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Synthesis and structural characterization of bulk and nanoalloys of Ni-B binary systems

Tekekaya, Halim Seçkin
Nanoalloys can exhibit features which are distinct than bulk form of the same alloys. Novel properties of Ni-B magnetic nanoalloys have been used in many application areas such as; biomedicine, catalysis, nano-electronics, targeted drug delivery, data storage, magnetic separation, magnetic recording media, etc. It has been also shown that boron notably affects and increases wear resistance and hardness in nickel based alloys. Nickel based alloys having boron content show a good resistance to abrasion and galling due to boride dispersion in the structure. This study aims to synthesize Ni60B40 alloys in the bulk and nanocrystalline-amorphous forms by means of arc melting and suction casting methods under controlled atmosphere and mechanical alloying technique, respectively and to provide comparable structural characterization of them. Initially, bulk Ni60B40 alloys were produced in equilibrium and nonequilibrium casting conditions and presence of Ni4B3 and Ni2B intermetallic phases have been detected by X-ray diffractometer, SEM-EDS and DSC analysis. On the basis of DSC analysis, the melting temperatures of the Ni4B3 and Ni2B crystalline phases have been founded as 1025 °C and 1142 °C, respectively, which coincide well with data published in the literature. After the completion of structural characterization of Ni60B40 bulk alloy, milling process was performed with Nickel and Boron powders up to 80 hours of milling time. The vials were filled with Argon inert gas before milling in order to prevent oxidation. Initial sub-micron sized particles have been reduced down to a size ranged in between 50-100 nm at the end of ball milling procedure. However, microstructure of Ni60B40 nanoalloy significantly depend on milling time; amorphization of alloy starts at 40 h milling time and Ni3B intermetallic phase starts to form at 60 h and grow with further increasing of milling time. DSC analysis of nanoalloy powders milled for 80 h milling time reveals that the Ni60B40 nanoalloy shows an amorphization behavior and glass transition and crystallization. temperatures have been determined as 388 °C and 502 °C, respectively. Also, on the basis of the particle size, it has been shown that the amount of nano-sized particles increases with further milling times. Magnetic properties of Ni60B40 bulk and nanoalloy powders have been measured by using vibrating-sample magnetometer. Magnetic measurements of Ni60B40 nanoalloys exhibit a general decrease in saturation magnetization and increase in coercivity with increasing milling time.