Browsing by Subject "Nano-crystalline NiTi"

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Open Access
Effects of milling and annealing on formation and structural characterization of nanocrystalline intermetallic compounds from Ni-Ti elemental powders
(2012) Ghadimi, M.; Shokuhfar, A.; Rostami H.R.; Ghaffari, M.
Nickel and Titanium elemental powders with a nominal composition Ni-50 at.%Ti were mechanical alloyed in a planetary high-energy ball mill in different milling conditions (5, 10, 20, 40 and 60 h). The investigation revealed that increasing milling time leads to a reduction in crystallite size, and after 60 h of milling, the Ti dissolved in Ni lattice and NiTi (B2) phase was obtained. With milling time, morphology of pre-alloyed powders changed from lamella to globular. Annealing of as-milled powders at 1173 K for 900 s led to formation of nanocrystalline NiTi (B19′), grain growth and release of internal strain. The results indicated that this technique is a powerful and high productive process for preparing NiTi intermetallic compound with nanocrystalline structure and appropriate morphology. © 2012 Elsevier B.V. All rights reserved.
Open Access
Formation of B19′, B2, and amorphous phases during mechano-synthesis of nanocrystalline NiTi intermetallics
(Elsevier BV, 2014-02) Amini, R.; Alijani, F.; Ghaffari, M.; Alizadeh, M.; Okyay, Ali Kemal
Ni-50Ti shape memory alloy was synthesized by mechanical alloying of the elemental powders mixture under an argon gas atmosphere. The structural and microstructural properties of the alloyed powders were evaluated by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Moreover, the Vickers microhardness of the powders was estimated at different milling times. According to the results, by milling progression, the amount of the amorphous phase increased considerably and after sufficient milling time (48h), the mechano-crystallization of the amorphous phase into the more stable crystalline phases (i.e. B2 and B19') occurred. It was found that the particles size and microhardness were significantly affected by the formation of the amorphous, B2, and B19' phases. It was also deduced that, by appropriate heating and cooling cycles, the B2 and thermally-induced B19' phases can be created. Furthermore, it was inferred that the formation of undesirable intermetallic phases (particularly NiTi2) during the heating cycle was considerably reduced by milling time evolution. © 2013 Elsevier B.V.