Research On Preparation And High Temperature Properties Of In-situ Synthesized High Si Mg-Zn-Si Matrix Composites

Magnesium matrix composites with lower density, higher specific strength,better resistant creep and other properties are widely applied in modern industries, such as automobile, communication, aeronautics, astronautics and so on, in which research on in-situ synthesized high volume fraction Mg₂Si reinforced Mg matrix composites is interested recently. Firstly,the present status and progress of in-situ synthesized and external reinforced magnesium matrix composites were reviewed. Then, the modification mechanisms of primary and eutectic Mg₂Si in in-situ synthesized high Si Mg-Zn-Si matrix composites were investigated by single element and multi-element alloying as well as high energy ultrasonic vibration. Furthermore, the combination preparation technology by alloying and high energy ultrasonic vibration, solidification characteristics, heat treatment, mechanical properties (room temperature and high temperature) and high temperature creep mechanism were systematically studied. The modification mechanisms, microstructure, interfacial microstructure and mechanical properties of the composites were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transition electron microscope (TEM), X-ray diffraction spectroscopy (XRD), energy dispersive spectroscopy (EDS), image processing software and electronic creep testing machine and theoretical calculation in detail in this thesis.The single element alloying modification results show that Ca and Ba could both modify the coarse primary dendrites Mg₂Si (ï¹¥100μm) and Chinese eutectic Mg₂Si to fine polyhedron particles (< 20μm). The size of primary/eutectic Mg₂Si and the amount of eutectic Mg₂Si decreased at first and then increase with increase of Ca or Ba content. The concentrating distribution of partial Ca or Ba atoms on the front zone of growing Mg₂Si caused"poisoning effect", and Ba alloying also resulted in the formation of tiny particles BaMg₂Si₂ which acted as the heterogeneous nuclei of primary Mg₂Si. But excessive Ca or Ba addition, which formed coarse CaMgSi or needle-like Ba₂Mg₃Si₄, caused over-modification.Combination alloying of Ca+Sb or Ba+Sb resulted in better modification effects. The size of Mg₂Si decreased with increase of alloying element contents. The size of primary Mg₂Si was modified as about 15μm particles in the composites with optimal binary alloying as 0.4%Ca+1.0%Sb or 1.0%Ba+1.0%Sb. The refining mechanism of binary alloying was the formation of CaMg2Sb₂ and Ba2Sb, which could be heterogeneous nuclei of the primary Mg₂Si. Multi-alloying of Ca, Ba, Sb and Sn resulted in further refinement of Mg₂Si by preferential formation of Ba2Sb,simultaneously Ba and Ca poisoned Mg₂Si growth.The effects of high energy ultrasonic vibration could transfer dendritic Mg₂Si to polyhedron particles, and facilitate their refinement and dispersion. Primary Mg₂Si became gradually smaller and spherical with the increasing ultrasonic vibration duration and power. With increasing vibration processing temperature, the size of primary Mg₂Si decreases at first and then increases. The homogeneous dispersion of primary Mg₂Si in the matrix resulted from capillarity effect and acoustic streaming shock wave enhanced by high intensity ultrasonic. The ultrasonic vibration altered the solidification conditions of Mg₂Si and promoted melt flow impact on them, which changed morphology of Mg₂Si from dendrite to particulate, and particularly restrained Mg₂Si growth rate. The results of T6 and T4 heat treatment show that eutectic Mg₂Si could be dissolved partially and spheroidized during solid-solution treatment. MgZn', MgZn₂ and Mg2Sn were precipitated in aging process. The optimal T6 heat treatment parameters were determined as solution treating at 400℃for 20 h, chilled in water and then aging at 200℃for 10 h.The results of tensile tests at room temperature and high temperature indicated that the tensile strength and elongation of the composites could be improved due to alloying or ultrasonic vibration processing. With increasing alloying element content, the tensile properties of single and binary element alloying composites increased at first and then decreased. As Mg₂Si modified to particles, the tensile fractures of the composites changed from quasi-cleavage fracture to mixed fracture with large amounts of dimples at room temperature. The tensile strength and elongation of ultrasonic vibrated composites increased with ultrasonic duration and power, and increased at first and then declined with rising vibration temperature. According to the microstructure and tensile properties of the composites, the combination of alloying component was optimized as 0.4%Ca+1.0%Ba+0.75%Sb+3.0%Sn, and the ultrasonic parameters were optimized as duration 60 s, temperature 680℃and power 0.6 kW.The microstructure and tensile properties of the composites, which were prepared by an alloying and vibration combination fabrication technique with optimal parameters of four element alloying and the ultrasonic, were further improved. The distribution of Mg₂Si particles in the composites were uniformed and fine (size about 10μm). The tensile strength of T6 treatment composites at room temperature and 150℃were determined as 211 MPa and 185 MPa, and elongation were 7.3 % and 12 % respectively. The tensile strengths were increased as 220%,208%,235% and 176%, compared with as-cast ZS64.The results of high temperature creep test reveal that Mg₂Si/Mg-Zn-Si composite, prepared by optimal alloying composition and ultrasonic vibration parameters, were of better creep resistance. The steady creep rate of T4-processed composites, mentioned above, at 200℃and 50 MPa was tested as 9.68×10-9 s^-1, and its creep fracture was observed as ductile fracture. Dynamic precipitation process during creep test was determined as: fine needle-likeβ1'(MgZn') phase gradually transformed to coarse MgZn2; Fine Mg2Sn particles also transformed to spherical. The ranges of strain hardening index (n) and creep activation energy (Q) of the composite were determined as 3.9⁵ and 88⁹3 kJ/mol at 150²25℃in tensile stress 30⁹0 MPa. Dislocation-climbing mechanism was revealed in creep process.