civilisation. Today, researchers are focusing on various matrix and reinforcement combinations with enhanced physical and mechanical properties. However, alloys are compositions made of two or more metallic elements
[1][2]. To guarantee the stability of the dispersion phase, the matrix phase is a continuous phase in which a composite is created in a microstructure of metals and completely envelops the dispersed phase. Although the dispersive phase differs depending on the material, the matrix phase’s significance remains constant in order to guarantee the appropriate execution of the dispersive phase. The strength of the bond in metals, which determines the rate of corrosion in Metal Matrix Composites (MMCs), can be strongly influenced by the effectiveness of the matrix phase
[3][4].
Figure 1a—cshows examples of Titanium matrix composite applications
[5][6][7].
Figure 1. (
a) Aircraft engine. (
b) Titanium-MMC crankshaft (
c) TMC cartilage implant
[5][6][7].
High-tech industries like aerospace, defence, automotive, and civil engineering frequently use metal matrix composites (MMCs) as structural materials
[8][9]. In particular, titanium alloy with reinforced particles is one such material. Particle-reinforced MMCs have the potential to provide superior mechanical qualities, such as increased specific strength and stiffness
[10][11]. In order to increase the titanium matrix’s stiffness, strength, hardness, and wear resistance while maintaining a quasi-isotropic behaviour that makes the standard reshaping process easier, ceramic materials are frequently utilised as reinforcement. Due to their superior mechanical qualities, silicon carbide (SiC) is one of the most often employed particles to reinforce titanium matrix
[12]. The shape
[13][14], size
[15], volume fraction
[16] and distribution of the reinforcements
[17], along with the characteristics of the reinforcements and matrix materials
[18][19], all influence the mechanical behaviour of particulate-reinforced MMCs. The mechanical characteristics of MMCs are influenced by the interfaces between the matrix and reinforcement materials
[20][21]. Therefore, the total mechanical properties of the MMCs are greatly influenced by the load-carrying capacity of reinforced ceramic particles in the metal matrix
[22] and the particle shape
[23][24]. The toughness, strength, and ductility
[25] of the composite are dramatically reduced by the debonding of reinforcements from the matrix. Cracks begin when the stress exceeds the interface’s support capacity, typically at the points where the largest stresses are produced, such as the spherical reinforcements poles or the corners of triangular or rectangular particles. The damage spreads as the fracture widens along the matrix/particle contact and reduces the amount of load that the matrix transmits to the reinforcement or the strengthening effect of reinforcements. Last but not least, cavities grow out of interface cracks, and fracture is caused by the amalgamation of interfacial cavities. These mechanisms are widely acknowledged in polymer- and metal matrix-based composite materials
[22][23], respectively
[26][27]. Although titanium was first discovered in the 18th century
[28], it was not until the middle of the 20th century that the titanium industry underwent substantial advancements. These modifications were brought about by the development of the gas turbine engine and resulted in the growth of industries specialising in the production of titanium sponges in the USA, Europe, and Japan
[29], as shown in
Figure 2a,b. Since then, the aerospace industry has dominated the use of titanium globally; both engines and airframe structures can use the metal. A very desirable combination of titanium’s qualities includes exceptional fatigue resistance, a high strength-to-weight ratio, and great corrosion resistance. Such characteristics permit broad applications; the major limitation on further deployment is the high cost of extraction and processing. Although
[30] the development of polymer-based composites and rising operating temperatures are issues for the aerospace industry, creative solutions like metal-matrix composites and titanium aluminides open up new possibilities for growth. Industries currently utilising these materials include biomedical, sports and marine sectors. Physical and mechanical properties are given in
Table 1 and
Table 2.
Figure 2.
(
a
) Titanium Sponge (
b
) TMC Engine blade ring of aircraft
.
Table 2. Mechanical properties of Titanium
[32].
Natural Occurrence |
Primordial |
Crystal Structure |
Hexagonal close packed |
Thermal Expansion |
8.6 µm/(m⋅K) (at 25 °C) |
Thermal Conductivity |
21.9 W/(m⋅K) |
Electrical Resistivity |
420 nω⋅m at 20 °C |
Magnetic Ordering |
Paramagnetic |
Young’s modulus |
116 GPa |
Shear Modulus |
44 GPa |
Poisson ratio |
0.32 |
Moh’s hardness |
6.0 |
Vicker’s hardness |
830–3420 MPa |
Brinell hardness |
716–2770 MPa |
2. Processing Methods of TMCs
Discontinuously reinforced titanium composites are produced majorly through the processes mentioned below.
2.1. Powder Metallurgy
Due to the strong chemical reactivity of titanium, standard ingot metallurgy methods are not suited for producing TMCs enhanced with ex situ additive particles. In order to create TMC components, powder metallurgy (PM) methods are frequently used. In fact, one of the best fabrication techniques for the creation of DRTCs is the PM processing route
[33][34][35]. Homogeneous powder mixing and dispersion are the two most crucial factors in assuring the optimal performance of composites during the PM processing of DRTCs. The surface coating may occasionally be used to improve or guarantee homogenous dispersion
[36]. The ultimate properties of the composites are determined by choice of distributed reinforcements, their size, shape, content, and the interfacial bonding between reinforcements and the matrix
[37]. Depending on how the additives interact with the matrix, the reinforcements can be introduced into the matrix using either an ex situ or an in situ processing method
[38][39]. Low porosity Ti/TiB composites have been synthesised using TiH
2 and TiB
2 powders via a hydrogen-assisted blended elemental powder metallurgy route. It resulted in the formation of highly dense structures because of incomplete in situ transformation of TiB
2 into the TiB phase
[40]. While fabricating β titanium alloy matrix composite, it was found that nucleation of TiB and TiC particulates causes the formation of hard β phases of TiB and TiC, causing a substantial increase in the hardness of the composites. Ref.
[41] evaluated the feasibility of using coated SiC and TiC as reinforcements with Titanium alloy in order to achieve homogenous mixing. It was noticed that hardness improved with an increase in sintering temperature, and the highest hardness value of 385 +/− 20 Hv was recorded. Ref.
[42] concluded that there was an increase in the density of the sintered Ti-
nanoAl
2O
3 composites with an increase in sintering time. Further, it also improved the hardness values and corrosion behaviour of the composite. Ref.
[43] have used Artificial Neural Networks to predict the wear behaviour of titanium-nano graphene platelets (Ti-(GNPs)-Si
3N
4 produced through powder metallurgy. They concluded that sintering temperature holds the highest impact on the surface roughness and hardness of the composites, followed by sintering time. Ref.
[44] fabricated titanium-graphene oxide (Ti-GO) through hot pressed sintering with varying (1–5%) Wt.% of graphene oxide. Because of the agglomeration of graphene oxide particles, there was a slight decrease in yield stress but a substantial improvement in hardness with a maximum hardness value of 457 Hv at 5 Wt.% of graphene oxide. Ref.
[45] studied the effect of process input parameters on the hardness of titanium matrix composites processed through direct energy deposition-based additive manufacturing. Crack-free titanium matrix composites with a hardness of 700 Hv and density of 99.1% were achieved with 5 Wt.% Titanium Boride as reinforcement material.
2.2. Ex Situ Processing Technique
Ex situ processing methods are used to include thermodynamically stable ceramics in titanium, such as SiC, TiC, TiB, and ZrC. Except when external impacts are applied on purpose, as in mechanical milling, both the particle size and morphology of the added particles before and after sintering essentially remain unchanged. This is because no new compounds are created during sintering and consolidation
[46]. In addition to producing DRTCs with improved mechanical qualities, ex situ production of these materials also enhances wear resistance and stabilises the friction coefficient during dry sliding
[47]. The ex situ approaches for creating DRTC have not attracted much attention because of their own drawbacks, such as the inadequate bonding between the matrix and the reinforcement
[48].
2.3. In Situ Synthesis Methods
During in situ processing, the titanium matrix’s high reactivity is combined with additive elements like boron, carbon, and nitrogen to create in situ stable particulates or needle-like reinforcements, which are dispersed through a solid-state reaction. TiB2, B4C, Cr3C2, and Si3N4 are common starter additions for in situ processing. As an illustration,
[49] titanium powder and TiB2 particles can interact during the sintering process to generate TiB whiskers, which are then disseminated throughout the matrix as reinforcement. In the case of in situ processing, increased interfacial bonding between the matrix and reinforcement leads to improved tribological performance. Additionally, composites created using in situ procedures have superior oxidation and creep resistance, high specific strength, and modulus
[50][51]. To prevent the creation of interfacial defects, the in situ reactions must be carefully controlled.
2.4. Rapid Solidification Process
Rapid solidification technology has improved greatly over the last 20 years to become a promising fabrication method for DRTC processing
[52][53][54]. Metallic powders are frequently produced in large quantities via the atomisation process. By atomising the melt with the reinforcements present, composite powder with the reinforcements can be created. One such instance was the remelting of a composite ingot made of Ti-6Al-4V (20 vol% TiC) employing induction heating and argon gas flow. The composite powder was then used to manufacture bulk composite material using hot isostatic pressing (HIP) at 900 and 950 °C, under 100 MPa for 4 h
[55]. For the production of titanium metal and titanium inter-metallic matrix composites, Martin Marietta Laboratories has created a unique ingot metallurgical method
[56]. During the casting process, the XDTM technology creates in situ ceramic reinforcements that are thermally stable, kinetically inert, and evenly spread within the melt. Ingots of Ti-48Al-2V and Ti-45Al with TiB2 reinforcement were created using this technology. Then, with the use of centrifugal atomisation and a rotating consumable electrode, powders of these composite materials were created. The atomisation procedure kept the ceramic particles that had formed in the ingot. It was discovered that the processing temperature had a significant impact on the size and scale of the TiB2 particle dispersion
[57]. Similar to this, by consolidating rapidly solidifying Ti64 alloy with various levels of boron addition,
[58] created in situ TiB-reinforced Ti64 composites. The Marko 5T melt-spinner was used to carry out the quick solidification procedure
[59][60]. This approach is only applicable to reinforcements that should have a density similar to that of the matrix material. The atomisation process can result in non-uniform distribution of reinforcements in the composite powder due to particle aggregation if the density of the reinforcements differs significantly from that of the matrix. Rapid solidification leads to non-equilibrium, which opens the door to new alloying techniques, such as the incorporation of rare-earth alloying elements in titanium. Recent research by
[61] has shown that plasma can be used as a chemical reactor to create in situ titanium composites using induction plasma technology. They stated that the chemical reaction between the components injected into the plasma could directly produce Ti64 reinforced with TiC or TiN particles.
Figure 3 shows DRTCs’ fabrication processes through the powder metallurgy route.
Figure 3. Powder metallurgy process.