Study On The Preparation And Crystallization Of Mg-based Bulk Amorphous Alloys With Low T_g

Mg-based amorphous alloys own excellent glass-forming ability (GFA). Mg₆₅ Cu₂₅ Y₁₀ alloy has the largest supercooled liquid regionΔT_x (=T_x-T_g, where T_x and T are the crystallization temperature and the glass transition temperature, respectively) and the best GFA in Mg-Ln-TM system. In the present paper, Cu and Y in Mg₆₅ Cu₂₅ Y₁₀ alloy have been replaced by Sn and Nd, respectively. Then, Mg-based amorphous alloy ribbons were prepared by single-roll melt spun method in Ar atmosphere, while bulk metallic glasses (BMG) were prepared by copper mold casting method. The glass-forming ability, crystallization behavior, melting behavior and hardness of Mg-based amorphous alloys were studied. And the plasticity of Mg-Cu-Nd-Y amorphous ribbons was tested. Mg₆₅ Cu₂₅ Nd₄Y₆ bulk amorphous alloy was heat-treated with isothermal annealing technique and the effect of different crystallization processes on the microstructure and the ambient-temperature ductility of alloys were studied.From experimental results, it is showed that the addition of Sn affects the GFA of Mg-based amorphous alloys hardly and the microhardness was found to decrease obviously with increasing subsituent of the above alloys for Cu. However, the addition of Nd affects the GFA of Mg-based amorphous alloys in different ways. When the content of Nd is 5%, metallic glass strip with thickness of 4．5 mm can be obtained. But the average microhardness change little. Otherwise, Mg-Cu-Nd-Y amorphous alloy ribbon has certain plastic deformation behavior in the supercooled liquid region, especially the Mg₆₅ Cu₂₅ Nd₇Y₃ amorphous alloy ribbon can obtain the greatest 350% elongation ratio under certain temperature and strain rate.With the different isothermal treatment, the Mg₆₅ Cu₂₅ Nd₄Y₆ BMG transits from the amorphous state to the crystal state, and the sharp crystal peaks superimose on the simple amorphous peak. As the result of the difference of temperature in the furnace, the intensity of crystallization peaks for the top and the bottom are different from each other. The SEM images and X-ray diffraction patterns for Mg₆₅ Cu₂₅ Nd₄Y₆ samples with different crystallization technique have been analysed. It can be seen that some crystalline phases emerge and they have been identified as Mg, Mg₂Cu and some unknown phases. The average microhardness of Mg₆₅ Cu₂₅ Nd₄Y₆ alloys after different crystallization improves obviously compared with the amorphous alloys. For the same crystallization technique, the microhardness of the bottom of amorphous alloy specimen are always larger than the top ones. The alloy is with the largest brittleness after being isothermally treated at 463K for 0．5h.