Research on the In-situ Fabrication of Network AlN_p and Its Strengthening Mechanisms to the Al Matrix at High Temperatures

Author:Ma Xia

Supervisor:liu xiang fa


Degree Year:2019





In this work,a new liquid-solid reaction method was proposed to in-situ synthesize the AlNp in A1 matrix.Through the adjustment of fabrication parameters,the control of AlNp size and distribution was fulfilled and the network structure of nano AlNp in Al matrix was constructed.A novel kind of Al composites with high strength and good high temperature resistance reinforced by network AINP was fabricated.The strengthening effect of network AlNp at room and high temperatures were both investigated in this work.The maximum tensile strength of the composites at 350℃ can reach up to 190 MPa.The strengthening mechanisms of the nano network AlNp on the Al matrix were unveiled according to the investigations on the microstructural evolution of the composites during tensile process at 350℃.Finally,the influences of microalloying and cryogenic treatment on the microstructure of the composites were investigated to further to optimize the performance of network AlNp/Al composites.The main research contents are as follows:(1)In-situ fabrication of AlNp and the formation of network AlNpA new liquid-solid reaction method was used to in situ fabricate the AlNp.By investigating the influnces of the pretreatment and alloying elements(Fe,Cu,Mn and Si)on the synthesis of AlNp,the control of AlNp size and distribution was fulfilled.The size of AlNp can be tailored from 20 nm to 5 μm,while their distribution can be homogenous,linear or network.The mechanical property test indicates that the nano A1NP with a network structure plays a better strengthening effect.The 3D network structure of AlNp was formed in the Al matrix and was systematically characterized by X-ray tomography and APT techniques.The connection of the network AlNp was also revealed by TEM.It is indicated that the 3D AlNp network is formed by a topology of nano A1NP chain,which is composed with some connected nano AlNp.The connection between AlN particles was revealed to be a(1013)AIN twinning bonding.Besides,the interfaces between A1NP and Al matrix are semi-coherent.(2)Mechanical properties and strengthening mechanisms of the AlNp/Al compositesNetwork AlNp/Al composites with a varying fraction from 4.1%to 16.4%were fabricated in this work and the microstructures were observed.Nano AINP with a network distribution is observed in the A1 matrix.The space between AINP nano chain decreases and the continunity of the network increases with the increasing fraction of AlNp.With the addition of AlNp,theμ-Al was refined to smaller than 1 μm,with a prefered orientation:{110}<111>.Mechanical property tests on the AlNp/Al composites at room and high temperature were conducted in this work.The tests indicate that the network AlNp presents a better reinforcing effect as compared with homogenous and linear distributed AlNp.The tensile strengths of 16.4AlNp/Al composites at room temperature and 350℃ are 520 MPa and 190 MPa,respectively.The microstructural evolution the AlNp/Al composites during the tensile deformation process at 350℃ was investigated in this to reveal the strengthening mechanism of the network AlNp.During the deformation,the texture of α-Al changes from a sheet texture to a fiber texture;the low angle grainboundaries(LGBs)decreased from 52.3%to 51%and the strain field increased at the early age of deformation and then decreased.The strengthening effects of the network AlNp can be analysized in two aspects:for the AlNp inside the grains,the twinning bonded network AlNp can impede the movement of dislocations,promote the formation of subgrains and thereby strengthening the A1 grains;for the AINP on the grain boundaries,the network AlNp can pin the grain boundaries and thereby strengthening the grain boundaries and avoiding the intergranular fracture at high temperature.(3)Optimizing the performance of AlNp/Al composites by microalloyingInfluence of microalloying elements(Cu,Si,Mn)on the microstructure and high temperature mechanical properties of 8.2AlNp/Al-0.3Fe composites were investigated in this work,in order to optimize the mechanical performance of the composites.The influnces of microalloying elements on the mechanical properties at room and high temperature are fifferent.For the mechanical properties at room temperature,the addition of Cu shows the highest strengh,while the addition of Si and Mn can both improve the strength and ductility.For the tensile properties at 350℃,the addition of Mn shows the best strengthening performance with a tensile strength of 135 MPa.The strengthening sequence of these three elements on the 8.2AlNp/Al-0.3Fe composites are Cu>Mn>Si at room temperature and Mn>Cu>Si at 350℃.Investigations reveal that the different influences of the microalloying elements on the high temperature properties of the composites are mainly related to the morphology of the Al-Fe intemeatlic compound and the segregation of element on the interface.The addition of Cu or Si will increased the size of needle-shaped Al3Fe and they are more likely to segregate on the AINp-Al interface;the addition of Mn can change the needle-shaped Al3Fe to the block Al3(Fe,Mn)with size smaller than 5 μm and there is no element segregation on the interface.In addition,Mn can delay the dynamic recrystallization and contribute to the stability of the composites at high temperature.As a result,the addition of Mn can improve the high temperature tensile properties of the network AlNp/Al-0.3Fe composites.(4)Optimizing the performance of AlNp/Al composites by cryogenic treatmentThe influences of water quenching and cryogenic treatment on the high temperature properties of the composites when cooling from 530℃ were investigated in this work.A new method to improve the ductility of the AlNp/Al composites by cryogenic treatment was proposed.The investigation indicates that the ductility of the composites after cryogenic treatment increase from 14%to 25%with just a small sacrifice of the strength.The microstructures of the composites after water quenching and cryogenic treatment were characterized and the mechanism of the improved ductility was unveiled.It is found that the density of dislocations inside the material is increased significantly.The dislocations would pile up when they slip to the AlNp network.With the increase of the piled dislocations,they tangled with each other and form the dislocation cell and finally form the subgrains.The subgrains can effectively disperse the plastic deformation and improve the uniform deformation of the composites,and thereby improving the ductility.