Controlling Theory Of Bulk Amorphous Forming Process And Preparation Of Al-based Bulk Amorphous With Large Size

To develop amorphous structure bars with strong amorphous forming ability is a research hot in field of Al-based amorphous alloys. The Al-based amorphous alloys have a higher specific strength, coupled with good toughness. The mechanical properties can be further improved for the composites with a part of nano-sized crystalline phase. One of the main factors restricting the application of Al-based amorphous alloys is poor glass-forming ability. The Al-based bulk amorphous bars with a dimeter more than 1 mm has not yet developed so far.In order to promote the development of Al-based amorphous structural materials, increasing the size of bulk amorphous is a problem that researchers must face. Now, there are two main ways being used to raise the size of bulk amorphous. The first is by changing the composition of the alloys to increase the amorphous forming ability; The second is by improving the preparation technique to increase the size of bulk amorphous. In the present work, the aim is to develop a new equipment and to prepare bulk Al-based amorphous bar with a dimeter of 1.5 mm using the new developed equipment in the case of maintaining constant composition.The following results are obtained.Based on the theory of heat conduction cooled melt, the relationship between the critical cooling rate Rc and the critical diameter and heat exchange coefficient h is derived, and the influence of the casting speed and air gas thickness on the heat exchange coefficient is also deduced, as shown in the following equations.According to the above equations, the Rc of 2.6叁103 K/s is calculated for Al86Ni6Y4.5Co2La1.5 alloy. Combined with practical requirement, namely the D of 1.5 mm, the h of 8 W/(m2·℃) is also calculated. A new chilling die-casting equipment is designed according to the factors influencing h and developed on the basis of selecting various specific components.To verify the usefulness of the device, the bars are prepared with the newly developed equipment and its structure is studied. The main results obtained are as followings.(1) An optimum casting temperature is 900℃ for the amorphous alloy cast using a die with a diameter of 1.5 mm as temperatures are 850,900,950 and 1000℃, respectively.(2) The amorphous is best formed at the casting temperature of 900℃ and speed of300 mm/s using a die with a diameter of 1.5 mm as casting speeds are 150,200,250 and 300℃, respectively.(3) The amorphous forming ability decreases as the diameter of casting mold increases from 1.5 mm to 2 and 3 mm.(4) With the increase of air gap thickness between the mold and the molten alloy, the effect of the preparation of the amorphous alloy bars become less.(5) A full amorphous bar with a diameter of 1.5 mm is obtained at the casting temperature of 900℃ and the casting speed of 300 mm/s.