Synthesis and characterization of chromium, silicon and niobium containing cobalt-iron based bulk metallic glasses

With the advancement in metallic glasses, bulk metallic glasses have found larger use in technological field. Metallic glasses do not possess definitive crystalline structures and said to have short range atomic order and require higher cooling rates in order to form their amorphous structures. Although early metallic glasses required higher cooling rates in the range 10-4-10-6 K/s, today, bulk metallic glasses can be obtained at lower cooling rates (0,1-100 K/s) due to their production methods and high number of alloying additions. Bulk metallic glasses have the advantages of superior corrosion and mechanical behaviour. In addition, bulk metallic glasses gives higher weight/strength ratio and therefore offer an advantage of lower weight factor in design depending on their compositional variation.In this study, it is the aim of this work that Co-Fe-Ta-B based metallic glasses, which shows superior mechanical strength, developed by A. Inoue, were attempted to produce with alloying elements (Cr, Si and Nb), higher its glass transition temperature, improve its glass forming ability and manufacturing property with the thicker cross section following the synthesis of bulk metallic glasses were attempted.In order to define bulk metallic glasses obtained through the route suggested above various analytical techniques were used to analyse its properties. Glass transition temperatures are characterized by DSC (Differential Scanning Calorimetry) and XRD technique were used to check its amorphous content and SEM (Scanning Electron Microscopy) and Optical microscope were used to characterize its microstructure and amorphous layer.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 Cr additions (at. 2%) have favorable effect on the amorphization of the base alloy. In addition, 5% silicon addition reduced melting temperature as well as enhancing the critical casting diameter of base alloy up to one and half times. When Cr and Si were added together to the base composition, critical casting thickness of the alloy increased aproximately two times, which Co41Cr2Fe20Ta2,75Nb2,75B26,5Si5 alloy is about 4 mm. As a result, new bulk metallic glass compositions with high critical casting thickness about 4 mm were determined. Also, hardness of Co43Fe20Ta5,5B31,5 alloy reported in theliterature was 1455 HV. Although modified compositions had higher glass forming ability, their hardness values (~1200 HV) were lower than than the base alloy.