Synthesis and character?zat?on of bulk metallic glasses

Co43Fe20Ta5.5B31.5 is one of the bulk metallic glass alloys which has the highest fracture strength (5185 MPa) that has been reported so far. However, this alloy has limited critical casting thickness up to 2 mm. In this study it was aimed to enhance glass forming ability of Co43Fe20Ta5.5B31.5 glassy alloy. For this purpose, effects of minor elements addition on glass forming ability (GFA) of Co43-xCuxFe20Ta5.5-z NbzB31.5-ySiy system were investigated. High purity powder components were used in experimental studies. Firstly, master alloys were prepared by ark melting under argon atmosphere. Then, appropriate amounts of master alloys were cast into a wedge and rod shaped copper molds. Casting processes were performed by induction and suction casting methods under argon atmosphere. The wedge shaped samples allowing to investigate each cross section at once were characterized by optical microscope, SEM and XRD analysis. By this way, compositions exhibiting enhanced glass forming ability were determined. Subsequently, these alloys selected were produced in rod form with diameters 2-6 mm by suction casting method. Thus, critical casting thickness of these alloys were determined. Finally, Vickers hardness of all glassy samples were measured under a load of 2.94 N. As a result, induction melting and casting experiments indicate that casting temperature has a large influence on the glass forming ability of the alloys. With the increase of casting temperature, glass forming ability of the alloy is enhanced. On the other hand, it was determined that minor copper additions (0.5-1 %) have favorable effect on the amorphization of the base alloy. In addition, 5% silicon addition reduced melting temperature as well as enhancing the glass forming ability of base alloy up to one and half times. When copper and silicon were added together to the base composition, critical casting thickness of the alloy increased aproximately three times, which is about 6 mm. GFA of Co42Cu1Fe20Ta2,75Nb2.75B26,5Si5 alloy is about 4 mm. As a result, new bulk metallic glass compositions with high critical casting thickness about 4-6 mm were determined. Also, hardness of Co43Fe20Ta5,5B31,5 alloy reported in the literature was 1455 Hv. Although modified compositions had higher glass forming ability, their hardness values (~1200 Hv) were lower than than the base alloy.