مجله مهندسی متالورژی
پیاپی 73 (بهار 1398)

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.

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 µm) the strength of 1279 MPa and elongation of 23% and for specimens with a fine austenite size (33 µm) the strength of 1231 MPa with a elongation of 19%, were obtained.

The creation of γ' phase uniform distribution is very important on the performance of 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 and modifying distribution of γ' 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 at other temperatures and times are discussed in the article.