Authors
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran
Abstract
Mechanical alloying of an elemental powder mixture with stoichiometric composition of Ti0.4Ni0.5Nb0.1 was conducted using planetary ball milling. The assessment of the formed phases was investigated using semi-empirical Miedema Model. In addition, the milled product was characterized using various techniques; X-ray diffraction studies revealed the formation of Ni (Ti) solid solution after the first hour of milling. However, dissolution of Nb was not found in neither of the other elements. After 7.5 h of milling, the formation of NiTi-based intermetallic compound begins while Nb is still remained as a free element. Finally, after 10h of milling, an amorphous phase was formed which remained for the next 10h. Field emission scanning electron microscopy investigations showed that a homogeneous structure is formed after 20h of milling, but further milling increases the product size due to agglomeration and cold welding of the relatively finer milling products. Transmission electron microscopy confirmed the presence of both the nano-crystalline NiTi intermetallic compound and amorphous phase. The experimental results were found to be in agreement with those of the thermodynamic calculations based on Miedema Model using Ni-Ti, Ni-Nb, Nb-Ti binary systems and Ni-Ti-Nb ternary system that predicted the formation of Ni (Ti) solid solution and immiscibility of Nb in Ti and Ni. In addition, a slight difference in the value of Gibss free energy of formation was found between the intermetallic compound and amorphous phase that confirms the their presence in the final milled product.
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