Preparation And Properties Of Nano-Al₂O₃ Reinforced Aluminum Matrix Composites By In Situ Method

In the study of the nano-alumina reinforced aluminum matrix composites,the Al2O3 reinforced particles prepared by in-situ method in the matrix has attracted the wide attention of the scholars all over the world due to its fine particles,uniform distribution,good interface with the matrix and excellent thermal stability.It also effectively avoids a series of problems existing in the ex-situ method.However,at present,there are still a series of unbroken problems in the preparation of nano-Al2O3 reinforced AMCs by in-situ method,which restrict the development of AMCs:① the in-situ reaction temperature is high,which is easy leads to incomplete reaction,and the reaction process is prone to produce brittle phase which is harmful to the matrix;② the content of nano-Al2O3 formed in-situ is uncontrollable,which leads to poor repeatability of mechanical properties;③ the natural oxidation process is performed during the preparation of aluminum powder.The densitometric alumina film prevents the substrate from further oxidation,which makes it difficult to increase the Al2O3 content in Al matrix.According to the characteristics of a dense nano-oxide film formed on the surface of aluminum can protect the internal metal from further oxidation.This paper proposes an innovative idea to increase the content of nano-Al2O3 by using a combination of high energy ball mill and staged oxidation.The purpose of this method is to control the natural oxidation process in the preparation of composite powder to achieve the controllable preparation of nano-Al2O3 in the matrix.Based on this,a combination of spark plasma sintering(SPS)technology and hot extrusion process was used to prepare AMCs with controllable reinforcement content.Through the characterization and analysis of the microstructure,interfacial bonding characteristics,room temperature mechanical properties and thermal stability of AMCs,the effect of in-situ y-Al2O3 content on the structure and properties of AMC was discussed,and the strengthening mechanism of nano-γ-Al2O3 reinforced AMCs was clarified.The experimental results show that the method used in this paper can significantly increase the γ-Al2O3 content in the matrix by controlling the oxidation process during the preparation of the composite powder,and achieve the controllable preparation of nano-Al2O3.The study found that the content of γ-Al2O3 increased with the increase of ball milling time and the number of staged oxidations.Under the condition of ball milling for 24 h and aeration for six times,the content of γ-Al2O3 in AMCs reached 7.9 vol.%.Through the observation of the microstructure of AMCs,it was found that nano-γ-Al2O3 generated in-situ was uniformly distributed in the form of short whiskers in the matrix.And the uniformly distributed nano-γ-Al2O3 can significantly refine the grain size of the matrix.With the increase of the amount of nano-γ-Al2O3 introduced,the grain size of the matrix is significantly reduced,and a good dispersion strengthening effect can be achieved.Combined with the performance test results of AMCs,with the increase of γ-Al2O3 content in the matrix,the mechanical properties of AMCs were significantly improved.When theγ-Al2O3 content reached 7.9 vol.%,the tensile strength of AMCs reached a maximum of 385 MPa,3.26 times of pure Al,and the yield strength improved to 2.9 times of pure Al,and the hardness of AMCs also showed the same trend,3.1 times of pure aluminum.Although the elongation gradually decreases while the strength of the composite material increases significantly,the elongation(8%)and electrical conductivity(40%IACS)still maintain a good performance compared to aluminum alloys and other AMCs at the maximum tensile strength.Through analysis of the interface bonding of the AMCs,it was found that the in-situ formed nano-γ-Al2O3 and Al matrix meet the coherent correspondence conditions.The interface bonding between the two phase is good,and the good interface bonding can play a positive role in load transfer during the deformation of AMCs,enhancing the strength of AMCs.The research on the thermal stability of γ-Al2O3/Al composites found that the composites still have excellent mechanical properties when annealed at 500℃ for 24 h.The yield strength,tensile strength and elongation still maintain at 305 MPa,382 MPa and 6%,respectively.When the annealing temperature is increased to 600℃,the strength of the material decreases slightly and the tensile strength decreases to 354 MPa.It is due to the release of residual stress inside the AMCs under long-term high-temperature annealing conditions.The main strengthening mechanisms of γ-Al2O3 enhanced AMCs include grain refinement strengthening,Orowan strengthening,load transfer and thermal mismatch strengthening.Among them,the Orowan strengthening contribution caused by the dispersion of small nano-reinforcement particles in the matrix is the most significant.The dispersed nano-γ-Al2O3 can play a significant pinning role during the annealing process,effectively inhibits the recrystallization process of AMCs during the annealing process,hinder the growth of the matrix grains,thus making the composite material can maintain good mechanical properties even at higher annealing temperatures and play a significant in strengthening the matrix.