Microstructure and mechanical properties of Al-B4C composite at elevated temperature strengthened with in situ Al2O3 network
Graduate School at Shenzhen,Tsinghua University
School of Materials Science and Engineering,Tsinghua University
Sate Key Laboratory of Nuclear Power Safety Monitoring Technology and Equipment,China Nuclear Power Engineering Co.,Ltd.
China Nuclear Powder Design Co.,Ltd.
作者简介：*Qiu-Lin Li,e-mail:liql@sz.tsinghua.edu.cn;
收稿日期：22 February 2018
基金：financially supported by Shenzhen Engineering Laboratory of Nuclear Materials and Service Safety;
Microstructure and mechanical properties of Al-B4C composite at elevated temperature strengthened with in situ Al2O3 network
Shang-Yang Feng Qiu-Lin Li Wei Liu Guo-Gang Shu Xin Wang
Graduate School at Shenzhen,Tsinghua University
School of Materials Science and Engineering,Tsinghua University
Sate Key Laboratory of Nuclear Power Safety Monitoring Technology and Equipment,China Nuclear Power Engineering Co.,Ltd.
China Nuclear Powder Design Co.,Ltd.
Abstract：
This study evaluated the mechanical properties and thermal properties of Al-12 vol%B4 C composite at elevated temperature strengthened with in situ Al2 O3 network.The composite was fabricated using powder metallurgy(PM) with raw materials of fine atomized aluminum powders,and the associated microstructures were observed.At 350 ℃,the composite had ultimate tensile strength of UTS=137 MPa,yield strength of YS0.2=118 MPa,and elongation of ε=4%.Besides,the mechanical properties of the composite remained unchanged at 350℃ after the long holding periods up to 1000 h.The excellent mechanical properties and thermal stability at 350℃ were secured by in situ am-Al2 O3 network that strengthened the grain boundaries.The interfacial debonding and brittle cracking of B4 C particles were the main fracture mechanisms of the composite.In addition,the influence of sintering temperature and rolling deformation on the microstructures and mechanical properties was studied.
Keyword：
Al-B4C; Alumina(Al2O3); Composite; Mechanical properties; Microstructure;
Received： 22 February 2018
1 Introduction
Metal matrix composites (MMC) reinforced by particulates are very promising materials,suitable for applications in automobile,aerospace and electronic industries due to their light-weight,high specific strength,superior wear and chemical resistance
Unfortunately,it is rarely reported that Al-B4C composites has been developed with expected service at elevated temperatures and to be thermally stable for long periods of time.Chen et al.
In recent years,nanosized particle-reinforced aluminum matrix composites (AMCs) received a tremendous attention due to their excellent mechanical properties and thermal stability at elevated temperatures.It has been reported
To develop neutron absorber material with high strength and thermal stability at elevated temperatures,the composite was fabricated using powder metallurgy (PM) with raw materials of fine atomized aluminum powders,and the associated microstructures were observed.The tensile properties and thermal stability of the composite at elevated temperatures were investigated.In addition,the influence of sintering temperature and rolling deformation on the microstructures and mechanical properties was studied.
2 Experimental
B4C particles (99%in purity,Jingangzuan Boron Carbide Co.,Ltd,China) and atomized aluminum powders(Xiangtian nanomaterials Shanghai Co.,Ltd,China) were used as the raw materials.The average sizes of B4C particles and aluminum powders were 4.1 and 1.5μm,respectively.The aluminum powders were mixed with about 12 vol%B4C particulates in alcohol medium for 2 h using an attritor at 150 r·min-l.The ball to powder weight ratio was 6:1.The mixed powders were dried and compacted by cold isostatic pressing (CIP) at 400 MPa.The green compacts were vacuum-sintered to remove free water and chemically bonded water which are associated with the oxide surfaces on the atomized aluminum powder at the temperatures of 120 and 420℃,respectively.To study the effect of temperature on sintering of the billets,the resulting billets were then finally vacuum-sintered at450,500,550 and 600℃,respectively,for 2 h under a vacuum of 2×10-3 Pa.Then,the samples with 10 mm in thickness were cut out from the sintered billet for hot rolling.The samples were directly rolled at 450℃,through multi-pass proceeded on 200-tons laboratory rolling mill.Before rolling,the samples were preheated for0.5 h in the resistance furnace,and the sample was reheated for 15 min between passes.They were hot rolled eleven times for total reduction corresponding to 70%.
Samples with dimension of 3 mm×10 mm×10 mm were polished by mechanical polishing and followed by ion beam (IB-09020CP,JEOL) polishing.Microstructure investigations of samples using scanning electron microscopy (SEM) were carried out using a TESCAN MIRA 3LMH instrument and a HITACH S4800 instrument,both equipped with energy-dispersive spectrometer (EDS).Meanwhile,for more detailed microstructural investigation,some samples were examined in FEI TECNAI G2 F30transmission electron microscope (TEM).X-ray diffraction(XRD,DLMAX 2500) was performed on solid samples to identify phase in the composite.
The densities of the as-sintered and rolled samples were measured by Archimedes’method.A prolonged annealing process up to 1000 h at 350℃was performed to elevate the long-term thermal stability of mechanical properties at elevated temperature.To be pointed out,the tensile tests for annealed treatment samples were performed at room temperature.
Tensile tests were carried out at a strain rate of1×10-3 s-1 on an MTS-Landmark 370.25 Servohydraulic test system equipped with Series 653 high-temperature furnace over the temperature range of 25-450℃.For high-temperature test,soaking time of 30 min was given after attaining the test temperature before testing and the test temperature was controlled within±1℃.Room and elevated temperatures tensile test samples were machined into rectangular specimens,conforming to ASTM E8 M and ASTM E21-09 standards.Three samples for repeated tensile tests were cut,and the tensile strength reported in the work was averaged from three tensile tests.The microhardness was measured by Vickers hardness machine at a load of 2.94 N and a dwelling time of 10 s on mechanical polished samples based on the ASTM E92standard.
3 Results and discussion
3.1 Microstructure
Figure 1 shows SEM images of the as-milled composite powders of Al-12 vol%B4C.B4C particles are distributed uniformly and the shape of spherical Al powder remains unchanged after the process of ball milling (wet milling).It should be noted that a short ball-milling time and low ballmilling speed in liquid environment of alcohol are the keys to maintaining the shape of spherical Al powder and achieving good dispersion of B4C particles.Besides,because there is no plastic deformation during the process of ball milling (wet milling),the composite powders show excellent compression performance,which is beneficial for reaching a high relative density of～95.0%compacted by cold isostatic pressing (CIP) at 400 MPa.
Figure 2 shows fracture surfaces of as-sintered samples before hot rolling.No obvious plastic deformation along the fracture surfaces can be observed when the sintering temperatures are below 600℃from Fig.2a-c.Besides,the spherical and polyhedrons aluminum powders can clearly be observed,indicating an incomplete sintering and poor interfacial bonding of aluminum powders.It can be seen from Fig.2d that typical ductile dimples appear along the fracture surface generated by plastic deformation of the matrix.The existence of in situ Al2O3 layers on the surface of aluminum powders hinders the diffusion during sintering process.Researches
Figure 3 shows the microstructure of as-sintered (sintered at 500℃) and as-rolled samples.Uniform distributions of B4C particles are evidently observed in both assintered and as-rolled samples.In Fig.3 a,micro-pores are generally observed in the as-sintered sample.More details can be seen with the higher magnification (Fig.3b):the spherical appearance of small aluminum powders can be observed,and original spherical aluminum powders deform only subtly during pressing while they form polyhedrons.It agrees with observations of the fracture surface (Fig.2b).After hot rolling deformation (Fig.3c),the most important feature is that the number of the micro-pores in the asrolled sample largely decreases compared to that of the sample before hot rolling deformation.The bonding between the aluminum substrates as well as the boron carbide/aluminum interface is greatly improved (Fig.3d).Combining XRD and EDS analysis results (Fig.3d-e),it can be concluded that Al and B4C phases exist in Al-12vol%B4C composite,and the darker phase in Figs.1,2 and3 is B4C phase.
TEM micros true tural characterization of the as-rolled sample (sintered at 500℃) is realized (Fig.4).Figure 4a shows the nearly continuous Al2O3 layers distributed at the grain boundary with a limited fracturing of Al2O3 layers so that a nearly continuous Al2O3 network-reinforced Al-12vol%B4C composite forms.And the detailed microstructural characterization of the in situ network (Fig.4b,c)reveals that the Al2O3 network is amorphous and mostly double layered with a thickness of～10 nm.The double layered Al2O3 network is formed in situ from native amAl2O3 layers of adjacent aluminum powder which contacted during cold isostatic pressing and hot rolling deformation.The interface is Al-Al2O3-Al2O3-Al or AlAl2O3-Al.
Fig.1 SEM images of as-milled composite powders of Al-12 vol%B4C:a low magnification and b high magnification
Fig.2 SEM images for fracture surfaces of as-sintered composites after tensile testing at sintering temperatures of a 450℃,b 500℃,c 550℃and d 600℃
The similar results had been reported in bulk aluminum materials by Balog et al.
For the structural and neutron absorber materials of Al-12 vol%B4C strengthened with in situ Al2O3 network,porosity,particle distribution and interface bonding strength directly affect the mechanical properties of the composite.Uniform distribution of particles and strong interfacial bonding are important to improve the mechanical properties of the composite.Usually,high-energy ballmilling process is used to distribute the fine ceramics particles in the aluminum powders with ceramics particles embedded in aluminum powders.As the particle size of aluminum powders is smaller than that of B4C particles,ball-milling process with short time and low speed in liquid environment of alcohol achieves a good dispersion of B4C,at the same time,brings the least impact on the shape of spherical aluminum powder.Uniform distributions of the B4C particles are evidently observed in both as-sintered and rolled samples.During solid phase sintering,higher sintering temperature can effectively decrease the number of pores and improve the interfacial bonding strength.However,in this composite,sintering at temperature of below 600℃in 2 h results in incomplete sintering.It is because that the existence of in situ am-Al2O3 layers on the surface of aluminum powders hinders the diffusion during sintering process.Hot rolling deformation can effectively eliminate the defects caused by incomplete sintering.As shown in Fig.3,the hot rolling deformation significantly decreases the number of the pores and improves the interfacial bonding strength between the Al substrates as well as the boron carbide/aluminum.Before hot rolling deformation,aluminum powders with larger sizes were easily pressed into polyhedrons,in contrast,aluminum powders with smaller sizes maintain the original spherical topography.Owing to a limited shear plastic deformation of as-rolled composite,as can be seem in Fig.4,aluminum powders show only limited fracturing of native Al2O3layers.Consequently,the micro structure of Al matrix strengthened with nearly continuous am-Al2O3 network forms in situ.
Fig.3 SEM images of a as-sintered and c as-rolled Al-12 vol%B4C composites;higher magnified SEM images of b as-sintered sample and d as-rolled sample with an insert of EDS result of B4C particle;e XRD pattern of Al-12 vol%B4C composite
Fig.4 Bright-field TEM images of as-rolled Al-12 vol%B4C composite:a am-Al2O3 network;b Al-Al interface with double layered am-Al2O3network with an insert of corresponding fast Fourier transform (FFT) pattern of am-Al2O3 in,and c Al-Al interface with single layered am-Al2O3network
3.2 Mechanical properties
Figure 5 shows the density evolution of Al-12 vol%B4C composite as a function of sintering temperature both before and after hot rolling deformation.It can be seen that the density changes are not sensitive to the sintering temperature of below 600℃,while the density increases a lot at the sintering temperature of 600℃.But no matter what the sintering temperature is,the density of the composite is higher after hot rolling deformation,which indicates that hot rolling deformation can improve the density of the composite.As for aluminum metal matrix composites prepared from fine atomized aluminum powders,it is necessary to consider the presence of oxide particles during the theoretical density calculation.The related methods had been given in details by Godfrey et al.
Fig.5 Density evolution of Al-12 vol%B4C composite as a function of sintering temperature
Figure 6 illustrates the compressive strength of Al-12vol%B4C composite before and after hot rolling deformation.Before hot rolling deformation,the composites sintered below the temperature of 600℃exhibit a large deviation of the measured strength and ductility.The composite sintered at 600℃shows brittle behavior and its tensile strength increases by～70 MPa than others’.It should be noted that all sintered composites fracture before yielding.The results agree with the observations of the fracture surfaces of the sintered composites as well as density evolution.After hot rolling deformation,the strengths of all composites increase significantly.It can be seen that the tensile strength of the samples sintered at500℃is 80 and 274 MPa before and after rolling,respectively.From Figs.3 and 4,the hot rolling deformation can eliminate the micro-pores and achieve a good metallurgical interface between the Al substrates as well as boron carbide/aluminum,thus inevitably improving the tensile strength of the composite.It is worth noting that the tensile strength of the sample sintered at 600℃after rolling decreases by about 8%,compared to that of the sample sintered at 500℃.Balog et al.
Fig.6 Compressive strength of Al-12 vol%B4C composite as a function of sintering temperature
Figure 7 shows the tensile properties of the as-rolled Al-12 vol%B4C composite (sintered at 500℃) as a function of temperature.For comparison,date of the Al-25vol%B4C and monolithic AA1100 alloy fabricated using liquid mixing process are also presented in Fig.8.The unique linear property/temperature profile of Al-12vol%B4C composite can be found.Besides,Al-12vol%B4C composite exhibits significantly higher strength than AA1100 alloy and Al-25 vol%B4C.It should be mentioned here that,as for high-temperature test,soaking time of 30 min was given after attaining the test temperature before testing and the test temperature was controlled within±1℃.As can be seen,room temperature UTS of Al-12 vol%B4C composite reaches about 274 MPa,which is improved by 94.2%and 187.2%compared to those of Al-25 vol%B4C and monolithic AA1100 alloy,respectively.At elevated temperatures of 350℃(0.67T/Tm,where Tm being absolute melting point of Al matrix),the UTSs of Al-25 vol%B4C and monolithic AA1100 alloy fall to a very low strength below 30 MPa,but UTS of Al-12 vol%B4C composite is reasonably maintained at137 MPa,which is higher than the room temperature UTS of unreinforced AA1100 alloy.
Fig.7 Effect of temperature on mechanical properties of as-rolled Al-12 vol%B4C composite (sintered at 500℃)
Fig.8 Effect of temperature on mechanical properties of AA1100-O
In Al-12 vol%B4C composite strengthened with in situ Al2O3 network,the fine grain and in situ Al2O3 network are the main strengthening factors.The composite has excellent mechanical properties at both room and elevated temperatures.At elevated temperature,the efficiency of grain boundaries is enhanced by nearly continuous double layered am-Al2O3 network,which maintains the HallPetch strengthening.
The grain size has a strong influence on metal strength since the grain boundaries can hinder the dislocation movement,which can be explained by the following expres sion:
whereσ0 is the flow stress for an infinite grain size,k is a constant determining the efficiency of grain boundaries as slip barriers,and d is the average grain size.From Figs.1,3 and 4,we can see that a nearly continuous double layered am-Al2O3 network with a thickness about 10 nm forms in situ at the grain boundary.At the same time,the fine grains of the original aluminum powders are well inherited to the final composite.
At room temperature,the distribution of B4C particle and the interfacial bonding between the B4C particles and matrix also affect the mechanical properties.As can be seen from Figs.3 and 4,uniform distributions of the B4C particles and good bonding boron carbide/aluminum interface are observed in the as-rolled composite,which can effectively transfer the load from the matrix to the hard particles,and the dislocations generated by thermal mismatch contribute to the matrix strength but those strengthening effects are weak at elevated temperature.
3.3 Thermal stability of the mechanical properties
Thermal stability of the as-rolled composites of Al-12vol%B4C (sintered at 500℃) were tested with the yield strength and microhardness with holding time of 10-1000 h at 350℃in Fig.9.It is well known that material loses its hardness and yield strength with a microstructure coarsening.And the results confirm that the composite exhibits no microhardness and yield strength changes after a prolonged annealing process up to 1000 h at 350℃,suggesting that microstructure is stable up to 350℃.The excellent thermal stability is secured by the in situ amAl2O3 network that pins grain boundaries and retards the recrystallization and grain growth at elevated temperatures.
Research reveals that the am-Al2O3 network remained stable after annealing at 400℃for 24 h and phase transformation of the am-Al2O3 network to crystalline y-Al2O3particles occurred at temperatures≥450℃
From Fig.4,the microstructure of Al-12 vol%B4C composite strengthened with nearly continuous double layered am-Al2O3 network with a thickness about 10 nm forms in situ at the grain boundary.So,whether the phase transformation of the am-Al2O3 network to crystallineγ-Al2O3 particles has been completed after a prolonged annealing process up to 1000 h at 350℃,the grain and subgrain structures of the Al-12 vol%B4C composite strengthened with am-Al2O3 network would remain unchanged.
3.4 Fractography
Fracture surfaces of as-rolled Al-12 vol%B4C composite (sintered at 500℃) tested at 350℃are shown in Fig.10.As shown in Fig.10a,a large number of dimples and tearing ridges can be clearly observed on the fracture surfaces.The angle between the tensile fracture surface and the specimen cross-section is 45°.Some of fractured B4C particles exist on the fracture surface(marked by the circles) and the high magnification image of the fracture B4C particles is displayed in Fig.10b.Also,the characteristics of holes after B4C particles pull-out are observed.It is worth pointing out that cracks and micro-holes are observed in the matrix.Both the tensile strength and strain elongation decrease with temperature increasing.The composite fractures when it reaches the ultimate tensile strength after the work-hardening stage at the tested temperature of350℃,and the elongation of the composite material is reduced to 4%.Balog et al.
Fig.9 Effect of holding time at 350℃on mechanical properties of Al-12 vol%B4C composite (sintered at 500℃),tested at 25℃
Fig.10 SEM images for fracture surface of as-rolled Al-12 vol%B4C composite (sintered at 500℃) after tested at 350℃:a low magnification and b high magnification;elemental distributions of c Al,d C and e B
Hard and brittle B4C and Al2O3 particles inhibit the plastic deformation of soft matrix;thus,the fracture of composites is a combination of micro-ductile fracture and macro-brittle fracture.The fracture mechanism of Al-12vol%B4C composite could be pided into the following categories:interface de-bonding,particle brittle cracking and matrix plastic damage
4 Conclusion
Al-12 vol%B4C composite strengthened with in situ amAl2O3 network was fabricated by powder metallurgy (PM)using fine atomized aluminum powders.Sintering temperature below 600℃has little effect on mechanical properties.Hot rolling results in decreasing the number of micro-pores and improving interfacial bonding strength;thus,the density and mechanical properties are improved.A nearly continuous double layered in situ Al2O3 network with a thickness of about 10 nm and uniform distributions of B4C particles are observed in the composite.The composite has the potential to be structural and neutron absorber materials for high-temperature application,due to the attractive mechanical properties and thermal stability at elevated temperature.At 350℃,the composite has UTS=137 MPa,YS0.2=118 MPa,ε=4%.Besides,the mechanical properties of the composite remain unchanged at 350℃after the long holding periods up to 1000 h.At elevated temperature,the major strengthening mechanism is the Hall-Petch strengthening.The excellent mechanical properties and thermal stability at 350℃are secured by in situ am-Al2O3 network that strengthens the grain boundaries.The interfacial de-bonding and brittle cracking of B4C particles are the main fracture mechanisms of the composite.
参考文献
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