| The Mg-Li alloys have great potential for use in many industrial fields seeking lightweight due to their low density, high specific strength, high specific modulus and excellent anti-shock and electromagnetic shielding properties. However, their low strength and poor corrosion resistance limit their widespread applications. Alloying and composite strengthening using ultra-fine YAl2 particles are the feasible ways to improve their comprehensive mechanical properties. In the present thesis, the effect of the additions of Y, Nd and Gd alloying element and the cold-rolling process on the microstructure and mechanical properties were investigated. In addition, The YAl2p/MgLiAl composite prepared by stir casting was initially forged and then rolled at 200?C to different thicknesses. The microstructural evolution in the composite during warm rolling was investigated by using transmission electron microscope(TEM). The main conclusions are highlighted as follows:(1) The effects of the additions of Y, Nd and Gd alloying element, the cold-rolling and heat treatment process on the microstructure were investigated. It is found that the amount of α phase decreases in the alloy which prepared by adding intermediate alloy in the traditional way, however, the amount of α phase increases by directly adding the mature WE43 alloy. Moreover, the Y-rich phase, Nd-rich phase, Mg12 Nd phase and Mg3 Re phase are found in the LZ91-Re alloy and distribute more uniform during cold rolling. Numerous dislocation cells are formed due to dislocation tangling during cold rolling. Furthermore, It is found that the LZ91-Re alloy after solid solution treated at 390℃ for 4h yields the smallest grain size with a large number of α phases. After aged, a mount of α phases in needle shape precipitated in situ in the alloy.(2) A MgLiAl(LA143) composite reinforced with YAl2 particles was fabricated by stir casting. The composite was initially forged at 200℃ and then subjected to rolling at the same temperature to different thicknesses. The microstructural evolution in the composite during rolling was investigated by using transmission electron microscope(TEM). It is found that increasing rolling reduction is conducive to the uniform distribution and refinement of the YAl2 particles. The rolling deformation promoted the precipitation of an α phase, and the α precipitate is semi-coherent to the matrix with an orientation relationship to the β matrix as:(0002)α‖(110)β. Moreover, Increasing reduction caused the production of micro-cracks in the YAl2 particles due to their poor coordination with the matrix, but the micro-cracks were filled by the LA143 matrix grain during rolling.(3) The microstructure of the composite was investigated by using transmission electron microscope(TEM) and high resolution transmission electron microscope(HRTEM) as the rolling reduction increases to 90%. It is found that many nano-sized YAl particles with a cubic shape were found in the matrix of the composite with a high rolling reduction due to the diffusion of Y from YAl2 to the matrix, which reacted with the Al in the matrix during warm rolling. In addition, the yield strength and tensile strength of YAl2p/LA143 composite are signi?cantly promoted by 31% and 23.5%, respectively, compared with matrix alloy, while a good ductility property is kept. Moreover, the fracture morphology of the composite and matrix alloy show a ductile type of fracture. |
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