稀有金属(英文版) 2019,38(12),1169-1177

Microstructure and mechanical properties of Cu-B₄C and CuAl-B₄C composites produced by hot pressing

Zulkuf Balalan Furkan Gulan

作者简介：*Zulkuf Balalan e-mail:zbalalan@bingol.edu.tr;

收稿日期：26 July 2018

基金：financially supported by the Bingol University Scientific Research Projects Coordination Unit (No.BAP-SBF.2017.00.001);

Microstructure and mechanical properties of Cu-B₄C and CuAl-B₄C composites produced by hot pressing

Zulkuf Balalan Furkan Gulan

Mechanical Engineering Department,Faculty of Engineering and Architecture,University of Bingol

Abstract：

Cu matrix composite materials reinforced with B₄C particle at four different contents(1.5 wt%,3.0 wt%,4.5 wt% and 6.0 wt%) and also CuAl matrix composites with 13 wt% Al reinforced with B₄C particle at four different contents(1.5 wt%,3.0 wt%,4.5 wt% and 6.0 wt%)were fabricated by hot pressing(HP) and a powder metallurgy(PM) process.Experimental samples were produced by keeping them at 880℃ at the constant pressure of 2.3×10⁸ Pa for 6 min.The density,microstructure and mechanical properties of the produced samples were examined.The microstructure and phase examinations were carried out by scanning electron microscope(SEM)and optical microscope(OM),energy-dispersive spectrometer(EDS) and X-ray diffractometer(XRD) analysis.The hardness measurements,three-point bending test and impact test were conducted to determine the mechanical properties.As a result of the examinations,it was observed that the relative density values decreased with the increasing content of B₄C and provided a relatively effective bonding.Moreover,it was homogeneously distributed in the produced specimens.Consequently,there was a considerable increase in the hardness and the bending strength of CuAl matrix specimens with Al addition.

Keyword：

Boron carbide; Hot pressing; Mechanical properties; Aluminum;

Received： 26 July 2018

1 Introduction

Copper (Cu) is a widely used metal since it has excellent properties such as corrosion resistance,ductility,formability,good thermal conductivity and electrical conductivity [ 1] .Cu is especially employed in applications where high electrical and thermal conductivity are needed.Main drawbacks of Cu and its alloys are low strength and poor wear resistance [ 2] .However,several applications have been investigated for Cu,and it is mainly (over 60%) used as an electrical conductor [ 1] .On the contrary to other high-conductive metals.commercially pure Cu is relatively soft.

Five major ways are applied in order to strengthen metallic materials,which are tension,dispersion hardening.sedimentation,solid solution and grain boundary [ 3] .Sedimentation and solid solution are related to adding particularly alloying elements to the metal which can enhance mechanical properties of the metal;however,this could decrease its electrical conductivity considerably [ 4] .On the other hand,tension,dispersion hardening and grain boundary techniques slightly reduce electrical conductivity of copper [ 5, 6, 7] .Powder metallurgy method pJ ays an important role in combining alloys and metal matrix composites and is highly encountered in the literature researches [ 8, 9, 10, 11, 12, 13, 14] .

Positive sides of aluminum and its alloys are good corrosion resistance,great strength-to-weight ratio and low-temperature properties.Hence,they are used in many engineering applications.On the other hand.aluminum has some negative sides as well,such as low wear resistance and resistance to high temperatures.In order to eliminate these negative sides,aluminum is reinforced with ceramic particles which have high mechanical properties [ 15, 16, 17] .Metal matrix composites have been used in many areas such as automobile,biotechnology,aerospace,defense and energy due to their fine mechanical properties with low weight [ 18, 19, 20, 21] .The properties of the matrix,the reinforcement and the reinforcement-matrix interface are the primarily factors that enable the properties of metal matrix composites to increase.In production of copper-like matrix composites,SiC,AI₂O₃,TiC and B₄C are great reinforcement materials [ 21, 22, 23, 24] .

Boron carbide (B₄C) is a rigid and high-strength material.Moreover,it has high melting point,good hardness and perfect oxidation resistance [ 22, 25, 26] .Hence,it is considered that production of Cu-B₄C and CuAl-B₄C composites will gain higher mechanical properties.However,until now,the reports on Cu-B₄C and CuAl-B₄C composites produced via hot pressing are seldom;particularly,the studies on relationships among Cu matrix,AI and B₄C ingredient,microstructure and mechanical properties are quite few.In this study,Cu and CuAl matrix composites were produced using different contents of B₄C reinforcement material through hot pressing process in order to compare both produced matrix composites in terms of microstructure and mechanical properties.

2 Experimental

Commercially imported Cu and Al powder (<75μm,99%pure) and B₄C (5μm,99.5%pure) were used as the matrix material and reinforcement,respectively.Figure 1 a-c shows SEM image of the Cu,A1 and B₄C powder used.In powder metallurgy,mechanical mixing is the most important operation for homogeneous dispersion of reinforcing phases with matrix even though later operations are also significant [ 27, 28, 29] .The B₄C powder illustrated irregular polygonal morphology with sharp edges,and most of them were wedge sheets.Furthermore,there was an aggregation of particles,indicating that B₄C powder had little dispersion.

It is seen in Fig.1a that the copper dust has a flare-like shape corners with a size between 20 and 35μm,irregular polygons without sharp edges and different particle size distributions.The A1 powder has the irregular infectious of different particle sizes,but dispersion of A1 was a little weak,and a few clusters could be observed in Fig.1c.B₄C was added to Cu matrix at four different contents as 1.5 wt%,3.0 wt%,4.5 wt%and 6.0 wt%,corresponding to S1,S2,S3and S4 samples.Cu+A1 13 wt%matrix composites materials were produced by reinforcing with different contents (1.5 wt%,3.0 wt%,4.5 wt%and 6.0 wt%) of B₄C particles via hot pressing (HP) method known as powder metallurgy (PM) process,corresponding to S5,S6,S7 and S8s amples.The mixture process was performed in the mixer for24 h by adding steel balls to prevent the agglomeration.The mixture powders were produced by being kept in the vacuum environment at the constant pressure of 2.3×10⁸ Pa at880℃for 6 min by using a graphite mold with size of40 mm×10 mm×4 mmby the hot pressing (Fig.2).The densities of the specimens were calculated using Redwap precision balance with the density measurement kit in accordance with Archimedes'principle.The hardness measurements were performed by applying a load of 500 N by using Vickers method in EMCO-TEST DuraScan 20model hardness tester.X-ray diffractometer (XRD) analysis was carried out with a scan rate of 0.005 (°)-s^-1 using Cu X-ray tube (λ=0.15405 nm) in Rigaku UltimaⅣX-Ray Diffractometer.The scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS) examinations were performed with JEOL JSM 6510 scanning electron microscope and IXRF 550 EDS system connected to this device.Optical microscopy (OM) examinations were performed with NIKON ECLIPSE MA 200 optical microscope device.The three-point bending test was performed in the Shimadzu AG-X 50 kN device in accordance with ASTM E8555standard.The impact tests were performed in the Terco Impact Tester MT3016 device using unnotched specimens with the sizes of 40 mm×10 mm×4 mm.

3 Results and discussion

3.1 Relative density

Figure 3 shows the relative density values of the composites produced after hot pressing.The lowest relative density was obtained in 6.0 wt%reinforced B₄C composite (S4).The highest relative density was obtained in the sample added with 1.5 wt%B₄C (S1).The average relative density value obtained in these specimens was 97.5%.Relative density of CuAl matrix composites decreased in comparison with Cu matrix composites due to the compounds formed (A1₃Cu,A1₃BC,A1B₂ and Al₂Cu).It was obtained that the decrease in the content of B₄C caused the relative density values to rise for the produced composites.

Fig.1 SEM images of a Cu powder,b B₄C reinforcement element and c Al powder

Fig.2 Sizes of produced composite components:a S1-S4 and b S5-S8 samples

Chen et al. [ 22] reached a green density of 86%by using the cold pressing process in the production of B₄C/6061A1neutron absorber panel composite by using the powder metallurgy method and then reached the density value of99%at the rate of 1.5 mm·s^-1 at the constant temperature of 490℃by using hot pressing.Also,Guo and Zhang found the tensile stress of the boron carbide-reinforced aluminum matrix composite produced as 480 MPa.Primarily,the decrease in the porosity and the increase in the relative density value by 94.5%contributed to the strength.They also suggested that the hot rolling was a simple and effective method to improve the mechanical properties [ 30] .Moreover,Yener et al. [ 31] determined the composite produced by cold pressing process with Cu 1 wt%,2 wt%and 3 wt%B₄C in accordance with the Archimedes'principle and stated that the relative density values decreased with the increase in the reinforcement content.The highest density (93.8%) was obtained from the composite with the reinforcement content of 1.0 wt%B₄C.An increase was observed in the hardness values with the increase in the reinforcement content.

Fig.3 Relative density values obtained as a result of hot pressing process

Figure 4 shows XRD patterns which illustrate the interaction between reinforcement and matrix in CuB₄C and CuAlB₄C composites produced.In XRD patterns,with the increase in B₄C reinforcement content,the intensity of peaks of B₄C increased;whereas,the intensity of Cu peaks partially decreased.It was observed in the specimens,with the determination limits of XRD device falling,that there was no copper oxide phase depending on the amount of B₄C and the dominant phase was composed of Cu and B₄C.It was seen that there were no low peaks in the produced B₄C specimen and the specimens were composed of B₄C carbide compound.As a result,it is obtained from XRD analyses that there is an interaction among particles in CuB₄C composites,but it is insufficient in Fig.4a.On the other hand,in CuAlB₄C composites,existence of Al compounds provided the interaction between reinforcement and matrix (wetting capability) in Fig.4b.6061/B₄C and1545 K/B₄C composites were investigated by SEM and XRD,and thus,resembling results can be found in Refs. [ 32, 33, 34, 35, 36, 37] .

3.2 Micros tructure

Figure 5 shows SEM images of B₄C composites with different reinforcement contents,and Fig.6 shows their OM images.B₄C reinforcement was distributed homogenously in the copper matrix.The pores formed in these specimens as in the specimens produced by the powder metallurgy and microcavities were not seen in the specimens produced with high density,but the presence of small cavities was determined in some specimens,and it is understood that thanks to the adequate mixing process,homogenous distribution ensured,and no agglomerate was found.

Fig.4 XRD patterns of produced composite pieces:a Cu-B₄C and b CuAl-B₄C

Fig.5 SEM images of Cu matrix composites reinforced with B₄C at different contents:a S1,b S5,c S4 and d S8 samples

As is clearly seen from Fig.5 that 1.5 wt%B₄C phase distributed uniformly and 1.5 wt%B₄C had a uniform distribution in Al phase in 1.5 wt%B₄CA1 sample.Gaps and cracks were not seen in S1 and S5 samples.Furthermore,it is seen from Fig.5 that particles of B₄C were a little bit in tendency to accumulation;this situation was more in S1.It is clear from Fig.5 that with B₄C content increasing,there is a downward tendency in bonding between B₄C phase and matrix phase,and there were gaps,fractures and cracks (see at S4 and S8).When a comparison made between images,phase-to-phase bonding was influenced negatively with B₄C content increasing and there was a downsizing in sizes of matrix phase with Al addition.

It is seen from Fig.6 that Al phase prevented homogeneous distribution of B₄C reinforcement and also caused accumulations of B₄C from place to place.It is clear from OM images that Al shows a better bonding behavior than B₄C.As can be seen in Fig.7,annealing twins formed in the composite reinforced with 4.5 wt%B₄C.The boundaries separating two zones which formed the mirror images of each other as a result of the symmetrically different orientations of crystal atomic planes were called as“twin boundaries”.The shear force affecting throughout the twin boundary during the heat treatment resulted in the formation of the annealing twins in thick lines by displacing the atoms.The annealing twins had different planes and directions according to each of the crystal structures and increased the strength of metals by making the shear difficult [ 38] .

The formation of pores occurred in some of the specimens even if it was a little.There was no agglomeration in the distribution of the large Cu grains;however,there was agglomeration in the dispersion of very small ones of the B₄C gains.All of the reinforcement particles were firmly caught by the matrix,as seen in Fig.8.Dole et al. [ 39] characterized the sintering behavior by increasing the temperature up to 2300℃and employing hot pressing and pressureless sintering techniques,and they observed the coarsening in the microstructure.Also,they reached 95%of theoretical density with the carbon reinforcement.As seen from EDS results given in Fig.9,the distribution of B,C and Cu in the composite material was seen for S2 and S5samples.In general point analysis,it was determined that there were 2.851 wt%C and 97.149 wt%Cu in the composition of the composite,and it was understood that the contents of these elements in the composite were compatible with those in the matrix.The presence of these elements (B,C and Cu) with Al were occured a carbide phase,and the main composition of this phase was the boron.It was evident that Al and Cu generated intermetallic compounds from EDS and XRD analysis.

Fig.6 OM images of Cu matrix composites reinforced with B₄C at different contents:a S1,b S5,c S4 and d S8 samples

Fig.7 SEM image taken from 4.5 wt%B₄C sample (S3) produced by hot pressing

Fig.8 SEM image taken from 6.0 wt%B₄C sample (S4) produced by hot pressing

3.3 Microhardness

There was a significant increase in the hardness together with the increase in B₄C content,as seen in Fig.10,and microhardness was proportionally increased.While the average hardness value of the specimens of the pure copper produced by this method was HV_0.1 45,the highest hardness value was obtained in the Cu matrix composite containing 6.0 wt%B₄C (S4) and the average value was HV_0.1 161.9.The hardness value of HV_0.1 352.82 was for S5 sample,the highest hardness values in CuAl matrix composites containing 6.0 wt%B₄C in S8 sample (HV_0.1478.38) were attained,and the average hardness value of HV_0.1 441.2 was reached in S7 sample.It is determined that hardness values of CuAlB₄C composites rose two times more than that of CuB₄C.The main reason for this rise based on EDS and XRD analysis is because of Al₂Cu and A1₄Cu₉ intermetallic compounds forming at interphases.Also,with the increase in B₄C content,the hardness increased.

Fig.9 SEM images and EDS results of a S2 and b S5 samples produced by hot pressing

The formation of the surface barrier around the boron carbide particles and the low rate of the reaction between B₄C and the melt alloy resulted in the limited increase in the hardness as a function of the period of heat treatments [ 40] .Altinsoy et al. [ 41] stated that the dislocation rate of high thermal expansion increased significantly during the sintering process when the copper had the low content of B₄C.Thus,they determined that the microhardness value of the composite also increased with the increase in the B₄C content.Thus,the hardness increased linearly with the increase in the reinforcement content of the composite.It was observed as HV 83 in the specimen reinforced with3.0 wt%B₄C.The fact that hardness results obtained in the present study were in parallel with the said study supports the present study.

Fig.10 Hardness values of composites produced by hot pressing

3.4 Three-point bending and impact energy

It was found that three-point bending test was applied to the specimens produced by hot pressing.Figure 11 shows the strength values obtained.The bending strength did not display a proportional decrease with the increase in B₄C content.It was determined that there was no linear increase in the strength values even if the hardness increased with the increase in B₄C content.In Cu matrix,the highest bending strength value (367 N·mm^-2) was observed in S2sample,while S4 sample had the lowest bending strength value of 263 N·mm^-2 and elongation value.On the other hand,in CuAl matrix composites,the highest bending strength value of 415.16 N·mm^-2 was measured in S6sample.The elongation rate increased due to the fact that both the strength values and the matrix (proportionately)were high in S1 and S2 samples with high density;whereas,the bending strength decreased in S3 and S4samples with low density.The results of the bending tests were low because of these values where the wetting was inadequate on the matrix-reinforcement element interface [ 42] .The mechanical properties of the materials were negatively affected by the heterogeneous distribution of the reinforcement particles in the matrix.The reinforcement particles were positioned as embedded to the matrix on the surface of matrix.

Fig.11 Strength values obtained from three-point bending test of composites produced by hot pressing process:a CuB₄C and b CuB₄C

As shown in Table 1,with the increase in B₄C content,it was seen that both bending strength and elongation value decreased.However,13 wt%Al addition rose the bending strength,but it reduced the elongation considerably.The rise of relative density affected the strength positively.As A1 addition improved the wetting capability between reinforcement and matrix,this situation contributed a favorable effect on the strength.

Alizadeh and Taheri [ 43] determined the increase of11%-17%in the yield stress of 2.0 wt%-4.0 wt%B₄C Al(B₄C reinforced aluminum matrix) matrix and the decrease of 38%-52%in the elongation compared to the Al nanostructure.Moreover,SEM images taken showed that B₄C particles used in the study had an irregular shape.It was observed that these irregularities resulted in the stress concentration and also caused both attenuation of the composite and the decrease in the mechanical properties as a result of the notch effect.

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Table 1 Density,stress and elongation of samples

The impact energy values of AlCu matrix composites were decreased with the increase in B₄C content.This was associated with insufficient wetting on the matrix-reinforcement element interface and the irregularity of the B₄C reinforcement powder,as shown in SEM images taken from the fracture surfaces after the impact test and threepoint bending tests.It was determined that A1 matrix was exposed to the plastic deformation during the deformation and the B₄C particles turned into hard and brittle structure with the effect of the applied load [ 29] .Larger B₄C particles were fractured under the high pressure easier compared with the small particles.The highest impact energy of 13.5 J·cm^-2 was obtained from S1 sample;on the other hand,the approximate value decreased by 90%in the S4sample (from 13.5 to 1.35 J·cm^-2) and that of S8 sample is0.55 J·cm^-2.

Figure 12 shows SEM images of the fracture surface after the impact test and three-point bending tests.It was seen from Fig.12a,c that a ductile fracture occurred in the specimens as a result of the dimple formation in the Cu matrix.Figure 12b and d shows that as B₄C was lower,finer dimples occurred and this led to stronger bonding between Cu matrix and B₄C.Wider and deeper dimples in the S1 and S4 samples under the constant load applied indicated that plastic deformation was much,and also lesser deformation occurred with the brittle structure in S5and S8 samples.Also,there was a fracture on the matrixreinforcement interface.It was observed in these zones that Cu matrix did not completely wet the reinforcement element.

As it is clearly seen from fracture surface SEM images in Fig.12,Al addition resulted in brittle fracture,but semiductile fracture was observed in B₄C reinforcement samples (S1 and S4).With B₄C content increasing,an entirely brittle fracture occurred in A1 samples.Although Al contributed a positive effect to B₄C in terms of strength,it was understood from tests results that A1 reduced toughness.The lowest resistivity was measured in S1 sample (1.25Ω·m),and the highest resistivity was measured in CuAl6.0 wt%B₄C matrix composite (1.67Ω·m).It is considered that the reason for high resistivity is that the intermetallic compounds took place rather than A1 addition.In the specimen with the lowest B₄C content,the copper caused the high thermal expansion in the specimens reinforced with low B₄C content and also led to the significant increase in the dislocation due to the high thermal expansion mismatch during the sintering.The highest conductivity was measured in the specimen with 1.0 wt%B₄C at the rate of 80%IACS (International Annealed Copper Standard).In addition,the porosities acted as a barrier between Cu particles and electrons,and the high porosity and the lower relative density resulted in the decrease in the electrical conductivity [ 44] .

Fig.12 SEM images taken from fracture surfaces after impact test and three-point bending:a S1,b S5,c S4 and d S8 samples

4 Conclusion

The production of the composites reinforced with B₄C in the Cu matrix by hot pressing and their characterization were studied in this study.The composites produced by the hot pressing process were determined to increase the relative density values at the low reinforcement content of B₄C.These high relative density values proved that the bending and impact energy were high.The hardness values increased with the increase in B₄C content.The strength values did not increase linearly together with the increase in the reinforcement content;however,the elongation decreased together with the increase in B₄C content.The highest bending strength value (367 N·mm^-2) was obtained for S2 sample and 415.16 N·mm^-2 for S6 sample reinforced with 3.0 wt%B₄C.However,while elongation value reached up to 10 mm in Cu matrix composites,it dropped to below 1 mm in CuAl matrix composites.The highest impact energy exhibited a ductile behavior with the effect of elongation and measured as 13.5 J·cm^-2 in S1sample.It was observed that the Cu matrix ensured an effective bonding with B₄C in the low reinforcement content;however,the composite was not wetted so that the composite could have the desired mechanical properties as the reinforcement content increased.It was found that the electrical resistivity rose with reinforcement content increasing,while the lowest resistivity was measured in S1sample.

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