Effect of TiCp volume fraction and WCp addition on the mechanical properties of TiCp/Cr8Mo2VSi composites

Titanium carbide particles (TiCp) is one of the most commonly used ceramic particles in ceramic- particle-reinforced metal matrix composites. The study prepared TiCp/Cr8Mo2VSi composites using the squeeze casting technique and investigated the effects of different TiCp volume fractions on the mechanical properties, including bending strength and impact toughness. Additionally, the study added tungsten carbide particles (WCp) powder to the preform to examine its effect on the mechanical properties of the composites. The study found that as the volume fraction of TiCp increased, the bending strength of the composites decreased gradually. The highest bending strength of 642.7 MPa was observed at 35% volume fraction. The impact toughness showed a small change, approximately 1.7 J cm−2. Upon the addition of WCp powder (with a mass fraction of 5.0 wt% and TiCp volume fraction of 50%), the bending strength and impact toughness of the composites were 375.2 MPa and 2.1 J cm−2, respectively. Compared to the composites without WCp powder (105.4 MPa, 1.7 J cm−2), the addition of WCp powder resulted in a 256.0% increase in bending strength and a 23.5% increase in impact toughness.


Introduction
Ceramic particle reinforced metal matrix composites are widely used in mining tools, cutting tools, and metal forming moulds due to their ability to combine the good toughness of metal matrix with the high hardness and wear resistance of ceramic particles [1][2][3].TiC [4][5][6], WC [7][8][9], aluminum oxide (Al 2 O 3 ) [10][11][12], and silicon carbide (SiC) [13][14][15] are currently among the most common ceramic particles used in such composites.TiC, a new type of carbide, is commonly used in steel matrices due to its high melting point, low density, high elastic modulus, and chemical stability [16,17].It is also a very effective ceramic-reinforcing particle, with high wettability with a steel matrix of approximately 40°.The resulting TiC-reinforced steel-matrix composites exhibit high hardness, wear resistance, chemical stability, and thermal conductivity [18,19].Li et al [20] prepared high manganese steel matrix composites reinforced with TiC using a powder metallurgy process.They investigated the effect of WC on the properties of the composites and found that the WC particles formed a heart-edge structure around the TiC particles.The hardness and bending properties increased with increasing WC content.TiC/Fe-based composites are commonly produced using powder metallurgical method [21][22][23][24].This method result in good metal-ceramic interfaces.However, the preparation of steel junction cemented carbides using these methods is expensive, inefficient, and involves a long preparation process.Additionally, it offers limited component size [25].Usually, powder metallurgy composites are brazed together with other parts.The strength of the joint is low, making it susceptible to failure under external loads.Additionally, welding defects may occur during brazing, which can further reduce the strength of the parts.
In order to solve these problems, this paper uses the squeeze casting method to prepare ceramic particles reinforced steel matrix composites.Compared with the powder metallurgy method, in the squeeze casting process, a preform was first prepared with ceramic particles, then preset at a certain position of the mould cavity for casting.Under the pressure, the liquid steel was poured into the cavity and infiltrated into the preform to make composite, at the same time the other sections of the part were formed, so that the composite and other sections of the part were integratedly formed.Therefore, the composite material prepared by this method does not need to be connected to the part twice, which can greatly enhance the bonding strength of the composite material and the part, and at the same time, shorten the whole preparation process of the part, which in turn improves the production efficiency.

Materials and preparation method
2.1.Test materials Figure 1 shows the microstructures of the TiC ceramic particles (99.9% purity) used as the reinforcing phase, with an average mesh size of 1250 (10 μm).To improve the interfacial bonding between the ceramic particles and the steel substrate, activated micronized WCp powder (99% purity) was used.Figure 2 displays the microstructure of the WCp powder with an average size of 1000-3000 mesh (4.8 μm-13 μm).Table 1 lists the chemical composition of the Cr8Mo2VSi mold steel matrix.The precast body is bonded using water glass as the adhesive agent.This is due to its strong adhesive properties, even at high temperatures, which provides the  preform with superior strength.This prevents the preform from collapsing during the squeeze casting process of the composites.Table 2 shows its basic parameters.

Preparation of composite materials
In the preparation of composites with added WC powder, TiC ceramic particles, activated micropowder WCp and reduced iron powder were first mixed using a ball mill.The ball mill used was a DQM-16 planetary ball mill with 8 mm diameter steel balls.The ball mill speed was 30 rpm, the ball-to-material ratio was 5:1, and the mixing time was 1 h.Next, water glass binder was added into the powder and mixed again for 20 min.The mixture was pressed into blocks measuring 55 mm × 25 mm × 15 mm using a small press (20 MPa).The preforms were then roasted in a high-temperature chamber furnace at 550 °C to eliminate residual moisture and improve strength.Finally, the composites were prepared using the squeeze casting infiltration technique.In preparing composites without added WC powder, the preparation process remained unchanged, except for the exclusion of reactive micronized powder during the ball milling and mixing process.
The squeeze casting process involved using an extrusion die preheated to 450 °C and a holding time of 2 h.Melting was carried out in a medium-frequency induction-melting furnace at a temperature of 1610 °C, with a pouring temperature of 1500 °C.The pressurised pressure was 45 tonnes, and the holding pressure was 10 min.The process parameters for squeeze casting are detailed in table 3.

Analysis of the composite materials 2.3.1. Microstructure
The microstructures of the specimens were observed at high magnification using a scanning electron microscope (ZEISS EVO18).The elemental composition and microstructure of the composite were further analyzed using its Energy-Dispersive X-ray Spectroscopy (EDS).

Hardness test
The specimens were polished before their hardness was tested using a Rockwell hardness tester (HR-150A).The Rockwell hardness test was in accordance with ASTM E18 of the American Society for Testing and Materials (ASTM).A ten-point measurement was performed to ensure the accuracy of the data, and the results were presented as the mean value.

Bending strength
The composite material was wire cut and polished into a rectangular specimen measuring 5 mm × 5 mm × 30 mm.The bending strength of the specimen was then tested using an electronic universal testing machine (CMT4503).The three-point bending test was in accordance with ASTM A370.The bending strength test parameters are listed in table 4. The bending strength calculation formula is shown in equation (1), and the schematic diagram of the three-point bending test is shown in figure 3. The bending strength was calculated using equation (1): where F denotes the load, b is the width, h is the thickness; L is the span.

Impact toughness
The composite material was wire cut and polished into a rectangular specimen measuring 10 mm × 10 mm × 55 mm.The impact toughness was then measured using an impact tester (PH750) with a maximum pendulum impact work of 750 J.The impact toughness test in accordance with ASTM E23.The formula for calculating the impact toughness test is shown in equation (2) and its schematic diagram is shown in figure 4.
The impact toughness was calculated using equation (2 where a k denotes the impact toughness value, A k is the impact power, F is the cross-section area.

Volume fraction of composite materials
In order to compare the properties of composites with different TiCp volume fractions, three volume fractions were selected in the preparation process, namely 35%, 50%, and 65%.One reason for this is that TiCp reinforced composites typically fall within the 0%-70% range [26].Another reason is that, in comparison to powder metallurgy methods, cast infiltration preparation of composites requires a lower TiCp volume fraction to ensure   full infiltration of the steel with the TiCp preforms.The highest range of volume fractions to be used in cast infiltration methods is around 65%. Table 5 presents the theoretical and actual volume fractions of the composite specimens, calculated using Image Pro Plus software.The difference between the theoretical and actual volume fractions of the composites is insignificant, meeting the experimental requirements.

Microstructure analysis of composite materials
Figure 5 shows microstructure images of the TiCp/Cr8Mo2VSi composites with different volume fractions.The Agglomeration of the TiC ceramic particles in the composites gradually increased with the volume fraction of the TiC ceramic particles, reaching a maximum at a volume fraction of 65%. Figure 6 shows the EDS surface scan of the TiCp/Cr8Mo2VSi composite when the volume fraction is 35%.From figure 6, it can be seen that the light grey part is the matrix, and the dark grey is the overlapping part of C element and Ti element for TiC ceramic particles.There exists some black substance at the interface bonding of TiC ceramic particles and matrix, which is found to be composed of elements such as Si, O, Ti, and Na by observing the energy spectrum of EDS surface.The composition of the black substance was further analyzed using EDS point scanning (figure 7).Plot 65 in figure 7 identifies the main constituent elements of the black substance as Ti, O, and Si.O derives from the water glass in the preform, which generates a glass phase at high temperatures.These results, therefore, identify the black substance as a glass phase consisting primarily of Ti, O, and Si, among other elements.fraction of ceramic particles increases, the impact toughness of the composites initially increases and then decreases, with minor fluctuations, all of which are significantly lower than the impact toughness of the Cr8Mo2VSi matrix (5.0 J cm −2 ).The composite specimen exhibits the highest impact toughness at a volume fraction of 50% composite material, reaching 1.7 J cm −2 .This suggests that the addition of ceramic particles reduces the impact toughness of the composite material.The table shows that the hardness of the composites increases gradually with the increase in the volume fraction of ceramic particles.At a volume fraction of 65%, the composite reaches a maximum hardness of 62.1 HRC, which is 16.3% higher than the matrix hardness (53.4 HRC).This suggests that the addition of ceramic particles can enhance the composite's hardness.Figure 8 shows the bending strength-displacement curves for different volume fractions of composites and Cr8Mo2VSi matrix in bending strength tests.The curves show a sharp decrease in bending strength after reaching the maximum, indicating a brittle fracture with no plastic phase.The bending strength of the 35% volume fraction is higher than the other fractions but lower than that of the matrix.

Mechanical properties of composite materials
Figure 9 shows the fracture micromorphology of the bending strength specimens with different volume fractions.Specifically, figure (a) illustrates the fracture morphology of the Cr8Mo2VSi matrix.Notably, the fracture exhibits river-like disintegration steps and a brittle fracture form.The bending fracture morphology of the composite material specimen can be observed in figures (b), (c), (d), and (e).The TiC particles fracture flat, leaving behind a small step where dissociated fracture occurred.The glassy phase material exists at the bonding interface between the TiC particles and the matrix.Cracks propagate along the expansion of the glassy phase material, resulting in brittle fracture of the composite material as a whole.The presence of glass phase material at the interface of ceramic particles and matrix reduces the bonding strength, making it susceptible to fracture under static load.As a result, the bending strength of the sample is not high.
Figure 10 shows the fracture morphology of impact toughness specimens with different volume fractions.Figure (a) illustrates the impact fracture morphology of the Cr8Mo2VSi matrix, which exhibits an obvious step of disintegration and a brittle fracture form.Figure (b) and (c) depict the impact fracture morphology of composites with a volume fraction of 35%, while figure (d) and (e) show the impact fracture morphology of composites with a volume fraction of 50%.The figure shows that the TiC particles fracture flat and have dissociated residual steps for brittle fracture.The interface between ceramic particles and matrix contains black glassy phase material, and the cracks mainly extend from this material.The cracks become longer and more pronounced under dynamic load, causing the sample to break easily and resulting in poor impact toughness.Combining the impact toughness test and fracture morphology analysis, the following conclusions can be drawn: The impact toughness of the composites is significantly lower than that of the matrix.Additionally, the impact toughness of composites with varying volume fractions initially increases and then decreases with increasing TiC volume fraction.Furthermore, all composite specimens with different volume fractions exhibit brittle fractures.

Effect of TiCp volume fraction on the mechanical properties of the composites
The mechanical properties of Fe-based composites are typically linearly related to the volume fraction of reinforcing particles [27], which increases as the volume fraction increases.However, in the composites studied in this paper, the mechanical properties decreased as the volume fraction of TiCp increased.In the experiments, water glass was added as a binder, resulting in the production of an amorphous brittle glassy phase material at the interface of TiCp and the matrix.The presence of this material greatly reduced the properties of the composites.As the TiCp content increased, so did the substance, ultimately leading to a reduction in the mechanical properties of the composites.

Effect of WCp powder addition on the mechanical properties of composites
Figure 11 shows microstructure images of the TiCp/Cr8Mo2VSi composites with different WCp mass fractions.The dark gray bulk phase in the figure is the TiC ceramic particles, the light gray is the Cr8Mo2VSi matrix, the bright color is the WC particles, and the black part at the interface between the ceramic particles and the matrix is the glass phase material.Figure 12 shows the EDS scans of the composites with 5.0 wt% WCp micropowder addition for the C, Ti and W elements.As can be seen from the figure, there is a thin layer of elemental W around the ceramic particles and the rest of elemental W is dispersed into the matrix.The ceramics with added WCp form a black core-white inner edge-grey outer edge structure through a dissolution-reprecipitation mechanism [20].A part of WCp is encapsulated in the edges of the ceramic particles to form a heart-edge structure, which improves the elastic modulus of the TiC ceramic particles and contributes to the improvement of the composite's impact fracture  resistance.There is also a part dispersed into the matrix to form a diffusely reinforced phase, which improves the properties of the composite.
Table 7 shows the results of the mechanical property tests carried out on TiCp/Cr8Mo2VSi composites with different mass fractions of WCp powder added, and figure 13 shows the bending strength-displacement curves of composites with different mass fractions of WCp powder added.According to the data presented in table 7 and figure 13, it can be concluded that the bending strength, impact toughness, and hardness of TiCp/ Cr8Mo2VSi composites increase with the addition of WCp powder.The maximum values of these properties are achieved at 5.0 wt%, which are 375.2MPa, 2.1 J cm −2 , and 57.6 HRC.
Figure 14 shows the fracture morphology of the bending strength specimens with different mass fractions of WCp powders, in which (a) and (b) are the fracture morphology of the composites with 0 wt% WCp powders, and it can be seen that the TiC ceramic particles in the fracture are left with step planes after deconvoluted fracture, and glassy phase substances exist around the interface of TiC, and the fracture is in brittle fracture; Fracture morphologies of composites with 5.0 wt% WCp powders added are shown in (c) and (d).TiC ceramic particles fracture flat, and glassy phase material is found at the interfacial bonding of TiC and matrix.The cracks extend along the interfacial bonding, and the ceramic particles are not well bonded with the matrix, resulting in brittle fracture.
Li et al [20] used powder metallurgy technology to prepare TiC reinforced high manganese steel matrix composites.They investigated the effect of WC addition on the composites' hardness, transverse fracture strength, and impact toughness.The results showed that the composites' hardness and transverse fracture strength increased with the addition of WC.The impact toughness value reached its maximum at 5 wt% WC addition.Previous research has determined that the optimal performance of WC composites is achieved with a 5 wt% addition.Therefore, this study focuses on composites prepared with the addition of 5 wt% WC.The  results indicate that WC is capable of forming a coating layer during the infiltration process, achieving the desired effect of the powder metallurgy method in a short amount of time.WC leads to the formation of a (Ti,W) C solid solution on the surface of TiC particles through the mechanism of dissolution precipitation.This results in the formation of a layer of W-rich coating on the surface of TiCp, which improves the wettability between TiCp and the matrix and enhances the interfacial bonding.This eliminates the influence of some glassy phase substances.The composite properties are improved by eliminating the influence of certain glassy phase substances.

Application prospects
This study can realize the efficient preparation of composite materials, so that the composite materials and other parts of the parts are formed integrally without the need for secondary connections, which can greatly enhance  the bonding strength of the composite materials and the parts, and at the same time shorten the entire preparation process of the parts, which in turn improves the production efficiency.It solves the disadvantages of high cost, long preparation process, low efficiency and limited size of components in the preparation of composites by powder metallurgy method.The prepared composites can be used as high-performance wearresistant materials with potential applications in hobbing knives for shield machines, rollers for roller presses, and hammer heads for crushers.

Conclusion
(1) The microstructure of the TiCp/Cr8Mo2VSi composites with different volume fractions showed a strong bonding between the matrix and ceramic particles.The ceramic particles were uniformly distributed, and no obvious casting defects were observed.However, agglomeration of the ceramic particles occurred when their volume fraction increased to 65%.
(2) The analysis conducted on TiCp/Cr8Mo2VSi composites with different volume fractions revealed that the bending strength of the composites decreased with increasing volume fraction of ceramic particles, reaching a maximum of 642.7 MPa at a volume fraction of 35%.The impact toughness of the specimens initially increased with increasing volume fraction of ceramic particles, but then decreased.The maximum impact toughness of 1.7 J cm −2 was observed at a volume fraction of 50%.The composite's hardness increased as the volume fraction increased, reaching a maximum of 62.1 HRC at a volume fraction of 65%.
(3) The analysis of TiCp/Cr8Mo2VSi composites with different mass fractions of WCp micropowders revealed that the bending strength, impact toughness and hardness of the composites increased as the content of micronised tungsten carbide powder increased, with the maximum values of bending strength, impact toughness and hardness being 375.2 MPa, 2.1 J cm −2 and 57.6 HRC.Compared to the mechanical properties of the composites without WCp powder (105.4MPa, 1.7 J cm −2 ), the bending strength was improved by about 256.0% and the impact toughness by about 23.5%.

Figure 1 .
Figure 1.Microstructure of the TiC ceramic particles.

Figure 13 .
Figure 13.Bending strength-displacement curves of composites with different mass fractions of WCp powders.

Table 2 .
Parameters of water glass.

Table 4 .
Bending strength test parameters.

Table 6
lists the mechanical properties of TiCp/Cr8Mo2VSi composites with varying volume fractions.The bending strength data shows that the bending strengths of composites with 50% and 65% volume fractions (105.4MPa,152.1 MPa) are approximately 509.8% and 322.6% lower, respectively, compared to the bending strength of composites with 35% volume fraction (642.7 MPa).At high volume fractions of ceramic particles, the composites exhibit poor flexural strength.The impact toughness data in table6shows that as the volume

Table 5 .
Actual and theoretical volume fractions of TiCp/Cr8Mo2VSi composite specimens.

Table 6 .
Mechanical properties of TiCp/Cr8Mo2VSi composites with different volume fractions.

Table 7 .
Mechanical properties of TiCp/Cr8Mo2VSi composites with different mass fractions of WCp powders.