Research On Manufacturing Of Bulk Mg-based Amorphous Alloys

In this paper,the forming mechanism and affecting favtor of bulk metallic glasses is systematically discussed from the aspects of thermodynamic, kinetic and microstructure . Thermodynamic factors which favor the amorphization of alloys analyzed, the glass forming range of ternary alloys system was proposed by using Miedema theory, Toop model and considering the influence of free energy of pure metals at different temperature. The approach was applied to predict the composition range of amorphous Mg-Cu-La alloys. Several methods(such as three empirical criterions ,binary alloys phase diagram , Miedema theory and Toop mode) were used to design a series of Mg65Cu25La10 ,Mg68Cu17La15 and Mg57Cu23La20 alloys, These bulk amorphous alloys with maximum diameter thickness reaching to 1mm were prepared by means of graphite-molding casting though the raw materials are of a low purity by conventional casting processes at a low cooling rate. The amorphous nature of the samples was identified by X-ray diffraction (XRD) ,and confirmed by differential scanning calorimeter (DSC) performing at a heating rate of 20K/min,comparing with multicomponent Mg-based Mg-Ln-TM bulk metallic glasses.The following results have been obtained: ①Mg-Cu-La bulk amorphous alloys have a higher glass forming ability (GFA). Thermal stability and glass-forming ability of Mg-Cu-La metallic glasses than these of Mg65Cu25Y10,Mg65Cu25Tb10 and Mg65Cu25Gd10 metallic glasses The glass transition temperature (Tg),the onset temperature (Tx) of crystallization and the supercooled liquid region (△Tx=Tx-Tg)are listed as below: ②Rare earth(RE) elements can increased ignition and oxygen resistances of Mg-based BMGs and have high affinity with other impurities such as H, S, and C which also have harmful effect on the GFA and the RE oxide nucleus with markedly different structure compared with that of Laves phase, and then the GFA is improved. ③Mul-ticomponent alloys element which have the appropriate large atomic size and electronegativity difference amomg the three constituent elements may induce highly dense packed microstructure can significantly improve the glass forming ability of bulk glass forming alloys. Consequently, the supercooled liquid can have a higher viscosity and solid/liquid interfacial energies leading to the suppression of nucleation and growth of crystalline phase during the solidification, and thus the GFA is improved. ④Glass transition from melt state to glassy state was controlled by thermodynamic and kinetic factor .But kinetic factor (such as viscosity)play a key role in the formation of the BMGs. ⑤The RE also has larger negative heat of mixing with Mg and Cu components in the alloy which can form more atomic pairs. Consequently, the packing density and viscous of the supercooled liquid are increased and the liquidus temperature is decreased with adequate amount of RE addition. Thus, the stability of supercooled liquid state of the alloys with low atomic diffusivity is enhanced, and then the GFA is improved.