Study On The Crystallization Behavior Of Mg-based Bulk Amorphous Alloy And Preparation Of Amorphous Matrix Composites

Materials can be classified into distinct species: crystal and amorphous body due to the ranging mode of atoms. Compared with crystalline alloys, amorphous alloys have many unique physical and chemical properties because of the short-range orders and long-range random packing mode of the atoms. The investigation on such bulk amorphous alloys has been the foreland of current domain of international materials. The Mg-based amorphous alloys and amorphous composites are prepared by a method of melt casting into copper mould in the air. And some of the bulk Mg-based amorphous alloys are heat-treated with isothermal annealing technics. All of the material are studied by the method of DSC, XRD, SEM, OM and so on.In this thesis, the effects of Zn on the glass-forming ability, crystallization behavior and melting behavior of Mg₆₅Cu_25-xZn_xY₁₀(x=3, 5, 7) alloys were studied first. With the x=5, glass-forming ability of the alloy was the best, Metallic glass strip with a thickness of 3.5 mm by the copper wedge mould was obtained, with the critical cooling rate 81.6K/s. At the same time, the glass transition temperature and the onset temperature of primary crystallization were the lowest, but the T_rg was the greatest, about 0.583. For x=3, the alloy has the best thermal stability, with the largest supercooled liquid region about 63.8K. Compared with other glass, the quaternary Mg₆₅Cu₂₀Zn₅Y₁₀ glass exhibits a more complex crystalliztion process, performed through four steps. But the melting, curve is a simple endothermic peak.The glass-forming ability is improved obviously, with the 5%(at) Y of Mg₆₅Cu_25-xZn_xY₁₀(x=3, 5) alloys substituted for Nd. Metallic glass strip with thickness of 4.5mm and 4mm can be obtained, with the critical cooling rate about 49.4K/s and 62.5K/s. The crystalliztion behavior of Mg₆₅Cu₂₂Zn₃Y₅ Nd₅ amorphous alloy shows a three-step process, and the metling process exhibits a narrow endothermic peak, about 17.6℃. Compared with the quaternary Mg₆₅Cu₂₀Zn₅Y₁₀ amorphous alloy, the glass transition temperature and the onset crystallization temperature of Mg₆₅Cu₂₂Zn₃Y₅Nd₅ amorhous alloy increase. With the isothermal treatment, the Mg₆₅Cu₂₂Zn₃Y₅ Nd₅ alloy transits from the amorphous state to the crystal state, and the sharp crystal peaks superimose on the simple amorphous peak. At the 448K, isothermal treated for 30min, the alloy has the largest brittleness; Treated at a higher temperature 468K, the microstructure of the alloy which has a lower brittleness is bigger compared with the others; Annealed the alloy step by step(448K, 15min+468K, 15min), the microstructure of the alloy changed more homogeneous and smaller. And the average microhardness is 268.2Hv, which exceed about 41Hv than the metallic glass.Mg₆₅Cu₂₃Al₂Y₁₀ and Mg₆₅Cu₂₀Al₅Y₁₀ metallic glass matrix composites is prepared in this subject. With the addition of Al, the sharp peaks from reinforced crystal-phases superimposed on the broad, diffuse scattering maxima from amorphous matrix. The glass transitting-stag eand crystallization exothermic peaks in DSC graph show that the matrix were glass-state. Compared with the metallic glass, the glass transition temperature T_g and the onset temperature of primary crystallizationT_xl of composites were reduced, with the supercooled liquid region narrowing. And the maxtrix of composites show a more complex crystalliztion process. Although the compositional difference of the alloys is very samll they show essentially different microstructure. For Mg₆₅Cu₂₃Al₂Y₁₀ alloy, needle-shape phases can be observed, but grain structure(about 100nm) formed in Mg₆₅Cu₂₀Al₅Y₁₀alloy. The average microhardness of Mg₆₅Cu₂₃Al₂Y₁₀ is 285.6Hv, which exceed 59.8Hv than the metallic glass. Compared with the amorphous alloys, the composites show a non-evidentbrittleness.