Preparation and Strengthening Mechanisms of Ultrasonic-Assisted Cr₃C₂ Particle-Reinforced Al Matrix Composite

Abstract

A novelty Al matrix composite reinforced with Cr₃C₂ particles was prepared by an ultrasound vibration treatment-assisted casting process. The Cr₃C₂ content on the microstructure and mechanical properties of Cr₃C_2p/Al composite were researched systematically. The results indicated that Cr₃C₂ particles were effectively distributed around the grain boundary and led to a great reduction in crystalline size. The density, porosity, and Vickers hardness of the composites rose simultaneously with the increase of Cr₃C₂ content. Meanwhile, the tensile strength and yield strength increased by 104.5% and 85.7%, respectively by adding 3.0 wt. % Cr₃C₂. The fracture morphologies showed that the fracture mechanism was plastic fracture for the pure Al and gradually transformed to brittle fracture as the content of Cr₃C₂ exceeded 3.0 wt. %. Moreover, the strengthening mechanism of the composite was also discussed.

1. Introduction

Particle-reinforced metal matrix composites (PRMMCs) are widely used for structure applications because of their high specific strength and stiffness compared with conventional alloys, as well as their isotropic properties and relatively straightforward processing compared with continuous fiber-reinforced composites [1]. Particulate ceramic-reinforced Al matrix composite [2,3,4,5] possesses low density, high strength, and outstanding corrosion resistance, which has led to it being considered as a promising material and applied in the fields of electronic engineering, spacecraft, and petroleum engineering. As is generally known, the adhesive strength, wettability, and interfacial reaction of the Al matrix and reinforced particles have a significant effect on the mechanical properties of the composite. A stronger interface would usually allow for effective load transfer from the Al matrix to the reinforced particle, leading to improved strength, stiffness, and resistance to the external environment. Indeed, the pair of their thermal expansion coefficients should be matching between a couple of composites, which can effectively decrease the micro-cracks in the interface and enhance bonding strength during the fabrication process.

Potential reinforcement particles such as Al₂O₃ [6], TiB₂ [7], Si₃N₄ [8] and TiC [9] were commonly used to improve the microstructure and mechanical properties of Al matrix composite and three preparation methods, namely powder metallurgy [10], in-situ synthesis [11], and stirring casting [12], were available. Guo et al. [13] researched the influence of ceramic particle dimension on the mechanical performance of the SiC_p/Al matrix composite. The composite reinforced by 13 μm SiC ceramic particles (high hardness and insulator) possessed the highest hardness, while the highest strength was held by the 5 μm particle of the specimen. The hardness of the specimen accompanied by the strength declined due to the increase in ceramic particle size. Gao et al. [3] prepared the Al–Cu composite reinforced with TiB₂ ceramic using the in situ synthesis method and studied the effect of the ultrasonic stirring time on the microstructure, strength, and toughness of the composite. The experimental results indicated that ultrasonic stirring improved the possibility of subsequent reactions and formed more dispersive TiB₂. TiB₂ ceramic particle agglomeration was reduced and the crystalline size of the material declined with the increase of ultrasonic stirring time. Besides this, the yield strength was also improved by extending the ultrasonic stirring time. The microstructure and mechanical performance of the particulate-reinforced Al matrix composite fabricated by strongly ultrasonic stirring were superior to the one prepared by mechanical stirring. The Cu element’s surface was coated with layer-structural Ti₃AlC₂ ceramic particles by Wang, which effectively improved the composite interface [14]. Then, the coated Ti₃AlC₂ was added into the Al matrix composite. The adhesive strength and mechanical properties were enhanced distinctly due to the generation of the Al₂Cu transitional phase. Moreover, the tribological properties of the material were also improved, which was attributed to the increase in mechanical performance.

It can be seen that TiB₂ and Ti₃AlC₂ ceramics both exhibited metal-like electrical conductivity through fitting the interface with aluminum. Chromium carbides had three different crystallographic structures: orthorhombic Cr₃C₂, hexagonal Cr₇C₃, and cubic Cr₂₃C₆. Orthorhombic Cr₃C₂ possesses the best mechanical properties. Cr₃C₂ ceramic phase, a highly metallic carbide, was an outstanding potential material with high modulus, high hardness, high melting point, outstanding wear and corrosion resistance. Cr₃C₂-Ni composites had been fabricated in situ in our previous researche [15,16,17] and the physical performance, tribological properties, and oxidation resistance were studied systematically. The studied results demonstrated that strong interfacial bonding forces and good wettability were present in Cr₃C₂ and Ni. Cr₃C₂ can act as the heterogeneous nucleation of Ni. Cr₃C₂ particles had no interface reaction with aluminum. The coefficient of thermal expansion of Cr₃C₂ ceramic was similar to metal, like Al and Ni, which can effectually promote the interfacial bonding of ceramic and metallic materials.

Certainly, highly metallic Cr₃C₂ particles would be possible options to reinforce the Al matrix which may have an impressive combination of mechanical properties and good resistance to corrosion. Based on our previous research, Cr₃C₂ ceramic particles added into the Al matrix composite by mechanical stirring can effectively eliminate large agglomerations, but this is impossible for 50–100 μm small agglomerations, especially for small particles. Ultrasonic vibration is an outstanding technology to deal with molten-liquid aluminum and magnesium alloys [18,19]. Ultrasonic vibration treatment (UVT) can be used to improve the particulate distribution of composites by dispersing particles to create more effective reinforcements due to Orowan strengthening [20]. Unfortunately, few studies about using UVT to improve the particulate distribution of aluminum matrix composites reinforced with high volumes of Cr₃C₂ particles were available so far. The smaller Cr₃C₂ particles are easily agglomerated. In order to improve the situation, we used ultrasonic vibration treatment, which allowed the Cr₃C₂ particles to be more evenly distributed in the Al matrix.

In this work, a novelty Al matrix composite reinforced with Cr₃C₂ particles was prepared by an ultrasound vibration treatment-assisted casting process. Micro scale particles (1–5 μm) of Cr₃C₂ and the influence of its content on microstructure and mechanical properties of Cr₂ particle-reinforced Al matrix composite are highlighted in this paper. Besides this, the strengthening mechanism of the composite is also discussed in detail.

2. Experimental Procedure

2.1. Fabrication of Cr₃C₂ Particle-Reinforced Al Matrix Composite

In this paper, Cr₃C₂ particle-reinforced Al matrix composite was fabricated using pure Al ingot (99.99%) and Cr₃C₂ powder by an ultrasound-assisted casting process at 760 °C for 30 min. The details of the sketch of the UVT system can be found in reference material [3]. The vibration frequency of the UVT system was 20 ± 1 kHz. The purity and crystalline sizes of Cr₃C₂ powder were 99.9% and 1–5 μm, respectively. The holding time of ultrasonic vibration stirring was 10 min. A certain amount of Cr₃C₂ particle (1.0 wt. %, 2.0 wt. %, 3.0 wt. % and 4.0 wt. %) was added in the matrix. The molten liquid was poured into the graphite die after removing the contaminants on the surface of the molten liquid. The ingot casting was cut to the standard dimension for microstructure characterization and mechanical properties tests. The sample was ground and polished to remove traces of the linear cutting by using the waterproof abrasive paper from 800# to 7000#. The sample was then slightly etched for 1 min using the Kohler reagent to observe the microstructure of the composite.

2.2. Mechanical Properties Test

The density and porosity were tested by the Archimedean drainage method which had also been reported in reference material [21]. Vickers hardness (HV) tests were conducted using the hardness tester equipped with the diamond cone indenter, and the angle was 120°. The load and loading time were 60 kg and 10 s, respectively. The strength and toughness of the Cr₃C₂ particle-reinforced Al matrix composite were tested using the tensile testing machine INSTRON 1195 (INSTRON (Shanghai) Test Equipment Trading Co., Ltd., Shanghai, China). The tensile rate was 0.5 mm/min and the average value of three groups of tests was confirmed as the final value.

2.3. Materials Characterization

The detailed phases of the initial powder and the Cr₃C₂ particle-reinforced Al matrix composite were detected and demarcated by X-Ray Diffraction (XRD). Scanning electron microscopy (SEM) was applied to research the microstructure morphologies of the Cr₃C₂ particle-reinforced Al matrix composite. The distribution of different elements was detected by Energy Disperse Spectroscopy (EDS). The crystalline grain was dyed and the crystalline grain size was calculated by the software of Image-Pro plus 6.0.

3. Results and Discussion

3.1. Microstructure of Cr₃C_2p/Al Composite

The SEM figure and EDS result of the initial Cr₃C₂ powder can be seen the Figure 1a,b. The crystalline grain sizes of the Cr₃C₂ powder were 1–5 μm and the distribution of the powder is homogeneous. The EDS results indicate high purity of the Cr₃C₂ powder. Figure 1c shows the particle size distribution of the initial Cr₃C₂ powder. The percentages of the Cr₃C₂ powder with 0–1 μm, 1–2 μm, 2–3 μm, 3–4 μm, and 4–5 μm are 26%, 38%, 24%, 7%, and 5%, respectively. The average particle size of the initial Cr₃C₂ powder was 1.83 μm after calculation.

Figure 2a shows the XRD results of Cr₃C₂ particle-reinforced Al matrix composite. The enlarged drawing for the 4.0 wt. % Cr₃C_2p/Al sample is shown in Figure 2b. Compared with the pure Al sample, the diffraction peaks of Cr₃C₂ can be detected for the Cr₃C_2p/Al samples. Moreover, the diffraction peaks of Al shift to the left side, which can be attributed to the mutual diffusion of Cr and Al elements. The diffusion of Cr in Al will lead to an increase in the lattice constant and interplanar crystal spacing (d) of Al atoms. According to the Bragg law (2d·sinθ = n·λ), the diffraction angle (θ) will decrease and the diffraction peaks of Al will shift to the left side.

The SEM morphologies and dyed results of Cr₃C₂ particle-reinforced Al matrix composite are shown in Figure 3a–j. The average crystalline size of the pure Al sample is about 105 μm. The distribution of the microstructure becomes homogeneous with the increase of Cr₃C₂ content. Besides, the average crystalline size of the composite declines at the first stage and then increases slightly with the increase of Cr₃C₂ content. Some etch pits of Cr₃C₂ ceramic particles can be seen in the microstructure because of the slightly corrosion of Kohler reagent, as shown in Figure 3d,e. The grain shapes of the big Cr₃C₂ ceramic particles are granular, which well corresponds with the initial Cr₃C₂ powder in Figure 1. According to the computed results by using the software of Image-Pro plus 6.0, the average crystalline sizes of the 2.0 wt. % and 4.0 wt. % Cr₃C₂ particle-reinforced Al matrix composite samples are about 21 μm and 51 μm, respectively. Therefore, their average crystalline sizes decrease by 80% and 51.4%, respectively. The declined average crystalline grain size can improve simultaneously the strength and toughness of the Cr₃C₂ particle-reinforced Al matrix composite via grain refinement.

The element distributions of the 1.0 wt. % Cr₃C_2p/Al composite sample are also shown in Figure 4. The grain boundaries are clear and homogeneous. It can be observed that the Al element as the matrix can be detected distinctly (Figure 4b). As shown in Figure 4c,d, Cr and C elements congregate among the grain boundaries, indicating that Cr₃C₂ ceramic particles serve as the heterogeneous nucleus regions for the Al matrix during the process of solidification. The average crystalline grain size can be refined.

3.2. Mechanical Properties of Cr₃C_2p/Al Composite

Figure 5a–c represents the effects of Cr₃C₂ content on the density, porosity, and Vickers hardness of Cr₃C₂ particle-reinforced Al matrix composite, respectively. The densities of Cr₃C₂ and Al are 6.68 and 2.7 g/cm³, respectively. As we know, the hardness of Cr₃C₂ is higher than that of Al matrix. Logically, the density and hardness of the Cr₃C_2p/Al composite will rise simultaneously with the increase in Cr₃C₂ content. In contrast, the increase in porosity can be attributed to the increase in the grain boundaries. Figure 5d–f shows the SEM morphologies of the indenter for the pure Al, 2.0 wt. %, and 4.0 wt. % Cr₃C_2p/Al composite samples. The outlines of the indenter are clear and the micro-cracks were not detected after the hardness tests, indicating excellent toughness of the Cr₃C_2p/Al composite. The areas of the indenter decrease obviously with the increase in Cr₃C₂ content, which can also indicate the higher hardness of the Cr₃C₂ particle-reinforced Al matrix composite compared to pure Al. From Figure 5f, it can be seen that the Cr₃C₂ ceramic particles are removed because of the corrosion action of the Kohler reagent. Moreover, the micro-cracks are also not detected at the grain boundaries of Cr₃C₂ and Al, which can be attributed to the bonding strength interface.

Figure 6a shows the stress–strain curves of the Cr₃C₂ particle-reinforced Al matrix composite with different Cr₃C₂ amounts added. Obviously, the stress rises, while the strain declines with the addition of Cr₃C₂ ceramic particles. Pure Al possesses the best toughness, and the toughness declines rapidly as the Cr₃C₂ content exceeds 3.0 wt. %. The 3.0 wt. % Cr₃C_2p/Al composite sample behaves with the best tensile strength and yield strength. As shown in Figure 6b,c, the tensile strengths of pure Al, 1.0 wt. %, 2.0 wt. %, 3.0 wt. %, and 4.0 wt. % Cr₃C_2p/Al composite samples are 44 MPa, 62 MPa, 73 MPa, 90 MPa, and 79 MPa, respectively. Their yield strengths are 26.3 MPa, 33.1 MPa, 37.4 MPa, 48.8 MPa, and 45.7 MPa, respectively. Compared with the pure Al sample, the tensile strengths of 1.0 wt. %, 2.0 wt. %, 3.0 wt. %, and 4.0 wt. % Cr₃C_2p/Al composite samples increase by 40.9%, 65.9%, 104.5%, and 79.5%, respectively. Therefore, it can be concluded that the strength is enhanced distinctly due to the addition of the Cr₃C₂ ceramic particles.

3.3. Fracture Morphology

The fracture morphologies of the Cr₃C₂ particle-reinforced Al matrix composite samples are shown in Figure 7a–j. Figure 7a shows the fracture morphologies of the pure Al sample. There is no impurity element for the pure Al sample and it can be seen that the fracture is smooth. From Figure 7a–e, it can be observed that the area of the fracture morphology rises distinctly with an increase of the Cr₃C₂ content, which is in good agreement with the result of the section shrinkage rate. Cr₃C₂ ceramic particles and large dimples can be seen in Figure 7g,h, indicating that the 1.0 wt. % and 2.0 wt. % Cr₃C_2p/Al composite samples exhibit excellent ductility. However, the sizes of the dimples decrease as the Cr₃C₂ content exceeds 3.0 wt. %. Moreover, the micro-cracks and cleavage steps exist on the fracture morphologies for the 3.0 wt. % and 4.0 wt. % Cr₃C_2p/Al composite samples, as labeled in Figure 7i,j. The generation and extension of the micro-cracks will result in fracture mechanism changes from ductile fracture to brittle fracture.

Figure 8 represents the effects of the Cr₃C₂ content on the elongation and section shrinkage rate of the Cr₃C₂ particle-reinforced Al matrix composite. The tensile strength and yield strength increase distinctly as mentioned above, while the elongation and section shrinkage rate decline with the addition of Cr₃C₂ ceramic particles. The 2.0 wt. % Cr₃C_2p/Al composite sample exhibits the best elongation and the 1.0 wt. % Cr₃C_2p/Al composite sample shows the best section shrinkage rate by adding Cr₃C₂ ceramic particles. From Figure 8a,b, it can be observed that the elongation and section shrinkage rate decline obviously when the addition of Cr₃C₂ ceramic particle exceeding 3.0 wt. %. The declining ductility will transform the fracture mechanism from ductile fracture to brittle fracture.

3.4. Strengthening Mechanism

There are four accessible strengthening mechanisms [22,23,24,25,26] for the enhancement of the yield strength of the Cr₃C₂ particle-reinforced Al matrix composite: grain refinement strengthening (σ_g), load-bearing strengthening (σ_Load), CTE mismatch strengthening (σ_CTE), and Orowan strengthening (σ_Orowan). Orowan strengthening is mainly because of the dislocation movement, which can go through or around the second phase particles. However, the limit of Orowan strengthening [27] is the crystalline grain sizes of the second phase particles, which should be less than 1 μm. In this study, the crystalline grain sizes of Cr₃C₂ ceramic particles are 1–5 μm. Therefore, Orowan strengthening is not appropriate for this study. The final predicted result of yield strength (σ_p) can be calculated by the following equation:

σ_p = σ_m + σ_g + σ_Load + σ_CTE

(1)

where σ_m is the yield strength of the Al matrix. In this study, σ_m is 26.3 MPa. As we know, grain refinement strengthening (σ_g) can be calculated by the Hall–Petch equation [28] as following:

σ_g = k(d_c^−1/2 − d_m^−1/2)

(2)

where k is the strengthening factor, d_c is the average crystalline grain size of the Cr₃C₂ particle-reinforced Al matrix composite, and d_m is the average crystalline grain size of the Al matrix. According to the reference [29], the strengthening factor of Al is 68 MPa μm^1/2. In this study, the average crystalline grain sizes of pure Al, 1.0 wt. %, 2.0 wt. %, 3.0 wt. %, and 4.0 wt. % Cr₃C_2p/Al composite samples are 105 μm, 23 μm, 21 μm, 34 μm, and 51 μm, respectively.

During the experimental process, the load has an important factor on the yield strength of the Cr₃C₂ particle-reinforced Al matrix composite. The load-bearing strengthening (σ_Load) can be calculated by the following equation [30]:

σ_Load = 1/2V_pσ_m

(3)

where V_p is volume fraction of Cr₃C₂ ceramic particles. In this study, the volume fractions of 1.0 wt. %, 2.0 wt. %, 3.0 wt. %, and 4.0 wt. % Cr₃C_2p/Al composite samples are 0.41%, 0.82%, 1.23%, and 1.66%, respectively.

The difference of the coefficient of thermal expansion (CTE) between Cr₃C₂ ceramic particles and the pure Al matrix will easily generate dislocations during the solidification process, which also has a significant effect on the yield strength of the Cr₃C₂ particle-reinforced Al matrix composite. In this study, the coefficient of thermal expansion of Cr₃C₂ ceramic particle and pure Al are 10.3 × 10⁻⁶ K⁻¹ and 23.6 × 10⁻⁶ K⁻¹, respectively. The CTE mismatch strengthening (σ_CTE) can be calculated by the following equations [31]:

σ_CTE = βG_mb(ρ_CTE)^1/2

(4)

ρ_CTE = 12ΔαΔTV_p/(bd_p(1 − V_p))

(5)

where β is the constant value of 1.25, and G_m is the shear modulus of pure Al; it can be calculated by G_m = E_m/(2(1 + v)), in which E_m and v are the Young modulus and Poisson’s ratio of pure Al, respectively. Δα is the difference value of the coefficient of thermal expansion of the pure Al and Cr₃C₂ ceramic particles, ΔT is the difference value of the casting temperature and room temperature, V_p is the volume fraction of Cr₃C₂ ceramic particle, b is the Burgers vector with the value of 0.286 nm, and d_p is the average crystalline grain size of the Cr₃C₂ particles. The detailed parameter values of them for the enhancement of the yield strength of Cr₃C₂ particle-reinforced Al matrix composite are shown in

Table 1. The detailed parameter values for the CTE mismatch strengthening of the Cr₃C_2p/Al composite.

Sample	dp/nm	Vp	β	Gm/GPa	Em/GPa	v	b/nm	Δα/10−6 K−1	ΔT/K
1.0 wt. % Cr3C2p/Al	3200	0.41%	1.25	26.3	70	0.33	0.286	13.3	735
2.0 wt. % Cr3C2p/Al		0.82%
3.0 wt. % Cr3C2p/Al		1.23%
4.0 wt. % Cr3C2p/Al		1.66%

.

The final predicted result and experimental result of the yield strength of Cr₃C_2p/Al composite are shown in

Table 2. The final predicted result and experimental result of the yield strength.

Sample	σm/MPa	σg/MPa	σLoad/MPa	σCTE/MPa	σp/MPa	Measured YS/MPa
1.0 wt. % Cr3C2p/Al	26.3	7.5	0.054	6.8	40.6	33.1
2.0 wt. % Cr3C2p/Al	26.3	8.2	0.108	9.7	44.3	37.4
3.0 wt. % Cr3C2p/Al	26.3	5.0	0.162	11.9	43.4	48.8
4.0 wt. % Cr3C2p/Al	26.3	2.9	0.218	13.8	43.2	45.7

. From this table, it can be seen that the predicted result of the yield strength is in good accordance with the experimental result for the Cr₃C_2p/Al composite samples. Moreover, it can be concluded that the main strengthening mechanisms for the improvement of the yield strength are grain refinement strengthening (σ_g) and CTE mismatch strengthening (σ_CTE).

4. Conclusions

In this paper, a novel Al matrix composite reinforced with Cr₃C₂ particles was prepared by an ultrasound-assisted casting process at 760 °C for 30 min. The effects of Cr₃C₂ content on the microstructure and mechanical properties of Cr₃C_2p/Al composite were researched systematically for the first time. The conclusions could be obtained as following:

• Compared with the pure Al sample, the average crystalline sizes of 1.0 wt. % Cr₃C_2p/Al composite decreased by 80%. The decreased average crystalline grain size can simultaneously improve the strength and toughness of the Cr₃C_2p/Al composite via grain refinement strengthening.
• The fracture morphologies showed that the fracture mechanism was plastic fracture for the pure Al, 1.0 wt. %, and 2.0 wt. % Cr₃C_2p/Al composite samples, which gradually transformed to brittle fracture as the content of Cr₃C₂ exceeded 3.0 wt. %.
• After calculation, the predicted result of the yield strength was in good accordance with the experimental result for the Cr₃C_2p/Al composite samples. The main strengthening mechanisms for the improvement of the yield strength were grain refinement strengthening (σ_g) and CTE mismatch strengthening (σ_CTE).