Fabrication And Mechanical Properties Of Al₂O₃ Nanoparticles Reinforced Aluminum Matrix Composite With Stacking Faults

In recent years,special attention has been devoted to development of light-weight structural materials driven by the requirements for weight reduction,fuel con-sumption and pollution-free in aerospace,military and transportation industries.Parti-cle reinforced aluminum matrix composites have been successfully used for structural application in these industries due to the unique combination of being light weight,possessing high strength and stiffness along with good friction resistance and fatigue resistance.Unfortunately,the trade-off between strength and ductility and sharply de-creasing of strength above 473 K limit their use in future cutting-edge application.The addition of nanoparticles to aluminum matrix can significantly improve the strength of the composite accompanying with well ductility,increasing volume frac-tion of nanoparticles in the composite has great potential to increase strength level at both room and elevated temperatures even further,However,the mechanical proper-ties and microstructure of nanoparticle reinforced aluminum matrix composites have not been well investigated owing to several tough problems arisen in fabrication pro-cess when the volume fraction of nanoparticles exceeds 4%,such as agglomeration of nanoparticles and non full densification of the composite,etc.The objective of this paper is to fabricate aluminum matrix composites reinforced with Al₂O₃ nanoparticles?>4 vol.%?using mechanical milling followed by vacuum hot pressing and to explore the structural evolution during mechanical milling,microstructures,mechanical prop-erties at room and elevated temperatures and corresponding deformation mechanism of Al₂O₃ nanoparticles reinforced aluminum matrix composites.The main contents and results included as following:?1?The morphologies and structural evolution during mechanical milling of Al₂O₃ nanoparticles reinforced pure aluminum composites were investigated.Results showed:The uniform distribution of Al₂O₃ nanoparticles within the aluminum matrix was achieved by mechanical milling;The presence of Al₂O₃ nanoparticles has great effect on accelerating milling process and decrease the particle size of the composite powders,the minor powder particle is obtained as the volume fraction of Al₂O₃ nano-particles exceeds 5%;The grain size of the Al matrix decreases rapidly in the first 5 h and leveled off at prolonged milling times,due to the incorporation of Al₂O₃ nanopar-ticles into the aluminum particles.The final grain size of the aluminum matrix de-creases as increasing volume fraction of Al₂O₃ nanoparticles.?2?The dense Al₂O₃ nanoparticles reinforced pure aluminum matrix composites were fabricated by vacuum hot pressing,and their microstructures were characterized.Results showed:For the Al-5%Al₂O₃ composite,the aluminum matrix is coarse-grained,the Al₂O₃ nanoparticles are uniformly distributed within the grain interior of the aluminum matrix,the Al₂O₃ nanoparticle and the aluminum matrix form a semi-coherent interface with the special relationship;the aluminum matrix contains a fairly large amount of stacking faults?SFs?/microtwins,but few or no full dislocation.These defects line up along two{111}planes of aluminum with an intersecting angle of⁷0^o,they are formed presumably via emission of Shockley partial dislocations.For the nanocomposites investigated here,partial dislocations were believed to have been nu-cleated at the Al/Al₂O₃ interfaces and/or Al grain boundaries,and then emitted and glide on a{111}Al plane to leave a SF in the wake.When such a SF was formed on the adjacent slip plane,a two-layer microtwin will be formed by dynamic overlapping of the SFs.This process can proceed further to form three-layer microtwin;as the vol-ume fraction of Al₂O₃ nanoparticles is 10,15 and 20%,the composites show similar microstructures:the aluminum matrix is ultrafine-grained,the Al₂O₃ naoparticle and the aluminum matrix form a incoherent interface;meanwhile,the density of the SFs/microtwins decreases as increasing the volume fraction of Al₂O₃ nanoparticles.?3?The Al-5%Al₂O₃ composite contains a fairly large amount of nano-sized strips within the aluminum matrix,which have the width with an average value of⁹.8 nm,and the length with an average value of⁷4.1 nm.The majority of strips embody large numbers of wide SFs,and a small amount of strips have a HCP lattice structure with lattice parameters:a=3.22?,c=4.63?and c/a=1.438.The formation mechanism of the strips is as following:when the density of wide SFs is high enough,wide SFs will accumulate and irregular overlapp,and then regular overlap with each other,based on minimizing both the bulk free energy and the total energy of SFs.The irregular overlapping of wide SFs on every second layer of{111}Al slip plans corre-sponds to the nano-sized aluminum strips containing several SFs,and the regular overlapping of wide SFs corresponds to the nano-sized aluminum strips with HCP lat-tice structure.?4?The compressive mechanical properties at ambient temperature of Al-5%Al₂O₃ composite were investigated.Results showed:the composite behaves supe-rior dynamic compressive properties compared to other Al-based materials,showing ultimate strength and failure-strain of 940 MPa and 9.8%,respectively.The composite exhibits weaken strain rate sensitivity at strain rate ranging from 600 to 1500/s,and negative strain rate sensitivity at strain rate ranging from 1500 to 5000/s;the high strength of the composite is attributed to load transformation mechanism and SFs/microtwins strengthening which involves SFs accumulation and impeding partial dislocation moving by Al₂O₃ nanoparticles,microtwins and nano-sized aluminum strips;the failure mode of the composite:ductile fracture at quasi-static state,brittle shear fracture at dynamic state and adiabatic shear band exists when the strain rate exceeds 1500/s.?5?The influence of volume fraction of Al₂O₃ nanoparticles on the compressive mechanical properties of the composites at ambient temperature was studied.When the volume fraction of Al₂O₃ nanoparticles increases,the strength and ductility of the composites decreases and increases,respectively.SFs/microtwins strengthening is the major strengthening mechanism,the density of SFs/microtwins decreases as increas-ing the volume fraction of Al₂O₃ nanoparticles,resulting in gradually weakening of strengthening contribution.Another reason is that the porosity of composite increases as increasing volume fraction of nanoparticles,which is deleterious to the mechanical properties of the composite.?6?The Al-5%Al₂O₃ composite exhibits remarkable high temperature strength compared to other Al-based materials ever reported.For example,it retains⁵50 MPa strength at 473 K(0.5₈)of Al),which is at least one time higher than an any other Al-based materials at the same temperature.Even at 0.72₈)?673 K?,the composite still has strength of 310 MPa,at least two times higher than other Al-based materials.The significantly high strength at elevated temperatures of the composite may be at-tributed to the following:High thermal stability of the microstructure of the composite due to coarse-grained aluminum matrix,the stable semi-coherent interface between Al₂O₃ nanoparticles and the aluminum matrix,as well as the existence of microtwins and nano-sized aluminum strips;strengthening from the SFs/microtwins via the inter-action between the SFs and impeding the partial dislocation motion.The power dissi-pation map shows that,the power dissipation by microstructure evolution is low,with a peak efficiency of 13%,in accordance with experiment observation that comparison of as-received sample with deformed sample reveals no obvious change in microstruc-tures.The instability map exhibits only one safe processing domain for hot working:in the temperature range of 523-568 K and at strain rates of 1×10^-4-2.7×10^-4/s.The failure mode of the composite:shear fracture along the 45^o direction from the com-pression axis at 473 K,and void nucleation,growth and coalescence at 673 K.