Study On Mechanical Properties Of B₄C/Al Composites By Pressureless Infiltration

The boron carbide particle reinforced aluminum matrix composite(B4C/Al)has the advantages of low density,high specific strength,high wear resistance and excellent neutron absorption performance.There are many preparation processes for B4C/Al composites.Pressureless infiltration is one of the ideal methods due to its simple process,low cost,easy control of particle distribution and easy access to large-scale.In this paper,B4C particles with average particle size of 2 ?m and 38 ?m were used to prepare B4C performs with particle-size distributions being 100%38?m,20%2.0?m+80%38?m and 100%2.O?m B4C(labeled as S,M,L),respectively.Correspondingly,three composites(denoted as 6S,6M and 6L)were prepared with 6061A1 being the matrix to study the effect of B4C particle size on the micro structure and mechanical properties of the composite.Then,2M,5M,6M and 7M B4C/Al composites were prepared with M performs being the reinforcement and 2024A1,5056A1,6061A1 and 7075A1 being the matrix,respectively.The effect of aluminum alloy composition on the microstructure and mechanical properties of the composites were investigated.Based on the mass and geometry of the preforms,the volume fractions of S,M and L preforms are calculated as 55.83%,67.78%and 53.18%,respectively.Firstly,the effect of B4C particle size on the microstructure and mechanical properties of the composites were studied.The results show that all of 6S,6M and 6L composites have the interfacial reaction in varying degrees,which is related to the volume fraction of small B4C particles.All of three B4C/6061A1 composites present the same phase constitution,including B4C,Al,Al3BC,AlB2 and Fe-Mn-rich phases.When the reinforcing phase is completely large particle,B4C particles are uniformly distributed in the composite.The degree of the interface reaction is weak,with the random distribution of the interface products,AlB2 and Al3BC.The hardness and flexural strength of the corresponding composite are 23.2 HRC and 406 MPa,respectively.Fine B4C particle possesses higher actual contact area with molten aluminum alloy than that of the large particle due to higher specific area of the fine particle resulting in the severe interfacial reaction.Therefore,with 20%of B4C fine particles being involved,the consumption of the aluminum matrix increases in the composite,wherein B4C large particle also present a uniform distribution characteristic.Within the region between the large particles,the remaining fine particles are uniformly distributed in the interface products.Due to the increasing fraction of the initial B4C particles and the interfacial products,the hardness of the composite increases to 40.02 HRC,and the flexural strength increases slightly to 425 MPa with a decreasing strain.When the reinforcing phase is completely fine particles,aluminum is consumed in a large amount by the severe interfacial reaction in the composite,wherein A13BC and AlB2 become the dominant phases,and the remaining B4C particles are uniformly distributed in the ceramic phase matrix.The hardness of the composite improves to 56.8 HRC.However,the flexural strength of the composite drops to 248 MPa,which is due to the presence of the micropore defects in 6S composites prepared by S preform.Secondly,the effects of aluminum alloy composition on the microstructure and mechanical properties of the composites were investigated.The results show that 2M,5M,6M and 7M composites contain Al,B4C,Al3BC and AlB2 phases.In addition,2M,5M and 7M composites also contain AlB10.For all of the composites,the severe interfacial reaction occurred in the aggregation areas of the small particles,with the formation of A13BC and AlB2.AlB10 is mainly distributed around large B4C particles in 2M,5M and 7M composites and is integrated with large B4C particles.The hardness of the four composites decreased in the sequence 2M(45.7 HRC),7M(43.1 HRC),5M(41.8 HRC)and 6M(40.02 HRC).The prepared B4C/Al composites are more close to be the ceramic materials due to the occurrence of the severe interfacial reaction.The micropores were observed in the 2M,5M and 7M composites,resulting in the decrease of the flexural strength and strain of the corresponding composites.Accordingly,7M composite possesses the lowest flexural strength,being about 296 MPa.Conversely,the flexural strength and strain of the 6M composite are the highest,being about 425 MPa and 0.183%,respectively.This could be mainly attributed to the evenly distribution of the B4C particles and the highest fraction of the residual aluminum alloy in the 6M composite.