Browsing by Subject "Phase transformation"

Now showing 1 - 4 of 4

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
Influence of diamond tool chamfer angle on surface integrity in ultra-precision turning of singe crystal silicon
(Springer, 2019) Karpat, Yiğit
Ultra precision diamond machining enables the economical production of freeform optics on infrared materials such as silicon. To produce optics with acceptable surface integrity, it is important to have a good understanding of process-work material interaction between diamond tool and brittle and hard single crystal IR materials. Chamfered cutting edges are known to have high strength, which makes them suitable for machining difficult-to-cut materials. This study investigates the influence of chamfer angle on the surface integrity of silicon. Diamond tool chamfer angles of − 20°, − 30°, and − 45° are considered under practical diamond turning conditions of single crystal silicon. State-of-the-art techniques were used to investigate the surface integrity of the machined silicon surfaces. The results show that chamfer angle of 30° yields more favorable results compared to 20° and 45° under the conditions tested. The results indicate the complex interplay between tool geometry and process parameters in reaching an acceptable level of surface integrity. A machinability map indicating ductile and brittle machining conditions for 30° chamfered diamond tool has been presented which includes directly transferable knowledge to the precision machining industry.
Open Access
Phase transformation during mechano-synthesis of nanocrystalline/amorphous Fe–32Mn–6Si alloys
(Elsevier, 2013) Amini, R.; Shamsipoor, A.; Ghaffari, M.; Alizadeh, M.; Okyay, Ali Kemal
Mechano-synthesis of Fe-32Mn-6Si alloy by mechanical alloying of the elemental powder mixtures was evaluated by running the ball milling process under an inert argon gas atmosphere. In order to characterize the as-milled powders, powder sampling was performed at predetermined intervals from 0.5 to 192 h. X-ray florescence analyzer, X-ray diffraction, scanning electron microscope, and high resolution transmission electron microscope were utilized to investigate the chemical composition, structural evolution, morphological changes, and microstructure of the as-milled powders, respectively. According to the results, the nanocrystalline Fe-Mn-Si alloys were completely synthesized after 48 h of milling. Moreover, the formation of a considerable amount of amorphous phase during the milling process was indicated by quantitative X-ray diffraction analysis as well as high resolution transmission electron microscopy image and its selected area diffraction pattern. It was found that the α-to-γ and subsequently the amorphous-to-crystalline (especially martensite) phase transformation occurred by milling development.
Open Access
Surface integrity of monocrystalline silicon nanostructured with engineered multi-tip diamond tools
(Springer, 2022-05) Karpat, Yiğit
The ability to fabricate micro/nanostructures on large surface areas would enhance product performance in optics and solar energy systems, where maintaining high productivity is also critical. Recently, diamond tools structured with nanoscale features have been used to machine ductile materials such as copper and electroless nickel. This study uses engineered diamond tools featuring multi-tip cutting edges to investigate nanoscale grooving of silicon. Multi-tip cutting edges create a certain level of pressure and temperature at the cutting zone, which leads to phase transformations in silicon. Experiments were performed using an ultra-precision machining setup to identify conditions leading to nanoscale ductile-mode machining of silicon. As nanogrooves reach 300 nm depth, hexagonal-Si (Si-IV) phase formation was observed based on laser Raman spectroscopy measurements. Hexagonal allotropes of silicon are known to improve light absorption of silicon. Additional experiments with non-structured diamond tools did not yield any Si-IV phase transformation, indicating the importance of obtaining necessary pressure and temperature conditions at the cutting zone.