Influence Of C Addition On The Glass Forming Of Amorphous Al-based Alloys

In this dissertation, Al₈₈Ni₆Y₆+x wt.%graphite/graphene（x=0,0.2,0.5and0.8）amorphous matrix composites have been successfully synthesized by mechanical alloying（MA）. The influences of simple substance carbon with different structure on the phasecomposition, microstructure, thermal stability and glass forming ability （GFA） for theas-milled powders have been investigated by a combination of X-ray diffractometry （XRD）,differential scanning calorimetry （DSC）, scanning electron microscope （SEM） and highresolution transmission electron microscopy （HRTEM）. A primary study of the amorphouspowders has been sintered by using spark plasma sintering （SPS） at different temperatures.The relative density, phase composition, microstructure, thermodynamic and microhardnessof the sintered samples are studied, indicating the influence of element C additions andsintering temperature on the mechanical properties of the bulk samples.The results show that the addition of graphite and graphene can significantly improve thethermal stability and GFA. The graphite additions and the increase of milling time cansignificantly change the crystallization behavior. Adding0.5and0.8wt.%graphite isbeneficial to shorten the amorphous process, while0.2wt.%graphite addition would prolongthe formation time of amorphous phases. The graphite addition can change the particlemicrostructure with clastic morphology, which can profit the refinement of powder paritcles,however, which can also cause the agglomerate phenomena. Graphite additions can greatlyenhance the values of the glass transition temperature （T_g）, onset crystallization temperature(T_x1), crystallization peak temperature （T_p） and supercooled liquid region （ΔT_x）. Propergraphite additions can increase the activation energy of crystallization Ea(when x=0.8after200h of MA time, E_x1and E_p1are500.6and489.4KJ/mol, respectively). With increasing themilling time, the values of Eabecome generally decreased. Moreover, graphite additions canreduce the crystallization rate constant （νT）, which can increase the GFA of the alloy. But thethermal stability and GFA of the alloy systems are not change in the same way, implying thedifferent influence mechanisms.The additions of graphene can change the crystallization behavior obviously, but theeffect of longer milling time on the crystallization behavior becomes weakened. Adding0.2 and0.5wt.%graphene has no significant effect on shortening the formation time ofamorphous phases, while0.8wt.%grapheme addition will prolong the amorphous process.The grapheme additions can change the particle microstructure with lamellar morphologies.Proper quality fractions can profit the refinement of microstructure, but excess additions canincrease the particle size and also cause the agglomerate phenomena. Moreover, grapheneadditions can greatly enhance the T_g, T_x1, T_pand ΔT_xvalues. With prolonging the milling time,the enhanced trend of characteristic thermodynamic parameters is not clear. Moreover, there isa negative correlation between the characteristic thermodynamic parameters and the enthalpyof the first exothermic peak. Proper graphene additions can increase the Eavalues (whenx=0.5, milled for200h, E_x1、E_p1are672.2KJ/mol、560.1KJ/mol, respectively). Withincreasing the milling time, the values Eabecome generally decreases. Besides, grapheneadditions can also reduce the νTvalue and broaden the region of supercooled liquid （whenx=0.8after200h of MA，ΔT_xis362K）, which can increase the GFA of the alloy.The results of Al₈₈Ni₆Y₆+0.8wt.%graphite/graphene sintered samples with differentsintering temperatures are shown as follow. All sintered samples are fully crystallized, whichimplys a failure of fabricating bulk amorphous matrix composite. The various trends ofrelative density and microhardness are similar. The sintered samples with graphite orgrapheme additions have a higher relative density and microhardness with increasing thesintering temperature. The bulk sample exhabits the best mechanical properties by0.8wt.%graphene addition and heating to1200℃. The density and Vickers hardness are4.24g/cm³and375HV, respectively. While with same sintering temperature, the sintered sample withgraphite additions shows the higher density and microhardness than that of the grapheneaddtions. The SEM results indicate that the compact degrees of particles are relatively low.The bonding strength between neighboring particles is mainly depends on the mechanicalforce come from outside. The lower Vickers hardness can be ascribed to the weakercombination among particles. The sintering parameters need further be explored andoptimized.