Research On The Preparation And High Temperature Creep Properties Of In Situ Particulate Reinforced A357 Matrix Composites

A357 aluminum alloy, which is a typical cast aluminum alloy, has been widely applied in automobile, aviation and military areas owing to its excellent castability of cast aluminum alloy. Nevertheless, the inferior creep properties of the aluminum alloy have limited the application in extreme environment such as high temperature and high load condition. Studies show that the dispersive particles induced into the aluminum can significantly improved the creep properties of alloy. Therefore, in-situ particles reinforced A357 matrix composites have been studied in the paper, and microstructure, room temperature mechanical properties and creep properties of high temperature of the composites were discussed.In situ A357 matrix composites from two reaction systems of A357-Na₂B₄O₇-K₂ZrF₆ and A357-KBF₄-K₂ZrF₆ were compared. Phases, microstructure and features of the in situ reinforcements were investigated by OM, XRD, SEM and TEM methods. Results show that as the frequency, electric current and strength of electromagnetic were 5Hz, 150 A and 0.023 T, the in situ reinforcements could be fabricated in both systems. However, the majority of the reinforcements of composites from A357-Na₂B₄O₇-K₂ZrF₆ was long-stripped Al3 Zr whose size was about 10⁴0μm. As volume fraction increased to 2%, ZrB2 reinforcements started to be synthesized with a small fraction and the size of the hexagon ZrB2 reinforcements was approximately 0.5¹μm; meanwhile, as volume fraction of the reinforcements rose from system A357-KBF₄-K₂ZrF₆, in situ system Zr B2 reinforcements gradually dispersed uniformly, whose size is about 100 nm and a part of them could measure 50nm; in addition, A large number of tiny ZrB2 reinforcements could act as nucleus to refine α-Al grains of the matrix; researches on fabrication show that as electromagnetic field frequency and strength increased to 14 Hz and 0.064 T and the electric current was 150 A, the number of Zr B2 reinforcements from system A357Al-Na₂B₄O₇-K₂ZrF₆ increased obviously, whose size decreased to about 200³00nm, while the number of Al3 Zr reinforcements exhibited an opposite trend, whose size reduced to approximately 10μm; dispersion of Zr B2 reinforcements was found more uniform.Tensile tests reveal that as electromagnetic field strength was 0.023 T, the tensile strength of in situ（Al3Zr+Zr B2）p/A357 matrix composites from system A357-Na₂B₄O₇-K₂ZrF₆ increased with particles volume fraction rising and as in situ particles volume fraction reached 2%, tensile strength got the maximum value, which was 280 MPa and it was 1.04 times of the tensile strength of the matrix; meanwhile, the tensile strength of in situ ZrB2np/A357 matrix composites from system A357-KBF₄-K₂ZrF₆ rose with in situ particle content increasing, in situ particles volume fraction reached 2%, tensile strength of the composites got 347 MPa and it was 1.23 times of the matrix; in addition, as electromagnetic field strength increased to 0.064 T, tensile strength of both in situ（Al₃Zr+ZrB₂）p/A357 matrix composites and in situ ZrB2np/A357 matrix composites increased to some extent and the tensile strength was 298 MPa and 347 MPa, respectively and they were 1.14 and 1.32 times of the matrix.Creep tests of the composites at high temperature exhibit that creep properties of the in situ ZrB2np/A357 matrix composites could be affected by the reinforcement content, temperature and applied stress. And as volume fraction of the reinforcement rose, the steady creep rate, apparent stress exponent and creep activation energy of the composites increased from 7.27×10-6s-1, 6.01 and 93KJ/mol to 1.29×10-7s-1, 14.96, and 152KJ/mol, respectively; steady creep rate of the composites rose with temperature or applied stress rising; a linear regression was carried out of Inε-Inσ and found out that the true stress exponent was 5, which meant the creep of the composites was controlled by dislocation climb and creep deformation was controlled by secondary particle mechanism. In addition, the threshold stress decreased from 15.57 MPa to 1.84 MPa while the temperature increased from 523 K to 573K; fracture analysis showed that the rupture characterization of the composites was ductile rupture.