**1. Introduction**

Over the last decades, investigations about composite materials have made great advances in understanding the importance of the starting materials and the manufacturing process, for the development of novel materials with outstanding properties [1]. In this regard, in the field of metal matrix composites, research studies have been conducted to achieve optimal bounding matrix reinforcement, improving the strength of the metal matrix composites (MMCs) [2]. Light metal matrix composites are valued, particularly in certain applications where low density should be balanced with mechanical requirements [3]. In particular, titanium composites (TMCs) offer these advantages over other light metal matrices [1, 4]. Their good corrosion behaviour and high specific properties make TMCs one of the most suitable candidates for aerospace applications [5].

Several authors have described several classifications of these materials. One of these classifications could be considered based on the kind of reinforcements: continuous or discontinuous reinforcement [6, 7]. Other classifications could be done according to the manufacturing route: traditional methods or powder metallurgy techniques [8–12]. An interesting route to promote the strengthening of the matrix is the "in situ" formation of secondary phases. This method allows a clean and well bounding between the matrix and the reinforcement [9]; consequently, better final behaviour of the TMCs may be expected [13, 14].

On the basis of previous and recent studies, this work focuses on TMCs which were manufactured employing discontinuous ceramic reinforcement. These ceramic phases were selected according to their reactivity with the titanium matrix. Many authors show the great variety of ceramic reinforcements; however, in this investigation TiC, TiB2, B, and B4C particles were studied. They were considered as precursors of secondary phase formation by in situ techniques [15–20].

From the manufacturing process point of view, powder metallurgy techniques of hot consolidation have proved useful at the study of in situ composites [21]. Therefore, for the development of TMCs, inductive hot pressing has been selected among other manufacturing processes. The experience of the authors in this technique had led to laying the basis of this study [22–28].

By a thorough analysis of the properties of the produced specimens via inductive hot pressing at different temperatures by the use of several starting material compositions, specific features of the TMCs could be studied. In this regard, the employment of the XRD technique is crucial in understanding the reaction phenomena between the matrix and the reinforcement. Furthermore, the behaviour of the ceramic particles in the matrix could be unpredictable and variable depending upon many factors; this study is the main objective to analyse the phenomena that could occur when factors as starting powder composition and processing parameters are varied and ultimately how these factors affect the final properties of the TMCs.

### **2. Materials and methods**

The interest in understanding the influence of the starting materials on the final behaviour of TMCs motived the study of three ceramic materials as reinforcements testing various concentrations in titanium matrices. Hence, for the TMC manufacture, diverse starting blends were prepared. The innovation of this investigation lies in the phase analysis carried out in specimens made from these blends. The employment of several operation temperatures and reinforcement typologies and concentrations allowed for the searching of significant differences, among the fabricated TMCs, while all these specimens have been processed at similar processing conditions.

The titanium grade 1 employed was manufactured by TLS GmbH (Bitterfeld, Germany). This titanium powder showed a spherical morphology and D50 below 45 μm. The four ceramic reinforcements were chosen considering their reactive behaviour in respect of the secondary phases' formation in titanium matrices. The supplier for TiC powder was H.C. STARK GmBH (Goslar, Germany) and for B4C powders was abcr GmbH (Karlsruhe, Germany), and the company for TiB2 was Treibacher Industrie AG (Althofen, Austria). The characterization of all the powders was carried out to verify the manufacturers' data about their size and morphology. The particle size distribution of the powders was measured by laser diffraction analysis (Mastersizer 2000, Malvern Instruments, Malvern, United Kingdom); these results are shown in **Table 1**.

X-ray powder diffraction analysis was done using a Bruker D8 Advance A25 (Billerica, Massachusetts, United States of America) with Cu-Kα radiation for the phase characterisation of as-received Ti, TiC, TiB2, and B4C powders and studying

**101**

**Figure 1.**

*XRD analysis of the starting powders: Ti grade 1, TiC, TiB2, and B4C.*

*In Situ Titanium Composites: XRD Study of Secondary Phases Tied to the Processing Conditions…*

**Material Morphology D50 (μm)** Ti Spherical 29.05 TiC Faceted 4.9 TiB2 Irregular 4.76 B4C Faceted 63.76

*DOI: http://dx.doi.org/10.5772/intechopen.88625*

*Characteristics of the starting materials.*

**Table 1.**

*In Situ Titanium Composites: XRD Study of Secondary Phases Tied to the Processing Conditions… DOI: http://dx.doi.org/10.5772/intechopen.88625*


**Table 1.**

*Inelastic X-Ray Scattering and X-Ray Powder Diffraction Applications*

behaviour of the TMCs may be expected [13, 14].

Several authors have described several classifications of these materials. One of these classifications could be considered based on the kind of reinforcements: continuous or discontinuous reinforcement [6, 7]. Other classifications could be done according to the manufacturing route: traditional methods or powder metallurgy techniques [8–12]. An interesting route to promote the strengthening of the matrix is the "in situ" formation of secondary phases. This method allows a clean and well bounding between the matrix and the reinforcement [9]; consequently, better final

On the basis of previous and recent studies, this work focuses on TMCs which

From the manufacturing process point of view, powder metallurgy techniques of hot consolidation have proved useful at the study of in situ composites [21]. Therefore, for the development of TMCs, inductive hot pressing has been selected among other manufacturing processes. The experience of the authors in this

By a thorough analysis of the properties of the produced specimens via inductive hot pressing at different temperatures by the use of several starting material compositions, specific features of the TMCs could be studied. In this regard, the employment of the XRD technique is crucial in understanding the reaction phenomena between the matrix and the reinforcement. Furthermore, the behaviour of the ceramic particles in the matrix could be unpredictable and variable depending upon many factors; this study is the main objective to analyse the phenomena that could occur when factors as starting powder composition and processing parameters are varied and ultimately how these factors affect the final properties of the TMCs.

The interest in understanding the influence of the starting materials on the final behaviour of TMCs motived the study of three ceramic materials as reinforcements testing various concentrations in titanium matrices. Hence, for the TMC manufacture, diverse starting blends were prepared. The innovation of this investigation lies in the phase analysis carried out in specimens made from these blends. The employment of several operation temperatures and reinforcement typologies and concentrations allowed for the searching of significant differences, among the fabricated TMCs, while all these specimens have been processed at similar processing conditions. The titanium grade 1 employed was manufactured by TLS GmbH (Bitterfeld, Germany). This titanium powder showed a spherical morphology and D50 below 45 μm. The four ceramic reinforcements were chosen considering their reactive behaviour in respect of the secondary phases' formation in titanium matrices. The supplier for TiC powder was H.C. STARK GmBH (Goslar, Germany) and for B4C powders was abcr GmbH (Karlsruhe, Germany), and the company for TiB2 was Treibacher Industrie AG (Althofen, Austria). The characterization of all the powders was carried out to verify the manufacturers' data about their size and morphology. The particle size distribution of the powders was measured by laser diffraction analysis (Mastersizer 2000, Malvern

Instruments, Malvern, United Kingdom); these results are shown in **Table 1**.

X-ray powder diffraction analysis was done using a Bruker D8 Advance A25 (Billerica, Massachusetts, United States of America) with Cu-Kα radiation for the phase characterisation of as-received Ti, TiC, TiB2, and B4C powders and studying

were manufactured employing discontinuous ceramic reinforcement. These ceramic phases were selected according to their reactivity with the titanium matrix. Many authors show the great variety of ceramic reinforcements; however, in this investigation TiC, TiB2, B, and B4C particles were studied. They were considered as

precursors of secondary phase formation by in situ techniques [15–20].

technique had led to laying the basis of this study [22–28].

**2. Materials and methods**

**100**

*Characteristics of the starting materials.*

**Figure 1.** *XRD analysis of the starting powders: Ti grade 1, TiC, TiB2, and B4C.*

the phase evolution of sintered TMCs. The reference intensity ratio (RIR) analysis was performed to semi-quantitatively determine the phases. This method is based on setting the diffraction data to the diffraction of standard reference materials. The intensity of a diffraction peak profile is a convolution of diverse factors, being the most representative of the concentration of the analysed species.

In **Figure 1**, the X-ray diffraction spectra of the starting materials (as-received) are shown. Based on the obtained diffraction patterns, these materials consist of only Ti, TiC, TiB2, and B4C, respectively.

Previously in the TMC consolidation, the raw material blends were prepared according to the fixed percentages in volume (see **Table 2**). The mixing procedure was described in previous authors' work [28]. Next, the specimens were sintered. To consolidate the TMCs, a self-made hot pressing machine, inductive hot pressing (iHP) equipment of RHP-Technology GmbH & Co. KG (Seibersdorf, Austria), was used. This machine enabled the operational cycle time to be reduced thanks to its advantageous high heating rate, which in turn is due to its special inductive heating setup. The prepared powders were inserted in the graphite die; it was lined with thin graphite paper and a protective coating of boron nitride (BN). The same procedure and die were used for all the iHP cycles (punch Ø 20 mm). Methods based on this rapid hot consolidation are considered as preferred techniques for in situ fabrication of nearly fully dense TMCs [29]. In **Table 2**, the processing conditions are shown. Likewise, the curves of the process cycle are represented graphically in


#### **Table 2.**

*Reinforcement percentages and processing parameters.*

**103**

consolidated especially at 1200°C.

*In Situ Titanium Composites: XRD Study of Secondary Phases Tied to the Processing Conditions…*

heating rate being 50°C/min. Following the consolidation, the specimens were

After a thorough metallographic preparation on the cross section of the specimens, the X-ray analysis and the microstructural study were performed. It was studied by means of SEM, using JEOL 6460LV (Tokyo, Japan) and FEI Teneo (Oregon, United States of America). Furthermore, the hardness measurement was carried out; seven indentations were performed on each specimen, avoiding the ceramic particles. A tester model, Struers Duramin A300 (Ballerup, Denmark), was employed to ascertain the Vickers hardness (HV2). An ultrasonic method (Olympus 38DL, Tokyo, Japan) was employed to calculate the Young modulus by measuring longitudinal and transversal propagation velocities of acoustic waves [30]. Archimedes' method (ASTM C373-14) was set for the density measurement.

This section has been structured according to the employed reinforcement, in order to present the results and to perform their discussion clearly and concisely. Therefore, there have been three subparts taking into account the ceramic materials

The X-ray diffraction spectra of TMCs made from TiC-Ti blends are shown in **Figure 3**. Based on the obtained diffraction patterns, these materials consist of Ti and TiCx phases. On the one hand, the X-ray analysis reveals that there are only Ti and TiC phases in specimens produced at 1000°C, regardless of whether the highest or the lowest TiC concentration (vol.%) was used in the starting blend. Likewise, only TiC stoichiometric phase is detected in specimens made from 10 vol.% of TiC, even in specimens produced at 1100 and 1200°C. On the other hand, the peak intensity of the Ti phase decreased; meanwhile, there was an apparition of slight diffraction peaks of nonstoichiometric TiC phase named TiC0.67. It suggested that there were possible reactions between the Ti and diffused C from the TiC particles at high concentrations (20 vol.% TiC and 30 vol.% TiC). The intensification of the nonstoichiometric TiC peaks from 1100 to 1200°C indicates the increase in the volume fraction of this phase, which can be confirmed by the RIR semi-quantification analysis. The results from RIR analysis are shown in **Table 3**. These results may lead to the assumption that the phenomenon of C diffusion was more significant at the highest TiC concentration (30 vol.%) and the highest operational temperature (1200°C). In agreement with the values of the RIR semi-quantification analysis in **Table 3**, the higher the TiC in starting materials was used, the higher the TiC phase values in the RIR analysis was detected. To clarify the distribution of the nonstoichiometric TiCx phases in TMCs, energy-dispersive X-ray spectroscopy (EDS) analysis was performed. In **Figure 4**, the EDS result revealed that there were concentration gradients between the centres of the TiC particles and the matrix. This clearly demonstrated the value of the temperature and the starting material compositions as influencing factors in the final behaviour of the TMCs. Moreover, it can be observed from **Figure 4** that C was diffused in the region, which is rich in titanium. This is in accordance with the slight displacement of the Ti peaks in the TMC patterns when the specimens were

bar), the cycle

**Figure 2**. The iHP equipment worked in vacuum conditions (5 10<sup>−</sup><sup>4</sup>

dislodged from the graphite die and cut in half vertically.

**3.1 X-ray diffraction analysis and microstructural study**

used as starting reinforcements in the TMC manufacturing.

*DOI: http://dx.doi.org/10.5772/intechopen.88625*

**3. Results and discussions**

*3.1.1 TiC*

#### **Figure 2.**

*Diagram of the inductive hot-pressing cycle: time vs. temperature and piston displacement.*

*In Situ Titanium Composites: XRD Study of Secondary Phases Tied to the Processing Conditions… DOI: http://dx.doi.org/10.5772/intechopen.88625*

**Figure 2**. The iHP equipment worked in vacuum conditions (5 10<sup>−</sup><sup>4</sup> bar), the cycle heating rate being 50°C/min. Following the consolidation, the specimens were dislodged from the graphite die and cut in half vertically.

After a thorough metallographic preparation on the cross section of the specimens, the X-ray analysis and the microstructural study were performed. It was studied by means of SEM, using JEOL 6460LV (Tokyo, Japan) and FEI Teneo (Oregon, United States of America). Furthermore, the hardness measurement was carried out; seven indentations were performed on each specimen, avoiding the ceramic particles. A tester model, Struers Duramin A300 (Ballerup, Denmark), was employed to ascertain the Vickers hardness (HV2). An ultrasonic method (Olympus 38DL, Tokyo, Japan) was employed to calculate the Young modulus by measuring longitudinal and transversal propagation velocities of acoustic waves [30]. Archimedes' method (ASTM C373-14) was set for the density measurement.
