In-situ Synthesis Of ?-Al₂O_3W/Al Composites By Powder Metallurgy And Their Mechanical Properties

Aluminum metal matrix composites?AMCs?emerged from the perpetual need for lighter weight,higher performance components in aerospace,aircraft and automotive industries.The common discontinuous reinforcements for AMCs include particulates,whiskers or short fibers,and platelets.Compared with other ceramic reinforcements,whiskers can slow down the degradation of ductility of the matrix,and improve the strength,which make the composites a promising substitution for aluminum alloys.According to the literature,?-Al₂O₃ has the same crystal structure as Al,and is considered to be a suitable reinforcement in Al matrix.In-situ synthesis was taken extensively attention,which can obtain cleaner interface,and enhance the composite material with more uniform phase distribution.In this work,H₃BO₃ powders?or B₂O₃?and pure Al powders were used as starting materials.Ball milling pretreatment combined with cold-pressed or hot-pressed method were performed to prepare the in-situ?-Al₂O_3W/Al composite.The thermodynamic conductions for the reaction were revealed.The effects of ball-milling on the growth of in-situ whisker were discussed.The interfacial bounding between whisker and Al was systematically examined.Moreover,the effects of the whisker content on the properties of the composites were also investigated,and composition proportions of the starting materials were optimized.The results indicate that?-Al₂O₃ whiskers,with diameters ranging from 5 nm to50 nm,were successfully in-situ synthesized by ball milling-cold pressing-hot extrusion method.The whiskers,possessing aspect ratio 10-20,are uniformly distributed and firmly bonded with the Al matrix,and penetrate into adjacent Al particles to form a network-like distribution,which bear and transfer extrinsic loads effectively.When the whisker content is 10 wt%,a tensile strength of 217 MPa was obtained.The strength is enhanced by 60.7%,and the ductility still maintains at11.0%.The?-Al₂O₃ whiskers,with diameters ranging from 20 nm to 500 nm,were successfully in-situ synthesized by ball milling-hot pressed sintering-hot extrusion method.The aspect ratio of whiskers is 10-20.When the whisker content is 15 wt%,the tensile strength reaches 261 MPa.In this case,the strength is enhanced by 89.1%,the ductility still keeps at 10.1%.The composite shows enhanced wear resistance and the wear width is only 30.4%of that of pure Al sample.The interfacial bounding between the in-situ whisker and the matrix is proven to be perfect.Owing to the advantage of in-situ synthesis,the interface between the whisker and the Al matrix is semi-coherent,and the mismatch is 0.69%.The interface is clean,smooth and free from impurities.That is why in-situ?-Al₂O₃whiskers can enhance Al matrix composite effectively.After hot extrusion,the tensile properties of the composites are significantly improved,which is attributed to the enhancement of the compactness.During extrusion,dynamic recrystallization occurs,a lot of sub-grains grow,dislocation density increases,and the composites experience dislocation strengthening.Additionally,the whiskers are orientated along the extrusion direction,which is beneficial to transferring load.