METALLURGICAL ENGINEERING
Year:2019 | Volume:22 | Issue:1 (73)|Papers Count:7

In this study, the kinetic of Au-based BMG (Au50Cu25. 5Ag7. 5Si17(at%)) at high temperature in supercooled liquid region was investigated. Crystallization kinetics of this amorphous alloy under non-isothermal (continuous heating) and isothermal conditions were investigated by differential scanning calorimetry (DSC). The results show that crystallization in this bulk metallic glass has one stage crystalline precipitation process during continuous heating. It was found that glass transition and crystallization kinetics are the function of continuous heating rate. Under non-isothermal conditions, activation energies corresponding to the characteristic temperatures were estimated by Kissinger equation. The calculated activation energies of glass transition, onset of crystallization and crystallization peak temperature are 246, 183 and 161 kJ/mol respectively. These activation energies revealed that the energy barrier for the glass transformation is higher than that for crystallization. The crystallization mechanism under isothermal condition was investigated by using Johnson– Mehl– Avrami (JMA) equation. The Avrami exponent is mainly in the range of 1 to 1. 6, which indicates heterogeneous nucleation with significant volume at the beginning of the transformation.

Amorphous Ti45Cu35Zr15Sn5(at. %) thin-films metallic glass was prepared by non-reactive magnetron sputteringin pure argon at room temperature. The microstructural evaluation shows that the as-deposited thin films have a glassy structure. In this paper, the kinetics of isochronal and isothermal differential scanning calorimetry of Ti45Cu35Zr15Sn5amorphousalloy was analyzed to understand the mechanism of crystallization. The crystallization process constituted of two exothermic transformations occurring from 723 to 863 K. The activation energy of the crystallizationwas obtained by the Kissinger-kahira-Sunose method. The results show that the activation energy ofthe Ti45Cu35Zr15Sn5film for the first and the second crystallization stepswere370 and 300 kJ/mol, respectively. In the case of isothermal crystallization, the crystallization kinetics was modeled by theJohnson– Mehl– Avrami-kohnogorov (JMAK) equation. The Avrami exponents were calculated to be about 2. The localAvrami exponents indicating that the crystallization mechanism of Ti45Cu35Zr15Sn5 freestandingthin films is three-dimensional growth with either a decreasing nucleation rate.

In the recent years attempts were made in order to develop nanostructured bainitic steels which have shown an outstanding combination of strength and toughness. The austenite grain size has a significant influence on phase transformation and mechanical properties of bainitic steels. The martensite treatment including cold rolling and subsequent annealing of microstructures with high percentage of martensite is one of the most effective thermo-mechanical methods for the grain refinement in steels. The present study aims to investigate the effects of thermomechanical and austempering processes on the microstructure and mechanical properties of the medium carbon high aluminium nanobainitic steel. In this regard, nanostructured bainitic steels with lower carbon content (0. 45 wt%) than conventional supper bainitic steel were cast and hot rolled. To obtain the bainitic structure the specimens were austenitized at 1030 ° C for 20 min and quenched in water. The specimens were subjected to 20% reduction in thickness by cold rolling and subsequent annealing at 600 ° C for 20 min. The specimens were austenitized and transformed isothermally at 340 ° C and finally were quenched in water. The microstructure was characterized by X-ray diffraction (XRD), optical (OM) and field emission scanning electron microscopy (FESEM) as well as tensile test for mechanical properties. The results showed that no significant changes were obtained in properties due to the increase in the rate of bainitic transformation and low changes in the austenite grain size. For specimens with a coarse austenite grain size (47 μ m) the strength of 1279 MPa and elongation of 23% and for specimens with a fine austenite size (33 μ m) the strength of 1231 MPa with a elongation of 19%, were obtained.

In this study recovery of rare earth elements from a NdFeB magnet was investigated. The magnet was firstly demagnetized at 450° C followed by grinding and leaching in 2 M sulfuric acid, wherein the rare earth elements were precipitated as their sodium double sulfate. However, iron and boron as the main impurities remained in the solution. The REEs double sulfate was subsequently treated with oxalic acid to obtain REEs oxalate followed by calcination of the rare-earth oxalates at 950° C for 2 hours to produce rare-earth oxides. After leaching the REEs oxide in 0. 5 M hydrochloric acid, solvent extraction was carried out using D2EHPA as the extractant. Results showed that selective extraction of Dy is possible at pH<0. 5, while the extraction of all four elements could be achieved efficiently in higher pH values. It was observed that the heavier elements were extracted with higher efficiency compared to the light elements for all pH values examined. Under the evaluated conditions, the extraction of Nd, Pr, and Dy at pH=3 was almost completed.

The creation of γ ’ phase uniform distribution is very important on the performanceof superalloys. The addition of secondary solution annealing stage to the conventional heat treatment process of the GTD-111 polycrystalline superalloy which is one of the useful industrial superalloys can lead to improving the microstructure of this alloy by decreasing the amount of eutectic phase and increasing the volume percent andmodifying distribution ofγ ’ phase. In this article, the effects of temperatures of 960, 980 and 1000 ° C and times of 1, 2, 4 and 8 hrs on the microstructure of GTD-111 superalloy are investigated. Microstructural investigations by OM and SEM methods show the secondary solution heat treatment at temperature of 980˚ C for 4 hrs leads to improving the microstructure with respect to decreasing the eutectic phase amount to 4 volume percent and increasing the γ ’ strengthening phase volume percent to 54%. Also, the morphology of γ ’ phase after this heat treatment is cuboidal with unimodal and homogenous distribution in the matrix phase. The microstructures of GTD-111 heat treated atother temperatures and times are discussed in the article.

Precipitation-hardened nickel based superalloys are very difficult to weld in a defect-free manner due to the formation of hot cracks in the weld metal as well as liquation cracks in the heat affected zone. Laser Cladding has been recognized as the one of the most attractive and promising state of the art welding to improve weld quality and repair high cost components such as industrial turbine blades. In this study, IN625 used as the filler was deposited on IN738LC mechanical samples by laser. The microstructure and metallurgical defects such as oxide phases, porosities and cracks were investigated by optical microscope and scanning electron microscope. The low oxide phase and porosity were observed at the deposited layer. Furthermore, the hardness and tensile properties of the base metal and the deposited samples were evaluated at room temperature. The results have shown that the yield stress and tensile strength of the deposited samples campare well with base material (1. 2%, 3. 2%, respectively) while the average values of Hardness and ductility decreased significantly (11. 0%, 33. 3%, respectively).

Titanium aluminum carbide was prepared employing mechanically activated self-propagating high-temperature synthesis process. The formation mechanism of Ti3AlC2 MAX phase using elemental titanium, aluminum and carbon (graphite) powders synthesized via two different preparation methods, wave propagation and thermal explosion synthesis techniques, were investigated. The combustion reaction products were characterized by differential thermal analysis, scanning electron microscopy and X-ray diffraction analysis. Although Ti3AlC2 was recognized as the dominant synthesis product, in both techniques, the formation of TiC was also verified as a byproduct. The MAX phase produced in the tubular furnace (thermal explosion mode) was purer than that synthesized in the reaction chamber (wave propagation mode). The results disclosed that the formation of TiC and TiAl compounds have significant roles on the combustion synthesis of Ti3AlC2 MAX phase.