Synthesis and characterization of copper based bulk amorphous alloys

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Tezin Türü: Yüksek Lisans

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Mühendislik Fakültesi, Metalurji ve Malzeme Mühendisliği Bölümü, Türkiye

Tezin Onay Tarihi: 2013

Tezin Dili: İngilizce

Öğrenci: ERDEM MERMER

Asıl Danışman (Eş Danışmanlı Tezler İçin): Mahmut Vedat Akdeniz

Eş Danışman: Amdulla Mehrabov

Özet:

Copper based bulk metallic glasses (BMG’s) are one of the promising advanced materials due to their unique mechanical properties, high glass forming ability (GFA) and low cost. In this thesis, the aim is to synthesize Cu-Hf based BMG’s and to investigate the crystallization behavior. Moreover, the crystallization kinetics by means of activation energies, critical cooling rate, nanocrystallization and Avrami exponents have been studied. Cu70Hf30 and Cu50Hf50 binary alloys were produced via arc melting with suction casting unit. However, amorphous structure could not be achieved because of the necessity of lower free energy arose from higher entropy. Therefore, aluminum was added to Cu-Hf system to stabilize the melt and to increase GFA which allowed producing amorphous samples.Thermal analysis studies showed that GFA parameters such as Trg, ΔTx are in good agreement with the results reported in the literature. It was observed that Cu49Hf42Al9 ternary alloy solidifies through eutectic reaction at 910 ºC under near equilibrium conditions. However, it was shown that suppression of this eutectic reaction under non-equilibrium cooling conditions lead to the formation of an amorphous phase. The critical cooling rate required for the suppression of the eutectic reaction was determined as 1.25 K/s by isochronal crystallization kinetics. Annealing of these amorphous samples at temperatures around Tx lead to single stage crystallization of CuHf2nanocrystals with an average size of about 20 nm.The average Avrami exponent value was estimated to be 2.3 implying the crystallization process was governed by diffusion controlled three-dimensional growth with decreasing nucleation rate.