Bulk amorphous/nanocrystalline materials: structural amorphous steels /

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2014
Yağmurlu, Bengi
Although conventional steels with crystalline structures have been extensively utilized by industries, bulk amorphous steels (BASs) show great potential to supersede these crystalline steels for some critical structural and functional applications because of their unusual combinations of engineering properties: these include higher strength and hardness, better wear and corrosion resistance. Moreover, compared with most other bulk amorphous alloy systems such as Zr- and Pd-based bulk metallic glasses, BASs offer some important advantages: much lower material cost, higher strength, better corrosion resistance, and higher thermal stability (the glass transition temperatures are close to or above 800 K). However, like all metallic glasses, usage of high purity constituent elements and advanced production methods, made commercial usage of BASs difficult due to production cost. Moreover, it is necessary to improve the glass forming abilities (GFA) of Fe-based alloys in order to enhance their ability to form bulk glassy steels under conventional industrial conditions. In this study, BASs design and production was successfully achieved from commercially available scrap cast irons by conventional centrifugal casting machine. For the first time, BAS production was succeeded from scrap precursors. Although the glassy phases of the initially produced base alloy had extremely wide supercooled liquid region (~120K), Tg (816K) needed to be improved for higher thermal stability. To destabilize the carbides formed in base alloy and to enhance the thermal properties of the BAS samples, different alloying additions with same amount of Mo substitutions were applied and Mn was determined as the best among the others. In the second part of the study, the Mn was content tried to be optimized by microalloying technique. As Mn content increased up to 7at.% in composition, improved GFA was obtained from samples; however, after that point samples showed a decrease in GFA. The samples with 7at.% Mn showed high Tg (863K) with relatively low ΔTx (25K), high glass forming ability with necessity of low critical cooling rate to suppress nucleation, extreme hardness near 1200HV and superior corrosion resistance in extreme environments.