The Study On Microstructures Of Interparticle Boundaries In Al And Al Alloys Prepared By Powder Metallurgy And Their Effecfs On The Mechanical Properties

Aluminum and aluminum alloys have become one of the most widely used non-ferrous metal materials due to their advantages such as light weight,high specific strength,and good ductility.At present,high-performance aluminum and aluminum alloys are widely used in energy-saving emission reduction,aerospace,automotive and other fields,and play an important role in the development of the national economy.Powder metallurgy can effectively reduce component segregation and refine the structure,which is one of the most promising methods for preparing high-performance aluminum and aluminum alloys.However,the oxide film on the surface of aluminum and aluminum alloy powder is difficult to dissolve and decompose during the sintering process,and it will hinder the diffusion of atoms,cause incomplete grain boundary(GBs)formed between the powder particles,and leave a lot of micropores and oxide films there,leading to the formation of interparticle boundary(IPB).The IPB of aluminum and aluminum alloy fabricated by powder metallurgy can easily become a preferential area for crack nucleation and expansion,which will seriously damage the mechanical properties of the material.In addition,the IPB is an extremely active area during sintering.The growth and precipitation behavior of grains and precipitates nearby must be different from those inside the powder particles,which has a large impact on the IPB structure and mechanical properties of the material.In this thesis,Spark Plasma Sintering(SPS)and SPS+Hot Extrusion(HE)were used to the thermomechanical consolidation of the Al powders with amorphous film and?-Al₂O₃film,nanocrystalline Al powder and Al-4.5wt.%Cu powder.By studying the evolution of the oxide film on the surface of the powders,the recrystallization and growth of the grains and the aging precipitation behavior of the nano-precipitated phase in IPB region during the thermomechanical consolidation process,the mechanism of the microstructure and structure evolution of the IPB was clarified,and the relationship between the IPB structure and mechanical properties was established.There are two kinds of oxide films on the surface of aluminum powder:amorphous oxide film(am-Al₂O₃)and?-Al₂O₃ film.For the am-Al₂O₃ film,the am-Al₂O₃ film underwent the phase transformation to?-Al₂O₃during the SPS process,and was broken into?-Al₂O₃ nanoparticles.At the same time,the exposed fresh metal surfaces combined with each other to form an Al-Al interface(GB).The final IPB is actually GBs dispersed with nano-oxide particles.During the tensile testing process,the nanopores are easy to nucleate at this kind of IPB,but it has little effect on the mechanical properties of the material,and the elongation can be as high as 53.3%.For the?-Al₂O₃ film,it did not undergo the phase transformation during the SPS process and will not be broken.Under the action of local high temperature on the powder surface,partial atomic bonding was achieved between the?-Al₂O₃ films.The final IPB is actually microporous continuous?-Al₂O₃ layer.This kind of IPB is a fast channel for crack nucleation and propagation,which can cause the material to fracture at a very low elongation of 3.8%and cause anisotropy of the mechanical properties of the SPS sample in the transverse and longitudinal directions,the plastic anisotropy factor can be as high as 2.76.SPS+HE can eliminate nanopores at the IPB,promote the continuous?-Al₂O₃ layer to break into nanoparticles and create a large number of Al-Al interfaces,which can improve the elongation of the material to 71.0%and eliminate the anisotropy of mechanical properties in the transverse and longitudinal directions,the plastic anisotropy factor was reduced to 1.00.Nanocrystalline aluminum powder was fabricated by high-energy ball milling of the aluminum powder,followed by thermomechanical consolidation of SPS to obtain ultrafine grained(UFG)aluminum.During the high-energy ball milling,the oxide film on the surface of the aluminum powder was broken and uniformly distributed inside the powder,which will not affect the structure of the IPB in the UFG aluminum.During the SPS process,local high temperature and local stress concentration existed on the powder surface,which resulted in rapid growth of some grains on the surface of the powder.The average diameter and area percentage of coarse grains are 1.8?m and 3.6%,respectively.The final IPB includes GBs of ultrafine grains,GBs of coarse grains and residual nanopores.Nanopores are the source of cracks,causing UFG aluminum to fracture along the IPB before yielding.SPS+HE and SPS+600?C annealing can eliminate nanopores at the IPB and promote the formation of a large number of coarse grains near the original IPB.The average diameter is 2.0?m and 2.2?m,and the area percentage is 10.3%and 54.6%,respectively.Coarse grains formed near the IPB can reduce the overall strength of the material,but can also reduce the flow stress and stress concentration at the IPB,so that the IPB is no longer a priority area for crack nucleation and expansion,and can generate and store a large number of dislocations,the elongation of the material is increased to 12.7%and 5.6%,respectively.During the SPS process of Al-4.5wt.%Cu powder,the oxide film on the surface of the powder was broken into Al₂O₃ nanoparticles,the Cu Al₂phase preferentially nucleated and precipitated at the IPB.The final IPB is actually the GB containing Al₂O₃ nanoparticles,Cu Al₂ phase and residual nanopores.Alumina particles,Cu Al₂ phase,and nanopores cause cracks to initiate and propagate at the IPB,reducing the plasticity of the material,and the elongation is only 10.0%.During the heat treatment,coarse Cu Al₂phases with a diameter of 150-600 nm preferentially precipitated at the IPB,and the precipitation free zone with a width of 40-60 nm was formed near the IPB,which seriously damaged the mechanical properties of the material,and the elongation was reduced to 6.0%.SPS+HE can eliminate nanopores at the IPB,increase the proportion of the Al-Al interface at the IPB,reduce the area percentage and width of the IPB perpendicular to the extrusion direction,and make the second phases at the IPB redistributed along the extrusion direction,which can strongly inhibit the initiation and propagation of cracks at the IPB,and greatly increase the elongation of the material to 21.6%.In summary,this thesis reveals the mechanism of phase transformation-induced fracture and fragmentation of the oxide film on the surface of aluminum powder during the SPS process:the amorphous alumina film on the surface of the aluminum powder undergoes a phase transformation during the sintering process and is converted into a?-Al₂O₃film,the oxide film shrinks and breaks during the phase transformation due to the higher density of?-Al₂O₃ compared with amorphous alumina.The precipitation behavior of the second phase at the IPB region and its influence on the mechanical properties of Al-4.5wt.%Cu alloy is clarified,it is found that the width of the precipitation free zone near the IPB is 40-60 nm,which is twice of that near the GB.The effect of thermomechanical deformation on the microstructure evolution of the IPB is illustrated:the hot extrusion promotes the disappearance of nanopores at the IPB,induces coarse grain growing and elongating along the extrusion direction near the IPB.These results can provide theoretical basis and useful reference for the IPB structure design and mechanical properties improvement of aluminum and aluminum alloys prepared by powder metallurgy.