M. Lütfi Öveçoğlu

Characterization investigations of a melt-spun ternary Al-8Si-5.1Cu (in wt.%) alloy

A ternary Al–8Si–5.1Cu (in wt.%) alloy was rapidly quenched from the melt at cooling rates between 10 6 and 10 7 K/s using the melt-spinning technique. The resulting ribbons were characterized using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and microhardness techniques. On the basis of the Al peak shifts measured in the XRD scans, a solid solubility extension value of 3.83 at.% Si in Al was determined. Whereas SEM investigations showed the presence of dendrites rich in Al, TEM investigations revealed nanosized spherically shaped Si crystals 20–25 nm in size. Both XRD and TEM investigations confirmed the absence of any intermetallic phase formation. The microhardness value of the melt-spun alloy was measured as 201 kg/mm 2 .

Synthesis of niobium borides by powder metallurgy methods using Nb2O5, B2O3 and Mg blends

Niobium boride powders having NbB, NbB2 and Nb3B4 phases in various amounts and single phase NbB powders were successfully synthesized by using powder metallurgy methods from related metal oxide raw materials in the presence of a strong reducing agent. Nb2O5, B2O3 and Mg powder blends were milled at room temperature by a high-energy ball mill for different time. Subsequently, undesired MgO phase was removed from the milled powders by HCl leaching to constitute NbB–NbB2–Nb3B4 as final products and they were subjected to an annealing process at 1500 °C for 4 h to observe probable boride transformation. Characterization was carried out by XRD, DSC, PSA, SEM/EDX, TEM and VSM. The effects of milling time (up to 9 h) on the formation, microstructure and thermal behavior of the final products were investigated. Reduction reaction took place after milling stoichiometric powder blends for 2 h. Nano-sized NbB–NbB2–Nb3B4 powders in high purity were obtained in the absence of any secondary phase and any impurity via mechanochemistry by milling for 5 h and leaching with 4 mol/L HCl. After annealing, pure and nano-sized NbB–NbB2–Nb3B4 powders transformed to a single NbB phase without leaving behind NbB2 and Nb3B4 phases.

Effects of La2O3 Addition on the Microstructure and Properties of Activated Sintered W-Ni Compacts

In order to improve the properties of Ni activated sintered W compacts, La2O3 was added to a W-1 wt.% Ni matrix alloy. W-1 wt.% Ni-0.5 wt.% La2O3 and W-1 wt.% Ni-1 wt.% La2O3 composites were fabricated by mechanical alloying and activated sintering methods. The effects of La2O3 content and mechanical alloying duration on the microstructural and physical properties of activated sintered W-Ni compacts were investigated. The results showed that La2O3 particles have a significant effect on the density/microhardness values and wear amounts of the samples. The relative density value of 96.39 % and microhardness value of 4.08±0.28 GPa of W-1 wt.% Ni samples increased to respectively 98.09 % and 5.45±0.29 GPa with the addition of 1 wt.% La2O3. Wear rate of 3.26±0.81 (mm3N-1m-1)x10-9 of the W-1 wt% Ni samples decreased to 2.10±0.24 (mm3N-1m-1)x10-9 with the addition of 0.5 wt.% La2O3. Furthermore, grain sizes decreased and microhardness values increased with increasing mechanical alloying duration.

Investigation of Mechanochemical Synthesized Tungsten Silicides from WO3, SiO2 and Mg Blends

In this study, tungsten silicide powders mechanochemically synthesized using WO3-SiO2-Mg powder blends. Stoichiometric proportions and excess amounts of initial powders were used to indicate the effects of final composition of synthesized tungsten silicide powders. Since the initial powder compositions affect the reaction times, all compositions were mechanically alloyed for 1 hour. In addition, thermodynamic calculations of all compositions were theoretically conducted. The dominant phases are WSi2 and MgO for all mechanically alloyed powders. Results show that the excess amount additions of initial powders directly effects the amount and formation of resultant phases in the synthesized powder compositions.

Characterization investigations of the mechanically alloyed and sequentially milled Al-12.6 wt.% Si eutectic alloy powders

Abstract This study reports on the detailed microstructural characterization of Al-12.6 wt.% Si eutectic alloy powders synthesized from Al and Si elemental starting materials via mechanical alloying (MA) for 4, 6, and 8 h, and subsequent cryomilling (CM). Mechanically alloyed (MA’d) powders were cryomilled (CM’d) for 10, 20, and 30 min in a freezer/mill using liquid N2 gas circulated externally around the polycarbonate milling vial containing stainless steel rods. SEM/EDS and TEM/EDS mapping analyses revealed the presence of some Si embedded in the Al-rich particles and some existed as free Si. As expected, with increasing MA and sequential milling (MA + CM) duration, strain values increased and crystallite sizes decreased. Si solubility in α-Al was enhanced with increasing MA durations and utilizing MA and CM consecutively (sequential milling) as indicated by DSC endothermic and XRD peak shifts. A maximum solubility of 2.25 at.% Si in α-Al or Si solid solubility extension of 0.65 at.% was estimated for the Al12.6Si eutectic alloy powders MA’d for 8 h.

Influence of the milling process on TiB2 particle reinforced Al-7 wt.-% Si matrix composites

Abstract Al-Si metal matrix composites have generally been manufactured using casting methods. Powder metallurgy has been used as an alternative manufacturing technique to obtain more homogeneous and segregation-free products. In this study, 2 wt.-% TiB2 particle reinforced Al-7 wt.-% Si composites were manufactured using high energy ball milling, cold pressing (at 450 MPa) and pressureless sintering (at 570 °C for 2 h under Ar flow) techniques. The effects of different milling processes, such as mechanical alloying at room temperature and/or cryomilling in an isolated polycarbonate cylinder soaked in liquid nitrogen or sequential milling, on the Al-7 wt.-% Si-2 wt.-% TiB2 powders and corresponding bulk products were investigated. The microstructural, physical and mechanical properties of the composites sintered from the mechanically alloyed, mechanically alloyed and cryomilled, and sequentially milled powders were significantly improved as compared with those of as-blended ones. The highest density, the highest microhardness and the lowest wear rate were obtained in a composite sintered from mechanically alloyed and cryomilled powders at 92.38 %, 214.14 ± 41.17 HV and 3.8 × 10−3 mm3·N−1 × m−1, respectively.

Synthesis and Characterization Investigations of Laboratory-Synthesized VB-VB 2 -V 3 B 4 Reinforced Al-7wt.% Si Composites via Mechanical Alloying and Pressureless Sintering

In this study, the effect of mechanical alloying (MA) on the microstructural, mechanical and physical properties of vanadium boride particulate reinforced Al-7 wt. % Si matrix composites were investigated. VB-VB2-V3B4 containing vanadium boride hybrid powders were mechanochemically synthesized for 5 h from the V2O5-B2O3-Mg powder blends and leached with hydrochloric acid (HCl) for purification. Laboratory-synthesized VB-VB2-V3B4 powders were incorporated into the Al-7wt. % Si matrix powders with the amount of 2 wt.% via MA for 4h in a SpexTM Mixer/Mill using hardened steel vial/balls with a ball-to-powder weight ratio of 7/1. After the MA process, phase analysis (X-ray diffraction), particle size analysis (laser particle size measurement), surface area analysis (Brunauer-Emmett-Teller measurement) and microstructural characterization (scanning electron microscope (SEM) micrograph) and thermal analysis (differential scanning calorimetry (DSC)) of the non-milled/milled Al-7 wt.% Si-2wt.% (VB-VB2-V3B4) powders were conducted. As-blended and MA’d powders were compacted at a uniaxial hydraulic press to obtain cylindrical compacts with a diameter of 12 mm under a pressure of 400 MPa. Green bodies were sintered at 570°C for 2 h under Ar gas flowing conditions. Microstructural characterizations of the sintered samples were carried out using XRD and optical microscope (OM). Physical and mechanical properties of the composites were investigated in terms of density (Archimedes method), Vickers microhardness and wear rate. The microhardness and wear rate of the 4h of MA’d and sintered sample respectively increased to 0.865±0.256 GPa and 0.0036 mm3/N.m as compared with those of as-blended and sintered sample.

Influence of Milling Media on the Mechanical Alloyed W-0.5 wt. Ti Powder Alloy

The effects of milling atmosphere and mechanical alloying (MA) duration on the effective lattice parameter, crystallite size, lattice strain, and amorphization rate of the W-0.5 wt. Ti powders were investigated. W-0.5 wt. Ti powders were mechanically alloyed (MA’d) for 10 h and 20 h in a high energy ball mill. Moreover, morphology of the powders for various MA was analyzed using SEM microscopy. Their powder density was also measured by helium pycnometer. The dry milled agglomerated powders have spherical particle, while wet milled powders have layered morphology. Milling media and increasing of milling time significantly reduce the crystallite size. The smallest crystallite size is 4.93 nm which belonged to the dry milled powders measured by Lorentzian method after 20 hours’ MA. However, after 20 hours, MA’d powders show the biggest crystallite size, as big as 57.07 nm, measured with the same method in ethanol.

Fabrication of TiC and ZrC reinforced Al-4 wt%Cu composite droplets using impulse atomization:

In this study, Al-4 wt%Cu alloy and its composites reinforced with different initial particle sizes of ZrC (8 µm and 157 µm) and TiC (13 µm and 93 µm) were fabricated by impulse atomization, and the microstructure of composite and unreinforced alloy droplets were examined by scanning electron microscopy, x-ray diffraction and cell size analyses. Scanning electron microscopic observations of composite droplets indicated that fine ZrC particles (8 µm) were mostly clustered while the other composites of the present investigation were distributed rather uniformly within the fine microstructure of droplets. The relationship between the cell size and the amount of carbide particles reveals that the presence of carbide particles in the alloy powders yields a finer structure. This could be due to the role of the carbide particles as heterogeneous sites for nucleation and to some extent due to particle restricted growth of matrix cells during solidification.